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Sample records for optically driven excitons

  1. Optically programmable excitonic traps

    PubMed Central

    Alloing, Mathieu; Lemaître, Aristide; Galopin, Elisabeth; Dubin, François

    2013-01-01

    With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable trapping potentials for indirect excitons of bilayer heterostructures. Our approach relies on the injection and spatial patterning of charges trapped in a field-effect device. We thereby imprint in-situ and on-demand electrostatic traps into which we optically inject cold and dense ensembles of excitons. This technique creates new opportunities to improve state-of-the-art technologies for the study of collective quantum behavior of excitons and also for the functionalisation of emerging exciton-based opto-electronic circuits. PMID:23546532

  2. Excitons and optical spectra of phosphorene nanoribbons

    NASA Astrophysics Data System (ADS)

    Nourbakhsh, Zahra; Asgari, Reza

    2016-07-01

    On the basis of many-body ab initio calculations, using the single-shot G0W0 method and Bethe-Salpeter equation, we study phosphorene nanoribbons (PNRs) in the two typical zigzag and armchair directions. The electronic structure, optical absorption, electron-hole (exciton) binding energy, exciton exchange splitting, and exciton wave functions are calculated for different sizes of PNRs. The typically strong splitting between singlet and triplet excitonic states make PNRs favorable systems for optoelectronic applications. Quantum confinement occurs in both kinds of PNRs, and it is stronger in the zPNRs, which behave like quasi-zero-dimensional systems. Scaling laws are investigated for the size-dependent behaviors of PNRs. The first bright excitonic state in PNRs is explored in detail.

  3. Excitons in the optical properties of nanotubes

    NASA Astrophysics Data System (ADS)

    Spataru, Catalin

    2006-03-01

    We present ab initio calculation of self-energy and electron-hole interaction (excitonic) effects on the optical spectra of single-walled carbon and BN nanotubes. We employed a many-electron Green's function approach that determines both the quasiparticle and optical excitations from first principles. We found important many-electron effects that explain many of the puzzling experimental findings in the optical spectrum of these quasi-one dimensional systems, and the calculated spectra are in excellent quantitative agreement with measurements. In carbon nanotubes, excitons can bind by as much as one eV in semiconducting nanotubes^a). We discovered that bound excitons also exist in metallic carbon nanotubes with binding energy of many tens of meVs^a). Excitonic effects are shown to be even more inportant in BN nanotubes than in carbon nanotubes. Unlike the carbon nanotubes, theory predicts that excitons in some BN nanotubes are comprised of coherent superposition of transitions from several different subband pairs^b). We have also calculated the radiative lifetime of excitons in semiconducting carbon nanotubes. Assuming a thermal occupation of bright and dark exciton bands, we find an effective radiative lifetime of the order of 10 ns at room temperature, in good accord with recent experiments^c). a) C.D. Spataru, S. Ismail-Beigi, L.X. Benedict and S.G. Louie, Phys. Rev. Lett. 92, 077402 (2004). b) C.-H. Park, C.D. Spataru and S.G. Louie, to be published. c) C.D. Spataru, S. Ismail-Beigi, R.B. Capaz and S.G. Louie, in press Phys. Rev. Lett.

  4. Electro-optical properties of Rydberg excitons

    NASA Astrophysics Data System (ADS)

    Zielińska-Raczyńska, Sylwia; Ziemkiewicz, David; Czajkowski, Gerard

    2016-07-01

    We show how to compute the electro-optical functions (absorption, reflection, and transmission) when Rydberg exciton-polaritons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. With the use of the real density matrix approach, numerical calculations applied for the Cu2O crystal are performed. We also examine in detail and explain the dependence of the resonance displacement on the state number and applied electric field strength. We report a fairly good agreement with recently published experimental data.

  5. Exciton Absorption in Semiconductor Quantum Wells Driven by a Strong Intersubband Pump Field

    NASA Technical Reports Server (NTRS)

    Liu, Ansheng; Ning, Cun-Zheng

    1999-01-01

    Optical interband excitonic absorption of semiconductor quantum wells (QW's) driven by a coherent pump field is investigated based on semiconductor Bloch equations. The pump field has a photon energy close to the intersubband spacing between the first two conduction subbands in the QW's. An external weak optical field probes the interband transition. The excitonic effects and pump-induced population redistribution within the conduction subbands in the QW system are included. When the density of the electron-hole pairs in the QW structure is low, the pump field induces an Autler-Townes splitting of the exciton absorption spectrum. The split size and the peak positions of the absorption doublet depend not only on the pump frequency and intensity but also on the carrier density. As the density of the electron-hole pairs is increased, the split contrast (the ratio between the maximum and minimum values) is decreased because the exciton effect is suppressed at higher densities due to the many-body screening.

  6. A measurable force driven by an excitonic condensate

    SciTech Connect

    Hakioğlu, T.; Özgün, Ege; Günay, Mehmet

    2014-04-21

    Free energy signatures related to the measurement of an emergent force (≈10{sup −9}N) due to the exciton condensate (EC) in Double Quantum Wells are predicted and experiments are proposed to measure the effects. The EC-force is attractive and reminiscent of the Casimir force between two perfect metallic plates, but also distinctively different from it by its driving mechanism and dependence on the parameters of the condensate. The proposed experiments are based on a recent experimental work on a driven micromechanical oscillator. Conclusive observations of EC in recent experiments also provide a strong promise for the observation of the EC-force.

  7. Optical control of charged exciton states in tungsten disulfide

    SciTech Connect

    Currie, M.; Hanbicki, A. T.; Jonker, B. T.; Kioseoglou, G.

    2015-05-18

    A method is presented for optically preparing WS{sub 2} monolayers to luminescence from only the charged exciton (trion) state–completely suppressing the neutral exciton. When isolating the trion state, we observed changes in the Raman A{sub 1g} intensity and an enhanced feature on the low energy side of the E{sup 1}{sub 2g} peak. Photoluminescence and optical reflectivity measurements confirm the existence of the prepared trion state. This technique also prepares intermediate regimes with controlled luminescence amplitudes of the neutral and charged exciton. This effect is reversible by exposing the sample to air, indicating the change is mitigated by surface interactions with the ambient environment. This method provides a tool to modify optical emission energy and to isolate physical processes in this and other two-dimensional materials.

  8. Realization of an all optical exciton-polariton router

    SciTech Connect

    Marsault, Félix; Nguyen, Hai Son; Tanese, Dimitrii; Lemaître, Aristide; Galopin, Elisabeth; Sagnes, Isabelle; Amo, Alberto

    2015-11-16

    We report on the experimental realization of an all optical router for exciton-polaritons. This device is based on the design proposed by Flayac and Savenko [Appl. Phys. Lett. 103, 201105 (2013)], in which a zero-dimensional island is connected through tunnel barriers to two periodically modulated wires of different periods. Selective transmission of polaritons injected in the island, into either of the two wires, is achieved by tuning the energy of the island state across the band structure of the modulated wires. We demonstrate routing of ps polariton pulses using an optical control beam which controls the energy of the island quantum states, thanks to polariton-exciton interactions.

  9. Nonlinear, driven-dissipative hydrodynamics and effective chiral description of an exciton-polariton superfluid

    NASA Astrophysics Data System (ADS)

    Kulkarni, Manas; Kolmakov, German

    2015-03-01

    Given recent remarkable experimental success on capturing hydrodynamic features of exciton-polariton condensates in optical microcavities and their potential implications for quantum and optical computing and information technologies, we present an effective chiral description for such systems. This description captures the fingerprints of hydrodynamics, namely, nonlinearity, dispersion and dissipation in the exciton-polariton system. The resulting chiral equation for the condensate perturbation wave dynamics is found to be of Burgers-type thereby providing a more transparent understanding of the complicated underlying coupled exciton-photon dynamics. By using analytical calculations and numerical simulations, we describe the phenomenon of polariton shock waves, solitons and defects in such systems. Our mapping is expected to have broad implications for other polariton and photon systems including dipolar exciton and magnon condensates. This mapping can further help one in engineering a delicate balance between the pump and damping to produce stable optical signals propagating in polariton circuits.

  10. All-optical depletion of dark excitons from a semiconductor quantum dot

    SciTech Connect

    Schmidgall, E. R.; Schwartz, I.; Cogan, D.; Gershoni, D.; Gantz, L.; Heindel, T.; Reitzenstein, S.

    2015-05-11

    Semiconductor quantum dots are considered to be the leading venue for fabricating on-demand sources of single photons. However, the generation of long-lived dark excitons imposes significant limits on the efficiency of these sources. We demonstrate a technique that optically pumps the dark exciton population and converts it to a bright exciton population, using intermediate excited biexciton states. We show experimentally that our method considerably reduces the dark exciton population while doubling the triggered bright exciton emission, approaching thereby near-unit fidelity of quantum dot depletion.

  11. PREFACE: International Conference on Optics of Excitons in Confined Systems

    NASA Astrophysics Data System (ADS)

    Viña, Luis; Tejedor, Carlos; Calleja, José M.

    2010-01-01

    The OECS11 (International Conference on Optics of Excitons in Confined Systems) was the eleventh of a very successful series of conferences that started in 1987 in Rome (Italy). Afterwards the conference was held at Naxos (Sicily, Italy, 1991), Montpellier (France, 1993), Cortona (Italy, 1995), Göttingen (Germany, 1997), Ascona (Switzerland, 1999), Montpellier (France, 2001), Lecce (Italy, 2003), Southampton (UK, 2005) and Patti (Sicily, Italy, 2007). It is addressed to scientists who lead fundamental and applied research on the optical properties of excitons in novel condensed-matter nanostructures. The 2009 meeting (7-11 September 2009) has brought together a large representation of the world leading actors in this domain, with the aim of stimulating the exchange of ideas, promoting international collaborations, and coordinating research on the newest exciton-related issues such as quantum information science and exciton quantum-collective phenomena. The meeting has included invited lectures, contributed oral presentations and posters, covering the following general topics: low-dimensional heterostructures: quantum wells, quantum wires and quantum dots polaritons quantum optics with excitons and polaritons many-body effects under coherent and incoherent excitation coherent optical spectroscopy quantum coherence and quantum-phase manipulation Bose-Einstein condensation and other collective phenomena excitons in novel materials The OECS 11 was held at the campus of the Universidad Autónoma de Madrid in Cantoblanco. The scientific program was composed of more than 200 contributions divided into 16 invited talks, 44 oral contributions and 3 poster sessions with a total of 150 presentations. The scientific level of the presentations was guaranteed by a selection process where each contribution was rated by three members of the Program Committee. The Conference has gathered 238 participants from 21 different countries, with the following distribution: Germany (43

  12. CDW-Exciton Condensate Competition and a Condensate Driven Force

    NASA Astrophysics Data System (ADS)

    Özgün, Ege; Hakioğlu, Tuğrul

    2016-08-01

    We examine the competition between the charge-density wave (CDW) instability and the excitonic condensate (EC) in spatially separated layers of electrons and holes. The CDW and the EC order parameters (OPs), described by two different mechanisms and hence two different transition temperatures TcCDW and TcEC, are self-consistently coupled by a microscopic mean field theory. We discuss the results in our model specifically focusing on the transition-metal dichalcogenides which are considered as the most typical examples of strongly coupled CDW/EC systems with atomic layer separations where the electronic energy scales are large with the critical temperatures in the range TcEC ˜ TcCDW ˜ 100-200 K. An important consequence of this is that the excitonic energy gap, hence the condensed free energy, vary with the layer separation resulting in a new type of force FEC. We discuss the possibility of this force as the possible driver of the structural lattice deformation observed in some TMDCs with a particular attention on the 1T-TiSe2 below 200 K.

  13. Influence of the dark exciton state on the optical and quantum optical properties of single quantum dots.

    PubMed

    Reischle, M; Beirne, G J; Rossbach, R; Jetter, M; Michler, P

    2008-10-01

    The dark exciton state strongly affects the optical and quantum optical properties of flat InP/GaInP quantum dots. The exciton intensity drops sharply compared to the biexciton with rising pulsed laser excitation power while the opposite is true with temperature. Also, the decay rate is faster for the exciton than the biexciton and the dark-to-bright state spin flip is enhanced with temperature. Furthermore, long-lived dark state related memory effects are observed in second-order cross-correlation measurements between the exciton and biexciton and have been simulated using a rate-equation model. PMID:18851549

  14. Influence of the Dark Exciton State on the Optical and Quantum Optical Properties of Single Quantum Dots

    NASA Astrophysics Data System (ADS)

    Reischle, M.; Beirne, G. J.; Roßbach, R.; Jetter, M.; Michler, P.

    2008-10-01

    The dark exciton state strongly affects the optical and quantum optical properties of flat InP/GaInP quantum dots. The exciton intensity drops sharply compared to the biexciton with rising pulsed laser excitation power while the opposite is true with temperature. Also, the decay rate is faster for the exciton than the biexciton and the dark-to-bright state spin flip is enhanced with temperature. Furthermore, long-lived dark state related memory effects are observed in second-order cross-correlation measurements between the exciton and biexciton and have been simulated using a rate-equation model.

  15. Exciton formation assisted by longitudinal optical phonons in monolayer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Thilagam, A.

    2016-09-01

    We examine a mechanism by which excitons are generated via the longitudinal optical (LO) phonon-assisted scattering process after optical excitation of monolayer transition metal dichalcogenides. The exciton formation time is computed as a function of the exciton center-of-mass wavevector, electron and hole temperatures, and carrier densities for known values of the Fröhlich coupling constant, LO phonon energy, lattice temperature, and the exciton binding energy in layered structures. For the monolayer MoS2, we obtain ultrafast exciton formation times on the sub-picosecond time scale at charge densities of 5 × 1011 cm-2 and carrier temperatures less than 300 K, in good agreement with recent experimental findings ( ≈0.3 ps). While excitons are dominantly created at zero center-of-mass wavevectors at low charge carrier temperatures ( ≈30 K), the exciton formation time is most rapid at non-zero wavevectors at higher temperatures ( ≥120 K) of charge carriers. The results show the inverse square-law dependence of the exciton formation times on the carrier density, consistent with a square-law dependence of photoluminescence on the excitation density. Our results show that excitons are formed more rapidly in exemplary monolayer selenide-based dichalcogenides (MoSe2 and WSe2) than sulphide-based dichalcogenides (MoS2 and WS2).

  16. The excitonic insulator route through a dynamical phase transition induced by an optical pulse

    NASA Astrophysics Data System (ADS)

    Brazovskii, S.; Kirova, N.

    2016-03-01

    We consider a dynamical phase transition induced by a short optical pulse in a system prone to thermodynamical instability. We address the case of pumping to excitons whose density contributes directly to the order parameter. To describe both thermodynamic and dynamic effects on equal footing, we adopt a view of the excitonic insulator for the phase transition and suggest a formation of the Bose condensate for the pumped excitons. The work is motivated by experiments in donor-acceptor organic compounds with a neutral- ionic phase transition coupled to the spontaneous lattice dimerization and to charge transfer excitons. The double nature of the ensemble of excitons leads to an intricate time evolution, in particular, to macroscopic quantum oscillations from the interference between the Bose condensate of excitons and the ground state of the excitonic insulator. The coupling of excitons and the order parameter also leads to self-trapping of their wave function, akin to self-focusing in optics. The locally enhanced density of excitons can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains that evolve through dynamical phase transitions and sequences of merging. The new circumstances in experiments and theory bring to life, once again, some remarkable inventions made by L.V. Keldysh.

  17. Optical lattices of excitons in InGaN/GaN quantum well systems

    SciTech Connect

    Chaldyshev, V. V. Bolshakov, A. S. Zavarin, E. E.; Sakharov, A. V.; Lundin, V. V.; Tsatsulnikov, A. F.; Yagovkina, M. A.

    2015-01-15

    Optical lattices of excitons in periodic systems of InGaN quantum wells with GaN barriers are designed, implemented, and investigated. Due to the collective interaction of quasi-two-dimensional excitons with light and a fairly high binding energy of excitons in GaN, optical Bragg reflection at room temperature is significantly enhanced. To increase the resonance optical response of the system, new structures with two quantum wells in a periodic supercell are designed and implemented. Resonance reflection of 40% at room temperatures for structures with 60 periods is demonstrated.

  18. Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes

    SciTech Connect

    Neumann, M. D.; Cobet, C.; Esser, N.; Laumer, B.; Wassner, T. A.; Eickhoff, M.; Feneberg, M.; Goldhahn, R.

    2011-07-01

    The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite Mg{sub x}Zn{sub 1-x}O grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model.

  19. Electrically driven optical metamaterials

    NASA Astrophysics Data System (ADS)

    Le-van, Quynh; Le Roux, Xavier; Aassime, Abdelhanin; Degiron, Aloyse

    2016-06-01

    The advent of metamaterials more than 15 years ago has offered extraordinary new ways of manipulating electromagnetic waves. Yet, progress in this field has been unequal across the electromagnetic spectrum, especially when it comes to finding applications for such artificial media. Optical metamaterials, in particular, are less compatible with active functionalities than their counterparts developed at lower frequencies. One crucial roadblock in the path to devices is the fact that active optical metamaterials are so far controlled by light rather than electricity, preventing them from being integrated in larger electronic systems. Here we introduce electroluminescent metamaterials based on metal nano-inclusions hybridized with colloidal quantum dots. We show that each of these miniature blocks can be individually tuned to exhibit independent optoelectronic properties (both in terms of electrical characteristics, polarization, colour and brightness), illustrate their capabilities by weaving complex light-emitting surfaces and finally discuss their potential for displays and sensors.

  20. Electrically driven optical metamaterials.

    PubMed

    Le-Van, Quynh; Le Roux, Xavier; Aassime, Abdelhanin; Degiron, Aloyse

    2016-06-22

    The advent of metamaterials more than 15 years ago has offered extraordinary new ways of manipulating electromagnetic waves. Yet, progress in this field has been unequal across the electromagnetic spectrum, especially when it comes to finding applications for such artificial media. Optical metamaterials, in particular, are less compatible with active functionalities than their counterparts developed at lower frequencies. One crucial roadblock in the path to devices is the fact that active optical metamaterials are so far controlled by light rather than electricity, preventing them from being integrated in larger electronic systems. Here we introduce electroluminescent metamaterials based on metal nano-inclusions hybridized with colloidal quantum dots. We show that each of these miniature blocks can be individually tuned to exhibit independent optoelectronic properties (both in terms of electrical characteristics, polarization, colour and brightness), illustrate their capabilities by weaving complex light-emitting surfaces and finally discuss their potential for displays and sensors.

  1. Electrically driven optical metamaterials.

    PubMed

    Le-Van, Quynh; Le Roux, Xavier; Aassime, Abdelhanin; Degiron, Aloyse

    2016-01-01

    The advent of metamaterials more than 15 years ago has offered extraordinary new ways of manipulating electromagnetic waves. Yet, progress in this field has been unequal across the electromagnetic spectrum, especially when it comes to finding applications for such artificial media. Optical metamaterials, in particular, are less compatible with active functionalities than their counterparts developed at lower frequencies. One crucial roadblock in the path to devices is the fact that active optical metamaterials are so far controlled by light rather than electricity, preventing them from being integrated in larger electronic systems. Here we introduce electroluminescent metamaterials based on metal nano-inclusions hybridized with colloidal quantum dots. We show that each of these miniature blocks can be individually tuned to exhibit independent optoelectronic properties (both in terms of electrical characteristics, polarization, colour and brightness), illustrate their capabilities by weaving complex light-emitting surfaces and finally discuss their potential for displays and sensors. PMID:27328976

  2. Electrically driven optical metamaterials

    PubMed Central

    Le-Van, Quynh; Le Roux, Xavier; Aassime, Abdelhanin; Degiron, Aloyse

    2016-01-01

    The advent of metamaterials more than 15 years ago has offered extraordinary new ways of manipulating electromagnetic waves. Yet, progress in this field has been unequal across the electromagnetic spectrum, especially when it comes to finding applications for such artificial media. Optical metamaterials, in particular, are less compatible with active functionalities than their counterparts developed at lower frequencies. One crucial roadblock in the path to devices is the fact that active optical metamaterials are so far controlled by light rather than electricity, preventing them from being integrated in larger electronic systems. Here we introduce electroluminescent metamaterials based on metal nano-inclusions hybridized with colloidal quantum dots. We show that each of these miniature blocks can be individually tuned to exhibit independent optoelectronic properties (both in terms of electrical characteristics, polarization, colour and brightness), illustrate their capabilities by weaving complex light-emitting surfaces and finally discuss their potential for displays and sensors. PMID:27328976

  3. Nano-optical imaging of WS e2 waveguide modes revealing light-exciton interactions

    NASA Astrophysics Data System (ADS)

    Fei, Z.; Scott, M. E.; Gosztola, D. J.; Foley, J. J.; Yan, J.; Mandrus, D. G.; Wen, H.; Zhou, P.; Zhang, D. W.; Sun, Y.; Guest, J. R.; Gray, S. K.; Bao, W.; Wiederrecht, G. P.; Xu, X.

    2016-08-01

    We report on a nano-optical imaging study of WS e2 thin flakes with scanning near-field optical microscopy (NSOM). The NSOM technique allows us to visualize in real space various waveguide photon modes inside WS e2 . By tuning the excitation laser energy, we are able to map the entire dispersion of these waveguide modes both above and below the A exciton energy of WS e2 . We found that all the modes interact strongly with WS e2 excitons. The outcome of the interaction is that the observed waveguide modes shift to higher momenta right below the A exciton energy. At higher energies, on the other hand, these modes are strongly damped due to adjacent B excitons or band-edge absorptions. The mode-shifting phenomena are consistent with polariton formation in WS e2 .

  4. Excitonic effects in the optical properties of alkaline earth chalcogenides from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Nejatipour, Hajar; Dadsetani, Mehrdad

    2015-08-01

    This paper studies excitonic effects in the optical properties of alkaline earth chalcogenides (AECs) by solving the equation of motion of the two-particle Green function, the Bethe-Salpeter equation (BSE). On the basis of quasi-particle states obtained by the GW approximation, (BSE + GW), the solution of BSE improves agreement with experiments. In these compounds, the main excitonic structures were reproduced appropriately. In the optical absorption spectra of AECs, the main excitonic structures originate in the direct transitions at X and Γ symmetry points, as confirmed by the experiments. In addition to real and imaginary parts of the dielectric functions, excitonic effects were studied in the electron energy loss functions of AECs. Moreover, the G0W0 approximation was used in order to determine the energy band gaps of AECs. This showed that except for MgO and BaO, the other AECs under study have indirect band gaps from Γ to X.

  5. Nature of the narrow optical band in H*-aggregates: Dozy-chaos-exciton coupling

    NASA Astrophysics Data System (ADS)

    Egorov, Vladimir V.

    2014-07-01

    Dozy chaos emerges as a combined effect of the collective chaotic motion of electrons and nuclei, and their chaotic electromagnetic interactions in the transient state of molecules experiencing quantum transitions. Following earlier discussions of the well-known Brönsted relations for proton-transfer reactions; the temperature-dependent electron transfer in Langmuir-Blodgett films; the shape of the optical bands of polymethine dye monomers, their dimers, and J-aggregates, this paper reports one more application of the dozy-chaos theory of molecular quantum transitions. The qualitative and quantitative explanations for shape of a narrow and blue-shifted optical absorption band in H*-aggregates is given on the basis of the dozy-chaos theory by taking into account the dozy-chaos-exciton coupling effect. It is emphasized that in the H*-aggregate chromophore (dimer of cyclic bis-thiacarbocyanines) there is a competition between two Frenkel exciton transitions through the chaotic reorganization motion of nuclear environment. As a result, the highly organized quantum transition to the upper exciton state becomes an exciton-induced source of dozy chaos for the low organized transition to the lower exciton state. This manifests itself in appearing the narrow peak and broad wing in the optical spectrum pattern of H*-aggregates. A similar enhancement in the H*-effect caused by the strengthening of the exciton coupling in H*-dimers, which could be achieved by synthesizing tertiary and quarternary thiacarbocyanine monomers, is predicted.

  6. Nonlinear optical spectra having characteristics of Fano interferences in coherently coupled lowest exciton biexciton states in semiconductor quantum dots

    SciTech Connect

    Gotoh, Hideki Sanada, Haruki; Yamaguchi, Hiroshi; Sogawa, Tetsuomi

    2014-10-15

    Optical nonlinear effects are examined using a two-color micro-photoluminescence (micro-PL) method in a coherently coupled exciton-biexciton system in a single quantum dot (QD). PL and photoluminescence excitation spectroscopy (PLE) are employed to measure the absorption spectra of the exciton and biexciton states. PLE for Stokes and anti-Stokes PL enables us to clarify the nonlinear optical absorption properties in the lowest exciton and biexciton states. The nonlinear absorption spectra for excitons exhibit asymmetric shapes with peak and dip structures, and provide a distinct contrast to the symmetric dip structures of conventional nonlinear spectra. Theoretical analyses with a density matrix method indicate that the nonlinear spectra are caused not by a simple coherent interaction between the exciton and biexciton states but by coupling effects among exciton, biexciton and continuum states. These results indicate that Fano quantum interference effects appear in exciton-biexciton systems at QDs and offer important insights into their physics.

  7. Influence of the Exciton Dark State on the Optical and Quantum Optical Properties of Single Quantum Dots

    NASA Astrophysics Data System (ADS)

    Reischle, M.; Beirne, G. J.; Roßbach, R.; Jetter, M.; Michler, P.

    2010-01-01

    The optical properties of single InP/GaInP quantum dots are strongly affected by the dark exciton state. The exciton intensity and decay time are strongly reduced at low temperatures. Additionally, memory effects have been observed in second-order autocorrelation and cross-correlation measurements that last over several excitation cycles. This behavior has been simulated using a rate equation model.

  8. In-situ optical transmission electron microscope study of exciton phonon replicas in ZnO nanowires by cathodoluminescence

    SciTech Connect

    Yang, Shize; Tian, Xuezeng; Wang, Lifen; Wei, Jiake; Qi, Kuo; Li, Xiaomin; Xu, Zhi E-mail: xdbai@iphy.ac.cn Wang, Wenlong; Zhao, Jimin; Bai, Xuedong E-mail: xdbai@iphy.ac.cn; Wang, Enge E-mail: xdbai@iphy.ac.cn

    2014-08-18

    The cathodoluminescence spectrum of single zinc oxide (ZnO) nanowires is measured by in-situ optical Transmission Electron Microscope. The coupling between exciton and longitudinal optical phonon is studied. The band edge emission varies for different excitation spots. This effect is attributed to the exciton propagation along the c axis of the nanowire. Contrary to free exciton emission, the phonon replicas are well confined in ZnO nanowire. They travel along the c axis and emit at the end surface. Bending strain increases the relative intensity of second order phonon replicas when excitons travel along the c-axis.

  9. Optically decomposed near-band-edge structure and excitonic transitions in Ga₂S₃.

    PubMed

    Ho, Ching-Hwa; Chen, Hsin-Hung

    2014-08-21

    The band-edge structure and band gap are key parameters for a functional chalcogenide semiconductor to its applications in optoelectronics, nanoelectronics, and photonics devices. Here, we firstly demonstrate the complete study of experimental band-edge structure and excitonic transitions of monoclinic digallium trisulfide (Ga₂S₃) using photoluminescence (PL), thermoreflectance (TR), and optical absorption measurements at low and room temperatures. According to the experimental results of optical measurements, three band-edge transitions of EA = 3.052 eV, EB = 3.240 eV, and EC = 3.328 eV are respectively determined and they are proven to construct the main band-edge structure of Ga₂S₃. Distinctly optical-anisotropic behaviors by orientation- and polarization-dependent TR measurements are, respectively, relevant to distinguish the origins of the EA, EB, and EC transitions. The results indicated that the three band-edge transitions are coming from different origins. Low-temperature PL results show defect emissions, bound-exciton and free-exciton luminescences in the radiation spectra of Ga₂S₃. The below-band-edge transitions are respectively characterized. On the basis of experimental analyses, the optical property of near-band-edge structure and excitonic transitions in the monoclinic Ga₂S₃ crystal is revealed.

  10. Optically decomposed near-band-edge structure and excitonic transitions in Ga2S3

    PubMed Central

    Ho, Ching-Hwa; Chen, Hsin-Hung

    2014-01-01

    The band-edge structure and band gap are key parameters for a functional chalcogenide semiconductor to its applications in optoelectronics, nanoelectronics, and photonics devices. Here, we firstly demonstrate the complete study of experimental band-edge structure and excitonic transitions of monoclinic digallium trisulfide (Ga2S3) using photoluminescence (PL), thermoreflectance (TR), and optical absorption measurements at low and room temperatures. According to the experimental results of optical measurements, three band-edge transitions of EA = 3.052 eV, EB = 3.240 eV, and EC = 3.328 eV are respectively determined and they are proven to construct the main band-edge structure of Ga2S3. Distinctly optical-anisotropic behaviors by orientation- and polarization-dependent TR measurements are, respectively, relevant to distinguish the origins of the EA, EB, and EC transitions. The results indicated that the three band-edge transitions are coming from different origins. Low-temperature PL results show defect emissions, bound-exciton and free-exciton luminescences in the radiation spectra of Ga2S3. The below-band-edge transitions are respectively characterized. On the basis of experimental analyses, the optical property of near-band-edge structure and excitonic transitions in the monoclinic Ga2S3 crystal is revealed. PMID:25142550

  11. Excitonic optical transitions characterized by Raman excitation profiles in single-walled carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Tran, H. N.; Blancon, J.-C.; Huntzinger, J.-R.; Arenal, R.; Popov, V. N.; Zahab, A. A.; Ayari, A.; San-Miguel, A.; Vallée, F.; Del Fatti, N.; Sauvajol, J.-L.; Paillet, M.

    2016-08-01

    We examine the excitonic nature of the E33 optical transition of the individual free-standing index-identified (23 ,7 ) single-walled carbon nanotube by means of the measurements of its radial-breathing-mode and G -mode Raman excitation profiles. We confirm that it is impossible to determine unambiguously the nature of its E33 optical transition (excitonic vs band to band) based only on the excitation profiles. Nevertheless, by combining Raman scattering, Rayleigh scattering, and optical absorption measurements on strictly the same individual (23 ,7 ) single-walled carbon nanotube, we show that the absorption, Rayleigh spectra, and Raman excitation profiles of the longitudinal and transverse G modes are best fitted by considering the nature of the E33 transition as excitonic. The fit of the three sets of data gives close values of the transition energy E33 and damping parameter Γ33. This comparison shows that the fit of the Raman excitation profiles provides with good accuracy the energy and damping parameter of the excitonic optical transitions in single-walled carbon nanotubes.

  12. Nature of the narrow optical band in H*-aggregates: Dozy-chaos–exciton coupling

    SciTech Connect

    Egorov, Vladimir V.

    2014-07-15

    Dozy chaos emerges as a combined effect of the collective chaotic motion of electrons and nuclei, and their chaotic electromagnetic interactions in the transient state of molecules experiencing quantum transitions. Following earlier discussions of the well-known Brönsted relations for proton-transfer reactions; the temperature-dependent electron transfer in Langmuir–Blodgett films; the shape of the optical bands of polymethine dye monomers, their dimers, and J-aggregates, this paper reports one more application of the dozy-chaos theory of molecular quantum transitions. The qualitative and quantitative explanations for shape of a narrow and blue-shifted optical absorption band in H{sup *}-aggregates is given on the basis of the dozy-chaos theory by taking into account the dozy-chaos–exciton coupling effect. It is emphasized that in the H{sup *}-aggregate chromophore (dimer of cyclic bis-thiacarbocyanines) there is a competition between two Frenkel exciton transitions through the chaotic reorganization motion of nuclear environment. As a result, the highly organized quantum transition to the upper exciton state becomes an exciton-induced source of dozy chaos for the low organized transition to the lower exciton state. This manifests itself in appearing the narrow peak and broad wing in the optical spectrum pattern of H{sup *}-aggregates. A similar enhancement in the H{sup *}-effect caused by the strengthening of the exciton coupling in H{sup *}-dimers, which could be achieved by synthesizing tertiary and quarternary thiacarbocyanine monomers, is predicted.

  13. Screening effect on the exciton mediated nonlinear optical susceptibility of semiconductor quantum dots.

    PubMed

    Bautista, Jessica E Q; Lyra, Marcelo L; Lima, R P A

    2014-11-17

    We study the exciton contribution to the third-order optical susceptibility of one-dimensional semiconductor quantum dots and show that the screening of the electron-hole interaction has a strong influence on the nonlinear optical properties in the weak confinement regime. Based on a density matrix formulation, we estimate the spectrum of the third-order optical susceptibility and its contribution to the refraction index and absorption coefficient. In particular, we show that the multipeaked spectrum of the nonlinear susceptibility, which results from the hydrogenoid character of the exciton eigenstates for a purely Coulombian electron-hole coupling, is reverted towards a single peaked structure as the interaction becomes strongly screened, thus leading to a substantial enhancement of the nonlinear optical properties of semiconductor quantum dots.

  14. Photocurrent spectroscopy of exciton and free particle optical transitions in suspended carbon nanotube pn-junctions

    SciTech Connect

    Chang, Shun-Wen; Theiss, Jesse; Hazra, Jubin; Aykol, Mehmet; Kapadia, Rehan; Cronin, Stephen B.

    2015-08-03

    We study photocurrent generation in individual, suspended carbon nanotube pn-junction diodes formed by electrostatic doping using two gate electrodes. Photocurrent spectra collected under various electrostatic doping concentrations reveal distinctive behaviors for free particle optical transitions and excitonic transitions. In particular, the photocurrent generated by excitonic transitions exhibits a strong gate doping dependence, while that of the free particle transitions is gate independent. Here, the built-in potential of the pn-junction is required to separate the strongly bound electron-hole pairs of the excitons, while free particle excitations do not require this field-assisted charge separation. We observe a sharp, well defined E{sub 11} free particle interband transition in contrast with previous photocurrent studies. Several steps are taken to ensure that the active charge separating region of these pn-junctions is suspended off the substrate in a suspended region that is substantially longer than the exciton diffusion length and, therefore, the photocurrent does not originate from a Schottky junction. We present a detailed model of the built-in fields in these pn-junctions, which, together with phonon-assistant exciton dissociation, predicts photocurrents on the same order of those observed experimentally.

  15. Excitons and spin-dependent optical effects in semiconductor quantum dots

    NASA Astrophysics Data System (ADS)

    Govorov, Alexander; Warburton, Richard; Karrai, Khaled

    2004-04-01

    We discuss the possibility of entangling a localized exciton in a quantum dot with delocalized electrons in adjacent continuum states. We present two mechanisms by which this can occur, both involving excitons in InAs self-assembled quantum dots. The first mechanism involves a tunnel hybridization between weakly-confined electron states in the quantum dot and delocalized states in the continuum [1]. By describing the hybridization with the Anderson Hamiltonian we discover theoretically the existence of novel exciton states which can be called Kondo excitons [1]. We predict several optical manifestations of Kondo excitons. The second mechanism corresponds to an intra-dot Auger-like process [2,3]. Experiments in a magnetic field strongly support the picture of Auger-like processes [2]. [1] A.O. Govorov, R. J. Warburton, and K. Karrai, Phys. Rev. B RC, 67, 241307 (2003). [2] R.J. Warburton, B. Urbaszek, E.J. McGhee, C. Schulhauser, A. Hogele, K. Karrai, A.O.Govorov, J.M. Garcia, B.D.Gerardot, and P.M. Petroff, Nature 427, 135 (2004). [3] A. O. Govorov, K. Karrai, R. J. Warburton, and A. V. Kalameitsev, Physica E, 295 (2004).

  16. New nonlinear optical effect: self-reflection phenomenon due to exciton-biexciton-light interaction in semiconductors

    NASA Astrophysics Data System (ADS)

    Khadzhi, P. I.; Lyakhomskaya, K. D.; Nadkin, L. Y.; Markov, D. A.

    2002-05-01

    The characteristic peculiarities of the self-reflection of a strong electromagnetic wave in a system of coherent excitons and biexcitons due to the exciton-photon interaction and optical exciton-biexciton conversion in semiconductors were investigated as one of the manifestations of nonlinear optical Stark-effect. It was found that a monotonously decreasing standing wave with an exponential decreasing spatial tail is formed in the semiconductor. Under the action of the field of a strong pulse, an optically homogeneous medium is converted, into the medium with distributed feedback. The appearance of the spatially separated narrow pears of the reflective index, extinction and reflection coefficients is predicted.

  17. Large polarization-dependent exciton optical Stark effect in lead iodide perovskites.

    PubMed

    Yang, Ye; Yang, Mengjin; Zhu, Kai; Johnson, Justin C; Berry, Joseph J; van de Lagemaat, Jao; Beard, Matthew C

    2016-01-01

    A strong interaction of a semiconductor with a below-bandgap laser pulse causes a blue-shift of the bandgap transition energy, known as the optical Stark effect. The energy shift persists only during the pulse duration with an instantaneous response time. The optical Stark effect has practical relevance for applications, including quantum information processing and communication, and passively mode-locked femtosecond lasers. Here we demonstrate that solution-processable lead-halide perovskites exhibit a large optical Stark effect that is easily resolved at room temperature resulting from the sharp excitonic feature near the bandedge. We also demonstrate that a polarized pump pulse selectively shifts one spin state producing a spin splitting of the degenerate excitonic states. Such selective spin manipulation is an important prerequisite for spintronic applications. Our result implies that such hybrid semiconductors may have great potential for optoelectronic applications beyond photovoltaics. PMID:27577007

  18. Large polarization-dependent exciton optical Stark effect in lead iodide perovskites

    DOE PAGES

    Yang, Ye; Yang, Mengjin; Zhu, Kai; Johnson, Justin C.; Berry, Joseph J.; van de Lagemaat, Jao; Beard, Matthew C.

    2016-08-31

    A strong interaction of a semiconductor with a below-bandgap laser pulse causes a blue-shift of the bandgap transition energy, known as the optical Stark effect. The energy shift persists only during the pulse duration with an instantaneous response time. The optical Stark effect has practical relevance for applications, including quantum information processing and communication, and passively mode-locked femtosecond lasers. Here we demonstrate that solution-processable lead-halide perovskites exhibit a large optical Stark effect that is easily resolved at room temperature resulting from the sharp excitonic feature near the bandedge. We also demonstrate that a polarized pump pulse selectively shifts one spinmore » state producing a spin splitting of the degenerate excitonic states. Such selective spin manipulation is an important prerequisite for spintronic applications. Lastly, our result implies that such hybrid semiconductors may have great potential for optoelectronic applications beyond photovoltaics.« less

  19. Large polarization-dependent exciton optical Stark effect in lead iodide perovskites

    NASA Astrophysics Data System (ADS)

    Yang, Ye; Yang, Mengjin; Zhu, Kai; Johnson, Justin C.; Berry, Joseph J.; van de Lagemaat, Jao; Beard, Matthew C.

    2016-08-01

    A strong interaction of a semiconductor with a below-bandgap laser pulse causes a blue-shift of the bandgap transition energy, known as the optical Stark effect. The energy shift persists only during the pulse duration with an instantaneous response time. The optical Stark effect has practical relevance for applications, including quantum information processing and communication, and passively mode-locked femtosecond lasers. Here we demonstrate that solution-processable lead-halide perovskites exhibit a large optical Stark effect that is easily resolved at room temperature resulting from the sharp excitonic feature near the bandedge. We also demonstrate that a polarized pump pulse selectively shifts one spin state producing a spin splitting of the degenerate excitonic states. Such selective spin manipulation is an important prerequisite for spintronic applications. Our result implies that such hybrid semiconductors may have great potential for optoelectronic applications beyond photovoltaics.

  20. Large polarization-dependent exciton optical Stark effect in lead iodide perovskites

    PubMed Central

    Yang, Ye; Yang, Mengjin; Zhu, Kai; Johnson, Justin C.; Berry, Joseph J.; van de Lagemaat, Jao; Beard, Matthew C.

    2016-01-01

    A strong interaction of a semiconductor with a below-bandgap laser pulse causes a blue-shift of the bandgap transition energy, known as the optical Stark effect. The energy shift persists only during the pulse duration with an instantaneous response time. The optical Stark effect has practical relevance for applications, including quantum information processing and communication, and passively mode-locked femtosecond lasers. Here we demonstrate that solution-processable lead-halide perovskites exhibit a large optical Stark effect that is easily resolved at room temperature resulting from the sharp excitonic feature near the bandedge. We also demonstrate that a polarized pump pulse selectively shifts one spin state producing a spin splitting of the degenerate excitonic states. Such selective spin manipulation is an important prerequisite for spintronic applications. Our result implies that such hybrid semiconductors may have great potential for optoelectronic applications beyond photovoltaics. PMID:27577007

  1. Quasiparticle energies, excitons, and optical spectra of few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Tran, Vy; Fei, Ruixiang; Yang, Li

    2015-12-01

    We report first-principles GW-Bethe-Salpeter-equation (BSE) studies of excited-state properties of few-layer black phosphorus (BP) (phosphorene). With improved GW computational methods, we obtained converged quasiparticle band gaps and optical absorption spectra by the single-shot (G0W0) procedure. Moreover, we reveal fine structures of anisotropic excitons, including the series of one-dimensional like wave functions, spin singlet-triplet splitting, and electron-hole binding energy spectra by solving BSE. An effective-mass model is employed to describe these electron-hole pairs, shedding light on estimating the exciton binding energy of anisotropic two-dimensional semiconductors without expensive ab initio simulations. Finally, the anisotropic optical response of BP is explained by using optical selection rules based on the projected single-particle density of states at band edges.

  2. Phonon coupling in optical transitions for singlet-triplet pairs of bound excitons in semiconductors

    NASA Astrophysics Data System (ADS)

    Pistol, M. E.; Monemar, B.

    1986-05-01

    A model is presented for the observed strong difference in selection rules for coupling of phonons in the one-phonon sideband of optical spectra related to bound excitons in semiconductors. The present treatment is specialized to the case of a closely spaced pair of singlet-triplet character as the lowest electronic states, as is common for bound excitons associated with neutral complexes in materials like GaP and Si. The optical transition for the singlet bound-exciton state is found to couple strongly only to symmetric A1 modes. The triplet state has a similar coupling strength to A1 modes, but in addition strong contributions are found for replicas corresponding to high-density-of-states phonons TAX, LAX, and TOX. This can be explained by a treatment of particle-phonon coupling beyond the ordinary adiabatic approximation. A weak mixing between the singlet and triplet states is mediated by the phonon coupling, as described in first-order perturbation theory. The model derived in this work, for such phonon-induced mixing of closely spaced electronic states, is shown to explain the observed phonon coupling for several bound-exciton systems of singlet-triplet character in GaP. In addition, the observed oscillator strength of the forbidden triplet state may be explained as partly derived from phonon-induced mixing with the singlet state, which has a much larger oscillator strength.

  3. Excitons in a mirror: Formation of “optical bilayers” using MoS{sub 2} monolayers on gold substrates

    SciTech Connect

    Mertens, Jan; Baumberg, Jeremy J.; Shi, Yumeng; Yang, Hui Ying; Molina-Sánchez, Alejandro; Wirtz, Ludger

    2014-05-12

    We report coupling of excitons in monolayers of molybdenum disulphide to their mirror image in an underlying gold substrate. Excitons at the direct band gap are little affected by the substrate whereas strongly bound C-excitons associated with a van-Hove singularity change drastically. On quartz substrates only one C-exciton is visible (in the blue) but on gold substrates a strong red-shifted extra resonance in the green is seen. Exciton coupling to its image leads to formation of a “mirror biexciton” with enhanced binding energy. Estimates of this energy shift in an emitter-gold system match experiments well. The absorption spectrum of MoS{sub 2} on gold thus resembles a bilayer of MoS{sub 2} which has been created by optical coupling. Additional top-mirrors produce an “optical bulk.”.

  4. Acoustically-Driven Trion and Exciton Modulation in Piezoelectric Two-Dimensional MoS2.

    PubMed

    Rezk, Amgad R; Carey, Benjamin; Chrimes, Adam F; Lau, Desmond W M; Gibson, Brant C; Zheng, Changxi; Fuhrer, Michael S; Yeo, Leslie Y; Kalantar-Zadeh, Kourosh

    2016-02-10

    By exploiting the very recent discovery of the piezoelectricity in odd-numbered layers of two-dimensional molybdenum disulfide (MoS2), we show the possibility of reversibly tuning the photoluminescence of single and odd-numbered multilayered MoS2 using high frequency sound wave coupling. We observe a strong quenching in the photoluminescence associated with the dissociation and spatial separation of electrons-holes quasi-particles at low applied acoustic powers. At the same applied powers, we note a relative preference for ionization of trions into excitons. This work also constitutes the first visual presentation of the surface displacement in one-layered MoS2 using laser Doppler vibrometry. Such observations are associated with the acoustically generated electric field arising from the piezoelectric nature of MoS2 for odd-numbered layers. At larger applied powers, the thermal effect dominates the behavior of the two-dimensional flakes. Altogether, the work reveals several key fundamentals governing acousto-optic properties of odd-layered MoS2 that can be implemented in future optical and electronic systems.

  5. Acoustically-Driven Trion and Exciton Modulation in Piezoelectric Two-Dimensional MoS2.

    PubMed

    Rezk, Amgad R; Carey, Benjamin; Chrimes, Adam F; Lau, Desmond W M; Gibson, Brant C; Zheng, Changxi; Fuhrer, Michael S; Yeo, Leslie Y; Kalantar-Zadeh, Kourosh

    2016-02-10

    By exploiting the very recent discovery of the piezoelectricity in odd-numbered layers of two-dimensional molybdenum disulfide (MoS2), we show the possibility of reversibly tuning the photoluminescence of single and odd-numbered multilayered MoS2 using high frequency sound wave coupling. We observe a strong quenching in the photoluminescence associated with the dissociation and spatial separation of electrons-holes quasi-particles at low applied acoustic powers. At the same applied powers, we note a relative preference for ionization of trions into excitons. This work also constitutes the first visual presentation of the surface displacement in one-layered MoS2 using laser Doppler vibrometry. Such observations are associated with the acoustically generated electric field arising from the piezoelectric nature of MoS2 for odd-numbered layers. At larger applied powers, the thermal effect dominates the behavior of the two-dimensional flakes. Altogether, the work reveals several key fundamentals governing acousto-optic properties of odd-layered MoS2 that can be implemented in future optical and electronic systems. PMID:26729449

  6. Electric field effect on optical harmonic generation at the exciton resonances in GaAs

    NASA Astrophysics Data System (ADS)

    Brunne, D.; Lafrentz, M.; Pavlov, V. V.; Pisarev, R. V.; Rodina, A. V.; Yakovlev, D. R.; Bayer, M.

    2015-08-01

    An electric field applied to a semiconductor reduces its crystal symmetry and modifies its electronic structure which is expected to result in changes of the linear and nonlinear response to optical excitation. In GaAs, we observe experimentally strong electric field effects on the optical second (SHG) and third (THG) harmonic generation. The SHG signal for the laser-light k vector parallel to the [001] crystal axis is symmetry forbidden in the electric-dipole approximation, but can be induced by an applied electric field in the vicinity of the 1 s exciton energy. Surprisingly, the THG signal, which is allowed in this geometry, is considerably reduced by the electric field. We develop a theory which provides good agreement with the experimental data. In particular, it shows that the optical nonlinearities for the 1 s exciton resonance are modified in an electric field by the Stark effect, which mixes the 1 s and 2 p exciton states of opposite parity. This mixing acts in opposite way on the SHG and THG processes, as it leads to the appearance of forbidden SHG in (001)-oriented GaAs and decreases the crystallographic THG.

  7. Optical spectroscopy and imaging of the higher energy excitons and bandgap of monolayer MoS2

    NASA Astrophysics Data System (ADS)

    Borys, Nicholas; Bao, Wei; Barnard, Edward; Ko, Changhyun; Tongay, Sefaatin; Wu, Junqiao; Yang, Li; Schuck, P. James

    Monolayer MoS2 (ML-MoS2) exhibits a rich manifold of excitons that dictate optoelectronic performance and functionality. Disentangling these states, which include the quasi-particle bandgap, is critical for developing 2D optoelectronic devices that operate beyond the optical bandgap. Whereas photoluminescence (PL) spectroscopy only probes the lowest-energy radiative state and absorption spectroscopy fails to discriminate energetically degenerate states, photoluminescence excitation (PLE) spectroscopy selectively probes only the excited states that thermalize to the emissive ground state exciton. Using PLE spectroscopy of ML-MoS2, we identify the Rydberg series of the exciton A and exciton B states as well as signatures of the quasi-particle bandgap and coupling between the indirect C exciton and the lowest-energy A exciton, which have eluded previous PLE studies. The assignment of these states is confirmed with density functional theory. Mapping the PLE spectrum reveals spatial variations of the higher-energy exciton manifold and quasi-particle bandgap which mirror the heterogeneity in the PL but also indicate variations in local exciton thermalization processes and chemical potentials.

  8. Tunable many-body interactions in semiconducting graphene: Giant excitonic effect and strong optical absorption

    NASA Astrophysics Data System (ADS)

    Dvorak, Marc; Wu, Zhigang

    2015-07-01

    Electronic and optical properties of graphene depend strongly on many-body interactions. Employing the highly accurate many-body perturbation approach based on Green's functions, we find a large renormalization over independent particle methods of the fundamental band gaps of semiconducting graphene structures with periodic defects. Additionally, their exciton binding energies are larger than 0.4 eV, suggesting significantly strengthened electron-electron and electron-hole interactions. Their absorption spectra show two strong peaks whose positions are sensitive to the defect fraction and distribution. The strong near-edge optical absorption and excellent tunability make these two-dimensional materials promising for optoelectronic applications.

  9. NONLINEAR OPTICAL PHENOMENA: Self-reflection in a system of excitons and biexcitons in semiconductors

    NASA Astrophysics Data System (ADS)

    Khadzhi, P. I.; Lyakhomskaya, K. D.

    1999-10-01

    The characteristic features of the self-reflection of a powerful electromagnetic wave in a system of coherent excitons and biexcitons in semiconductors were investigated as one of the manifestations of the nonlinear optical skin effect. It was found that a monotonically decreasing standing wave with an exponentially falling spatial tail is formed in the surface region of a semiconductor. Under the influence of the field of a powerful pulse, an optically homogeneous medium is converted into one with distributed feedback. The appearance of spatially separated narrow peaks of the refractive index, extinction coefficient, and reflection coefficient is predicted.

  10. Excitonic optical properties of wurtzite ZnS quantum dots under pressure

    SciTech Connect

    Zeng, Zaiping; Garoufalis, Christos S.; Baskoutas, Sotirios; Bester, Gabriel

    2015-03-21

    By means of atomistic empirical pseudopotentials combined with a configuration interaction approach, we have studied the optical properties of wurtzite ZnS quantum dots in the presence of strong quantum confinement effects as a function of pressure. We find the pressure coefficients of quantum dots to be highly size-dependent and reduced by as much as 23% in comparison to the bulk value of 63 meV/GPa obtained from density functional theory calculations. The many-body excitonic effects on the quantum dot pressure coefficients are found to be marginal. The absolute gap deformation potential of quantum dots originates mainly from the energy change of the lowest unoccupied molecular orbital state. Finally, we find that the exciton spin-splitting increases nearly linearly as a function of applied pressure.

  11. Temperature-dependent excitonic effects in the optical properties of single-layer MoS2

    NASA Astrophysics Data System (ADS)

    Molina-Sánchez, Alejandro; Palummo, Maurizia; Marini, Andrea; Wirtz, Ludger

    2016-04-01

    Temperature influences the performance of two-dimensional (2D) materials in optoelectronic devices. Indeed, the optical characterization of these materials is usually realized at room temperature. Nevertheless, most ab initio studies are still performed without including any temperature effect. As a consequence, important features are thus overlooked, such as the relative height of the excitonic peaks and their broadening, directly related to the temperature and to the nonradiative exciton relaxation time. We present ab initio calculations of the optical response of single-layer MoS2, a prototype 2D material, as a function of temperature using density functional theory and many-body perturbation theory. We compute the electron-phonon interaction using the full spinorial wave functions, i.e., fully taking into account the effects of spin-orbit interaction. We find that bound excitons (A and B peaks) and resonant excitons (C peak) exhibit different behavior with temperature, displaying different nonradiative linewidths. We conclude that the inhomogeneous broadening of the absorption spectra is mainly due to electron-phonon scattering mechanisms. Our calculations explain the shortcomings of previous (zero-temperature) theoretical spectra and match well with the experimental spectra acquired at room temperature. Moreover, we disentangle the contributions of acoustic and optical phonon modes to the quasiparticles and exciton linewidths. Our model also allows us to identify which phonon modes couple to each exciton state, which is useful for the interpretation of resonant Raman-scattering experiments.

  12. Electro optical tuning of Tamm-plasmon exciton-polaritons

    SciTech Connect

    Gessler, J.; Baumann, V.; Emmerling, M.; Amthor, M.; Winkler, K.; Schneider, C.; Kamp, M.; Höfling, S.

    2014-11-03

    We report on electro optical tuning of the emission from GaAs quantum wells resonantly coupled to a Tamm-plasmon mode in a hybrid metal/dielectric structure. The structures were studied via momentum resolved photoluminescence and photoreflectance spectroscopy, and the surface metal layer was used as a top gate, which allowed for a precise tuning of the quantum well emission via the quantum confined Stark effect. By tuning the resonance, we were able to observe the characteristic anticrossing behavior of a polaritonic emission in the strong light-matter coupling regime, yielding a Rabi splitting of (9.2 ± 0.2) meV.

  13. Nanofabrication and applications of subwavelength optical probes: Chemical and biological sensors, light sources and exciton probes

    SciTech Connect

    Tan, W.

    1993-01-01

    The author has developed a new and controllable nanofabrication technique, photo-nanofabrication, based on near-field photo-chemical synthesis and nanometer optical sources. Photo-nanofabrication can produce subwavelength light and exciton probes with or without specific chemical or biological sensitivity. By applying near-field optics, the author has successfully demonstrated a new concept of near-field photochemical synthesis, in which the dimension of a product is solely determined by the size of the light source. The most successful application to date is the development of the smallest fiberoptic chemical sensors. Specifically, a thousandfold miniaturization of an immobilized fiberoptic pH sensor has been achieved, leading to at least a millionfold decrease in necessary sample volume and to at least a hundredfold shorter response time. The sensors have high fluorescence intensity and excellent detection limit. New internal calibration methods have also been developed for accurate pH quantification. The newly developed optical sensors have been used in real time measurements of pH on individual, viable, intact rat conceptuses during the period of organogenesis. The sensors can discriminate pH changes of less than 0.1 pH unit in the physiologic pH range. Static determinations of pH in rat conceptuses of varying gestational ages show decreasing pH with conceptal age. Chemical dynamic alterations in pH of intact rat conceptuses, in response to several variations in their environmental conditions, have been measured. Passive and active subwavelength light sources have been constructed with both micropipettes and fiberoptic tips. They have been used as exciton and light sources and in preliminary probe-to-sample distance regulated, Foerster energy transfer studies as well as in studies of the probe-to-sample interfacial Kasha effect. They were also used in supertip development for near-field scanning optical microscopy and for molecular exciton microscopy.

  14. Magneto-optical properties and recombination dynamics of isoelectronic bound excitons in ZnO

    SciTech Connect

    Chen, S. L.; Chen, W. M.; Buyanova, I. A.

    2014-02-21

    Magneto-optical and time-resolved photoluminescence (PL) spectroscopies are employed to evaluate electronic structure of a bound exciton (BX) responsible for the 3.364 eV line (labeled as I{sub 1}{sup *}) in bulk ZnO. From time-resolved PL spectroscopy, I{sub 1}{sup *} is concluded to originate from the exciton ground state. Based on performed magneto-PL studies, the g-factors of the involved electron and hole are determined as being g{sub e} = 1.98 and g{sub h}{sup ∥}(g{sub h}{sup ⊥}) = 1.2(1.62), respectively. These values are nearly identical to the reported g-factors for the I{sup *} line in ZnO (Phys. Rev. B 86, 235205 (2012)), which proves that I{sub 1}{sup *} should have a similar origin as I{sup *} and should arise from an exciton bound to an isoelectronic center with a hole-attractive potential.

  15. Transformation Optics Approach to Plasmon-Exciton Strong Coupling in Nanocavities.

    PubMed

    Li, Rui-Qi; Hernángomez-Pérez, D; García-Vidal, F J; Fernández-Domínguez, A I

    2016-09-01

    We investigate the conditions yielding plasmon-exciton strong coupling at the single emitter level in the gap between two metal nanoparticles. Inspired by transformation optics ideas, a quasianalytical approach is developed that makes possible a thorough exploration of this hybrid system incorporating the full richness of its plasmonic spectrum. This allows us to reveal that by placing the emitter away from the cavity center, its coupling to multipolar dark modes of both even and odd parity increases remarkably. This way, reversible dynamics in the population of the quantum emitter takes place in feasible implementations of this archetypal nanocavity. PMID:27636492

  16. Transformation Optics Approach to Plasmon-Exciton Strong Coupling in Nanocavities

    NASA Astrophysics Data System (ADS)

    Li, Rui-Qi; Hernángomez-Pérez, D.; García-Vidal, F. J.; Fernández-Domínguez, A. I.

    2016-09-01

    We investigate the conditions yielding plasmon-exciton strong coupling at the single emitter level in the gap between two metal nanoparticles. Inspired by transformation optics ideas, a quasianalytical approach is developed that makes possible a thorough exploration of this hybrid system incorporating the full richness of its plasmonic spectrum. This allows us to reveal that by placing the emitter away from the cavity center, its coupling to multipolar dark modes of both even and odd parity increases remarkably. This way, reversible dynamics in the population of the quantum emitter takes place in feasible implementations of this archetypal nanocavity.

  17. Decoherence processes during optical manipulation of excitonic qubits in semiconductor quantum dots

    NASA Astrophysics Data System (ADS)

    Wang, Q. Q.; Muller, A.; Bianucci, P.; Rossi, E.; Xue, Q. K.; Takagahara, T.; Piermarocchi, C.; MacDonald, A. H.; Shih, C. K.

    2005-07-01

    Using photoluminescence spectroscopy, we have investigated the nature of Rabi oscillation damping during optical manipulation of excitonic qubits in self-assembled quantum dots. Rabi oscillations were recorded by varying the pulse amplitude for fixed pulse durations between 4ps and 10ps . Up to five periods are visible, making it possible to quantify the excitation dependent damping. We find that this damping is more pronounced for shorter pulse widths and show that its origin is the nonresonant excitation of carriers in the wetting layer, most likely involving bound-to-continuum and continuum-to-bound transitions.

  18. Excitonic optical nonlinearity induced by internal field screening in (211) oriented strained-layer superlattices

    SciTech Connect

    Sela, I.; Watkins, D.E.; Laurich, B.K.; Smith, D.L. ); Subbanna, S.; Kroemer, H. )

    1991-02-18

    The nonlinear optical properties of a new class of strained-layer superlattices (intrinsic Stark effect superlattices) have been investigated. Specifically, we have compared the nonlinear transmission of Ga{sub 1{minus}{ital x}}In{sub {ital x}}As-GaAs strained-layer superlattices grown along the (211) axis to identical superlattices grown along the (100) axis, and found that the optical nonlinearity in the (211) sample is about one order of magnitude greater than in the (100) sample. A blue shift of the exciton resonance and an increase in the exciton absorption strength in the (211) sample with increasing light intensity was observed (attributed to screening of the intrinsic Stark effect fields by photogenerated carriers), resulting in the stronger optical nonlinearity. The maximum of the nonlinear absorption index, {vert bar}{alpha}{sub 2}{vert bar}, in the (211) sample was 54 cm/W ({vert bar}Im {chi}{sup 3}{vert bar}=0.33 esu) whereas in the (100) sample the maximum of {vert bar}{alpha}{sub 2}{vert bar} was 6.9 cm/W ({vert bar}Im {chi}{sup 3}{vert bar}=0.042 esu). The measured carrier recovery time in both samples was 2 ns.

  19. Dynamic quasi-energy-band modulation and exciton effects in biased superlattices driven by a two-color far-infrared field: Disappearance of dynamic localization

    NASA Astrophysics Data System (ADS)

    Yashima, Kenta; Hino, Ken-Ichi; Toshima, Nobuyuki

    2003-12-01

    A theoretical study of the optical and electronic properties of semiconductor superlattices in ac-dc fields, termed the dynamic Wannier-Stark ladder (DWSL), is done. The biased superlattices are driven by two far-infrared fields with different frequencies and relative phase of δ. Here, the frequency of the first laser is equal to the Bloch frequency ωB of the system under study, while that of the second laser is equal to 2ωB. Quasienergies of the DWSL are calculated based on the Floquet theorem, and the associated linear photoabsorption spectra are evaluated. For δ=0, a gourd-shaped quasi-energy structure characteristic of both dynamic localization (DL) and delocalization (DDL), similar to the usual DWSL driven by a single laser, appears. By changing the ratio of the two laser strengths, however, the width of the quasi-energy band and the locations of both DL and DDL vary noticeably. As for δ≠0, on the other hand, band collapse and the associated DL do not necessarily follow. In fact, DL vanishes and the quasi-energy degeneracy is lifted in a certain range of δ. Just DDL remains over the entire range of the laser strength, eventually resulting in a plateaulike band structure in the linear absorption spectra. The basic physics underlying this phenomenon, which can be readily interpreted in terms of a closed analytical expression, is that all quasi-energies for given crystal momenta are out of phase with each other as a function of laser strength without converging to a single point of energy. This is a feature of this DWSL which sharply distinguishes it from a conventional DWSL generated using a single laser to drive it. Furthermore, an exciton effect is incorporated with the above noninteracting problem, so that exciton dressed states are formed. It is found that this effect gives rise to more involved quasi-energy structures and a more pronounced release of the energy degeneracy of DL, leading again to the formation of a band structure in the absorption

  20. Electronic and optical properties of single excitons and biexcitons in type-II quantum dot nanocrystals

    SciTech Connect

    Koc, Fatih; Sahin, Mehmet E-mail: mehsahin@gmail.com

    2014-05-21

    In this study, a detailed investigation of the electronic and optical properties (i.e., binding energies, absorption wavelength, overlap of the electron-hole wave functions, recombination oscillator strength, etc.) of an exciton and a biexciton in CdTe/CdSe core/shell type-II quantum dot heterostructures has been carried out in the frame of the single band effective mass approximation. In order to determine the electronic properties, we have self-consistently solved the Poisson-Schrödinger equations in the Hartree approximation. We have considered all probable Coulomb interaction effects on both energy levels and also on the corresponding wave functions for both single exciton and biexciton. In addition, we have taken into account the quantum mechanical exchange-correlation effects in the local density approximation between same kinds of particles for biexciton. Also, we have examined the effect of the ligands and dielectric mismatch on the electronic and optical properties. We have used a different approximation proposed by Sahin and Koc [Appl. Phys. Lett. 102, 183103 (2013)] for the recombination oscillator strength of the biexciton for bound and unbound cases. The results obtained have been presented comparatively as a function of the shell thicknesses and probable physical reasons in behind of the results have been discussed in a detail.

  1. Exciton scattering approach for branched conjugated molecules and complexes. IV. Transition dipoles and optical spectra.

    PubMed

    Li, Hao; Malinin, Sergey V; Tretiak, Sergei; Chernyak, Vladimir Y

    2010-03-28

    The electronic excitation energies and transition dipole moments are the essential ingredients to compute an optical spectrum of any molecular system. Here we extend the exciton scattering (ES) approach, originally developed for computing excitation energies in branched conjugated molecules, to the calculation of the transition dipole moments. The ES parameters that characterize contributions of molecular building blocks to the total transition dipole can be extracted from the quantum-chemical calculations of the excited states in simple molecular fragments. Using these extracted parameters, one can then effortlessly calculate the oscillator strengths and optical spectra of various large molecular structures. We illustrate application of this extended ES approach using an example of phenylacetylene-based molecules. Absorption spectra predicted by the ES approach show close agreement with the results of the reference quantum-chemical calculations. PMID:20370110

  2. Band-edge optical transitions in a nonpolar-plane GaN substrate: exciton-phonon coupling and temperature effects

    NASA Astrophysics Data System (ADS)

    Wang, M. Z.; Xu, S. J.

    2016-09-01

    We present a detailed investigation of the band-edge optical transitions involving the interacting exciton-phonon system, especially first-order longitudinal optical (LO) phonon-assisted luminescence of bound and free excitons in m- and c-plane GaN substrates in a low temperature range from 4 K to 40 K. The main luminescence features of all of the three kinds of excitons can be well described by the theoretical models that take exciton-LO-phonon coupling into account. The effective Bohr radii of the excitons play a key role in determining the Huang-Rhys factor characterizing the exciton-LO-phonon coupling strength in GaN. An interesting oscillatory structure is found to appear in the low-temperature luminescence spectra of the nonpolar-plane GaN substrate, which needs to be clarified by further investigations.

  3. Signatures of Quantum Coherences in Rydberg Excitons

    NASA Astrophysics Data System (ADS)

    Grünwald, P.; Aßmann, M.; Heckötter, J.; Fröhlich, D.; Bayer, M.; Stolz, H.; Scheel, S.

    2016-09-01

    Coherent optical control of individual particles has been demonstrated both for atoms and semiconductor quantum dots. Here we demonstrate the emergence of quantum coherent effects in semiconductor Rydberg excitons in bulk Cu2O . Because of the spectral proximity between two adjacent Rydberg exciton states, a single-frequency laser may pump both resonances with little dissipation from the detuning. As a consequence, additional resonances appear in the absorption spectrum that correspond to dressed states consisting of two Rydberg exciton levels coupled to the excitonic vacuum, forming a V -type three-level system, but driven only by one laser light source. We show that the level of pure dephasing in this system is extremely low. These observations are a crucial step towards coherently controlled quantum technologies in a bulk semiconductor.

  4. Ultrafast Optical Studies of Multiple Exciton Generation in Lead Chalcogenide Quantum Dots

    NASA Astrophysics Data System (ADS)

    Midgett, Aaron G.

    2011-12-01

    Providing affordable, clean energy is one of the major challenges facing society today, and one of the promising solutions is third generation solar energy conversion. Present day, first and second-generation solar cells can at most convert each absorbed photon into a single electron hole pair, thereby establishing a theoretical limit to the power conversion efficiency. The process of multiple exciton generation (MEG) in semiconductor quantum dots increases that theoretical efficiency from 33% to 42% by utilizing the excess energy of high energy photons that is otherwise wasted as heat to excite a second electron-hole pair, thereby boosting the potential photocurrent. This thesis explores the benefits of MEG in quantum confined systems and shows that quantum dots are more efficient at generating multiple excitons from a single photon than bulk semiconductors. The variations in optical measurements of MEG have raised skepticism and brought into question the validity of these experiments. The two important questions that this thesis attempts to address are (1) what are the enhanced QYs in isolated PbSe QDs and (2) does quantum confinement enhance MEG over bulk semiconductors. Experimental variations in the enhanced QYs are partially explained by the production of a long-lived photocharged state that increases the apparent photon-to-exciton QYs. A procedure is detailed that decreases the possibility of producing this charged state. By studying the production of these states, conditions are found that minimize their effect and produce less variation in the reported QYs. Variations in the MEG efficiency were studied in films of chemically treated PbSe quantum dots where a different mechanism was responsible for an apparent decrease of the measured QYs. Finally, for the first time, a quantum dot size-dependence in the MEG efficiency was found in colloidal PbSe, PbS, and PbSxSe1-x quantum dot solutions and is attributed to the increased Coulomb interaction in materials

  5. Ultrafast exciton dynamics and light-driven H2 evolution in colloidal semiconductor nanorods and Pt-tipped nanorods.

    PubMed

    Wu, Kaifeng; Zhu, Haiming; Lian, Tianquan

    2015-03-17

    distributions. The effect of the exciton dynamics on photoreduction reactions is illustrated using methyl viologen (MV(2+)) as a model electron acceptor. The steady-state MV(2+) photoreduction quantum yield of CdSe/CdS dot-in-rod NRs approaches unity under rod excitation, much larger than CdSe QDs and CdSe/CdS core/shell QDs. Detailed time-resolved studies show that in quasi-type II CdSe/CdS NRs and type II ZnSe/CdS NRs strong quantum confinement in the radial direction facilitates fast electron transfer and hole removal, whereas the fast carrier mobility along the axial direction enables long distance charge separation and slow charge recombination, which is essential for efficient MV(2+) photoreduction. The NR/MV(2+) relay system can be coupled to Pt nanoparticles in solution for light-driven H2 generation. Alternatively, Pt-tipped CdS and CdSe/CdS NRs provide fully integrated all inorganic systems for light-driven H2 generation. In CdS-Pt and CdSe/CdS-Pt hetero-NRs, ultrafast hole trapping on the CdS rod surface or in CdSe core enables efficient electron transfer from NRs to Pt tips by suppressing hole and energy transfer. It is shown that the quantum yields of photodriven H2 generation using these heterostructures correlate well with measured hole transfer rates from NRs to sacrificial donors, revealing that hole removal is the key efficiency-limiting step. These findings provide important insights for designing more efficient quantum confined NR and NR-Pt based systems for solar-to-fuel conversion. PMID:25682713

  6. Simultaneous monitoring of singlet and triplet exciton variations in solid organic semiconductors driven by an external static magnetic field

    SciTech Connect

    Ding, Baofu Alameh, Kamal

    2014-07-07

    The research field of organic spintronics has remarkably and rapidly become a promising research area for delivering a range of high-performance devices, such as magnetic-field sensors, spin valves, and magnetically modulated organic light emitting devices (OLEDs). Plenty of microscopic physical and chemical models based on exciton or charge interactions have been proposed to explain organic magneto-optoelectronic phenomena. However, the simultaneous observation of singlet- and triplet-exciton variations in an external magnetic field is still unfeasible, preventing a thorough theoretical description of the spin dynamics in organic semiconductors. Here, we show that we can simultaneously observe variations of singlet excitons and triplet excitons in an external magnetic field, by designing an OLED structure employing a singlet-exciton filtering and detection layer in conjunction with a separate triplet-exciton detection layer. This OLED structure enables the observation of a Lorentzian and a non-Lorentzian line-shape magnetoresponse for singlet excitons and triplet excitons, respectively.

  7. Interplay of Cu and oxygen vacancy in optical transitions and screening of excitons in ZnO:Cu films

    SciTech Connect

    Darma, Yudi; Rusydi, Andrivo; Seng Herng, Tun; Marlina, Resti; Fauziah, Resti; Ding, Jun

    2014-02-24

    We study room temperature optics and electronic structures of ZnO:Cu films as a function of Cu concentration using a combination of spectroscopic ellipsometry, photoluminescence, and ultraviolet-visible absorption spectroscopy. Mid-gap optical states, interband transitions, and excitons are observed and distinguishable. We argue that the mid-gap states are originated from interactions of Cu and oxygen vacancy (Vo). They are located below conduction band (Zn4s) and above valence band (O2p) promoting strong green emission and narrowing optical band gap. Excitonic states are screened and its intensities decrease upon Cu doping. Our results show the importance of Cu and Vo driving the electronic structures and optical transitions in ZnO:Cu films.

  8. Optical spectroscopy study of the phase of the reflection coefficient of a single quantum well in the exciton resonance region

    NASA Astrophysics Data System (ADS)

    Malpuech, G.; Kavokin, A.; Leymarie, J.; Disseix, P.; Vasson, A.

    1999-11-01

    Using reflectivity and thermally detected optical absorption spectra of a (In,Ga)As/GaAs single quantum well (QW) sample we have restored the complex amplitude reflection coefficient of the QW in the vicinity of the exciton resonance region. Our method has allowed experimental determination of the phase of the reflection coefficient of a QW. Knowing this phase we were able to distinguish between inhomogeneous and homogeneous contributions to the exciton broadening. A model based on the nonlocal dielectric response theory and assuming the in-plane component of the wave vector of light to be conserved has provided an excellent agreement with the data. The time-resolved reflection spectra of a single QW have been obtained by the numerical Fourier transform of the complex reflection coefficient of the QW. They have shown pronounced oscillations caused by an inhomogeneous broadening of the single exciton resonance.

  9. Interaction Driven Subgap Spin Exciton in the Kondo Insulator SmB6

    SciTech Connect

    Fuhrman, W. T.; Leiner, Jonathan C.; Nikolić, P.; Granroth, Garrett E.; Stone, Matthew B.; Lumsden, Mark D.; DeBeer-Schmitt, Lisa M.; Alekseev, Pavel A.; Mignot, Jean-Michel; Koohpayeh, S. M.; Cottingham, P.; Phelan, William Adam; Schoop, L.; McQueen, T. M.; Broholm, C.

    2015-01-21

    In this paper, using inelastic neutron scattering, we map a 14 meV coherent resonant mode in the topological Kondo insulator SmB6 and describe its relation to the low energy insulating band structure. The resonant intensity is confined to the X and R high symmetry points, repeating outside the first Brillouin zone and dispersing less than 2 meV, with a 5d-like magnetic form factor. We present a slave-boson treatment of the Anderson Hamiltonian with a third neighbor dominated hybridized band structure. This approach produces a spin exciton below the charge gap with features that are consistent with the observed neutron scattering. Finally, we find that maxima in the wave vector dependence of the inelastic neutron scattering indicate band inversion.

  10. Interaction Driven Subgap Spin Exciton in the Kondo Insulator SmB6

    DOE PAGES

    Fuhrman, W. T.; Leiner, Jonathan C.; Nikolić, P.; Granroth, Garrett E.; Stone, Matthew B.; Lumsden, Mark D.; DeBeer-Schmitt, Lisa M.; Alekseev, Pavel A.; Mignot, Jean-Michel; Koohpayeh, S. M.; et al

    2015-01-21

    In this paper, using inelastic neutron scattering, we map a 14 meV coherent resonant mode in the topological Kondo insulator SmB6 and describe its relation to the low energy insulating band structure. The resonant intensity is confined to the X and R high symmetry points, repeating outside the first Brillouin zone and dispersing less than 2 meV, with a 5d-like magnetic form factor. We present a slave-boson treatment of the Anderson Hamiltonian with a third neighbor dominated hybridized band structure. This approach produces a spin exciton below the charge gap with features that are consistent with the observed neutron scattering.more » Finally, we find that maxima in the wave vector dependence of the inelastic neutron scattering indicate band inversion.« less

  11. Coherent dynamics of exciton orbital angular momentum transferred by optical vortex pulses

    NASA Astrophysics Data System (ADS)

    Shigematsu, K.; Yamane, K.; Morita, R.; Toda, Y.

    2016-01-01

    The coherent dynamics of the exciton center-of-mass motion in bulk GaN are studied using degenerate four-wave-mixing (FWM) spectroscopy with Laguerre-Gaussian (LG) mode pulses. We evaluate the exciton orbital angular momentum (OAM) dynamics from the degree of OAM, which is derived from the distributions of OAM (topological charge) of the FWM signals. When excitons are excited with two single-mode LG pulses, the exciton OAM decay time significantly exceeds the exciton dephasing time, which can be attributed to high uniformity of the exciton dephasing in our bulk sample because the decoherence of the exciton OAM is governed by the angular variation in the exciton dephasing. We also analyze the topological charge (ℓ ) dependence of the OAM decay using a multiple-mode LG pump pulse, which allows us to simultaneously observe the dynamics of the exciton OAM for different ℓ values under the same excitation conditions. The OAM decay times of the ℓ =1 component are usually longer than those of the ℓ =0 component. The ℓ -dependent OAM decay is supported by a phenomenological model which takes into account the local nonuniformity of the exciton dephasing.

  12. Excitons and the lifetime of organic semiconductor devices

    PubMed Central

    Forrest, Stephen R.

    2015-01-01

    While excitons are responsible for the many beneficial optical properties of organic semiconductors, their non-radiative recombination within the material can result in material degradation due to the dumping of energy onto localized molecular bonds. This presents a challenge in developing strategies to exploit the benefits of excitons without negatively impacting the device operational stability. Here, we will briefly review the fundamental mechanisms leading to excitonic energy-driven device ageing in two example devices: blue emitting electrophosphorescent organic light emitting devices (PHOLEDs) and organic photovoltaic (OPV) cells. We describe strategies used to minimize or even eliminate this fundamental device degradation pathway. PMID:25987572

  13. Valence-band ordering and magneto-optic exciton fine structure in ZnO

    NASA Astrophysics Data System (ADS)

    Lambrecht, Walter R.; Rodina, Anna V.; Limpijumnong, Sukit; Segall, B.; Meyer, Bruno K.

    2002-02-01

    Using first-principles linear muffin-tin orbital density functional band structure calculations, the ordering of the states in the wurtzite ZnO valence-band maximum, split by crystal-field and spin-orbit coupling effects, is found to be Γ7(5)>Γ9(5)>Γ7(1), in which the number in parentheses indicates the parent state without spin-orbit coupling. This results from the negative spin-orbit splitting, which in turn is due to the participation of the Zn 3d band. The result is found to be robust even when effects beyond the local density approximation on the Zn 3d band position are included. Using a Kohn-Luttinger model parametrized by our first-principles calculations, it is furthermore shown that the binding energies of the excitons primarily derived from each valence band differ by less than the valence-band splittings even when interband coupling effects are included. The binding energies of n=2 and n=1 excitons, however, are not in a simple 1/4 ratio. Our results are shown to be in good agreement with the recent magneto-optical experimental data by Reynolds et al. [Phys. Rev. B 60, 2340 (1999)], in spite of the fact that on the basis of these data these authors claimed that the valence-band maximum would have Γ9 symmetry. The differences between our and Reynolds' analysis of the data are discussed and arise from the sign of the Landé g factor for holes, which is here found to be negative for the upper Γ7 band.

  14. Optical absorption by Dirac excitons in single-layer transition-metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Trushin, Maxim; Goerbig, Mark Oliver; Belzig, Wolfgang

    2016-07-01

    We develop an analytically solvable model able to qualitatively explain nonhydrogenic exciton spectra observed recently in two-dimensional (2D) semiconducting transition-metal dichalcogenides. Our exciton Hamiltonian explicitly includes additional angular momentum associated with the pseudospin degree of freedom unavoidable in 2D semiconducting materials with honeycomb structure. We claim that this is the key ingredient for understanding the nonhydrogenic exciton spectra that was missing so far.

  15. Modulation optical spectroscopy of excitons in structures with GaAs multiple quantum wells separated by tunneling-nontransparent barriers

    SciTech Connect

    Chaldyshev, V. V. Shkol'nik, A. S.; Evtikhiev, V. P.; Holden, T.

    2007-12-15

    Contactless optical electroreflectance measurements at different temperatures are used to study exciton states in a structure involving a periodic system of 36 GaAs quantum wells separated by tunneling-nontransparent AlGaAs barriers with thickness 104 nm. In the structure, the width of 32 of the quantum wells is 15 nm, while the width of the remaining four quantum wells, numbered 5, 14, 23, and 32, is 20 nm. The periodicity of the structure corresponds to the Bragg interference condition at the excitonic frequency in quantum wells at the angle of incidence of light {approx}43 deg. From the quantitative analysis of the shape of the contactless electroreflectance line, the parameters of the exciton ground states and excited states are determined for both types of quantum wells. It is established that, for the system of four 20-nm-wide quantum wells separated by a distance of 830 nm, the size-quantization energy in the ground state is 8.4 {+-} 0.1 meV, and the parameter of broadening of the excitonic peak is 1.8 {+-} 0.1 meV at 17 K and increases with temperature up to 2.0 {+-} 0.1 meV at 80 K. For the system of 32 wells with the width 15 nm, the quantum confinement energy in the ground state is 14.9 {+-} 0.1 meV, and the parameter of broadening of the excitonic peak is 2.2 {+-} 0.1 and 2.6 {+-} 0.1 meV at 17 and 80 K, respectively. The possible causes of radiative and nonradiative broadening of exciton states in the systems are discussed.

  16. Quantum confinement in semiconductor nanofilms: Optical spectra and multiple exciton generation

    NASA Astrophysics Data System (ADS)

    Khmelinskii, Igor; Makarov, Vladimir I.

    2016-04-01

    We report optical absorption and photoluminescence (PL) spectra of Si and SnO2 nanocrystalline films in the UV-vis-NIR range, featuring discrete bands resulting from transverse quantum confinement, observed in the optical spectra of nanofilms for the first time ever. The film thickness ranged from 3.9 to 12.2 nm, depending on the material. The results are interpreted within the particle-in-a-box model, with infinite walls. The calculated values of the effective electron mass are independent on the film thickness and equal to 0.17mo (Si) and 0.21mo (SnO2), with mo the mass of the free electron. The second calculated model parameter, the quantum number n of the HOMO (valence band), was also thickness-independent: 8.00 (Si) and 7.00 (SnO2). The transitions observed in absorption all start at the level n and correspond to Δn = 1, 2, 3, …. The photoluminescence bands exhibit large Stokes shifts, shifting to higher energies with increased excitation energy. In effect, nanolayers of Si, an indirect-gap semiconductor, behave as a direct-gap semiconductor, as regards the transverse-quantized level system. A prototype Si-SnO2 nanofilm photovoltaic cell demonstrated photoelectron quantum yields achieving 2.5, showing clear evidence of multiple exciton generation, for the first time ever in a working nanofilm device.

  17. Symmetry breaking and excitonic effects on optical properties of defective nanographenes.

    PubMed

    Noguchi, Yoshifumi; Sugino, Osamu

    2015-02-14

    We investigate optical properties of the nanographene family and predict a defect induced effect by utilizing the all-electron first-principles GW+Bethe-Salpeter equation (BSE) method based on the many-body perturbation theory. As an accuracy check of the GW+BSE, photoabsorption spectra are calculated for a grossly warped nanographene (C80H30), which was very recently synthesized [Kawasumi et al., Nat. Chem. 5, 739-744 (2013)]. The calculated spectra are found to faithfully reproduce the shape, height, and position of the measured peaks. Then the method is applied to the flat nanographene without defect (C24H12 and C38H16), the curved ones with single defect (C20H10, C28H14, and C32H16), and fragments of C80H30 with double defect (C36H16 and C42H20). The existence of the defects significantly changes the optical spectra. In particular, the interaction between the defects is found to break the symmetry of the atomic geometries and enhance the excitonic effect, thereby generating the extra peaks at the lower photon energy side of the main peak. The present results might help explain the origin of the first two peaks experimentally observed for C80H30. PMID:25681912

  18. Influence of the sign of the coupling on the temperature dependence of optical properties of one-dimensional exciton models

    NASA Astrophysics Data System (ADS)

    Cruzeiro, L.

    2008-10-01

    A new physical cause for a temperature-dependent double peak in exciton systems is put forward within a thermal equilibrium approach for the calculation of optical properties of exciton systems. Indeed, it is found that one-dimensional exciton systems with only one molecule per unit cell can have an absorption spectrum characterized by a double peak provided that the coupling between excitations in different molecules is positive. The two peaks, whose relative intensities vary with temperature, are located around the exciton band edges, being separated by an energy of approximately 4V, where V is the average coupling between nearest neighbours. For small amounts of diagonal and off-diagonal disorder, the contributions from the intermediate states in the band are also visible as intermediate structure between the two peaks, this being enhanced for systems with periodic boundary conditions. At a qualitative level, these results correlate well with experimental observations in the molecular aggregates of the thiacarbocyanine dye THIATS and in the organic crystals of acetanilide and N-methylacetamide.

  19. Natural Atomic Orbital Representation for Optical Spectra Calculations in the Exciton Scattering Approach.

    PubMed

    Li, Hao; Chernyak, Vladimir Y; Tretiak, Sergei

    2012-12-20

    The exciton scattering (ES) method allows efficient calculations of spectroscopic observables in large low-dimensional conjugated molecular systems. To compute the transition dipoles between the ground and excited electronic states, we should extract the ES dipole parameters from quantum chemistry calculations in simple molecular fragments. In this manuscript, we show how to retrieve these parameters from any reference quantum chemistry model that uses an arbitrary nonorthogonal and possibly overcomplete atomic orbital basis set. Our approach relies on the natural atomic orbital (NAO) representation, in which the basis functions are orthonormal and the atom-like character is preserved. We apply the ES approach, combined with the NAO analysis to optical spectra of branched phenylacetylene oligomers. Absorption spectra predicted by the ES method demonstrate close agreement with the results of direct quantum chemistry calculations, when the Time-Dependent Density Functional Theory (TD-DFT) being used as a reference. This testifies applicability of a variety of quantum-chemical techniques, where the NAO population analysis can be conducted, for the ES framework. PMID:26291103

  20. Natural Atomic Orbital Representation for Optical Spectra Calculations in the Exciton Scattering Approach.

    PubMed

    Li, Hao; Chernyak, Vladimir Y; Tretiak, Sergei

    2012-12-20

    The exciton scattering (ES) method allows efficient calculations of spectroscopic observables in large low-dimensional conjugated molecular systems. To compute the transition dipoles between the ground and excited electronic states, we should extract the ES dipole parameters from quantum chemistry calculations in simple molecular fragments. In this manuscript, we show how to retrieve these parameters from any reference quantum chemistry model that uses an arbitrary nonorthogonal and possibly overcomplete atomic orbital basis set. Our approach relies on the natural atomic orbital (NAO) representation, in which the basis functions are orthonormal and the atom-like character is preserved. We apply the ES approach, combined with the NAO analysis to optical spectra of branched phenylacetylene oligomers. Absorption spectra predicted by the ES method demonstrate close agreement with the results of direct quantum chemistry calculations, when the Time-Dependent Density Functional Theory (TD-DFT) being used as a reference. This testifies applicability of a variety of quantum-chemical techniques, where the NAO population analysis can be conducted, for the ES framework.

  1. Optical properties of halide and oxide compounds including the excitonic effects

    NASA Astrophysics Data System (ADS)

    Shwetha, G.; Kanchana, V.

    2014-04-01

    We have studied the optical properties of alkali halide and alkaline-earth oxide compounds including the excitonic effects by using the newly developed bootstrap kernel approximation for the exchange-correlation kernel of the Time-Dependent Density Functional Theory (TD-DFT) implemented in Full-Potential Linearized Augmented Plane Wave (FP-LAPW) method in the elk code. The bootstrap calculations are computationally less expensive and give results the same quality as the Bethe-Salpeter equation. We found improved results when compared to normal Density Functional Theory calculations, and observed results are comparable with the experiments. The lower energy peak of imaginary part of dielectric spectra shifts to lower energy regions as we move from MgO to BaO indicating the decrease in the band gap of these compounds from MgO to BaO. In all the studied compounds, the lower energy peak of the imaginary part of dielectric function is due to the transition from halogen p or oxide p states to metal derived s/d states.

  2. Optical Characterization of Strong UV Luminescence Emitted from the Excitonic Edge of Nickel Oxide Nanotowers

    PubMed Central

    Ho, Ching-Hwa; Kuo, Yi-Ming; Chan, Ching-Hsiang; Ma, Yuan-Ron

    2015-01-01

    NiO had been claimed to have the potential for application in transparent conducting oxide, electrochromic device for light control, and nonvolatile memory device. However, the detailed study of excitonic transition and light-emission property of NiO has rarely been explored to date. In this work, we demonstrate strong exciton-complex emission of high-quality NiO nanotowers grown by hot-filament metal-oxide vapor deposition with photoluminescence as an evaluation tool. Fine and clear emission features coming from the excitonic edge of the NiO are obviously observed in the photoluminescence spectra. A main excitonic emission of ~3.25 eV at 300 K can be decomposed into free exciton, bound excitons, and donor-acceptor-pair irradiations at lowered temperatures down to 10 K. The band-edge excitonic structure for the NiO nanocrystals has been evaluated and analyzed by transmission and thermoreflectacne measurements herein. All the experimental results demonstrate the cubic NiO thin-film nanotower is an applicable direct-band-gap material appropriate for UV luminescence and transparent-conducting-oxide applications. PMID:26506907

  3. Symmetry breaking and excitonic effects on optical properties of defective nanographenes

    SciTech Connect

    Noguchi, Yoshifumi Sugino, Osamu

    2015-02-14

    We investigate optical properties of the nanographene family and predict a defect induced effect by utilizing the all-electron first-principles GW+Bethe-Salpeter equation (BSE) method based on the many-body perturbation theory. As an accuracy check of the GW+BSE, photoabsorption spectra are calculated for a grossly warped nanographene (C{sub 80}H{sub 30}), which was very recently synthesized [Kawasumi et al., Nat. Chem. 5, 739–744 (2013)]. The calculated spectra are found to faithfully reproduce the shape, height, and position of the measured peaks. Then the method is applied to the flat nanographene without defect (C{sub 24}H{sub 12} and C{sub 38}H{sub 16}), the curved ones with single defect (C{sub 20}H{sub 10}, C{sub 28}H{sub 14}, and C{sub 32}H{sub 16}), and fragments of C{sub 80}H{sub 30} with double defect (C{sub 36}H{sub 16} and C{sub 42}H{sub 20}). The existence of the defects significantly changes the optical spectra. In particular, the interaction between the defects is found to break the symmetry of the atomic geometries and enhance the excitonic effect, thereby generating the extra peaks at the lower photon energy side of the main peak. The present results might help explain the origin of the first two peaks experimentally observed for C{sub 80}H{sub 30}.

  4. Resonant optical reflection by a periodic system of the quantum well excitons at the second quantum state

    SciTech Connect

    Chaldyshev, V. V.; Poddubny, A. N.; Vasil'ev, A. P.; Chen Yuechao; Liu Zhiheng

    2011-02-14

    A periodic multiple quantum well GaAs/AlGaAs structure was designed, grown, and characterized in order to reveal resonant features in optical spectra when the Bragg resonance was tuned to the second quantum state x(e2-hh2) of the heavy-hole exciton-polaritons in the multiple quantum wells. This double resonance was demonstrated by tuning the incident angle of the light as well as by comparison with a single quantum well structure. A significant enhancement of the light-matter interaction was observed, which manifests itself by strong resonant optical reflection and electroreflection.

  5. Arnold diffusion in a driven optical lattice.

    PubMed

    Boretz, Yingyue; Reichl, L E

    2016-03-01

    The effect of time-periodic forces on matter has been a topic of growing interest since the advent of lasers. It is known that dynamical systems with 2.5 or more degrees of freedom are intrinsically unstable. As a consequence, time-periodic driven systems can experience large excursions in energy. We analyze the classical and quantum dynamics of rubidium atoms confined to a time-periodic optical lattice with 2.5 degrees of freedom. When the laser polarizations are orthogonal, the system consists of two 1.5 uncoupled dynamical systems. When laser polarizations are turned away from orthogonal, an Arnold web forms and the dynamics undergoes a fundamental change. For parallel polarizations, we find huge random excursions in the rubidium atom energies and significant entanglement of energies in the quantum dynamics. PMID:27078351

  6. Optical and spin properties of localized and free excitons in GaBi x As1-x /GaAs multiple quantum wells

    NASA Astrophysics Data System (ADS)

    Balanta, M. A. G.; Kopaczek, J.; Orsi Gordo, V.; Santos, B. H. B.; Rodrigues, A. D.; Galeti, H. V. A.; Richards, R. D.; Bastiman, F.; David, J. P. R.; Kudrawiec, R.; Galvão Gobato, Y.

    2016-09-01

    Raman spectroscopy and magneto-photoluminescence measurements under high magnetic fields were used to investigate the optical and spin properties of GaBiAs/GaAs multiple quantum wells (MQWs). An anomalous negative diamagnetic energy shift was observed at higher temperatures and higher laser intensities, which was associated to a sign inversion of hole effective mass in these structures. In addition, an enhancement of the polarization degree with decreasing of laser intensity was observed (experimental condition where the emission is dominated by localized excitons). This effect was explained by changes of spin relaxation and exciton recombination times due to exciton localization by disorder.

  7. Optical and spin properties of localized and free excitons in GaBi x As1‑x /GaAs multiple quantum wells

    NASA Astrophysics Data System (ADS)

    Balanta, M. A. G.; Kopaczek, J.; Orsi Gordo, V.; Santos, B. H. B.; Rodrigues, A. D.; Galeti, H. V. A.; Richards, R. D.; Bastiman, F.; David, J. P. R.; Kudrawiec, R.; Galvão Gobato, Y.

    2016-09-01

    Raman spectroscopy and magneto-photoluminescence measurements under high magnetic fields were used to investigate the optical and spin properties of GaBiAs/GaAs multiple quantum wells (MQWs). An anomalous negative diamagnetic energy shift was observed at higher temperatures and higher laser intensities, which was associated to a sign inversion of hole effective mass in these structures. In addition, an enhancement of the polarization degree with decreasing of laser intensity was observed (experimental condition where the emission is dominated by localized excitons). This effect was explained by changes of spin relaxation and exciton recombination times due to exciton localization by disorder.

  8. Optical spectroscopy of free excitons in a CuInS{sub 2} chalcopyrite semiconductor compound

    SciTech Connect

    Mudryi, A. V. Ivanyukovich, A. V.; Yakushev, M. V.; Martin, R.; Saad, A.

    2008-01-15

    The spectra of reflectance and luminescence of high-quality CuInS{sub 2} single crystals grown by oriented crystallization are studied at the temperature 4.2 K. In the region of the fundamental absorption edge, the two excitonic resonance reflectance peaks, nondegenerate peak A at the energy {approx}1.5356 eV and doubly degenerate peak BC at the energy {approx}1.5567 eV, and the luminescence signal produced by free and bound excitons are observed. The luminescence lines, A{sub UPB} at {approx}1.5361 eV and A{sub LPB} at {approx}1.5347 eV, with a half-width {approx}1 meV, are attributed to exciton-polariton recombination. From the experimentally observed energy position of the exciton ground state and excited states, the binding energy of free excitons is determined to be {approx}18.5 meV. In studying the photoluminescence in magnetic fields up to 10 T, a diamagnetic shift of the ground state of free excitons A is observed.

  9. Optical Absorption Spectra and Excitons of Dye-Substrate Interfaces: Catechol on TiO2(110).

    PubMed

    Mowbray, Duncan John; Migani, Annapaola

    2016-06-14

    Optimizing the photovoltaic efficiency of dye-sensitized solar cells (DSSC) based on staggered gap heterojunctions requires a detailed understanding of sub-band gap transitions in the visible from the dye directly to the substrate's conduction band (CB) (type-II DSSCs). Here, we calculate the optical absorption spectra and spatial distribution of bright excitons in the visible region for a prototypical DSSC, catechol on rutile TiO2(110), as a function of coverage and deprotonation of the OH anchoring groups. This is accomplished by solving the Bethe-Salpeter equation (BSE) based on hybrid range-separated exchange and correlation functional (HSE06) density functional theory (DFT) calculations. Such a treatment is necessary to accurately describe the interfacial level alignment and the weakly bound charge transfer transitions that are the dominant absorption mechanism in type-II DSSCs. Our HSE06 BSE spectra agree semiquantitatively with spectra measured for catechol on anatase TiO2 nanoparticles. Our results suggest deprotonation of catechol's OH anchoring groups, while being nearly isoenergetic at high coverages, shifts the onset of the absorption spectra to lower energies, with a concomitant increase in photovoltaic efficiency. Further, the most relevant bright excitons in the visible region are rather intense charge transfer transitions with the electron and hole spatially separated in both the [110] and [001] directions. Such detailed information on the absorption spectra and excitons is only accessible via periodic models of the combined dye-substrate interface. PMID:27183273

  10. Exciton storage in type-II quantum dots using the optical Aharonov-Bohm effect

    SciTech Connect

    Climente, Juan I.; Planelles, Josep

    2014-05-12

    We investigate the bright-to-dark exciton conversion efficiency in type-II quantum dots subject to a perpendicular magnetic field. To this end, we take the exciton storage protocol recently proposed by Simonin and co-workers [Phys. Rev. B 89, 075304 (2014)] and simulate its coherent dynamics. We confirm the storage is efficient in perfectly circular structures subject to weak external electric fields, where adiabatic evolution is dominant. In practice, however, the efficiency rapidly degrades with symmetry lowering. Besides, the use of excited states is likely unfeasible owing to the fast decay rates. We then propose an adaptation of the protocol which does not suffer from these limitations.

  11. Exciton-related nonlinear optical properties in cylindrical quantum dots with asymmetric axial potential: combined effects of hydrostatic pressure, intense laser field, and applied electric field

    PubMed Central

    2012-01-01

    The exciton binding energy of an asymmetrical GaAs-Ga1−xAlxAs cylindrical quantum dot is studied with the use of the effective mass approximation and a variational calculation procedure. The influence on this quantity of the application of a direct-current electric field along the growth direction of the cylinder, together with that of an intense laser field, is particularly considered. The resulting states are used to calculate the exciton-related nonlinear optical absorption and optical rectification, whose corresponding resonant peaks are reported as functions of the external probes, the quantum dot dimensions, and the aluminum molar fraction in the potential barrier regions. PMID:22971418

  12. Exciton-related nonlinear optical properties in cylindrical quantum dots with asymmetric axial potential: combined effects of hydrostatic pressure, intense laser field, and applied electric field.

    PubMed

    Zapata, Alejandro; Acosta, Ruben E; Mora-Ramos, Miguel E; Duque, Carlos A

    2012-01-01

    : The exciton binding energy of an asymmetrical GaAs-Ga1-xAlxAs cylindrical quantum dot is studied with the use of the effective mass approximation and a variational calculation procedure. The influence on this quantity of the application of a direct-current electric field along the growth direction of the cylinder, together with that of an intense laser field, is particularly considered. The resulting states are used to calculate the exciton-related nonlinear optical absorption and optical rectification, whose corresponding resonant peaks are reported as functions of the external probes, the quantum dot dimensions, and the aluminum molar fraction in the potential barrier regions. PMID:22971418

  13. Si dielectric function in a local basis representation: Optical properties, local field effects, excitons, and stopping power

    NASA Astrophysics Data System (ADS)

    Gómez, M.; González, P.; Ortega, J.; Flores, F.

    2014-11-01

    An atomiclike basis representation is used to analyze the dielectric function ɛ (q ⃗+G ⃗,q ⃗+G⃗';ω ) of Si. First, we show that a s p3d5 local basis set yields good results for the electronic band structure of this crystal and, then, we analyze the Si optical properties including local field and excitonic effects. In our formulation, we follow Hanke and Sham [W. Hanke and L. J. Sham, Phys. Rev. B 12, 4501 (1975), 10.1103/PhysRevB.12.4501; Phys. Rev. B 21, 4656 (1980), 10.1103/PhysRevB.21.4656], and introduce excitonic effects using a many-body formulation that incorporates a static screened electron-hole interaction. Dynamical effects in this interaction are also analyzed and shown to introduce non-negligible corrections in the optical spectrum. Our results are found in reasonable agreement with the experimental evidence and with other theoretical results calculated with the computationally more demanding plane-wave representation. Finally, calculations for the stopping power of Si are also presented.

  14. Theoretical and experimental analysis of an optical driven servo system

    NASA Astrophysics Data System (ADS)

    Lu, F.; Wang, X. J.; Huang, J. H.; Liu, Y. F.

    2016-09-01

    An optical driven servo system model based on single-type PLZT ceramic is proposed in this paper. The control equation of the proposed servo system is derived based on the mathematical model of PLZT with coupled multi-physics fields. The parameters of photodeformation of the PLZT actuator during both the illumination phase and light off phase are identified through the static experiment. Then displacement response of optical driven servo system is numerically simulated based on the control equation presented in this paper. After that, the closed-loop control experiment of optical driven servo system based on PLZT single-type ceramic with a simple on-off method is carried out. The experimental results show that the optical driven servo system with simple on-off method can achieve the target displacement by applying UV light to the PLZT actuator. Furthermore, an improved on-off control strategy is proposed to decrease the undesirable fluctuation around the target displacement.

  15. Theoretical and experimental analysis of an optical driven servo system

    NASA Astrophysics Data System (ADS)

    Lu, F.; Wang, X. J.; Huang, J. H.; Liu, Y. F.

    2016-09-01

    An optical driven servo system model based on single-type PLZT ceramic is proposed in this paper. The control equation of the proposed servo system is derived based on the mathematical model of PLZT with coupled multi-physics fields. The parameters of photodeformation of the PLZT actuator during both the illumination phase and light off phase are identified through the static experiment. Then displacement response of optical driven servo system is numerically simulated based on the control equation presented in this paper. After that, the closed-loop control experiment of optical driven servo system based on PLZT single-type ceramic with a simple on–off method is carried out. The experimental results show that the optical driven servo system with simple on–off method can achieve the target displacement by applying UV light to the PLZT actuator. Furthermore, an improved on–off control strategy is proposed to decrease the undesirable fluctuation around the target displacement.

  16. Using bound exciton transitions to optically resolve neutral donor hyperfine states of various donor species in Silicon-28

    NASA Astrophysics Data System (ADS)

    Salvail, Jeff; Dluhy, Phillip; Saeedi, Kamyar; Szech, Michael; Riemann, Helge; Abromisov, Nikolai; Becker, Peter; Pohl, Hans-Joachim; Thewalt, Michael

    2014-03-01

    Phosphorus in silicon is established as a promising resource for use in quantum information processing tasks. The neutral donor hyperfine states have been shown to have record long coherence times, high fidelity gates via RF pulses, and projective readout via optical bound exciton transitions. As Shannon's theory of information tells us, we can process more information in an alphabet of more symbols, so there is motivation to look at donors with higher nuclear spin than the I = 1 / 2 of 31P, which provide access to Hilbert spaces of dimension greater than two. In this talk I will describe optical studies of the donors 75As (I = 3 / 2), 121Sb (I = 5 / 2), and 209Bi (I = 9 / 2) in 28Si.

  17. Exciton dynamicsstudied via internal THz transitions

    SciTech Connect

    Kaindl, R.A.; Hagele, D.; Carnahan, M.A.; Lovenich, R.; Chemla,D.S.

    2003-02-26

    We employ a novel, ultrafast terahertz probe to investigatethe dynamical interplay of optically-induced excitons and unboundelectron-hole pairs in GaAs quantum wells. Resonant creation ofheavy-hole excitons induces a new low-energy oscillator linked totransitions between the internal exciton degrees of freedom. The timeresolved terahertz optical conductivity is found to be a probe wellsuited for studies of fundamental processes such as formation, relaxationand ionization of excitons.

  18. Optical absorption edge in α-Fe2O3: The exciton-magnon structure

    NASA Astrophysics Data System (ADS)

    Galuza, A. I.; Beznosov, A. B.; Eremenko, V. V.

    1998-10-01

    Transmission spectra of synthetic and natural hematite (α-Fe2O3) crystals are measured at temperatures 10, 25, and 300 K in the wavelength range 500-1100 nm, and the absorption spectra are computed. Pure exciton and exciton-magnon d-d transition bands are revealed, the corresponding wavelengths at 10 K being λ0=1020 nm and λ1=965 nm respectively. The half-widths and oscillator forces are g0=84 cm-1, f0=4×10-9, g1=60 cm-1, f1=1.4×10-7 for 10 K, g0=85 cm-1, f0=5×10-9, g1=110 cm-1, f1=2.1×10-7 for 25 K. The mechanisms of band formation for weakly allowed d-d transitions in hematite are analyzed.

  19. Repulsively bound exciton-biexciton states in high-spin fermions in optical lattices

    SciTech Connect

    Argueelles, A.; Santos, L.

    2011-03-15

    We show that the interplay between spin-changing collisions and quadratic Zeeman coupling provides a mechanism for the formation of repulsively bound composites in high-spin fermions, which we illustrate by considering spin flips in an initially polarized hard-core one-dimensional Mott insulator of spin-3/2 fermions. We show that after the flips the dynamics is characterized by the creation of two types of exciton-biexciton composites. We analyze the conditions for the existence of these bound states and discuss their intriguing properties. In particular we show that the effective mass and stability of the composites depends nontrivially on spin-changing collisions, on the quadratic Zeeman effect, and on the initial exciton localization. Finally, we show that the composites may remain stable against inelastic collisions, opening the possibility of interesting quantum composite phases.

  20. Electrodynamic and excitonic intertube interactions in semiconducting carbon nanotube aggregates.

    PubMed

    Crochet, Jared J; Sau, Jay D; Duque, Juan G; Doorn, Stephen K; Cohen, Marvin L

    2011-04-26

    The optical properties of selectively aggregated, nearly single chirality single-wall carbon nanotubes were investigated by both continuous-wave and time-resolved spectroscopies. With reduced sample heterogeneities, we have resolved aggregation-dependent reductions of the excitation energy of the S(1) exciton and enhanced electron-hole pair absorption. Photoluminescence spectra revealed a spectral splitting of S(1) and simultaneous reductions of the emission efficiencies and nonradiative decay rates. The observed strong deviations from isolated tube behavior are accounted for by enhanced screening of the intratube Coulomb interactions, intertube exciton tunneling, and diffusion-driven exciton quenching. We also provide evidence that density gradient ultracentrifugation can be used to structurally sort single-wall carbon nanotubes by aggregate size as evident by a monotonic dependence of the aforementioned optical properties on buoyant density.

  1. Dissipation-Induced Symmetry Breaking in a Driven Optical Lattice

    SciTech Connect

    Gommers, R.; Bergamini, S.; Renzoni, F.

    2005-08-12

    We analyze the atomic dynamics in an ac driven periodic optical potential which is symmetric in both time and space. We experimentally demonstrate that in the presence of dissipation the symmetry is broken, and a current of atoms through the optical lattice is generated as a result.

  2. Optical driven electromechanical transistor based on tunneling effect.

    PubMed

    Jin, Leisheng; Li, Lijie

    2015-04-15

    A new electromechanical transistor based on an optical driven vibrational ring structure has been postulated. In the device, optical power excites the ring structure to vibrate, which acts as the shuttle transporting electrons from one electrode to the other forming the transistor. The electrical current of the transistor is adjusted by the optical power. Coupled opto-electro-mechanical simulation has been performed. It is shown from the dynamic analysis that the stable working range of the transistor is much wider than that of the optical wave inside the cavity, i.e., the optical resonance enters nonperiodic states while the mechanical vibration of the ring is still periodic.

  3. Correlated structural-optical study of single nanocrystals in a gap-bar antenna: effects of plasmonics on excitonic recombination pathways.

    PubMed

    Wang, Feng; Karan, Niladri S; Nguyen, Hue Minh; Ghosh, Yagnaseni; Sheehan, Chris J; Hollingsworth, Jennifer A; Htoon, Han

    2015-06-01

    We performed time-correlated single-photon counting experiments on individual silica coated CdSe/CdS core/thick-shell nanocrystal quantum dots (a.k.a., giant NQDs [g-NQDs]), placed on the plasmonic gap-bar antennas. Optical properties were directly correlated with the scanning electron microscopy (SEM) images of g-NQD-plasmonic antenna coupled structures. The structures, in which the g-NQDs are located in the gap of the antenna, afford a coupling with up to 9.6 fold enhancement of radiative recombination rates. These coupled g-NQDs are also characterized by a strong enhancement of bi-exciton emission efficiency that increases with their radiative enhancement factor. By analysing these findings with a simple model, we show that the plasmonic field of the antenna does not alter the Auger recombination processes of the bi-exciton states. As a result, enhancements of the single and bi-exciton radiative recombination rates lead directly to bi-exciton emission enhancement. These findings suggest that a plasmonic field can be utilized effectively in achieving a strong bi-exciton emission that is needed for photon pair generation and plasmon-assisted lasing.

  4. Laser-driven polyplanar optic display

    SciTech Connect

    Veligdan, J.T.; Biscardi, C.; Brewster, C.; DeSanto, L.; Beiser, L.

    1998-01-01

    The Polyplanar Optical Display (POD) is a unique display screen which can be used with any projection source. This display screen is 2 inches thick and has a matte-black face which allows for high contrast images. The prototype being developed is a form, fit and functional replacement display for the B-52 aircraft which uses a monochrome ten-inch display. The new display uses a 200 milliwatt green solid-state laser (532 nm) as its optical source. In order to produce real-time video, the laser light is being modulated by a Digital Light Processing (DLP) chip manufactured by Texas Instruments, Inc. A variable astigmatic focusing system is used to produce a stigmatic image on the viewing face of the POD. In addition to the optical design, the authors discuss the DLP chip, the optomechanical design and viewing angle characteristics.

  5. Optically driven actuators using poly(vinylidene difluoride)

    NASA Astrophysics Data System (ADS)

    Mizutani, Yasuhiro; Otani, Yukitoshi; Umeda, Norihiro

    2008-05-01

    Optically driven actuators have a feature of a non-contact method supplied by light energy. A new method is proposed with three poly(vinylidene difluoride) (PVDF) cantilevers as the legs and a polymer film as the body. The PVDF cantilevers are coated with silver on one surface. When one side of the cantilever is irradiated by a laser beam, an electric field is produced along a cross-section of the cantilever by the pyroelectric effect and a mechanical displacement occurs by the piezoelectric effect. Its response time and its generated force are measured experimentally. Two types of optically driven actuators using PVDF film are proposed to move using different characteristics.

  6. Thermally driven continuous-wave and pulsed optical vortex.

    PubMed

    Ding, Yitian; Xu, Miaomiao; Zhao, Yongguang; Yu, Haohai; Zhang, Huaijin; Wang, Zhengping; Wang, Jiyang

    2014-04-15

    We demonstrated a continuous-wave (cw) and pulsed optical vortex with topological charges driven by heat generated during the lasing process without introducing the astigmatism effect and reducing lasing efficiency. During the lasing process, the topological charges were changeable by the thermal-induced lens and selected by the mode-matching between the pump and oscillating beams. With a graphene sample as the saturable absorber, a pulsed optical vortex was achieved at a wavelength of 1.36 μm, which identified that graphene could be used as a pulse modulator for the generation of a pulsed optical vortex. Thermally driven cw and pulsed optical vortexes should have various promising applications based on the compact structure, changeable topological charges, and specific wavelength. PMID:24978994

  7. Photovoltaic driven multiple quantum well optical modulator

    NASA Technical Reports Server (NTRS)

    Maserjian, Joseph (Inventor)

    1990-01-01

    Multiple quantum well (MQW) structures (12) are utilized to provide real-time, reliable, high-performance, optically-addressed spatial-light modulators (SLM) (10). The optically-addressed SLM comprises a vertical stack of quantum well layers (12a) within the penetration depth of an optical write signal 18, a plurality of space charge barriers (12b) having predetermined tunneling times by control of doping and thickness. The material comprising the quantum well layers has a lower bandgap than that of the space charge barrier layers. The write signal modulates a read signal (20). The modulation sensitivity of the device is high and no external voltage source is required. In a preferred embodiment, the SLM having interleaved doped semiconductor layers for driving the MQW photovoltaically is characterized by the use of a shift analogous to the Moss-Burnstein shift caused by the filling of two-dimensional states in the multiple quantum wells, thus allowing high modulation sensitivity in very narrow wells. Arrays (30) may be formed with a plurality of the modulators.

  8. Thermo-optically driven adaptive mirror

    NASA Astrophysics Data System (ADS)

    Reinert, Felix; Lüthy, Willy

    2006-02-01

    The ideal adaptive optical mirror combines large aperture with high spatial and temporal resolution and a phase shift of at least 2π. Further, a simple low-cost solution is preferred. No adaptive system can perfectly fulfill all these requirements. We present a system that has the potential to reach this goal with the exception of high temporal resolution. But even with a moderate temporal resolution of one second such a system can find practical applications. For example as a laser resonator mirror that allows to modify the intensity distribution of the emission, or to correct slowly varying aberrations of optical systems. Two possible mechanisms can be used to change the optical path length of the adaptive mirror: thermal expansion of the mirror substrate or the thermally induced change of the refractive index (thermal dispersion) of a medium in front of the mirror. Both mechanisms have been shown to lead to promising results. In both cases heating was performed by irradiation of light in the active medium. The thermal dispersion based adaptive mirror is built with a thin layer of a liquid in front of a mirror. To allow a modification of the refractive index by irradiation with a diode laser at 808 nm, a suitable absorber is dissolved in the water. With chopped irradiation a resolution of 3.8 Hz at 30 % contrast is measured. This mirror has been used in a laser resonator to modify the output distribution of the laser. The thermal expansion based adaptive mirror is built with a thin layer of a silicon elastomer with a gold coated front side. We present a preparation method to produce thin films of Sylgard on sapphire. With an irradiated intensity of only 370 mW/cm2 surface modulations of up to 350 nm are obtained. With a test pattern a resolution of 1.6 line-pairs per millimeter at 30 % contrast is measured. The temporal resolution is better than one second.

  9. Emulation of lossless exciton-polariton condensates by dual-core optical waveguides: stability, collective modes, and dark solitons.

    PubMed

    Salasnich, Luca; Malomed, Boris A; Toigo, Flavio

    2014-10-01

    We propose a possibility to simulate the exciton-polariton (EP) system in the lossless limit, which is not currently available in semiconductor microcavities, by means of a simple optical dual-core waveguide, with one core carrying the nonlinearity and operating close to the zero-group-velocity-dispersion point, and the other core being linear and dispersive. Both two-dimensional (2D) and one-dimensional (1D) EP systems may be emulated by means of this optical setting. In the framework of this system, we find that, while the uniform state corresponding to the lower branch of the nonlinear dispersion relation is stable against small perturbations, the upper branch is always subject to the modulational instability. The stability and instability are verified by direct simulations too. We analyze collective excitations on top of the stable lower-branch state, which include a Bogoliubov-like gapless mode and a gapped one. Analytical results are obtained for the corresponding sound velocity and energy gap. The effect of a uniform phase gradient (superflow) on the stability is considered too, with a conclusion that the lower-branch state becomes unstable above a critical wave number of the flux. Finally, we demonstrate that the stable 1D state may carry robust dark solitons. PMID:25375613

  10. Emulation of lossless exciton-polariton condensates by dual-core optical waveguides: stability, collective modes, and dark solitons.

    PubMed

    Salasnich, Luca; Malomed, Boris A; Toigo, Flavio

    2014-10-01

    We propose a possibility to simulate the exciton-polariton (EP) system in the lossless limit, which is not currently available in semiconductor microcavities, by means of a simple optical dual-core waveguide, with one core carrying the nonlinearity and operating close to the zero-group-velocity-dispersion point, and the other core being linear and dispersive. Both two-dimensional (2D) and one-dimensional (1D) EP systems may be emulated by means of this optical setting. In the framework of this system, we find that, while the uniform state corresponding to the lower branch of the nonlinear dispersion relation is stable against small perturbations, the upper branch is always subject to the modulational instability. The stability and instability are verified by direct simulations too. We analyze collective excitations on top of the stable lower-branch state, which include a Bogoliubov-like gapless mode and a gapped one. Analytical results are obtained for the corresponding sound velocity and energy gap. The effect of a uniform phase gradient (superflow) on the stability is considered too, with a conclusion that the lower-branch state becomes unstable above a critical wave number of the flux. Finally, we demonstrate that the stable 1D state may carry robust dark solitons.

  11. Excitonic surface lattice resonances

    NASA Astrophysics Data System (ADS)

    Humphrey, A. D.; Gentile, M. J.; Barnes, W. L.

    2016-08-01

    Electromagnetic resonances are important in controlling light at the nanoscale. The most studied such resonance is the surface plasmon resonance that is associated with metallic nanostructures. Here we explore an alternative resonance, the surface exciton-polariton resonance, one based on excitonic molecular materials. Our study is based on analytical and numerical modelling. We show that periodic arrays of suitable molecular nanoparticles may support surface lattice resonances that arise as a result of coherent interactions between the particles. Our results demonstrate that excitonic molecular materials are an interesting alternative to metals for nanophotonics; they offer the prospect of both fabrication based on supramolecular chemistry and optical functionality arising from the way the properties of such materials may be controlled with light.

  12. Relationship between molecular stacking and optical properties of 9,10-bis((4-N,N-dialkylamino)styryl) anthracene crystals: the cooperation of excitonic and dipolar coupling.

    PubMed

    Li, Feng; Gao, Na; Xu, Hai; Liu, Wei; Shang, Hui; Yang, Wenjun; Zhang, Ming

    2014-08-01

    Five 9,10-bis((4-N,N-dialkylamino)styryl) anthracene derivatives (DSA-C1-DSA-C7) with different length alkyl chains were synthesized. They showed the same color in dilute solutions but different colors in crystals. The absorption, photoluminescence, and fluorescence decay indicate that there exist both excitonic and dipolar coupling in crystals of DSA-C1-DSA-C7. X-ray crystallographic analysis revealed that all the crystals belong to the triclinic space group P1 with one molecule per unit cell and that the molecules in every crystal have the identical orientation. This offers ideal samples to investigate the impact of the molecular stacking on the optical properties of the crystals. For the first time, the cooperation of excitonic and dipolar coupling has been comprehensively studied, and the contribution to the spectral shift from the excitonic and dipolar couplings quantitatively obtained. The experiments of amplified spontaneous emission (ASE) together with measurements of the quantum efficiency further confirmed this interpretation. The results suggest that the excitonic and dipolar couplings between the adjacent molecules are both important and jointly induce the spectral shifts of the crystals.

  13. Azimuthons and pattern formation in annularly confined exciton-polariton Bose-Einstein condensates

    NASA Astrophysics Data System (ADS)

    Li, Guangyao

    2016-01-01

    We present numerical analysis of steady states in a two-component (spinor) driven-dissipative quantum fluid formed by condensed exciton polaritons in an annular optically induced trap. We demonstrate that an incoherent ring-shaped optical pump creating the exciton-polariton confinement supports the existence of stationary and rotating azimuthon steady states with azimuthally modulated density associated with Josephson vortices. Such states can be imprinted by coherent light pulses with a defined orbital angular momentum, as well as generated spontaneously in the presence of thermal noise.

  14. Phase diagram of degenerate exciton systems.

    PubMed

    Lai, C W; Zoch, J; Gossard, A C; Chemla, D S

    2004-01-23

    Degenerate exciton systems have been produced in quasi-two-dimensional confined areas in semiconductor coupled quantum well structures. We observed contractions of clouds containing tens of thousands of excitons within areas as small as (10 micron)2 near 10 kelvin. The spatial and energy distributions of optically active excitons were determined by measuring photoluminescence as a function of temperature and laser excitation and were used as thermodynamic quantities to construct the phase diagram of the exciton system, which demonstrates the existence of distinct phases. Understanding the formation mechanisms of these degenerate exciton systems can open new opportunities for the realization of Bose-Einstein condensation in the solid state.

  15. Tangled nonlinear driven chain reactions of all optical singularities

    NASA Astrophysics Data System (ADS)

    Vasil'ev, V. I.; Soskin, M. S.

    2012-03-01

    Dynamics of polarization optical singularities chain reactions in generic elliptically polarized speckle fields created in photorefractive crystal LiNbO3 was investigated in details Induced speckle field develops in the tens of minutes scale due to photorefractive 'optical damage effect' induced by incident beam of He-Ne laser. It was shown that polarization singularities develop through topological chain reactions of developing speckle fields driven by photorefractive nonlinearities induced by incident laser beam. All optical singularities (C points, optical vortices, optical diabolos,) are defined by instantaneous topological structure of the output wavefront and are tangled by singular optics lows. Therefore, they have develop in tangled way by six topological chain reactions driven by nonlinear processes in used nonlinear medium (photorefractive LiNbO3:Fe in our case): C-points and optical diabolos for right (left) polarized components domains with orthogonally left (right) polarized optical vortices underlying them. All elements of chain reactions consist from loop and chain links when nucleated singularities annihilated directly or with alien singularities in 1:9 ratio. The topological reason of statistics was established by low probability of far enough separation of born singularities pair from existing neighbor singularities during loop trajectories. Topology of developing speckle field was measured and analyzed by dynamic stokes polarimetry with few seconds' resolution. The hierarchy of singularities govern scenario of tangled chain reactions was defined. The useful space-time data about peculiarities of optical damage evolution were obtained from existence and parameters of 'islands of stability' in developing speckle fields.

  16. Distributed fibre optic strain measurements on a driven pile

    NASA Astrophysics Data System (ADS)

    Woschitz, Helmut; Monsberger, Christoph; Hayden, Martin

    2016-05-01

    In civil engineering pile systems are used in unstable areas as a foundation of buildings or other structures. Among other parameters, the load capacity of the piles depends on their length. A better understanding of the mechanism of load-transfer to the soil would allow selective optimisation of the system. Thereby, the strain variations along the loaded pile are of major interest. In this paper, we report about a field trial using an optical backscatter reflectometer for distributed fibre-optic strain measurements along a driven pile. The most significant results gathered in a field trial with artificial pile loadings are presented. Calibration results show the performance of the fibre-optic system with variations in the strain-optic coefficient.

  17. Cross-polarized excitons in carbon nanotubes.

    PubMed

    Kilina, Svetlana; Tretiak, Sergei; Doorn, Stephen K; Luo, Zhengtang; Papadimitrakopoulos, Fotios; Piryatinski, Andrei; Saxena, Avadh; Bishop, Alan R

    2008-05-13

    Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E(22) van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active "bright" excitonic band polarized parallel to the tube axis and several optically "weak" cross-polarized excitons. The rest are optically (near) forbidden "dark" transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E(11) transition are more localized along the circumference of a tube, compared with others related to the E(12) and E(21) cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data. PMID:18463293

  18. Cross-polarized excitons in carbon nanotubes

    PubMed Central

    Kilina, Svetlana; Tretiak, Sergei; Doorn, Stephen K.; Luo, Zhengtang; Papadimitrakopoulos, Fotios; Piryatinski, Andrei; Saxena, Avadh; Bishop, Alan R.

    2008-01-01

    Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E22 van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active “bright” excitonic band polarized parallel to the tube axis and several optically “weak” cross-polarized excitons. The rest are optically (near) forbidden “dark” transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E11 transition are more localized along the circumference of a tube, compared with others related to the E12 and E21 cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data. PMID:18463293

  19. Electronic band gaps and exciton binding energies in monolayer M oxW1 -xS2 transition metal dichalcogenide alloys probed by scanning tunneling and optical spectroscopy

    NASA Astrophysics Data System (ADS)

    Rigosi, Albert F.; Hill, Heather M.; Rim, Kwang Taeg; Flynn, George W.; Heinz, Tony F.

    2016-08-01

    Using scanning tunneling spectroscopy (STS) and optical reflectance contrast measurements, we examine band-gap properties of single layers of transition metal dichalcogenide (TMDC) alloys: Mo S2 , M o0.5W0.5S2 , M o0.25W0.75S2 , M o0.1W0.9S2 , and W S2 . The quasiparticle band gap, spin-orbit separation of the excitonic transitions at the K /K' point in the Brillouin zone, and binding energies of the A exciton are extracted from STS and optical data. The exciton binding energies change roughly linearly with tungsten concentration. For our samples on an insulating substrate, we report quasiparticle band gaps from 2.17 ± 0.04 eV (Mo S2) to 2.38 ± 0.06 eV (W S2) , with A exciton binding energies ranging from 310 to 420 meV.

  20. Excitons at the (001) surface of anatase: Spatial behavior and optical signatures

    SciTech Connect

    Giorgi, Giacomo; Yamashita, Koichi; Palummo, Maurizia; Chiodo, Letizia

    2011-08-15

    Within an ab initio study, based on the application of Many-Body Perturbation Theory approaches on top of ground-state Density Functional Theory calculations, we study the optical behavior of the TiO{sub 2} anatase (001) surface. We focus on the (1 x 1) and the (1 x 4) reconstructions, both experimentally observed, which reveal a different optical response and an anisotropy, in the (001) plane, not present in the bulk phase. The determination of the spatial behavior of the electron-hole photoexcited couple provides a possible explanation of the observed enhanced photocatalytic activity of TiO{sub 2} anatase nanostructures with a high percentage of (001)-(1 x 1) exposed facets.

  1. Exciton-polariton patterns in coherently pumped semiconductor microcavities

    NASA Astrophysics Data System (ADS)

    Werner, Albrecht; Egorov, Oleg A.; Lederer, Falk

    2014-06-01

    We theoretically study spatially periodic exciton-polariton patterns in a semiconductor microcavity coherently driven by an optical pump. Patterns grow spontaneously in a uniformly pumped cavity above a certain threshold (Turing instability). By means of a perturbation analysis in the vicinity of this bifurcation point, we determine the conditions for pattern formation and predict their properties. The shape of those patterns depends strongly on the detuning of the pump frequency from the system's resonances. For instance, near the bottom of the lower branch of the polariton dispersion relation, a hexagonal pattern is the most favorable periodic solution similar to pattern formation in optical cavities endowed with a defocusing Kerr nonlinearity. Towards the excitonic resonance, the influence of the upper polariton branch becomes important giving rise to a diversity of other solutions, such as roll, labyrinthine, and honeycomb patterns. Our numerical simulations reveal patterns with point and line defects and the formation of disordered patterns of a soliton gas.

  2. Exciton size and quantum transport in nanoplatelets

    SciTech Connect

    Pelzer, Kenley M. Gray, Stephen K.; Darling, Seth B.; Schaller, Richard D.

    2015-12-14

    Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.

  3. Theoretical Investigation Optical Properties of Si12C12 Clusters and Oligomers having Potential as Excitonic Materials

    NASA Astrophysics Data System (ADS)

    Duan, Xiaofeng; Burggraf, Larry

    2015-03-01

    SiC clusters may have potential in 2-D exciton circuits. We determined the most stable SinCn isomer structures (n <=12) out of hundreds to thousands isomers using a method combining Stochastic Potential Surface Search and Pseududopotential Plane-Wave Density Functional Theory Car-Parinello Molecular Dynamics simulated annealing (PSPW-CPMD-SA). Four low-energy Si12C12 isomer structures are discussed to illustrate the varying optical properties of clusters with structures: i) cage type with C- and Si- segregations, ii) symmetric type formed having π-stacked C aromatic rings and exterior Si regions, iii) nearly planar bowl with C fullerene fragment surrounded by Si atoms, and iv) symmetrical SiC cluster having alternate SiC bonding in the structure. We employed B3LYP and PBE0 functionals and both cc-pVTZ and aug-cc-pVTZ basis sets to perform TDDFT calculations of excitation energies and photo-absorption spectra to show how structure and bonding patterns affect photo excitations in different types of SiC clusters. The electron and the hole charge distribution patterns in excitation were calculated for major photoabsorption transitions, reported for the most stable isomer, closo Si12C12. To understand electric field effects we also calculated dynamical polarizabilities for all the four structures using Coupled Perturbed Hartree-Fock (CPHF) at B3LYP/aug-cc-pVTZ and PBE0/aug-cc-pVTZ level of theory. We gratefully acknowledge support from the Air Force Office of Scientific Research in a program managed by Dr Michael Berman.

  4. Properties of Type-II ZnTe/ZnSe Submonolayer Quantum Dots Studied via Excitonic Aharonov- Bohm Effect and Polarized Optical Spectroscopy

    NASA Astrophysics Data System (ADS)

    Ji, Haojie

    In this thesis I develop understanding of the fundamental physical and material properties of type-II ZnTe/ZnSe submonolayer quantum dots (QDs), grown via combination of molecular beam epitaxy (MBE) and migration enhanced epitaxy (MEE). I use magneto-photoluminescence, including excitonic Aharonov-Bohm (AB) effect and polarized optical spectroscopy as the primary tools in this work. I present previous studies as well as the background of optical and magneto-optical processes in semiconductor nanostructures and introduce the experimental methods in Chapters 1 - 3. In Chapter 4 I focus on the excitonic AB effect in the type-II QDs. I develop a lateral tightly-bound exciton model for ZnTe/ZnSe type-II QDs, using analytical methods and numerical calculations. This explained the magneto-PL observation and allowed for establishing the size and density of the QDs in each sample based on the results of PL and magneto-PL measurements. For samples with larger QDs, I observe behaviors that fall between properties of quantum-dot and quantum-well-like systems due to increased QD densities and their type-II nature. Finally, the decoherence mechanisms of the AB excitons are investigated via the temperature dependent studies of the magneto-PL. It is determined that the AB exciton decoherence is due to transport-like (acoustic phonon) scattering of the electrons moving in the ZnSe barriers, but with substantially smaller magnitude of electron-phonon coupling constant due to relatively strong electron-hole coupling within these type-II QDs. In Chapter 5 I discuss the results of circularly polarized magneto-PL measurements. A model with ultra-long spin-flip time of holes confined to submonolayer QDs is proposed. The g-factor of type-II excitons was extracted from the Zeeman splitting and the g-factor of electrons was obtained by fitting the temperature dependence of the degree of circular polarization (DCP), from which g-factor of holes confined within ZnTe QDs was found. It is shown

  5. Comparison of magneto-optical properties of various excitonic complexes in CdTe and CdSe self-assembled quantum dots

    NASA Astrophysics Data System (ADS)

    Kobak, J.; Smoleński, T.; Goryca, M.; Rousset, J.-G.; Pacuski, W.; Bogucki, A.; Oreszczuk, K.; Kossacki, P.; Nawrocki, M.; Golnik, A.; Płachta, J.; Wojnar, P.; Kruse, C.; Hommel, D.; Potemski, M.; Kazimierczuk, T.

    2016-07-01

    We present a comparative study of two self-assembled quantum dot (QD) systems based on II-VI compounds: CdTe/ZnTe and CdSe/ZnSe. Using magneto-optical techniques we investigated a large population of individual QDs. The systematic photoluminescence studies of emission lines related to the recombination of neutral exciton X, biexciton XX, and singly charged excitons (X+, X-) allowed us to determine average parameters describing CdTe QDs (CdSe QDs): X-XX transition energy difference 12 meV (24 meV); fine-structure splitting δ1=0.14 meV (δ1=0.47 meV); g-factor g  =  2.12 (g  =  1.71) diamagnetic shift γ=2.5 μeV T-2 (γ =1.3 μeV T-2). We find also statistically significant correlations between various parameters describing internal structure of excitonic complexes.

  6. The dynamics of radiation-driven, optically thick winds

    NASA Astrophysics Data System (ADS)

    Shen, Rong-Feng; Nakar, Ehud; Piran, Tsvi

    2016-06-01

    Recent observation of some luminous transient sources with low colour temperatures suggests that the emission is dominated by optically thick winds driven by super-Eddington accretion. We present a general analytical theory of the dynamics of radiation pressure-driven, optically thick winds. Unlike the classical adiabatic stellar wind solution whose dynamics are solely determined by the sonic radius, here the loss of the radiation pressure due to photon diffusion also plays an important role. We identify two high mass-loss rate regimes (dot{M} > L_Edd/c^2). In the large total luminosity regime, the solution resembles an adiabatic wind solution. Both the radiative luminosity, L, and the kinetic luminosity, Lk, are super-Eddington with L < Lk and L ∝ L_k^{1/3}. In the lower total luminosity regime, most of the energy is carried out by the radiation with Lk < L ≈ LEdd. In a third, low mass-loss regime (dot{M} < L_Edd/c^2), the wind becomes optically thin early on and, unless gas pressure is important at this stage, the solution is very different from the adiabatic one. The results are independent from the energy generation mechanism at the foot of the wind; therefore, they are applicable to a wide range of mass ejection systems, from black hole accretion, to planetary nebulae, and to classical novae.

  7. Optical Frequency Domain Visualization of Electron Beam Driven Plasma Wakefields

    NASA Astrophysics Data System (ADS)

    Zgadzaj, Rafal; Downer, M. C.; Muggli, Patric; Yakimenko, Vitaly; Babzien, Marcus; Kusche, Karl; Fedurin, Mikhail

    2010-11-01

    Beam-driven plasma wakefield accelerators (PWFA), such as the ``plasma afterburner,'' are a promising approach for significantly increasing the particle energies of conventional accelerators. The study and optimization of PWFA would benefit from an experimental correlation between the parameters of the drive bunch, the accelerated bunch and the corresponding, accelerating plasma wave structure. However, the plasma wave structure has not yet been observed directly in PWFA. We will report our current work on noninvasive optical Frequency Domain Interferometric (FDI) and Holographic (FDH) visualization of beam-driven plasma waves. Both techniques employ two laser pulses (probe and reference) co-propagating with the particle drive-beam and its plasma wake. The reference pulse precedes the drive bunch, while the probe overlaps the plasma wave and maps its longitudinal and transverse structure. The experiment is being developed at the BNL/ATF Linac to visualize wakes generated by two and multi-bunch drive beams.

  8. All optical controlled-NOT gate based on an exciton-polariton circuit

    NASA Astrophysics Data System (ADS)

    Solnyshkov, D. D.; Bleu, O.; Malpuech, G.

    2015-07-01

    We propose an implementation of a CNOT gate for quantum computing based on a patterned microcavity polariton system, which can be manufactured using the modern technological facilities. The qubits are encoded in the spin-coherent polariton states. The structure consists of two wire cavities oriented at 45° with a micropillar between them. The polariton spin rotates due to the Longitudinal-Transverse splitting between polarization eigenstates in the wires. In the pillar, the optically generated circularly polarized polariton macrooccupied state plays the role of the control qubit. Because of the spin-anisotropic polariton interaction, it induces an effective magnetic field along the Z-direction with a sign depending on the qubit value.

  9. Suppression of quantum decoherence via infrared-driven coherent exciton-plasmon coupling: Undamped field and Rabi oscillations

    SciTech Connect

    Sadeghi, S. M.; Patty, K. D.

    2014-02-24

    We show that when a semiconductor quantum dot is in the vicinity of a metallic nanoparticle and driven by a mid-infrared laser field, its coherent dynamics caused by interaction with a visible laser field can become free of quantum decoherence. We demonstrate that this process, which can offer undamped Rabi and field oscillations, is the result of coherent normalization of the “effective” polarization dephasing time of the quantum dot (T{sub 2}{sup *}). This process indicates formation of infrared-induced coherently forced oscillations, which allows us to control the value of T{sub 2}{sup *} using the infrared laser. The results offer decay-free ultrafast modulation of the effective field experienced by the quantum dot when neither the visible laser field nor the infrared laser changes with time.

  10. Floquet engineering with quasienergy bands of periodically driven optical lattices

    NASA Astrophysics Data System (ADS)

    Holthaus, Martin

    2016-01-01

    A primer on the Floquet theory of periodically time-dependent quantum systems is provided, and it is shown how to apply this framework for computing the quasienergy band structure governing the dynamics of ultracold atoms in driven optical cosine lattices. Such systems are viewed here as spatially and temporally periodic structures living in an extended Hilbert space, giving rise to spatio-temporal Bloch waves whose dispersion relations can be manipulated at will by exploiting ac-Stark shifts and multiphoton resonances. The elements required for numerical calculations are introduced in a tutorial manner, and some example calculations are discussed in detail, thereby illustrating future prospects of Floquet engineering.

  11. Influence of excitonic oscillator strengths on the optical properties of GaN and ZnO

    NASA Astrophysics Data System (ADS)

    Mallet, E.; Réveret, F.; Disseix, P.; Shubina, T. V.; Leymarie, J.

    2014-07-01

    We report on an extensive study of the excitonic properties of GaN and ZnO bulk samples with an accurate determination of excitonic parameters by linear and nonlinear spectroscopies. The in-depth comparative study is carried out between these two competitive wide band gap semiconductors for a better understanding of damping processes. In GaN, it is shown that due to microscopic disorder, such as lattice strain fluctuations, inhomogeneous broadening prevails over homogeneous broadening at low temperature. The opposite situation occurs in ZnO, where the homogeneous broadening dominates due to resonant Rayleigh scattering of exciton polaritons and their interaction with acoustic phonons. This comparative study also allows us to highlight the influence of oscillator strengths on spectrally resolved four-wave mixing and time-integrated four-wave mixing.

  12. Optical density of states in ultradilute GaAsN alloy: Coexistence of free excitons and impurity band of localized and delocalized states

    SciTech Connect

    Bhuyan, Sumi; Pal, Bipul; Bansal, Bhavtosh; Das, Sanat K.; Dhar, Sunanda

    2014-07-14

    Optically active states in liquid phase epitaxy-grown ultra-dilute GaAsN are studied. The feature-rich low temperature photoluminescence spectrum has contributions from excitonic band states of the GaAsN alloy, and two types of defect states—localized and extended. The degree of delocalization for extended states both within the conduction and defect bands, characterized by the electron temperature, is found to be similar. The degree of localization in the defect band is analyzed by the strength of the phonon replicas. Stronger emission from these localized states is attributed to their giant oscillator strength.

  13. Magneto-optical spectrum and the effective excitonic Zeeman splitting energies of Mn and Co-doped CdSe nanowires

    SciTech Connect

    Xiong, Wen; Chen, Wensuo

    2013-12-21

    The electronic structure of Mn and Co-doped CdSe nanowires are calculated based on the six-band k·p effective-mass theory. Through the calculation, it is found that the splitting energies of the degenerate hole states in Mn-doped CdSe nanowires are larger than that in Co-doped CdSe nanowires when the concentration of these two kinds of magnetic ions is the same. In order to analysis the magneto-optical spectrum of Mn and Co-doped CdSe nanowires, the four lowest electron states and the four highest hole states are sorted when the magnetic field is applied, and the 10 lowest optical transitions between the conduction subbands and the valence subbands at the Γ point in Mn and Co-doped CdSe nanowires are shown in the paper, it is found that the order of the optical transitions at the Γ point almost do not change although two different kinds of magnetic ions are doped in CdSe nanowires. Finally, the effective excitonic Zeeman splitting energies at the Γ point are found to increase almost linearly with the increase of the concentration of the magnetic ions and the magnetic field; meanwhile, the giant positive effective excitonic g factors in Mn and Co-doped CdSe nanowires are predicted based on our theoretical calculation.

  14. Subnanosecond control of excitons in coupled quantum well nanostructures: Photonic storage and Exciton Conveyer devices

    NASA Astrophysics Data System (ADS)

    Winbow, Alexander Graham

    Indirect excitons in GaAs coupled quantum well nanostructures are a versatile system for fundamental study of cold neutral bosonic gases and demonstration of novel optoelectronic devices based on excitons --- a bound electron--hole pair --- rather than electrons. Indirect exciton lifetimes range from nanoseconds to microseconds and cool rapidly after photoexcitation to the lattice temperature. Lithographically-patterned electrodes enable design of potential energy landscapes, and both energy and lifetime can be controlled in situ, rapidly, on timescales much shorter than the exciton lifetime. Such intrinsically optoelectronic devices can operate at speeds relevant to optical networks, and later be fabricated in other semiconductors for higher-temperature operation. Two different kinds of devices are demonstrated: Photon storage --- an optical memory --- with 250 ps rise time of the readout optical signal and storage time reaching microseconds was implemented with indirect excitons in CQW. The storage and release of photons was controlled by the gate voltage pulse, and the transient processes in the CQW studied by measuring the kinetics of the exciton emission spectra. This control of excitons on timescales much shorter than the exciton lifetime demonstrates the feasibility of studying excitons in in situ controlled electrostatic traps. The Exciton Conveyer is a laterally moving electrostatic lattice potential for actively transporting excitons. Generated by laterally modulated electrodes, the potential velocity and depth are controlled in situ by frequency and voltage. We observed exciton transport characterized by average exciton cloud spatial extension over several tens of microns, and observed dynamical localization--delocalization transitions for the excitons in the conveyer: In the localization regime of deeper potentials and moderate exciton density, excitons are moved by the conveyer; in the delocalized regime of shallower lattice potential or high exciton

  15. Spatially indirect excitons in coupled quantum wells

    SciTech Connect

    Lai, Chih-Wei Eddy

    2004-03-01

    observed. The spatial and energy distributions of optically active excitons were used as thermodynamic quantities to construct a phase diagram of the exciton system, demonstrating the existence of distinct phases. Optical and electrical properties of the CQW sample were examined thoroughly to provide deeper understanding of the formation mechanisms of these cold exciton systems. These insights offer new strategies for producing cold exciton systems, which may lead to opportunities for the realization of BEC in solid-state systems.

  16. Optical Frequency Domain Visualization of Electron Beam Driven Plasma Wakefields

    NASA Astrophysics Data System (ADS)

    Zgadzaj, Rafal; Downer, Michael C.; Muggli, Patric; Yakimenko, Vitaly; Kusche, Karl; Fedurin, Michhail; Babzien, Marcus

    2010-11-01

    Bunch driven plasma wakefield accelerators (PWFA), such as the "plasma afterburner," are a promising emerging method for significantly increasing the energy output of conventional particle accelerators [1]. The study and optimization of this method would benefit from an experimental correlation of the drive bunch parameters and the accelerated particle parameters with the corresponding plasma wave structure. However, the plasma wave structure has not been observed directly so far. We will report ongoing development of a noninvasive optical Frequency Domain Interferometric (FDI) [2] and Holographic (FDH) [3] diagnostics of bunch driven plasma wakes. Both FDI and FDH have been previously demonstrated in the case of laser driven wakes. These techniques employ two laser pulses co-propagating with the drive particle bunch and the trailing plasma wave. One pulse propagates ahead of the drive bunch and serves as a reference, while the second is overlapped with the plasma wave and probes its structure. The multi-shot FDI and single-shot FDH diagnostics permit direct noninvasive observation of longitudinal and transverse structure of the plasma wakes. The experiment is being developed at the 70 MeV Linac in the Accelerator Test Facility at Brookhaven National Laboratory to visualize wakes generated by two [4] and multi-bunch [5] drive beams.

  17. Contrastive analysis of multiple exciton generation theories

    NASA Astrophysics Data System (ADS)

    Tan, Hengyu; Chang, Qing

    2015-10-01

    Multiple exciton generation (MEG) is an effect that semiconductor nanocrystals (NCs) quantum dots (QDs) generate multiple excitons (electron-hole pairs) through absorbing a single high energy photon. It can translate the excess photon energy of bandgap (Eg) into new excitons instead of heat loss and improve the photovoltaic performance of solar cells. However, the theories of MEG are not uniform. The main MEG theories can be divided into three types. The first is impact ionization. It explains MEG through a conventional way that a photogenerated exciton becomes multiple excitons by Coulomb interactions between carriers. The Second is coherent superposition of excitonic states. Multiple excitons are generated by the coherent superposition of single photogenerated exciton state with enough excess momentum and the two-exciton state with the same momentum. The third is excitation via virtual excitonic states. The nanocrystals vacuum generates a virtual biexciton by coulomb coupling between two valence band electrons. The virtual biexciton absorbing a photon with an intraband optical transition is converted into a real biexciton. This paper describes the MEG influence on solar photoelectric conversion efficiency, concludes and analyzes the fundamentals of different MEG theories, the MEG experimental measure, their merits and demerits, calculation methods of generation efficiency.

  18. Exciton spin dynamics in GaSe

    SciTech Connect

    Tang, Yanhao; Xie, Wei; McGuire, John A. Lai, Chih Wei; Mandal, Krishna C.

    2015-09-21

    We analyze exciton spin dynamics in GaSe under nonresonant circularly polarized optical pumping with an exciton spin-flip rate-equation model. The model reproduces polarized time-dependent photoluminescence measurements in which the initial circular polarization approaches unity even when pumping with 0.15 eV excess energy. At T = 10 K, the exciton spin relaxation exhibits a biexponential decay with sub-20 ps and >500 ps time constants, which are also reproduced by the rate-equation model assuming distinct spin-relaxation rates for hot (nonequilibrium) and cold band-edge excitons.

  19. Two-Dimensional Superfluidity of Exciton Polaritons Requires Strong Anisotropy

    NASA Astrophysics Data System (ADS)

    Altman, Ehud; Sieberer, Lukas M.; Chen, Leiming; Diehl, Sebastian; Toner, John

    2015-01-01

    Fluids of exciton polaritons, excitations of two-dimensional quantum wells in optical cavities, show collective phenomena akin to Bose condensation. However, a fundamental difference from standard condensates stems from the finite lifetime of these excitations, which necessitates continuous driving to maintain a steady state. A basic question is whether a two-dimensional condensate with long-range algebraic correlations can exist under these nonequilibrium conditions. Here, we show that such driven two-dimensional Bose systems cannot exhibit algebraic superfluid order except in low-symmetry, strongly anisotropic systems. Our result implies, in particular, that recent apparent evidence for Bose condensation of exciton polaritons must be an intermediate-scale crossover phenomenon, while the true long-distance correlations fall off exponentially. We obtain these results through a mapping of the long-wavelength condensate dynamics onto the anisotropic Kardar-Parisi-Zhang equation.

  20. Laser-driven Sisyphus cooling in an optical dipole trap

    SciTech Connect

    Ivanov, Vladyslav V.; Gupta, Subhadeep

    2011-12-15

    We propose a laser-driven Sisyphus-cooling scheme for atoms confined in a far-off resonance optical dipole trap. Utilizing the differential trap-induced ac Stark shift, two electronic levels of the atom are resonantly coupled by a cooling laser preferentially near the trap bottom. After absorption of a cooling photon, the atom loses energy by climbing the steeper potential, and then spontaneously decays preferentially away from the trap bottom. The proposed method is particularly suited to cooling alkaline-earth-metal-like atoms where two-level systems with narrow electronic transitions are present. Numerical simulations for the cases of {sup 88}Sr and {sup 174}Yb demonstrate the expected recoil and Doppler temperature limits. The method requires a relatively small number of scattered photons and can potentially lead to phase-space densities approaching quantum degeneracy in subsecond time scales.

  1. Intrinsic optical bistability in a strongly driven Rydberg ensemble

    NASA Astrophysics Data System (ADS)

    de Melo, Natalia R.; Wade, Christopher G.; Šibalić, Nikola; Kondo, Jorge M.; Adams, Charles S.; Weatherill, Kevin J.

    2016-06-01

    We observe and characterize intrinsic optical bistability in a dilute Rydberg vapor. The bistability is characterized by sharp jumps between states of low and high Rydberg occupancy with jump-up and -down positions displaying hysteresis depending on the direction in which the control parameter is changed. We find that the shift in frequency of the jump point scales with the fourth power of the principal quantum number. Also, the width of the hysteresis window increases with increasing principal quantum number, before reaching a peak and then closing again. The experimental results are consistent with predictions from a simple theoretical model based on semiclassical Maxwell-Bloch equations including the effects of interaction-induced broadening and level shifts. These results provide insight into the dynamics of driven dissipative systems.

  2. Optically driven Archimedes micro-screws for micropump application.

    PubMed

    Lin, Chih-Lang; Vitrant, Guy; Bouriau, Michel; Casalegno, Roger; Baldeck, Patrice L

    2011-04-25

    Archimedes micro-screws have been fabricated by three-dimensional two-photon polymerization using a Nd:YAG Q-switched microchip laser at 532nm. Due to their small sizes they can be easily manipulated, and made to rotate using low power optical tweezers. Rotation rates up to 40 Hz are obtained with a laser power of 200 mW, i.e. 0.2 Hz/mW. A photo-driven micropump action in a microfluidic channel is demonstrated with a non-optimized flow rate of 6 pL/min. The optofluidic properties of such type of Archimedes micro-screws are quantitatively described by the conservation of momentum that occurs when the laser photons are reflected on the helical micro-screw surface.

  3. Rate equations model and optical external efficiency of optically pumped electrically driven terahertz quantum cascade lasers

    NASA Astrophysics Data System (ADS)

    Hamadou, A.; Thobel, J.-L.; Lamari, S.

    2016-10-01

    A four level rate equations model for a terahertz optically pumped electrically driven quantum cascade laser is here introduced and used to model the system both analytically and numerically. In the steady state, both in the presence and absence of the terahertz optical field, we solve the resulting nonlinear system of equations and obtain closed form expressions for the levels occupation, population inversion as well as the mid-infrared pump threshold intensity in terms of the device parameters. We also derive, for the first time for this system, an analytical formula for the optical external efficiency and analyze the simultaneous effects of the cavity length and pump intensity on it. At moderate to high pump intensities, we find that the optical external efficiency scales roughly as the reciprocal of the cavity length.

  4. Excitations, optical absorption spectra, and optical excitonic gaps of heterofullerenes: I. C60, C59N+ and C48N12

    SciTech Connect

    Xie, R; Bryant, G W; Sun, G; C.Nicklaus, M; Heringer, D; Frauenheim, T; Manaa, M R; Smith, Jr., V H; Araki, Y; Ito, O

    2003-10-02

    Low-energy excitations and optical absorption spectrum of C{sub 60} are computed by using time-dependent (TD) Hartree-Fock (HF), TD-density functional theory (TD-DFT), TD-DFT-based tight-binding (TD-DFT-TB) and a semiempirical ZINDO method. A detailed comparison of experiment and theory for the excitation energies, optical gap and absorption spectrum of C{sub 60} is presented. It is found that electron correlations and collective effects of exciton pairs play important roles in assigning accurately the spectral features of C{sub 60} and the TD-DFT method with non-hybrid functionals or a local spin density approximation leads to more accurate excitation energies than with hybrid functionals. The level of agreement between theory and experiment for C{sub 60} justifies similar calculations of the excitations and optical absorption spectrum of a monomeric azafullerene cation C{sub 59}N{sup +} exhibits distinguishing spectral features different from C{sub 60}: (1) the first singlet is dipole-allowed and the optical gap is redshifted by 1.44 eV; (2) several weaker absorption maxima occur in the visible region; (3) the transient triplet-triplet absorption at 1.60 eV (775 nm) is much broader and the decay of the triplet state is much faster. The calculated spectra of C{sub 59}N{sup +} characterize and explain well our measured ultraviolet-visible (UV-vis) and transient absorption spectra of the carborane anion salt [C{sub 59}N][Ag(CB{sub 11}H{sub 6}Cl{sub 6}){sub 2}]. For the most stable isomer of C{sub 48}N{sub 12}, we predict that the first singlet is dipole-allowed, the optical gap is redshifted by 1.22 eV relative to that of C{sub 60}, and optical absorption maxima occur at 585, 528, 443, 363, 340, 314 and 303 nm. We point out that the characterization of the UV-vis and transient absorption spectra of C{sub 48}N{sub 12} isomers is helpful in distinguishing the isomer structures required for applications in molecular electronics. For C{sub 59}N{sup +} and C{sub 48}N

  5. Ultrafast dynamical response of the lower exciton-polariton branch in CdZnTe

    NASA Astrophysics Data System (ADS)

    Lohrenz, J.; Melzer, S.; Ruppert, C.; Akimov, I. A.; Mariette, H.; Reichelt, M.; Trautmann, A.; Meier, T.; Betz, M.

    2016-02-01

    We investigate the transient optical response in high-quality Cd0.88Zn0.12Te crystals in the regime of slow light propagation on the lower exciton-polariton branch. Femtosecond photoexcitation leads to very substantial transmission changes in a ˜10 -meV broad spectral range within the transparency window of the unexcited semiconductor. These nonlinear optical signatures decay on picosecond time scales governed by carrier thermalization and recombination. The temporal and spectral dependence indicate the dynamical optical response as arising from excitation-induced dephasing and perturbed free induction decay. Model simulations for the optical response taking into account the actual exciton-polariton dispersion and excitation-induced dephasing of a nonlinearly driven two-level system support this interpretation.

  6. Optically driven resonance of nanoscale flexural oscillators in liquid.

    PubMed

    Verbridge, Scott S; Bellan, Leon M; Parpia, Jeevak M; Craighead, H G

    2006-09-01

    We demonstrate the operation of radio frequency nanoscale flexural resonators in air and liquid. Doubly clamped string, as well as singly clamped cantilever resonators, with nanoscale cross-sectional dimensions and resonant frequencies as high as 145 MHz are driven in air as well as liquid with an amplitude modulated laser. We show that this laser drive technique can impart sufficient energy to a nanoscale resonator to overcome the strong viscous damping present in these media, resulting in a mechanical resonance that can be measured by optical interference techniques. Resonance in air, isopropyl alcohol, acetone, water, and phosphate-buffered saline is demonstrated for devices having cross-sectional dimensions close to 100 nm. For operation in air, quality factors as high as 400 at 145 MHz are demonstrated. In liquid, quality factors ranging from 3 to 10 and frequencies ranging from 20 to 100 MHz are observed. These devices, and an all-optical actuation and detection system, may provide insight into the physics of the interaction of nanoscale mechanical structures with their environments, greatly extending the viscosity range over which such small flexural resonant devices can be operated. PMID:16968035

  7. Optically driven quantum networks: Applications in molecular electronics

    NASA Astrophysics Data System (ADS)

    Körner, H.; Mahler, G.

    1993-07-01

    Progress in nanostructuring tends to provide us with synthetic structures for which, for example, energy or time scales can be adjusted in such a way that quantum systems with unusual physical properties emerge. The challenge of molecular electronics is to make these properties represent computer functions. We investigate a quantum network model consisting of a modular array of localized few-level subsystems. When driven optically, a diagonal (energy renormalizing) interaction among these subsystems is shown to lead to a complex stochastic dynamics, which may be interpreted as a highly parallel Monte-Carlo-type simulation ``programmed'' by the external light field. A first application is demonstrated in terms of a two-dimensional kinetic Ising model with J(Rn-Rm)~||Rn-Rm||-3. In another application the nonlocal nonlinear optical properties are exploited in specific pump and probe scenarios: Under certain conditions simple image processing tasks are performed. A possible realization of such quantum network models by an array of charge-transfer quantum dots is discussed.

  8. Theory of core excitons

    SciTech Connect

    Dow, J. D.; Hjalmarson, H. P.; Sankey, O. F.; Allen, R. E.; Buettner, H.

    1980-01-01

    The observation of core excitons with binding energies much larger than those of the valence excitons in the same material has posed a long-standing theoretical problem. A proposed solution to this problem is presented, and Frenkel excitons and Wannier excitons are shown to coexist naturally in a single material. (GHT)

  9. A silicon-nanowire memory driven by optical gradient force induced bistability

    SciTech Connect

    Dong, B.; Cai, H. Gu, Y. D.; Kwong, D. L.; Chin, L. K.; Ng, G. I.; Ser, W.; Huang, J. G.; Yang, Z. C.; Liu, A. Q.

    2015-12-28

    In this paper, a bistable optical-driven silicon-nanowire memory is demonstrated, which employs ring resonator to generate optical gradient force over a doubly clamped silicon-nanowire. Two stable deformation positions of a doubly clamped silicon-nanowire represent two memory states (“0” and “1”) and can be set/reset by modulating the light intensity (<3 mW) based on the optical force induced bistability. The time response of the optical-driven memory is less than 250 ns. It has applications in the fields of all optical communication, quantum computing, and optomechanical circuits.

  10. Transmutation of skyrmions to half-solitons driven by the nonlinear optical spin Hall effect.

    PubMed

    Flayac, H; Solnyshkov, D D; Shelykh, I A; Malpuech, G

    2013-01-01

    We show that the spin domains, generated in the linear optical spin Hall effect by the analog of spin-orbit interaction for exciton polaritons, are associated with the formation of a Skyrmion lattice. In the nonlinear regime, the spin anisotropy of the polariton-polariton interactions results in a spatial compression of the domains and in a transmutation of the Skyrmions into oblique half-solitons. This phase transition is associated with both the focusing of the spin currents and the emergence of a strongly anisotropic emission pattern.

  11. Bright and dark excitons in semiconductor carbon nanotubes

    SciTech Connect

    Tretiak, Sergei

    2008-01-01

    We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low

  12. Simulation of Multi-Dimensional Signals in the Optical Domain: Quantum-Classical Feedback in Nonlinear Exciton Propagation.

    PubMed

    Richter, Martin; Fingerhut, Benjamin P

    2016-07-12

    We present an algorithm for the simulation of nonlinear 2D spectra of molecular systems in the UV-vis spectral region from atomistic molecular dynamics trajectories subject to nonadiabatic relaxation. We combine the nonlinear exciton propagation (NEP) protocol, that relies on a quasiparticle approach with the surface hopping methodology to account for quantum-classical feedback during the dynamics. Phenomena, such as dynamic Stokes shift due to nuclear relaxation, spectral diffusion, and population transfer among electronic states, are thus naturally included and benchmarked on a model of two electronic states coupled to a harmonic coordinate and a classical heatbath. The capabilities of the algorithm are further demonstrated for the bichromophore diphenylmethane that is described in a fully microscopic fashion including all 69 classical nuclear degrees of freedom. We demonstrate that simulated 2D signals are especially sensitive to the applied theoretical approximations (i.e., choice of active space in the CASSCF method) where population dynamics appears comparable. PMID:27248511

  13. Excitons in the Fractional Quantum Hall Effect

    DOE R&D Accomplishments Database

    Laughlin, R. B.

    1984-09-01

    Quasiparticles of charge 1/m in the Fractional Quantum Hall Effect form excitons, which are collective excitations physically similar to the transverse magnetoplasma oscillations of a Wigner crystal. A variational exciton wavefunction which shows explicitly that the magnetic length is effectively longer for quasiparticles than for electrons is proposed. This wavefunction is used to estimate the dispersion relation of these excitons and the matrix elements to generate them optically out of the ground state. These quantities are then used to describe a type of nonlinear conductivity which may occur in these systems when they are relatively clean.

  14. Cavity-Enhanced Transport of Excitons

    NASA Astrophysics Data System (ADS)

    Schachenmayer, Johannes; Genes, Claudiu; Tignone, Edoardo; Pupillo, Guido

    2015-05-01

    We show that exciton-type transport in certain materials can be dramatically modified by their inclusion in an optical cavity: the modification of the electromagnetic vacuum mode structure introduced by the cavity leads to transport via delocalized polariton modes rather than through tunneling processes in the material itself. This can help overcome exponential suppression of transmission properties as a function of the system size in the case of disorder and other imperfections. We exemplify massive improvement of transmission for excitonic wave packets through a cavity, as well as enhancement of steady-state exciton currents under incoherent pumping. These results may have implications for experiments of exciton transport in disordered organic materials. We propose that the basic phenomena can be observed in quantum simulators made of Rydberg atoms, cold molecules in optical lattices, as well as in experiments with trapped ions.

  15. Excitonic polarons in low-dimensional transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Thilagam, A.

    2015-05-01

    We examine the excitonic polaron properties of common monolayer transition metal dichalcogenides (MoS2, MoSe2, WS2 and WSe2). The excitonic polaron is formed when excitons interact with acoustic or optical phonons via coupling to the deformation potentials associated with the conduction and valence bands. A unitary transformation which performs an approximate diagonalization of the exciton-phonon operator is used to evaluate the ground state energy of the excitonic polaron. We derive analytical expressions of the changes in the excitonic polaron energy and mass at small exciton wavevectors involving the deformation potential due to optical phonons. The polaronic effect of the monolayer transition metal dichalcogenides is examined by comparing changes in the energy gap shift and effective masses based on known deformation potential constants for carrier-phonon interactions. Our results indicate the occurrence of comparable energy shifts when the ground state exciton interacts with optical or acoustic phonons. We extend our calculations to explore the influence of exciton-lattice interactions on the binding energies and the self-trapping of excitons in two-dimensional layers of transition metal dichalcogenides.

  16. Optical and transport properties correlation driven by amorphous/crystalline disorder in InP nanowires.

    PubMed

    Kamimura, H; Gouveia, R C; Carrocine, S C; Souza, L D; Rodrigues, A D; Teodoro, M D; Marques, G E; Leite, E R; Chiquito, A J

    2016-11-30

    Indium phosphide nanowires with a single crystalline zinc-blend core and polycrystalline/amorphous shell were grown from a reliable route without the use of hazardous precursors. The nanowires are composed by a crystalline core covered by a polycrystalline shell, presenting typical lengths larger than 10 μm and diameters of 80-90 nm. Raman spectra taken from as-grown nanowires exhibited asymmetric line shapes with broadening towards higher wave numbers which can be attributed to phonon localization effects. It was found that optical phonons in the nanowires are localized in regions with average size of 3 nm, which seems to have the same order of magnitude of grain sizes in the polycrystalline shell. Regardless of the fact that the nanowires exhibit a crystalline core, any considerable degree of disorder can lead to a localized behaviour of carriers. In consequence, the variable range hopping was observed as the main transport instead of the usual thermal excitation mechanisms. Furthermore the hopping length was ten times smaller than nanowire cross-sections, confirming that the nanostructures do behave as a 3D system. Accordingly, the V-shape observed in PL spectra clearly demonstrates a very strong influence of the potential fluctuations on the exciton optical recombination. Such fluctuations can still be observed at low temperature regime, confirming that the amorphous/polycrystalline shell of the nanowires affects the exciton recombination in every laser power regime tested. PMID:27662434

  17. Optical and transport properties correlation driven by amorphous/crystalline disorder in InP nanowires.

    PubMed

    Kamimura, H; Gouveia, R C; Carrocine, S C; Souza, L D; Rodrigues, A D; Teodoro, M D; Marques, G E; Leite, E R; Chiquito, A J

    2016-11-30

    Indium phosphide nanowires with a single crystalline zinc-blend core and polycrystalline/amorphous shell were grown from a reliable route without the use of hazardous precursors. The nanowires are composed by a crystalline core covered by a polycrystalline shell, presenting typical lengths larger than 10 μm and diameters of 80-90 nm. Raman spectra taken from as-grown nanowires exhibited asymmetric line shapes with broadening towards higher wave numbers which can be attributed to phonon localization effects. It was found that optical phonons in the nanowires are localized in regions with average size of 3 nm, which seems to have the same order of magnitude of grain sizes in the polycrystalline shell. Regardless of the fact that the nanowires exhibit a crystalline core, any considerable degree of disorder can lead to a localized behaviour of carriers. In consequence, the variable range hopping was observed as the main transport instead of the usual thermal excitation mechanisms. Furthermore the hopping length was ten times smaller than nanowire cross-sections, confirming that the nanostructures do behave as a 3D system. Accordingly, the V-shape observed in PL spectra clearly demonstrates a very strong influence of the potential fluctuations on the exciton optical recombination. Such fluctuations can still be observed at low temperature regime, confirming that the amorphous/polycrystalline shell of the nanowires affects the exciton recombination in every laser power regime tested.

  18. Optical and transport properties correlation driven by amorphous/crystalline disorder in InP nanowires

    NASA Astrophysics Data System (ADS)

    Kamimura, H.; Gouveia, R. C.; Carrocine, S. C.; Souza, L. D.; Rodrigues, A. D.; Teodoro, M. D.; Marques, G. E.; Leite, E. R.; Chiquito, A. J.

    2016-11-01

    Indium phosphide nanowires with a single crystalline zinc-blend core and polycrystalline/amorphous shell were grown from a reliable route without the use of hazardous precursors. The nanowires are composed by a crystalline core covered by a polycrystalline shell, presenting typical lengths larger than 10 μm and diameters of 80-90 nm. Raman spectra taken from as-grown nanowires exhibited asymmetric line shapes with broadening towards higher wave numbers which can be attributed to phonon localization effects. It was found that optical phonons in the nanowires are localized in regions with average size of 3 nm, which seems to have the same order of magnitude of grain sizes in the polycrystalline shell. Regardless of the fact that the nanowires exhibit a crystalline core, any considerable degree of disorder can lead to a localized behaviour of carriers. In consequence, the variable range hopping was observed as the main transport instead of the usual thermal excitation mechanisms. Furthermore the hopping length was ten times smaller than nanowire cross-sections, confirming that the nanostructures do behave as a 3D system. Accordingly, the V-shape observed in PL spectra clearly demonstrates a very strong influence of the potential fluctuations on the exciton optical recombination. Such fluctuations can still be observed at low temperature regime, confirming that the amorphous/polycrystalline shell of the nanowires affects the exciton recombination in every laser power regime tested.

  19. Laser pulse induced multiple exciton kinetics in molecular ring structures

    NASA Astrophysics Data System (ADS)

    Hou, Xiao; Wang, Luxia

    2016-11-01

    Multiple excitons can be formed upon strong optical excitation of molecular aggregates and complexes. Based on a theoretical approach on exciton-exciton annihilation dynamics in supramolecular systems (May et al., 2014), exciton interaction kinetics in ring aggregates of two-level molecules are investigated. Excited by the sub-picosecond laser pulse, multiple excitons keep stable in the molecular ring shaped as a regular polygon. If the symmetry is destroyed by changing the dipole of a single molecule, the excitation of different molecules becomes not identical, and the changed dipole-dipole interaction initiates subsequent energy redistribution. Depending on the molecular distance and the dipole configuration, the kinetics undergo different types of processes, but all get stable within some hundreds of femtoseconds. The study of exciton kinetics will be helpful for further investigations of the efficiency of optical devices based on molecular aggregates.

  20. Exciton-Polariton Dynamics in Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Bondarev, Igor

    2007-03-01

    This work addresses theoretically the nonlinear response of phonon-coupled excitons[1] in carbon nanotubes to an external electromagnetic field. The photon Green's function approach developed recently to quantize the electromagnetic field in the presence of quasi-1D absorbing bodies[2],[3] is being used to study the dynamics of phonon-coupled excitonic states interacting with the surface photonic modes excited by the external electromagnetic field in semiconductor carbon nanotubes. The formation of the new elementary excitations, exciton-polaritons, representing the eigen states of the full photon-matter Hamiltonian has been studied for small-diameter nanotubes under strong exciton-photon coupling. Time-resolved simulations have been performed of the coherent exciton- polariton dynamics with the exciton-phonon interactions taken into account. The criteria for the coherent control of the excitonic states population in optically excited carbon nanotubes have been formulated. [1]F.Plentz et al, Phys. Rev. Lett. 95, 247401 (2005). [2]I.V.Bondarev and Ph.Lambin, Phys. Rev. B 72, 035451 (2005). [3]I.V.Bondarev and Ph.Lambin, in: Trends in Nanotubes Reasearch (NovaScience, New York, 2006), p.139.

  1. Exciton radiative lifetime in transition metal dichalcogenide monolayers

    NASA Astrophysics Data System (ADS)

    Robert, C.; Lagarde, D.; Cadiz, F.; Wang, G.; Lassagne, B.; Amand, T.; Balocchi, A.; Renucci, P.; Tongay, S.; Urbaszek, B.; Marie, X.

    2016-05-01

    We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For MoS e2 monolayer, we measure τrad0=1.8 ±0.2 ps at T =7 K that we interpret as the intrinsic radiative recombination time. Similar values are found for WS e2 monolayers. Our detailed analysis suggests the following scenario: at low temperature (T ≲50 K ), the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton-phonon interactions. Following this first nonthermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and nonradiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.

  2. Excitons in boron nitride single layer

    NASA Astrophysics Data System (ADS)

    Galvani, Thomas; Paleari, Fulvio; Miranda, Henrique P. C.; Molina-Sánchez, Alejandro; Wirtz, Ludger; Latil, Sylvain; Amara, Hakim; Ducastelle, François

    2016-09-01

    Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed in the bulk and still stronger effects are predicted for single layers. We present here a detailed study of these properties by combining ab initio calculations and a tight-binding Wannier analysis in both real and reciprocal space. Due to the simplicity of the band structure with single valence (π ) and conduction (π*) bands the tight-binding analysis becomes quasiquantitative with only two adjustable parameters and provides tools for a detailed analysis of the exciton properties. Strong deviations from the usual hydrogenic model are evidenced. The ground-state exciton is not a genuine Frenkel exciton, but a very localized tightly bound one. The other ones are similar to those found in transition-metal dichalcogenides and, although more localized, can be described within a Wannier-Mott scheme.

  3. Exciton absorption in narrow armchair graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Monozon, B. S.; Schmelcher, P.

    2016-11-01

    We develop an analytical approach to the exciton optical absorption for narrow gap armchair graphene nanoribbons (AGNR). We focus on the regime of dominant size quantization in combination with the attractive electron-hole interaction. An adiabatic separation of slow and fast motions leads via the two-body Dirac equation to the isolated and coupled subband approximations. Discrete and continuous exciton states are in general coupled and form quasi-Rydberg series of purely discrete and resonance type character. The corresponding oscillator strengths and widths are derived. We show that the exciton peaks are blue-shifted, become broader and increase in magnitude upon narrowing the ribbon. At the edge of a subband the singularity related to the 1D density of states is transformed into finite absorption via the presence of the exciton. Our analytical results are in good agreement with those obtained by other methods including numerical approaches. Estimates of the expected experimental values are provided for realistic AGNR.

  4. Revealing and Characterizing Dark Excitons through Coherent Multidimensional Spectroscopy.

    PubMed

    Tollerud, Jonathan O; Cundiff, Steven T; Davis, Jeffrey A

    2016-08-26

    Dark excitons are of fundamental importance in a broad range of contexts but are difficult to study using conventional optical spectroscopy due to their weak interaction with light. We show how coherent multidimensional spectroscopy can reveal and characterize dark states. Using this approach, we identify parity-forbidden and spatially indirect excitons in InGaAs/GaAs quantum wells and determine details regarding lifetimes, homogeneous and inhomogeneous linewidths, broadening mechanisms, and coupling strengths. The observations of coherent coupling between these states and bright excitons hint at a role for a multistep process by which excitons in the barrier can relax into the quantum wells. PMID:27610881

  5. Revealing and Characterizing Dark Excitons through Coherent Multidimensional Spectroscopy

    NASA Astrophysics Data System (ADS)

    Tollerud, Jonathan O.; Cundiff, Steven T.; Davis, Jeffrey A.

    2016-08-01

    Dark excitons are of fundamental importance in a broad range of contexts but are difficult to study using conventional optical spectroscopy due to their weak interaction with light. We show how coherent multidimensional spectroscopy can reveal and characterize dark states. Using this approach, we identify parity-forbidden and spatially indirect excitons in InGaAs/GaAs quantum wells and determine details regarding lifetimes, homogeneous and inhomogeneous linewidths, broadening mechanisms, and coupling strengths. The observations of coherent coupling between these states and bright excitons hint at a role for a multistep process by which excitons in the barrier can relax into the quantum wells.

  6. Binding energies of indirect excitons in double quantum well systems

    NASA Astrophysics Data System (ADS)

    Rossokhaty, Alex; Schmult, Stefan; Dietsche, Werner; von Klitzing, Klaus; Kukushkin, Igor

    2011-03-01

    A prerequisite towards Bose-Einstein condensation is a cold and dense system of bosons. Indirect excitons in double GaAs/AlGaAs quantum wells (DQWs) are believed to be suitable candidates. Indirect excitons are formed in asymmetric DQW structures by mass filtering, a method which does not require external electric fields. The exciton density and the electron-hole balance can be tuned optically. Binding energies are measured by a resonant microwave absorption technique. Our results show that screening of the indirect excitons becomes already relevant at densities as low as ~ 5 × 109 cm-2 and results in their destruction.

  7. Optically induced excitonic electroabsorption in a periodically delta-doped InGaAs/GaAs multiple quantum well structure

    NASA Technical Reports Server (NTRS)

    Larsson, A.; Maserjian, J.

    1991-01-01

    Large optically induced Stark shifts have been observed in a periodically delta-doped InGaAs/GaAs multiple quantum well structure. With an excitation intensity of 10 mW/sq cm, an absolute quantum well absorption change of 7000/cm was measured with a corresponding differential absorption change as high as 80 percent. The associated maximum change in the quantum well refractive index is 0.04. This material is promising for device development for all-optical computing and signal processing.

  8. Micro-Ball-Lens Optical Switch Driven by SMA Actuator

    NASA Technical Reports Server (NTRS)

    Yang, Eui-Hyeok

    2003-01-01

    The figure is a simplified cross section of a microscopic optical switch that was partially developed at the time of reporting the information for this article. In a fully developed version, light would be coupled from an input optical fiber to one of two side-by-side output optical fibers. The optical connection between the input and the selected output fiber would be made via a microscopic ball lens. Switching of the optical connection from one output fiber to another would be effected by using a pair of thin-film shape-memory-alloy (SMA) actuators to toggle the lens between two resting switch positions. There are many optical switches some made of macroscopic parts by conventional fabrication techniques and some that are microfabricated and, hence, belong to the class of microelectromechanical systems (MEMS). Conventionally fabricated optical switches tend to be expensive. MEMS switches can be mass-produced at relatively low cost, but their attractiveness has been diminished by the fact that, heretofore, MEMS switches have usually been found to exhibit high insertion losses. The present switch is intended to serve as a prototype of low-loss MEMS switches. In addition, this is the first reported SMA-based optical switch. The optical fibers would be held in V grooves in a silicon frame. The lens would have a diameter of 1 m; it would be held by, and positioned between, the SMA actuators, which would be made of thin films of TiNi alloy. Although the SMA actuators are depicted here as having simple shapes for the sake of clarity of illustration, the real actuators would have complex, partly net-like shapes. With the exception of the lens and the optical fibers, the SMA actuators and other components of the switch would be made by microfabrication techniques. The components would be assembled into a sandwich structure to complete the fabrication of the switch. To effect switching, an electric current would be passed through one of the SMA actuators to heat it above

  9. Determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer quantum dots via spectral analysis of optical signature of the Aharanov-Bohm excitons

    SciTech Connect

    Ji, Haojie; Dhomkar, Siddharth; Roy, Bidisha; Kuskovsky, Igor L.; Shuvayev, Vladimir; Deligiannakis, Vasilios; Tamargo, Maria C.; Ludwig, Jonathan; Smirnov, Dmitry; Wang, Alice

    2014-10-28

    For submonolayer quantum dot (QD) based photonic devices, size and density of QDs are critical parameters, the probing of which requires indirect methods. We report the determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer QDs, based on spectral analysis of the optical signature of Aharanov-Bohm (AB) excitons, complemented by photoluminescence studies, secondary-ion mass spectroscopy, and numerical calculations. Numerical calculations are employed to determine the AB transition magnetic field as a function of the type-II QD radius. The study of four samples grown with different tellurium fluxes shows that the lateral size of QDs increases by just 50%, even though tellurium concentration increases 25-fold. Detailed spectral analysis of the emission of the AB exciton shows that the QD radii take on only certain values due to vertical correlation and the stacked nature of the QDs.

  10. Excitonic gap formation and condensation in the bilayer graphene structure

    NASA Astrophysics Data System (ADS)

    Apinyan, V.; Kopeć, T. K.

    2016-09-01

    We have studied the excitonic gap formation in the Bernal Stacked, bilayer graphene (BLG) structures at half-filling. Considering the local Coulomb interaction between the layers, we calculate the excitonic gap parameter and we discuss the role of the interlayer and intralayer Coulomb interactions and the interlayer hopping on the excitonic pair formation in the BLG. Particularly, we predict the origin of excitonic gap formation and condensation, in relation to the furthermost interband optical transition spectrum. The general diagram of excitonic phase transition is given, explaining different interlayer correlation regimes. The temperature dependence of the excitonic gap parameter is shown and the role of the chemical potential, in the BLG, is discussed in details.

  11. Microgravity-Driven Optic Nerve/Sheath Biomechanics Simulations

    NASA Technical Reports Server (NTRS)

    Ethier, C. R.; Feola, A.; Myers, J. G.; Nelson, E.; Raykin, J.; Samuels, B.

    2016-01-01

    Visual Impairment and Intracranial Pressure (VIIP) syndrome is a concern for long-duration space flight. Current thinking suggests that the ocular changes observed in VIIP syndrome are related to cephalad fluid shifts resulting in altered fluid pressures [1]. In particular, we hypothesize that increased intracranial pressure (ICP) drives connective tissue remodeling of the posterior eye and optic nerve sheath (ONS). We describe here finite element (FE) modeling designed to understand how altered pressures, particularly altered ICP, affect the tissues of the posterior eye and optic nerve sheath (ONS) in VIIP. METHODS: Additional description of the modeling methodology is provided in the companion IWS abstract by Feola et al. In brief, a geometric model of the posterior eye and optic nerve, including the ONS, was created and the effects of fluid pressures on tissue deformations were simulated. We considered three ICP scenarios: an elevated ICP assumed to occur in chronic microgravity, and ICP in the upright and supine positions on earth. Within each scenario we used Latin hypercube sampling (LHS) to consider a range of ICPs, ONH tissue mechanical properties, intraocular pressures (IOPs) and mean arterial pressures (MAPs). The outcome measures were biomechanical strains in the lamina cribrosa, optic nerve and retina; here we focus on peak values of these strains, since elevated strain alters cell phenotype and induce tissue remodeling. In 3D, the strain field can be decomposed into three orthogonal components, denoted as first, second and third principal strains. RESULTS AND CONCLUSIONS: For baseline material properties, increasing ICP from 0 to 20 mmHg significantly changed strains within the posterior eye and ONS (Fig. 1), indicating that elevated ICP affects ocular tissue biomechanics. Notably, strains in the lamina cribrosa and retina became less extreme as ICP increased; however, within the optic nerve, the occurrence of such extreme strains greatly increased as

  12. Streaked Optical Pyrometer System for Laser-Driven Shock-Wave Experiments on OMEGA

    SciTech Connect

    Miller, J.E.; Boehly, T.R.; Melchior, Meyerhofer, D.D.; Celliers, P.M.; Eggert, J.H.; Hicks, D.G.; Sorce, C.M.; Oertel, J.A.; Emmel, P.M.

    2007-03-23

    The temperature of laser-driven shock waves is of interest to inertial confinement fusion and high-energy-density physics. We report on a streaked optical pyrometer that measures the self-emission of laser-driven shocks simultaneously with a velocity interferometer system for any reflector (VISAR). Together these diagnostics are used to obtain the temporally and spatially resolved temperatures of ~Mbar shocks driven by the OMEGA laser. We provide a brief description of the diagnostic and how it is used with VISAR. Key spectral calibration results are discussed and important characteristics of the recording system are presented.

  13. Streaked optical pyrometer system for laser-driven shock-wave experiments on OMEGA.

    PubMed

    Miller, J E; Boehly, T R; Melchior, A; Meyerhofer, D D; Celliers, P M; Eggert, J H; Hicks, D G; Sorce, C M; Oertel, J A; Emmel, P M

    2007-03-01

    The temperature of laser-driven shock waves is of interest to inertial confinement fusion and high-energy-density physics. We report on a streaked optical pyrometer that measures the self-emission of laser-driven shocks simultaneously with a velocity interferometer system for any reflector (VISAR). Together these diagnostics are used to obtain the temporally and spatially resolved temperatures of approximately megabar shocks driven by the OMEGA laser. We provide a brief description of the diagnostic and how it is used with VISAR. Key spectral calibration results are discussed and important characteristics of the recording system are presented. PMID:17411209

  14. Exciton Emission under Strong Exciton-Plasmon Coupling in Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Bondarev, Igor; Woods, Lilia; Tatur, Kevin

    2010-03-01

    We study theoretically the interactions of excitonic states with surface electromagnetic modes of small-diameter (˜1nm) semiconducting single-walled carbon nanotubes (CNs). We show that these interactions can result in strong exciton-interband-surface-plasmon coupling in individual CNs. This results in the exciton emission line (Rabi) splitting ˜0.1eV as the exciton energy is tuned to the nearest interband plasmon resonance of the CN [1]. The exciton-plasmon coupling strength we predict for individual CNs is close to that previously reported for hybrid plasmonic nanostructures artificially fabricated of organic semiconductors on metallic films [2]. The quantum confined Stark effect with an electrostatic field applied perpendicular to the CN axis can be used to control the exciton-plasmon coupling, and the exciton emission accordingly [3]. We expect this effect to open up paths to new tunable optoelectronic device applications of small-diameter semiconducting CNs.[4pt] [1] I.V.Bondarev, K.Tatur, L.M.Woods, Optics Commun. 282, 661 (2009). [2] J.Bellessa, et al., Phys. Rev. Lett. 93, 036404 (2004). [3] I.V.Bondarev, L.M.Woods, K.Tatur, Phys. Rev. B 80, 085407 (2009).

  15. Optical field emission from resonant gold nanorods driven by femtosecond mid-infrared pulses

    SciTech Connect

    Kusa, F.; Echternkamp, K. E.; Herink, G.; Ropers, C.; Ashihara, S.

    2015-07-15

    We demonstrate strong-field photoelectron emission from gold nanorods driven by femtosecond mid-infrared optical pulses. The maximum photoelectron yield is reached at the localized surface plasmon resonance, indicating that the photoemission is governed by the resonantly-enhanced optical near-field. The wavelength- and field-dependent photoemission yield allows for a noninvasive determination of local field enhancements, and we obtain intensity enhancement factors close to 1300, in good agreement with finite-difference time domain computations.

  16. Optically driven oscillations of ellipsoidal particles. Part I: experimental observations.

    PubMed

    Mihiretie, B M; Snabre, P; Loudet, J-C; Pouligny, B

    2014-12-01

    We report experimental observations of the mechanical effects of light on ellipsoidal micrometre-sized dielectric particles, in water as the continuous medium. The particles, made of polystyrene, have shapes varying between near disk-like (aspect ratio k = 0.2) to very elongated needle-like (k = 8). Rather than the very tightly focused beam geometry of optical tweezers, we use a moderately focused laser beam to manipulate particles individually by optical levitation. The geometry allows us varying the longitudinal position of the particle, and to capture images perpendicular to the beam axis. Experiments show that moderate-k particles are radially trapped with their long axis lying parallel to the beam. Conversely, elongated (k > 3) or flattened (k < 0.3) ellipsoids never come to rest, and permanently "dance" around the beam, through coupled translation-rotation motions. The oscillations are shown to occur in general, be the particle in bulk water or close to a solid boundary, and may be periodic or irregular. We provide evidence for two bifurcations between static and oscillating states, at k ≈ 0.33 and k ≈ 3 for oblate and prolate ellipsoids, respectively. Based on a recently developed 2-dimensional ray-optics simulation (Mihiretie et al., EPL 100, 48005 (2012)), we propose a simple model that allows understanding the physical origin of the oscillations. PMID:25577402

  17. Excitonic correlation in the Mott crossover regime in Ge

    NASA Astrophysics Data System (ADS)

    Sekiguchi, Fumiya; Shimano, Ryo

    2015-04-01

    Exciton Mott transition (EMT) in Ge was investigated by using optical-pump and terahertz-probe spectroscopy. From the quantitative analysis of optical conductivity and dielectric function, we evaluated the densities of unbound electron-hole pairs and excitons after the photoexcitation, from which we determined the ionization ratio of excitons α. The Mott crossover density region in Ge was elucidated from the density dependence of α in the temperature range above the critical temperature of electron-hole droplets. The 1 s -2 p excitonic transition energy hardly shifted with increasing density toward the EMT. Combined with the similar results recently observed in bulk Si, we suggest that the robustness of excitonic correlation against the Coulomb screening is a universal feature in bulk semiconductors in the Mott crossover regime.

  18. Bias activated dielectric response of excitons and excitonic Mott transition in quantum confined lasers structures.

    NASA Astrophysics Data System (ADS)

    Bhunia, Amit; Bansal, Kanika; Datta, Shouvik; Alshammari, Marzook S.; Henini, Mohamed

    In contrast to the widely reported optical techniques, there are hardly any investigations on corresponding electrical signatures of condensed matter physics of excitonic phenomena. We studied small signal steady state capacitance response in III-V materials based multi quantum well (AlGaInP) and MBE grown quantum dot (InGaAs) laser diodes to identify signatures of excitonic presence. Conductance activation by forward bias was probed using frequency dependent differential capacitance response (fdC/df), which changes characteristically with the onset of light emission indicating the occurrence of negative activation energy. Our analysis shows that it is connected with a steady state population of exciton like bound states. Calculated average energy of this bound state matches well with the binding energy of weakly confined excitons in this type of structures. Further increase in charge injection decreases the differential capacitive response in AlGaInP based diodes, indicating a gradual Mott transition of excitonic states into electron hole plasma. This electrical description of excitonic Mott transition is fully supplemented by standard optical spectroscopic signatures of band gap renormalization and phase space filling effects.

  19. Polarization-dependent exciton linewidth in semiconductor quantum wells: A consequence of bosonic nature of excitons

    NASA Astrophysics Data System (ADS)

    Singh, Rohan; Suzuki, Takeshi; Autry, Travis M.; Moody, Galan; Siemens, Mark E.; Cundiff, Steven T.

    2016-08-01

    The exciton coherent signal decay rate in GaAs quantum wells, as measured in four-wave mixing experiments, depends on the polarization of the excitation pulses. Using polarization-dependent two-dimensional coherent spectroscopy, we show that this behavior is due to the bosonic character of excitons. Interference between two different quantum mechanical pathways results in a smaller decay rate for cocircular and colinear polarization of the optical excitation pulses. This interference does not exist for cross-linearly polarized excitation pulses resulting in a larger decay rate. Our result shows that the bosonic nature of excitons must be considered when interpreting ultrafast spectroscopic studies of exciton dephasing in semiconductors. This behavior should be considered while interpreting results of ultrafast spectroscopy experiments involving bosonlike excitations.

  20. Exciton management in organic photovoltaic multidonor energy cascades.

    PubMed

    Griffith, Olga L; Forrest, Stephen R

    2014-05-14

    Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures. PMID:24702468

  1. Exciton energy recycling from ZnO defect levels: towards electrically driven hybrid quantum-dot white light-emitting-diodes.

    PubMed

    Zhao, Xin; Liu, Weizhen; Chen, Rui; Gao, Yuan; Zhu, Binbin; Demir, Hilmi Volkan; Wang, Shijie; Sun, Handong

    2016-03-21

    An electrically driven quantum-dot hybrid white light-emitting diode is fabricated via spin coating CdSe quantum dots onto a GaN/ZnO nanorod matrix. For the first time, quantum dots are excited by fluorescence resonance energy transfer from the carriers trapped at surface defect levels. The prototype device exhibits achromatic emission, with a chromaticity coordinate of (0.327, 0.330), and correlated color temperature similar to sunlight.

  2. Metamaterial-driven lens optics for new beam forming patterns

    NASA Astrophysics Data System (ADS)

    Zaghloul, Amir I.; Weiss, Steven J.

    2011-06-01

    This paper discusses the general concept of using metamaterials in microwave lenses. The different optics afforded by the inclusion of metamaterials in the lens structure produce new features such as reduced size and new beam formations. The use of negative refractive index materials is discussed in reference to the original concept of the perfect lens, leading to the Rotman lens and the Luneburg lens. In Rotman lens, negative refractions help reducing the lens size and a broadband electromagnetic band gap (EBG) surface is used to prevent reflections off the sidewalls. Verification of negative refraction and simulation of isotropic material performance are presented, as well as an example of broadening the band of an EBG surface.

  3. A Modular, IGBT Driven, Ignitron Switched, Optically Controlled Power Supply

    NASA Astrophysics Data System (ADS)

    Carroll, Evan; von der Linden, Jens; You, Setthivoine

    2013-10-01

    An experiment to investigate the dynamics of canonical flux tubes at the University of Washington uses two high energy pulsed power supplies to generate and sustain the plasma discharge. A modular 240 μF , 12 kV DC capacitor based power supply, discharged by ignitron, has been developed specifically for this application. Design considerations include minimizing inductance, rapid switching, fast rise times, and electrically isolated control. An optically coupled front panel and fast IGBT ignitron drive circuit, sequenced manually or by software, control the charge and discharge of the power supply. A complete, sequenced charge/discharge has been successfully tested with a dummy load, producing a peak current of 100 kA and a rise time of 25 μs . This work was sponsored in part by the US DOE Grant DE-SC0010340.

  4. Exciton Storage in a Nanoscale Aharonov-Bohm Ring with Electric Field Tuning

    SciTech Connect

    Fischer, Andrea M.; Roemer, Rudolf A.; Campo, Vivaldo L. Jr.; Portnoi, Mikhail E.

    2009-03-06

    We study analytically the optical properties of a simple model for an electron-hole pair on a ring subjected to perpendicular magnetic flux and in-plane electric field. We show how to tune this excitonic system from optically active to optically dark as a function of these external fields. Our results offer a simple mechanism for exciton storage and readout.

  5. Excitonic luminescence upconversion in a two-dimensional semiconductor

    NASA Astrophysics Data System (ADS)

    Jones, Aaron M.; Yu, Hongyi; Schaibley, John R.; Yan, Jiaqiang; Mandrus, David G.; Taniguchi, Takashi; Watanabe, Kenji; Dery, Hanan; Yao, Wang; Xu, Xiaodong

    2016-04-01

    Photon upconversion is an elementary light-matter interaction process in which an absorbed photon is re-emitted at higher frequency after extracting energy from the medium. This phenomenon lies at the heart of optical refrigeration in solids, where upconversion relies on anti-Stokes processes enabled either by rare-earth impurities or exciton-phonon coupling. Here, we demonstrate a luminescence upconversion process from a negatively charged exciton to a neutral exciton resonance in monolayer WSe2, producing spontaneous anti-Stokes emission with an energy gain of 30 meV. Polarization-resolved measurements find this process to be valley selective, unique to monolayer semiconductors. Since the charged exciton binding energy closely matches the 31 meV A1' optical phonon, we ascribe the spontaneous excitonic anti-Stokes to doubly resonant Raman scattering, where the incident and outgoing photons are in resonance with the charged and neutral excitons, respectively. In addition, we resolve a charged exciton doublet with a 7 meV splitting, probably induced by exchange interactions, and show that anti-Stokes scattering is efficient only when exciting the doublet peak resonant with the phonon, further confirming the excitonic doubly resonant picture.

  6. A photo-driven dual-frequency addressable optical device of banana-shaped molecules

    NASA Astrophysics Data System (ADS)

    Krishna Prasad, S.; Lakshmi Madhuri, P.; Hiremath, Uma S.; Yelamaggad, C. V.

    2014-03-01

    We propose a photonic switch employing a blend of host banana-shaped liquid crystalline molecules and guest photoisomerizable calamitic molecules. The material exhibits a change in the sign of the dielectric anisotropy switching from positive to negative, at a certain crossover frequency of the probing field. The consequent change in electric torque can be used to alter the orientation of the molecules between surface-determined and field-driven optical states resulting in a large change in the optical transmission characteristics. Here, we demonstrate the realization of this feature by an unpolarized UV beam, the first of its kind for banana-shaped molecules. The underlying principle of photoisomerization eliminates the need for a second driving frequency. The device also acts as a reversible conductance switch with an order of magnitude increase of conductivity brought about by light. Possible usage of this for optically driven display devices and image storage applications is suggested.

  7. All-optically driven system in ultrasonic wave-based structural health monitoring

    NASA Astrophysics Data System (ADS)

    Bi, Siwen; Wu, Nan; Zhou, Jingcheng; Zhang, Haifeng; Wang, Xingwei

    2016-04-01

    Ultrasonic wave based structural health monitoring (SHM) is an innovative method for nondestructive detection and an area of growing interest. This is due to high demands for wireless detection in the field of structural engineering. Through optically exciting and detecting ultrasonic waves, electrical wire connections can be avoided, and non-contact SHM can be achieved. With the combination of piezoelectric transducer (PZT) (which possesses high heat resistance) and the noncontact detection, this system has a broad range of applications, even in extreme conditions. This paper reports an all-optically driven SHM system. The resonant frequencies of the PZT transducers are sensitive to a variety of structural damages. Experimental results have verified the feasibility of the all-optically driven SHM system.

  8. A photo-driven dual-frequency addressable optical device of banana-shaped molecules

    SciTech Connect

    Krishna Prasad, S. Lakshmi Madhuri, P.; Hiremath, Uma S.; Yelamaggad, C. V.

    2014-03-17

    We propose a photonic switch employing a blend of host banana-shaped liquid crystalline molecules and guest photoisomerizable calamitic molecules. The material exhibits a change in the sign of the dielectric anisotropy switching from positive to negative, at a certain crossover frequency of the probing field. The consequent change in electric torque can be used to alter the orientation of the molecules between surface-determined and field-driven optical states resulting in a large change in the optical transmission characteristics. Here, we demonstrate the realization of this feature by an unpolarized UV beam, the first of its kind for banana-shaped molecules. The underlying principle of photoisomerization eliminates the need for a second driving frequency. The device also acts as a reversible conductance switch with an order of magnitude increase of conductivity brought about by light. Possible usage of this for optically driven display devices and image storage applications is suggested.

  9. Optical-parametric-oscillator solitons driven by the third harmonic

    NASA Astrophysics Data System (ADS)

    Lutsky, Vitaly; Malomed, Boris A.

    2004-12-01

    We introduce a model of a lossy second-harmonic-generating (χ(2)) cavity externally pumped at the third harmonic, which gives rise to driving terms of a new type, corresponding to a cross-parametric gain. The equation for the fundamental-frequency (FF) wave may also contain a quadratic self-driving term, which is generated by the cubic nonlinearity of the medium. Unlike previously studied phase-matched models of χ(2) cavities driven at the second harmonic or at FF, the present one admits an exact analytical solution for the soliton, at a special value of the gain parameter. Two families of solitons are found in a numerical form, and their stability area is identified through numerical computation of the perturbation eigenvalues (stability of the zero solution, which is a necessary condition for the soliton’s stability, is investigated in an analytical form). One family is a continuation of the special analytical solution. At given values of the parameters, one soliton is stable and the other one is not; they swap their stability at a critical value of the mismatch parameter. The stability of the solitons is also verified in direct simulations, which demonstrate that an unstable pulse rearranges itself into a stable one, or into a delocalized state, or decays to zero. A soliton which was given an initial boost C starts to move but quickly comes to a halt, if the boost is smaller than a critical value Ccr . If C>Ccr , the boost destroys the soliton (sometimes, through splitting into two secondary pulses). Interactions between initially separated solitons are investigated, too. It is concluded that stable solitons always merge into a single one. In the system with weak loss, it appears in a vibrating form, slowly relaxing to the static shape. With stronger loss, the final soliton emerges in the stationary form.

  10. Exciton effective mass enhancement in coupled quantum wells in electric and magnetic fields

    NASA Astrophysics Data System (ADS)

    Wilkes, J.; Muljarov, E. A.

    2016-02-01

    We present a calculation of exciton states in semiconductor coupled quantum wells in the presence of electric and magnetic fields applied perpendicular to the QW plane. The exciton Schrödinger equation is solved in real space in three-dimensions to obtain the Landau levels of both direct and indirect excitons. Calculation of the exciton energy levels and oscillator strengths enables mapping of the electric and magnetic field dependence of the exciton absorption spectrum. For the ground state of the system, we evaluate the Bohr radius, optical lifetime, binding energy and dipole moment. The exciton mass renormalization due to the magnetic field is calculated using a perturbative approach. We predict a non-monotonous dependence of the exciton ground state effective mass on magnetic field. Such a trend is explained in a classical picture, in terms of the ground state tending from an indirect to a direct exciton with increasing magnetic field.

  11. Arsenic-bound excitons in diamond

    NASA Astrophysics Data System (ADS)

    Barjon, J.; Jomard, F.; Morata, S.

    2014-01-01

    A set of new excitonic recombinations is observed in arsenic-implanted diamond. It is composed of two groups of emissions at 5.355/5.361 eV and at 5.215/5.220/5.227 eV. They are respectively attributed to the no-phonon and transverse-optical phonon-assisted recombinations of excitons bound to neutral arsenic donors. From the Haynes rule, an ionization energy of 0.41 eV is deduced for arsenic in diamond, which shows that arsenic is a shallower donor than phosphorus (0.6 eV), in agreement with theory.

  12. Probing excitonic dark states in single-layer tungsten disulphide

    NASA Astrophysics Data System (ADS)

    Ye, Ziliang; Cao, Ting; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G.; Zhang, Xiang

    2014-09-01

    Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism, doping-dependent charged excitons and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (~0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many

  13. Time Dependent Study of Multiple Exciton Generation in Nanocrystal Quantum Dots

    NASA Astrophysics Data System (ADS)

    Damtie, Fikeraddis A.; Wacker, Andreas

    2016-03-01

    We study the exciton dynamics in an optically excited nanocrystal quantum dot. Multiple exciton formation is more efficient in nanocrystal quantum dots compared to bulk semiconductors due to enhanced Coulomb interactions and the absence of conservation of momentum. The formation of multiple excitons is dependent on different excitation parameters and the dissipation. We study this process within a Lindblad quantum rate equation using the full many-particle states. We optically excite the system by creating a single high energy exciton ESX in resonance to a double exciton EDX. With Coulomb electron-electron interaction, the population can be transferred from the single exciton to the double exciton state by impact ionisation (inverse Auger process). The ratio between the recombination processes and the absorbed photons provide the yield of the structure. We observe a quantum yield of comparable value to experiment assuming typical experimental conditions for a 4 nm PbS quantum dot.

  14. How to Draw Energy Level Diagrams in Excitonic Solar Cells.

    PubMed

    Zhu, X-Y

    2014-07-01

    Emerging photovoltaic devices based on molecular and nanomaterials are mostly excitonic in nature. The initial absorption of a photon in these materials creates an exciton that can subsequently dissociate in each material or at their interfaces to give charge carriers. Any attempt at mechanistic understanding of excitonic solar cells must start with drawing energy level diagrams. This seemingly elementary exercise, which is described in textbooks for inorganic solar cells, has turned out to be a difficult subject in the literature. The problem stems from conceptual confusion of single-particle energy with quasi-particle energy and the misleading practice of mixing the two on the same energy level diagram. Here, I discuss how to draw physically accurate energy diagrams in excitonic solar cells using only single-particle energies (ionization potentials and electron affinities) of both ground and optically excited states. I will briefly discuss current understanding on the electronic energy landscape responsible for efficient charge separation in excitonic solar cells.

  15. Permanent Rabi oscillations in coupled exciton-photon systems with PT-symmetry.

    PubMed

    Chestnov, Igor Yu; Demirchyan, Sevak S; Alodjants, Alexander P; Rubo, Yuri G; Kavokin, Alexey V

    2016-01-21

    We propose a physical mechanism which enables permanent Rabi oscillations in driven-dissipative condensates of exciton-polaritons in semiconductor microcavities subjected to external magnetic fields. The method is based on stimulated scattering of excitons from the incoherent reservoir. We demonstrate that permanent non-decaying oscillations may appear due to the parity-time symmetry of the coupled exciton-photon system realized in a specific regime of pumping to the exciton state and depletion of the reservoir. At non-zero exciton-photon detuning, robust permanent Rabi oscillations occur with unequal amplitudes of exciton and photon components. Our predictions pave way to realization of integrated circuits based on exciton-polariton Rabi oscillators.

  16. Permanent Rabi oscillations in coupled exciton-photon systems with PT -symmetry

    PubMed Central

    Chestnov, Igor Yu.; Demirchyan, Sevak S.; Alodjants, Alexander P.; Rubo, Yuri G.; Kavokin, Alexey V.

    2016-01-01

    We propose a physical mechanism which enables permanent Rabi oscillations in driven-dissipative condensates of exciton-polaritons in semiconductor microcavities subjected to external magnetic fields. The method is based on stimulated scattering of excitons from the incoherent reservoir. We demonstrate that permanent non-decaying oscillations may appear due to the parity-time symmetry of the coupled exciton-photon system realized in a specific regime of pumping to the exciton state and depletion of the reservoir. At non-zero exciton-photon detuning, robust permanent Rabi oscillations occur with unequal amplitudes of exciton and photon components. Our predictions pave way to realization of integrated circuits based on exciton-polariton Rabi oscillators. PMID:26790534

  17. Permanent Rabi oscillations in coupled exciton-photon systems with PT -symmetry

    NASA Astrophysics Data System (ADS)

    Chestnov, Igor Yu.; Demirchyan, Sevak S.; Alodjants, Alexander P.; Rubo, Yuri G.; Kavokin, Alexey V.

    2016-01-01

    We propose a physical mechanism which enables permanent Rabi oscillations in driven-dissipative condensates of exciton-polaritons in semiconductor microcavities subjected to external magnetic fields. The method is based on stimulated scattering of excitons from the incoherent reservoir. We demonstrate that permanent non-decaying oscillations may appear due to the parity-time symmetry of the coupled exciton-photon system realized in a specific regime of pumping to the exciton state and depletion of the reservoir. At non-zero exciton-photon detuning, robust permanent Rabi oscillations occur with unequal amplitudes of exciton and photon components. Our predictions pave way to realization of integrated circuits based on exciton-polariton Rabi oscillators.

  18. Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons.

    PubMed

    Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio

    2016-03-17

    Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ≈ 250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics.

  19. Deformation Measurement of a Driven Pile Using Distributed Fibre-optic Sensing

    NASA Astrophysics Data System (ADS)

    Monsberger, Christoph; Woschitz, Helmut; Hayden, Martin

    2016-03-01

    New developments in distributed fibre-optic sensing allow the measurement of strain with a very high precision of about 1 µm / m and a spatial resolution of 10 millimetres or even better. Thus, novel applications in several scientific fields may be realised, e. g. in structural monitoring or soil and rock mechanics. Especially due to the embedding capability of fibre-optic sensors, fibre-optic systems provide a valuable extension to classical geodetic measurement methods, which are limited to the surface in most cases. In this paper, we report about the application of an optical backscatter reflectometer for deformation measurements along a driven pile. In general, pile systems are used in civil engineering as an efficient and economic foundation of buildings and other structures. Especially the length of the piles is crucial for the final loading capacity. For optimization purposes, the interaction between the driven pile and the subsurface material is investigated using pile testing methods. In a field trial, we used a distributed fibre-optic sensing system for measuring the strain below the surface of an excavation pit in order to derive completely new information. Prior to the field trial, the fibre-optic sensor was investigated in the laboratory. In addition to the results of these lab studies, we briefly describe the critical process of field installation and show the most significant results from the field trial, where the pile was artificially loaded up to 800 kN. As far as we know, this is the first time that the strain is monitored along a driven pile with such a high spatial resolution.

  20. Optical gain in 1.3-μm electrically driven dilute nitride VCSOAs

    PubMed Central

    2014-01-01

    We report the observation of room-temperature optical gain at 1.3 μm in electrically driven dilute nitride vertical cavity semiconductor optical amplifiers. The gain is calculated with respect to injected power for samples with and without a confinement aperture. At lower injected powers, a gain of almost 10 dB is observed in both samples. At injection powers over 5 nW, the gain is observed to decrease. For nearly all investigated power levels, the sample with confinement aperture gives slightly higher gain. PMID:24417791

  1. Thermo-optically driven adaptive mirror based on thermal expansion: preparation and resolution

    NASA Astrophysics Data System (ADS)

    Reinert, Felix; Lüthy, W.

    2005-12-01

    A thermo-optically driven adaptive mirror is presented. It is based on the thermal expansion of a thin film heated with a light pattern. We describe a procedure for the preparation of a silicon elastomer with a high-quality optical surface. This material, Sylgard 184, has a high linear thermal expansion coefficient of 3.1μ10-4 K-1. Surface modulations are recorded in an interferometer. Modulations of 350 nm result at an intensity of 370 mW/cm2. The resolution is measured with a line pattern. The contrast drops to 30 % at 1.6 line pairs per millimeter (lp/mm).

  2. Thermo-optically driven adaptive mirror based on thermal expansion: preparation and resolution.

    PubMed

    Reinert, Felix; Lüthy, W

    2005-12-26

    A thermo-optically driven adaptive mirror is presented. It is based on the thermal expansion of a thin film heated with a light pattern. We describe a procedure for the preparation of a silicon elastomer with a high-quality optical surface. This material, Sylgard 184, has a high linear thermal expansion coefficient of 3.110-4 K-1. Surface modulations are recorded in an interferometer. Modulations of 350 nm result at an intensity of 370 mW/cm2. The resolution is measured with a line pattern. The contrast drops to 30 % at 1.6 line pairs per millimeter (lp/mm).

  3. Mapping the exciton diffusion in semiconductor nanocrystal solids.

    PubMed

    Kholmicheva, Natalia; Moroz, Pavel; Bastola, Ebin; Razgoniaeva, Natalia; Bocanegra, Jesus; Shaughnessy, Martin; Porach, Zack; Khon, Dmitriy; Zamkov, Mikhail

    2015-03-24

    Colloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for device applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. Owing to the lack of theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a subdiffraction spatial sensitivity and a picosecond temporal resolution. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation at known distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of predissociation hops, the rate of interparticle energy transfer, and the exciton diffusivity. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling.

  4. Storing excitons in transition-metal dichalcogenides using dark states

    NASA Astrophysics Data System (ADS)

    Gunlycke, Daniel; Tseng, Frank; Simsek, Ergun

    Monolayer transition-metal dichalcogenides exhibit strongly bound excitons confined to two dimensions. One challenge in exploiting these excitons is that they have a finite life time and collapse through electron-hole recombination. We propose that the exciton life time could be extended by transitioning the exciton population into dark states. The symmetry of these dark states require the electron and hole to be spatially separated, which not only causes these states to be optically inactive but also inhibits electron-hole recombination. Based on an atomistic model we call the Triangular Lattice Exciton (3ALE) model, we derive transition matrix elements and approximate selection rules showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing. This work was supported by the Office of Naval Research, directly and through the Naval Research Laboratory.

  5. Measurement of an Explosively Driven Hemispherical Shell Using 96 Points of Optical Velocimetry

    SciTech Connect

    Danielson, J. R.; Daykin, E P; Diaz, A. B.; Doty, D. L.; Frogget, B. C.; Furlanetto, M. R.; Gallegos, C. H.; Gibo, M; Garza, A; Holtkamp, D B; Hutchins, M S; Perez, C; Perez, C; Pena, M; Romero, V T; Shinas, M A; Teel, M G; Tabaka, L J

    2014-04-01

    We report the measurement of the surface motion of a hemispherical copper shell driven by high explosives. This measurement was made using three 32-channel multiplexed photonic Doppler velocimetry (PDV) systems, in combination with a novel compound optical probe. Clearly visible are detailed features of the motion of the shell over time, enhanced by spatial correlation. Significant non-normal motion is apparent, and challenges in measuring such a geometry are discussed.

  6. Excitations, optical absorption spectra, and optical excitonic gaps of heterofullerenes. I. C60, C59N+, and C48N12: theory and experiment.

    PubMed

    Xie, Rui-Hua; Bryant, Garnett W; Sun, Guangyu; Nicklaus, Marc C; Heringer, David; Frauenheim, Th; Manaa, M Riad; Smith, Vedene H; Araki, Yasuyuki; Ito, Osamu

    2004-03-15

    Low-energy excitations and optical absorption spectrum of C(60) are computed by using time-dependent (TD) Hartree-Fock, TD-density functional theory (TD-DFT), TD DFT-based tight-binding (TD-DFT-TB), and a semiempirical Zerner intermediate neglect of diatomic differential overlap method. A detailed comparison of experiment and theory for the excitation energies, optical gap, and absorption spectrum of C(60) is presented. It is found that electron correlations and correlation of excitations play important roles in accurately assigning the spectral features of C(60), and that the TD-DFT method with nonhybrid functionals or a local spin density approximation leads to more accurate excitation energies than with hybrid functionals. The level of agreement between theory and experiment for C(60) justifies similar calculations of the excitations and optical absorption spectrum of a monomeric azafullerene cation C(59)N(+), to serve as a spectroscopy reference for the characterization of carborane anion salts. Although it is an isoelectronic analogue to C(60), C(59)N(+) exhibits distinguishing spectral features different from C(60): (1) the first singlet is dipole-allowed and the optical gap is redshifted by 1.44 eV; (2) several weaker absorption maxima occur in the visible region; (3) the transient triplet-triplet absorption at 1.60 eV (775 nm) is much broader and the decay of the triplet state is much faster. The calculated spectra of C(59)N(+) characterize and explain well the measured ultraviolet-visible (UV-vis) and transient absorption spectra of the carborane anion salt [C(59)N][Ag(CB(11)H(6)Cl(6))(2)] [Kim et al., J. Am. Chem. Soc. 125, 4024 (2003)]. For the most stable isomer of C(48)N(12), we predict that the first singlet is dipole-allowed, the optical gap is redshifted by 1.22 eV relative to that of C(60), and optical absorption maxima occur at 585, 528, 443, 363, 340, 314, and 303 nm. We point out that the characterization of the UV-vis and transient absorption

  7. Excitations, optical absorption spectra, and optical excitonic gaps of heterofullerenes. I. C60, C59N+, and C48N12: Theory and experiment

    NASA Astrophysics Data System (ADS)

    Xie, Rui-Hua; Bryant, Garnett W.; Sun, Guangyu; Nicklaus, Marc C.; Heringer, David; Frauenheim, Th.; Manaa, M. Riad; Smith, Vedene H.; Araki, Yasuyuki; Ito, Osamu

    2004-03-01

    Low-energy excitations and optical absorption spectrum of C60 are computed by using time-dependent (TD) Hartree-Fock, TD-density functional theory (TD-DFT), TD DFT-based tight-binding (TD-DFT-TB), and a semiempirical Zerner intermediate neglect of diatomic differential overlap method. A detailed comparison of experiment and theory for the excitation energies, optical gap, and absorption spectrum of C60 is presented. It is found that electron correlations and correlation of excitations play important roles in accurately assigning the spectral features of C60, and that the TD-DFT method with nonhybrid functionals or a local spin density approximation leads to more accurate excitation energies than with hybrid functionals. The level of agreement between theory and experiment for C60 justifies similar calculations of the excitations and optical absorption spectrum of a monomeric azafullerene cation C59N+, to serve as a spectroscopy reference for the characterization of carborane anion salts. Although it is an isoelectronic analogue to C60, C59N+ exhibits distinguishing spectral features different from C60: (1) the first singlet is dipole-allowed and the optical gap is redshifted by 1.44 eV; (2) several weaker absorption maxima occur in the visible region; (3) the transient triplet-triplet absorption at 1.60 eV (775 nm) is much broader and the decay of the triplet state is much faster. The calculated spectra of C59N+ characterize and explain well the measured ultraviolet-visible (UV-vis) and transient absorption spectra of the carborane anion salt [C59N][Ag(CB11H6Cl6)2] [Kim et al., J. Am. Chem. Soc. 125, 4024 (2003)]. For the most stable isomer of C48N12, we predict that the first singlet is dipole-allowed, the optical gap is redshifted by 1.22 eV relative to that of C60, and optical absorption maxima occur at 585, 528, 443, 363, 340, 314, and 303 nm. We point out that the characterization of the UV-vis and transient absorption spectra of C48N12 isomers is helpful in

  8. Bright and dark excitons in semiconductor carbon nanotubes: insights from electronic structure calculations.

    PubMed

    Kilina, Svetlana; Badaeva, Ekaterina; Piryatinski, Andrei; Tretiak, Sergei; Saxena, Avadh; Bishop, Alan R

    2009-06-01

    We review electronic structure calculations of finite-length semiconducting carbon nanotubes using time-dependent density functional theory (TD-DFT) and the time dependent Hartree-Fock (TD-HF) approach coupled with semi-empirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and AM1 calculations overbind excitons, inaccurately predicting the lowest energy state as a bright exciton. Changing the AM1 with the ZINDO Hamiltonian in TD-HF calculations predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functionals with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands: the lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ( approximately 0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leading to the rapid population of the lowest dark exciton. This rationalizes the low

  9. Exciton Transfer in Carbon Nanotube Aggregates for Energy Harvesting Applications

    NASA Astrophysics Data System (ADS)

    Davoody, Amirhossein; Karimi, Farhad; Knezevic, Irena

    Carbon nanotubes (CNTs) are promising building blocks for organic photovoltaic devices, owing to their tunable band gap, mechanical and chemical stability. We study intertube excitonic energy transfer between pairs of CNTs with different orientations and band gaps. The optically bright and dark excitonic states in CNTs are calculated by solving the Bethe-Salpeter equation. We calculate the exciton transfer rates due to the direct and exchange Coulomb interactions, as well as the second-order phonon-assisted processes. We show the importance of phonons in calculating the transfer rates that match the measurements. In addition, we discuss the contribution of optically inactive excited states in the exciton transfer process, which is difficult to determine experimentally. Furthermore, we study the effects of sample inhomogeneity, impurities, and temperature on the exciton transfer rate. The inhomogeneity in the CNT sample dielectric function can increase the transfer rate by about a factor of two. We show that the exciton confinement by impurities has a detrimental effect on the transfer rate between pairs of similar CNTs. The exciton transfer rate increases monotonically with increasing temperature. Support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0008712.

  10. Bound Exciton Complexes

    NASA Astrophysics Data System (ADS)

    Meyer, B. K.

    In the preceding chapter, we concentrated on the properties of free excitons. These free excitons may move through the sample and hit a trap, a nonradiative or a radiative recombination center. At low temperatures, the latter case gives rise to either deep center luminescence, mentioned in Sect. 7.1 and discussed in detail in Chap. 9, or to the luminescence of bound exciton complexes (BE or BEC). The chapter continues with the most prominent of these BECs, namely A-excitons bound to neutral donors. The next aspects are the more weakly BEs at ionized donors. The Sect. 7.4 treats the binding or localization energies of BEC from a theoretical point of view, while Sect. 7.5 is dedicated to excited states of BECs, which contain either holes from deeper valence bands or an envelope function with higher quantum numbers. The last section is devoted to donor-acceptor pair transitions. There is no section devoted specifically to excitons bound to neutral acceptors, because this topic is still partly controversially discussed. Instead, information on these A0X complexes is scattered over the whole chapter, however, with some special emphasis seen in Sects. 7.1, 7.4, and 7.5.

  11. Intraband effects in excitonic second-harmonic generation

    NASA Astrophysics Data System (ADS)

    Pedersen, Thomas Garm

    2015-12-01

    A theory for the nonlinear excitonic optical response of semiconductors is developed. By adopting the length gauge, intraband effects are rigorously taken into account. We show that the second-order nonlinear response mixing intra- and interband transitions can be expressed in terms of generalized derivatives of the exciton Green's function. The theory is applied to hexagonal boron-nitride monolayers. For both the linear and nonlinear response, a dramatic influence of excitons is found. Hence, new discrete resonances appear as well as pronounced changes in the continuum spectrum.

  12. Confocal shift interferometry of coherent emission from trapped dipolar excitons

    SciTech Connect

    Repp, J.; Schinner, G. J.; Schubert, E.; Rai, A. K.; Wieck, A. D.; Reuter, D.; Wurstbauer, U.; Holleitner, A. W.; and others

    2014-12-15

    We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.

  13. Exciton and multi-exciton dynamics in CdSe/Cd1-xZnxS quantum dots

    NASA Astrophysics Data System (ADS)

    Righetto, Marcello; Minotto, Alessandro; Bozio, Renato

    2016-04-01

    The outstanding optical properties of Semiconductor Quantum Dots (QDs) have attracted much interest for over two decades. The development of synthetic methods for the production of core-shell QDs has opened the way to attaining almost ideal emitting properties. Their implementation in opto-electronic devices, such as light emitting diodes (LEDs) and lasers, requires a full understanding of the fine details of their photophysics. The exciton dynamics of core and coreshell QDs was extensively studied by means of pump and probe (P and P) and transient photoluminescence (TRPL) spectroscopies. Nevertheless, the wealth of possible exciton and multi-exciton decay mechanisms, operating on comparable time-scales, results in complex signals. In this work, the exciton dynamics of a complete CdSe/Cd1-xZnxS series is investigated, with a focus on exciton trapping processes. Insights into the energy distribution of exciton traps are unveiled by wavelength resolve QY measurements. Multicolor P and P measurements give a deeper insight into the dynamics of exciton trapping and Auger recombinations. An inversion method is proposed as a powerful tool for separating different contribution in complex P and P transients. The outcomes of this work clarify the role of core/shell interfaces and surfaces in modulating the optical properties and suggest possible routes for their improvement.

  14. Optically and electrically driven organic thin film transistors with diarylethene photochromic channel layers.

    PubMed

    Hayakawa, Ryoma; Higashiguchi, Kenji; Matsuda, Kenji; Chikyow, Toyohiro; Wakayama, Yutaka

    2013-05-01

    We achieved drain-current switching of diarylethene-channel field-effect transistors with light- and electric-field effects. The drain current was reversibly changed by alternating ultraviolet and visible light irradiation. Stress is placed on the fact that the on/off ratio realized by light irradiation was 1 × 10(2) (1 × 10(4)%) and this value is much larger than those in other photochromism-based transistors. These results indicate that the drain current was effectively controlled by light irradiation. Furthermore, the on and off states modulated by light were maintained without light irradiation even after 1 week, exhibiting that our transistor works as an optical memory. We clarified that the light-driven modulation can be attributed to the transformation in the π-conjugation system accompanied by photoisomerization. These findings have the potential to attain high-performance optoelectrical organic devices including optical sensors, optical memory, and photoswitching transistors.

  15. Optically controlled dense current structures driven by relativistic plasma aperture-induced diffraction

    NASA Astrophysics Data System (ADS)

    Gonzalez-Izquierdo, Bruno; Gray, Ross J.; King, Martin; Dance, Rachel J.; Wilson, Robbie; McCreadie, John; Butler, Nicholas M. H.; Capdessus, Remi; Hawkes, Steve; Green, James S.; Borghesi, Marco; Neely, David; McKenna, Paul

    2016-05-01

    The collective response of charged particles to intense fields is intrinsic to plasma accelerators and radiation sources, relativistic optics and many astrophysical phenomena. Here we show that a relativistic plasma aperture is generated in thin foils by intense laser light, resulting in the fundamental optical process of diffraction. The plasma electrons collectively respond to the resulting laser near-field diffraction pattern, producing a beam of energetic electrons with a spatial structure that can be controlled by variation of the laser pulse parameters. It is shown that static electron-beam and induced-magnetic-field structures can be made to rotate at fixed or variable angular frequencies depending on the degree of ellipticity in the laser polarization. The concept is demonstrated numerically and verified experimentally, and is an important step towards optical control of charged particle dynamics in laser-driven dense plasma sources.

  16. Using dark states for exciton storage in transition-metal dichalcogenides.

    PubMed

    Tseng, Frank; Simsek, Ergun; Gunlycke, Daniel

    2016-01-27

    We explore the possibility of storing excitons in excitonic dark states in monolayer semiconducting transition-metal dichalcogenides. In addition to being optically inactive, these dark states require the electron and hole to be spatially separated, thus inhibiting electron/hole recombination and allowing exciton lifetimes to be extended. Based on an atomistic exciton model, we derive transition matrix elements and an approximate selection rule showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on a population analysis for different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing.

  17. Using dark states for exciton storage in transition-metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Tseng, Frank; Simsek, Ergun; Gunlycke, Daniel

    2016-01-01

    We explore the possibility of storing excitons in excitonic dark states in monolayer semiconducting transition-metal dichalcogenides. In addition to being optically inactive, these dark states require the electron and hole to be spatially separated, thus inhibiting electron/hole recombination and allowing exciton lifetimes to be extended. Based on an atomistic exciton model, we derive transition matrix elements and an approximate selection rule showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on a population analysis for different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing.

  18. Anomalous magnetization of a carbon nanotube as an excitonic insulator

    NASA Astrophysics Data System (ADS)

    Rontani, Massimo

    2014-11-01

    We show theoretically that an undoped carbon nanotube might be an excitonic insulator—the long-sought phase of matter proposed by Keldysh, Kohn, and others fifty years ago. We predict that the condensation of triplet excitons, driven by intervalley exchange interaction, spontaneously occurs at equilibrium if the tube radius is sufficiently small. The signatures of exciton condensation are its sizable contributions to both the energy gap and the magnetic moment per electron. The increase of the gap might have already been measured, albeit with a different explanation [V. V. Deshpande, B. Chandra, R. Caldwell, D. S. Novikov, J. Hone, and M. Bockrath, Science 323, 106 (2009), 10.1126/science.1165799]. The enhancement of the quasiparticle magnetic moment is a pair-breaking effect that counteracts the weak paramagnetism of the ground-state condensate of excitons. This property could rationalize the anomalous magnitude of magnetic moments recently observed in different devices close to charge neutrality.

  19. Trapping and transport of indirect excitons in coupled quantum wells

    NASA Astrophysics Data System (ADS)

    Wuenschell, Jeffrey K.

    Spatially indirect excitons are optically generated composite bosons with a radiative lifetime sufficient to reach thermal equilibrium. This work explores the physics of indirect excitons in coupled quantum wells in the GaAs/AlGaAs system, specifically in the low-temperature, high-density regime. Particular attention is paid to a technique whereby a spatially inhomogeneous strain field is used as a trapping potential. In the process of modeling the trapping profile in wide quantum wells, dramatic effects due to intersubband coupling were observed at high strain. Experimentally, this regime coincides with the abrupt appearance of a dark population of indirect excitons at trap center, an effect originally suspected to be related to Bose-Einstein condensation. Here, the role of band mixing due to the strain-induced distortion of the crystal symmetry will be explored in detail in the context of this effect. Experimental studies presented here and in the literature suggest that Bose-Einstein condensation in indirect exciton systems may be difficult to detect with optical means (e.g., coherence measurements, momentum-space narrowing), possibly due to the strong dipole interaction between indirect excitons. Due to similarities between this system and liquid helium, it may be more fruitful to look for transport-related signatures of condensation, such as super fluidity. Here, a method for performing transport measurements on optically generated indirect excitons is also outlined and preliminary results are presented.

  20. Miniaturized magnetic-driven scanning probe for endoscopic optical coherence tomography.

    PubMed

    Pang, Ziwei; Wu, Jigang

    2015-06-01

    We designed and implemented a magnetic-driven scanning (MDS) probe for endoscopic optical coherence tomography (OCT). The probe uses an externally-driven tiny magnet in the distal end to achieve unobstructed 360-degree circumferential scanning at the side of the probe. The design simplifies the scanning part inside the probe and thus allows for easy miniaturization and cost reduction. We made a prototype probe with an outer diameter of 1.4 mm and demonstrated its capability by acquiring OCT images of ex vivo trachea and artery samples from a pigeon. We used a spectrometer-based Fourier-domain OCT system and the system sensitivity with our prototype probe was measured to be 91 dB with an illumination power of 850 μW and A-scan exposure time of 1 ms. The axial and lateral resolutions of the system are 6.5 μm and 8.1 μm, respectively. PMID:26114041

  1. Miniaturized magnetic-driven scanning probe for endoscopic optical coherence tomography.

    PubMed

    Pang, Ziwei; Wu, Jigang

    2015-06-01

    We designed and implemented a magnetic-driven scanning (MDS) probe for endoscopic optical coherence tomography (OCT). The probe uses an externally-driven tiny magnet in the distal end to achieve unobstructed 360-degree circumferential scanning at the side of the probe. The design simplifies the scanning part inside the probe and thus allows for easy miniaturization and cost reduction. We made a prototype probe with an outer diameter of 1.4 mm and demonstrated its capability by acquiring OCT images of ex vivo trachea and artery samples from a pigeon. We used a spectrometer-based Fourier-domain OCT system and the system sensitivity with our prototype probe was measured to be 91 dB with an illumination power of 850 μW and A-scan exposure time of 1 ms. The axial and lateral resolutions of the system are 6.5 μm and 8.1 μm, respectively.

  2. Coherent and Incoherent Coupling Dynamics between Neutral and Charged Excitons in Monolayer MoSe2

    NASA Astrophysics Data System (ADS)

    Hao, Kai; Xu, Lixiang; Nagler, Philipp; Singh, Akshay; Tran, Kha; Dass, Chandriker Kavir; Schüller, Christian; Korn, Tobias; Li, Xiaoqin; Moody, Galan

    2016-08-01

    The optical properties of semiconducting transition metal dichalcogenides are dominated by both neutral excitons (electron-hole pairs) and charged excitons (trions) that are stable even at room temperature. While trions directly influence charge transport properties in optoelectronic devices, excitons may be relevant through exciton-trion coupling and conversion phenomena. In this work, we reveal the coherent and incoherent nature of exciton-trion coupling and the relevant timescales in monolayer MoSe2 using optical two-dimensional coherent spectroscopy. Coherent interaction between excitons and trions is definitively identified as quantum beating of cross-coupling peaks that persists for a few hundred femtoseconds. For longer times up to 10 ps, surprisingly, the relative intensity of the cross-coupling peaks increases, which is attributed to incoherent energy transfer likely due to phonon-assisted up-conversion and down-conversion processes that are efficient even at cryogenic temperature.

  3. Characterization of laser-driven shock waves in solids using a fiber optic pressure probe

    DOE PAGES

    Cranch, Geoffrey A.; Lunsford, Robert; Grun, Jacob; Weaver, James; Compton, Steve; May, Mark; Kostinski, Natalie

    2013-11-08

    Measurement of laser-driven shock wave pressure in solid blocks of polymethyl methacrylate is demonstrated using fiber optic pressure probes. Three probes based on a fiber Fabry–Perot, fiber Bragg grating, and interferometric fiber tip sensor are tested and compared. Shock waves are generated using a high-power laser focused onto a thin foil target placed in close proximity to the test blocks. The fiber Fabry–Perot sensor appears capable of resolving the shock front with a rise time of 91 ns. As a result, the peak pressure is estimated, using a separate shadowgraphy measurement, to be 3.4 GPa.

  4. Photon Statistics of a Two-Level Atom in a Driven Optical Cavity

    NASA Astrophysics Data System (ADS)

    Clemens, James; Rice, Perry

    1997-10-01

    We consider the second-order intensity correlation function g^(τ ) for a single two-level atom in an optical cavity driven by a classical field. Previous results are extended beyond the weak-field limit, using a quantum trajectory method. Manifestly quantum behavior is observed, and we compare our results to recent experiments by Mielke et. al. ( S. L. Mielke, G. T. Foster, and L. A. Orozco, submitted to Physical Review Letters.) More information can be found at http://muohio.edu/ ~ ricepr/research.htm.

  5. Non-adiabatic effects on the optical response of driven systems

    NASA Astrophysics Data System (ADS)

    Fregoso, Benjamin M.; Kolodrubetz, Michael; Moore, Joel

    Periodically driven systems have received renewed interest due to their capacity to engineer non-trivial effective Hamiltonians. A characteristic of such systems is how they respond to weak periodicity-breaking drive, as for example when a laser is pulsed instead of continuous wave. We develop semi-classical equations of motion of a wave packet in the presence of electric and magnetic fields which are turned on non-adiabatically. We then show the emergence of significant corrections to electronic collective excitations and optical responses of topological insulator surface states, Weyl metals and semiconductor mono-chalcogenides.

  6. Fluctuation-driven topological transition of binary condensates in optical lattices

    NASA Astrophysics Data System (ADS)

    Suthar, K.; Roy, Arko; Angom, D.

    2015-04-01

    We show the emergence of a third Goldstone mode in binary condensates at phase separation in quasi-one-dimensional (quasi-1D) optical lattices. We develop the coupled discrete nonlinear Schrödinger equations using Hartree-Fock-Bogoliubov theory with the Popov approximation in the Bose-Hubbard model to investigate the mode evolution at zero temperature, in particular, as the system is driven from the miscible to the immiscible phase. We demonstrate that the position exchange of the species in the 87Rb-85Rb system is accompanied by a discontinuity in the excitation spectrum. Our results show that, in quasi-1D optical lattices, the presence of the fluctuations dramatically changes the geometry of the ground-state density profile of two-component Bose-Einstein condensates.

  7. High-Q silica zipper cavity for optical radiation pressure driven MOMS switch

    SciTech Connect

    Tetsumoto, Tomohiro; Tanabe, Takasumi

    2014-07-15

    We design a silica zipper cavity that has high optical and mechanical Q (quality factor) values and demonstrate numerically the feasibility of a radiation pressure driven micro opto-mechanical system (MOMS) directional switch. The silica zipper cavity has an optical Q of 4.0 × 10{sup 4} and an effective mode volume V{sub mode} of 0.67λ{sup 3} when the gap between two cavities is 34 nm. The mechanical Q (Q{sub m}) is determined by thermo-elastic damping and is 2.0 × 10{sup 6} in a vacuum at room temperature. The opto-mechanical coupling rate g{sub OM} is as high as 100 GHz/nm, which allows us to move the directional cavity-waveguide system and switch 1550-nm light with 770-nm light by controlling the radiation pressure.

  8. Development of dielectric elastomer driven micro-optical zoom lens system

    NASA Astrophysics Data System (ADS)

    Kim, Hyunseok; Park, Jongkil; Chuc, Nguyen Huu; Choi, H. R.; Nam, J. D.; Lee, Y.; Jung, H. S.; Koo, J. C.

    2007-04-01

    Normally, various micro-scale devices adopt electromechanical actuators for their basic mechanical functions. Those types of actuators require a complicated power transfer system even for generating a tiny scale motion. Since the mechanical power transfer system for the micro-scale motion may require many components, the system design to fit those components into a small space is always challenging. Micro-optical zoom lens systems are recently popularly used for many portable IT devices such as digital cameras, camcorder, and cell phones, Noting the advantages of EAP actuators over the conventional electromechanical counterparts in terms of simple actuator mechanisms, a micro-optic device that is driven with the EAP actuator is introduced in the present work. EAP material selection, device design and fabrication will be also delineated.

  9. Control of Exciton Photon Coupling in Nano-structures

    NASA Astrophysics Data System (ADS)

    Liu, Xiaoze

    In this thesis, we study the interaction of excitons with photons and plasmons and methods to control and enhance this interaction. This study is categorized in three parts: light-matter interaction in microcavity structures, direct dipole-dipole interactions, and plasmon-exciton interaction in metal-semiconductor systems. In the microcavity structures, the light-matter interactions become significant when the excitonic energy is in resonance with microcavity photons. New hybrid quantum states named polariton states will be formed if the strong coupling regime is achieved, where the interaction rate is faster than the average decay rate of the excitons and photons. Polaritons have been investigated in zinc oxide (ZnO) nanoparticles based microcavity at room temperature and stimulated emission of the polaritons has also been observed with a low optical pump threshold. Exictons in organic semiconductors (modeled as Frenkel excitons) are tightly bound to molecular sites, and differ considerably from loosely bound hydrogen atom-like inorganic excitons (modeled as Wannier-Mott excitons). This fundamental difference results in distinct optoelectronic properties. Not only strongly coupled to Wannier-Mott excitons in ZnO, the microcavity photons have also been observed to be simultaneously coupled to Frenkel excitons in 3,4,7,8-naphthalene tetracarboxylic dianhydride (NTCDA). The photons here act like a glue combining Wannier-Mott and Frenkel excitons into new hybrid polaritons taking the best from both constituents. Two-dimensional (2D) excitons in monolayer transition metal dichalcogenides (TMDs) have emerged as a new and fascinating type of Wannier-Mott-like excitons due to direct bandgap transition, huge oscillator strength and large binding energy. Monolayer molybdenum disulfide (MoS2) has been incorporated into the microcavity structure and 2D exciton-polaritons have been observed for the first time with directional emission in the strong coupling regime. Valley

  10. Optimization of torque on an optically driven micromotor by manipulation of the index of refraction

    NASA Astrophysics Data System (ADS)

    Wing, Frank M., III; Mahajan, Satish; Collett, Walter

    2004-12-01

    Since the 1970"s, the focused laser beam has become a familiar tool to manipulate neutral, dielectric micro-objects. A number of authors, including Higurashi and Gauthier, have described the effects of radiation pressure from laser light on microrotors. Collett, et al. developed a wave, rather than a ray optic, approach in the calculation of such forces on a microrotor for the first time. This paper describes a modification to the design of a laser driven, radiation pressure microrotor, intended to improve the optically generated torque. Employing the wave approach, the electric and magnetic fields in the vicinity of the rotor are calculated using the finite difference time domain (FDTD) method, which takes into account the wave nature of the incident light. Forces are calculated from the application of Maxwell"s stress tensor over the surfaces of the rotor. Results indicate a significant increase in torque when the index of refraction of the microrotor is changed from a single value to an inhomogeneous profile. The optical fiber industry has successfully employed a variation in the index of refraction across the cross section of a fiber for the purpose of increasing the efficiency of light transmission. Therefore, it is hoped that various fabrication methods can be utilized for causing desired changes in the index of refraction of an optically driven microrotor. Various profiles of the index of refraction inside a microrotor are considered for optimization of torque. Simulation methodology and results of torque on a microrotor for various profiles of the index of refraction are presented. Guidelines for improvised fabrication of efficient microrotors may then be obtained from these profiles.

  11. Singlet exciton fission photovoltaics.

    PubMed

    Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A

    2013-06-18

    Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses

  12. Excitons in Semiconductor Quantum Wells Studied Using Two-Dimensional Coherent Spectroscopy

    NASA Astrophysics Data System (ADS)

    Singh, Rohan

    Correlated electron-hole pairs, or excitons, in semiconductor nanostructures have been studied extensively over the past few decades. The optical response of excitons is complicated due to inhomogeneous broadening, presence of multiple states, and exciton-exciton interactions. In this work we bring new perspectives to exciton physics in semiconductor quantum wells (QWs) through two-dimensional coherent spectroscopy (2DCS). The effect of QW growth direction on the optical properties of excitons is explored by studying (110)-oriented GaAs QWs. The homogeneous and inhomogeneous linewidths of the heavy-hole exciton resonance are measured. By probing the optical nonlinear response for polarization along the in-plane crystal axes [110] and [001], we measure different homogeneous linewidths for the two orthogonal directions. This difference is found to be due to anisotropic excitation-induced dephasing, caused by a crystal-axis-dependent absorption coefficient. The extrapolated zero-excitation density homogeneous linewidth exhibits an activation-like temperature dependence. Spectral diffusion of excitons in (001)-oriented QWs has been studied. We show that the spectral diffusion characteristics depend strongly on the sample temperature. Spectral diffusion is generally assumed to follow the strong-redistribution approximation, partly because of lack of any evidence to the contrary. We find that this assumption is violated at low sample temperatures for excitons in QWs; high-energy excitons preferentially relax due to a negligible phonon population at low temperatures. The frequency-frequency correlation function is measured through a numerical fitting procedure to quantify spectral diffusion for sample temperatures >20 K. Exciton-exciton interactions affect the light-matter interactions in QWs significantly. We present an intuitive and simple model for these interactions by treating excitons as interacting bosons. We show that the polarization-dependent exciton dephasing

  13. Strong Quantum Coherence between Fermi Liquid Mahan Excitons.

    PubMed

    Paul, J; Stevens, C E; Liu, C; Dey, P; McIntyre, C; Turkowski, V; Reno, J L; Hilton, D J; Karaiskaj, D

    2016-04-15

    In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called "Mahan excitons." The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the optical Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system.

  14. Optical design of the Fresnel lens for LED-driven flashlight.

    PubMed

    Chen, Yi-Cheng; Nian, Shih-Chih; Huang, Ming-Shyan

    2016-02-01

    The Fresnel lens is composed of micrometer-sized v-groove structures that determine the maximum illuminance and brightness uniformity of LED-driven flashlights, which are used in high-quality photography. The fabrication quality of the microstructures and the accuracy of the geometrical curvature of the Fresnel lens affect the optical characteristics of the emitted light traveling through the lens, which in turn determines the maximum illuminance and brightness uniformity. This paper presents a systematic design procedure for fabricating the Fresnel lens and investigates the influence of geometrical design and fabrication process on optical performance. The optical analysis was performed using the commercial software TracePro. The results revealed that a small tip radius of the v-groove microstructure facilitates brightness uniformity. Furthermore, both the simulation and the experimental results revealed that Fresnel lenses fabricated through injection molding or injection compression molding have either errors of microstructure height more than 3%-6% or curvature errors higher than 6%, which would affect the optical performance, especially the brightness uniformity. PMID:26836072

  15. Optical design of the Fresnel lens for LED-driven flashlight.

    PubMed

    Chen, Yi-Cheng; Nian, Shih-Chih; Huang, Ming-Shyan

    2016-02-01

    The Fresnel lens is composed of micrometer-sized v-groove structures that determine the maximum illuminance and brightness uniformity of LED-driven flashlights, which are used in high-quality photography. The fabrication quality of the microstructures and the accuracy of the geometrical curvature of the Fresnel lens affect the optical characteristics of the emitted light traveling through the lens, which in turn determines the maximum illuminance and brightness uniformity. This paper presents a systematic design procedure for fabricating the Fresnel lens and investigates the influence of geometrical design and fabrication process on optical performance. The optical analysis was performed using the commercial software TracePro. The results revealed that a small tip radius of the v-groove microstructure facilitates brightness uniformity. Furthermore, both the simulation and the experimental results revealed that Fresnel lenses fabricated through injection molding or injection compression molding have either errors of microstructure height more than 3%-6% or curvature errors higher than 6%, which would affect the optical performance, especially the brightness uniformity.

  16. Negative activation energy and dielectric signatures of excitons and excitonic Mott transitions in quantum confined laser structures

    NASA Astrophysics Data System (ADS)

    Bhunia, Amit; Bansal, Kanika; Henini, Mohamed; Alshammari, Marzook S.; Datta, Shouvik

    2016-10-01

    Mostly, optical spectroscopies are used to investigate the physics of excitons, whereas their electrical evidences are hardly explored. Here, we examined a forward bias activated differential capacitance response of GaInP/AlGaInP based multi-quantum well laser diodes to trace the presence of excitons using electrical measurements. Occurrence of "negative activation energy" after light emission is understood as thermodynamical signature of steady state excitonic population under intermediate range of carrier injections. Similar corroborative results are also observed in an InGaAs/GaAs quantum dot laser structure grown by molecular beam epitaxy. With increasing biases, the measured differential capacitance response slowly vanishes. This represents gradual Mott transition of an excitonic phase into an electron-hole plasma in a GaInP/AlGaInP laser diode. This is further substantiated by more and more exponentially looking shapes of high energy tails in electroluminescence spectra with increasing forward bias, which originates from a growing non-degenerate population of free electrons and holes. Such an experimental correlation between electrical and optical properties of excitons can be used to advance the next generation excitonic devices.

  17. Excitonic effects in oxyhalide scintillating host compounds

    SciTech Connect

    Shwetha, G.; Kanchana, V.; Valsakumar, M. C.

    2014-10-07

    Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X = F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.

  18. Dimensional crossover of free exciton diffusion in etched GaAs wire structures

    NASA Astrophysics Data System (ADS)

    Bieker, S.; Stühler, R.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.

    2015-09-01

    We report on low-temperature spatially resolved photoluminescence spectroscopy to study the diffusion of free excitons in etched wire structures of high-purity GaAs. We assess the stationary diffusion profiles by the free exciton second LO-phonon replica to circumvent the inherent interpretation ambiguities of the previously investigated free exciton zero-phonon line. Moreover, strictly resonant optical excitation prevents the distortion of the diffusion profiles due to local heating in the carrier system. We observe a dimensional crossover from 2D to 1D exciton diffusion when the lateral wire width falls below the diffusion length.

  19. Excitons in asymmetric quantum wells

    NASA Astrophysics Data System (ADS)

    Grigoryev, P. S.; Kurdyubov, A. S.; Kuznetsova, M. S.; Ignatiev, I. V.; Efimov, Yu. P.; Eliseev, S. A.; Petrov, V. V.; Lovtcius, V. A.; Shapochkin, P. Yu.

    2016-09-01

    Resonance dielectric response of excitons is studied for the high-quality InGaAs/GaAs heterostructures with wide asymmetric quantum wells (QWs). To highlight effects of the QW asymmetry, we have grown and studied several heterostructures with nominally square QWs as well as with triangle-like QWs. Several quantum confined exciton states are experimentally observed as narrow exciton resonances. A standard approach for the phenomenological analysis of the profiles is generalized by introducing different phase shifts for the light waves reflected from the QWs at different exciton resonances. Good agreement of the phenomenological fit to the experimentally observed exciton spectra for high-quality structures allowed us to reliably obtain parameters of the exciton resonances: the exciton transition energies, the radiative broadenings, and the phase shifts. A direct numerical solution of the Schrödinger equation for the heavy-hole excitons in asymmetric QWs is used for microscopic modeling of the exciton resonances. Remarkable agreement with the experiment is achieved when the effect of indium segregation is taken into account. The segregation results in a modification of the potential profile, in particular, in an asymmetry of the nominally square QWs.

  20. Many-body effects and excitonic features in 2D biphenylene carbon

    NASA Astrophysics Data System (ADS)

    Lüder, Johann; Puglia, Carla; Ottosson, Henrik; Eriksson, Olle; Sanyal, Biplab; Brena, Barbara

    2016-01-01

    The remarkable excitonic effects in low dimensional materials in connection to large binding energies of excitons are of great importance for research and technological applications such as in solar energy and quantum information processing as well as for fundamental investigations. In this study, the unique electronic and excitonic properties of the two dimensional carbon network biphenylene carbon were investigated with GW approach and the Bethe-Salpeter equation accounting for electron correlation effects and electron-hole interactions, respectively. Biphenylene carbon exhibits characteristic features including bright and dark excitons populating the optical gap of 0.52 eV and exciton binding energies of 530 meV as well as a technologically relevant intrinsic band gap of 1.05 eV. Biphenylene carbon's excitonic features, possibly tuned, suggest possible applications in the field of solar energy and quantum information technology in the future.

  1. Two-dimensional excitons in three-dimensional hexagonal boron nitride

    SciTech Connect

    Cao, X. K.; Lin, J. Y. Jiang, H. X.; Clubine, B.; Edgar, J. H.

    2013-11-04

    The recombination processes of excitons in hexagonal boron nitride (hBN) have been probed using time-resolved photoluminescence. It was found that the theory for two-dimensional (2D) exciton recombination describes well the exciton dynamics in three-dimensional hBN. The exciton Bohr radius and binding energy deduced from the temperature dependent exciton recombination lifetime is around 8 Å and 740 meV, respectively. The effective masses of electrons and holes in 2D hBN deduced from the generalized relativistic dispersion relation of 2D systems are 0.54m{sub o}, which are remarkably consistent with the exciton reduced mass deduced from the experimental data. Our results illustrate that hBN represents an ideal platform to study the 2D optical properties as well as the relativistic properties of particles in a condensed matter system.

  2. Giant optical enhancement of strain gradient in ferroelectric BiFeO3 thin films and its physical origin

    DOE PAGES

    Li, Yuelin; Adamo, C.; Chen, Pice; Evans, Paul G.; Nakhmanson, Serge M.; Parker, William; Rowland, Clare E.; Schaller, Richard D.; Schlom, Darrell G.; Walko, Donald A.; et al

    2015-11-20

    Through mapping of the spatiotemporal strain profile in ferroelectric BiFeO3 epitaxial thin films, we report an optically initiated dynamic enhancement of the strain gradient of 105–106 m-1 that lasts up to a few ns depending on the film thickness. Correlating with transient optical absorption measurements, the enhancement of the strain gradient is attributed to a piezoelectric effect driven by a transient screening field mediated by excitons. In conclusion, these findings not only demonstrate a new possible way of controlling the flexoelectric effect, but also reveal the important role of exciton dynamics in photostriction and photovoltaic effects in ferroelectrics.

  3. Giant optical enhancement of strain gradient in ferroelectric BiFeO3 thin films and its physical origin

    PubMed Central

    Li, Yuelin; Adamo, Carolina; Chen, Pice; Evans, Paul G.; Nakhmanson, Serge M.; Parker, William; Rowland, Clare E.; Schaller, Richard D.; Schlom, Darrell G.; Walko, Donald A.; Wen, Haidan; Zhang, Qingteng

    2015-01-01

    Through mapping of the spatiotemporal strain profile in ferroelectric BiFeO3 epitaxial thin films, we report an optically initiated dynamic enhancement of the strain gradient of 105–106 m−1 that lasts up to a few ns depending on the film thickness. Correlating with transient optical absorption measurements, the enhancement of the strain gradient is attributed to a piezoelectric effect driven by a transient screening field mediated by excitons. These findings not only demonstrate a new possible way of controlling the flexoelectric effect, but also reveal the important role of exciton dynamics in photostriction and photovoltaic effects in ferroelectrics. PMID:26586421

  4. Probing excitonic dark states in single-layer tungsten disulphide.

    PubMed

    Ye, Ziliang; Cao, Ting; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G; Zhang, Xiang

    2014-09-11

    Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism, doping-dependent charged excitons and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (∼0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many

  5. Resonant surface plasmon-exciton interaction in hybrid MoSe2@Au nanostructures.

    PubMed

    Abid, I; Bohloul, A; Najmaei, S; Avendano, C; Liu, H-L; Péchou, R; Mlayah, A; Lou, J

    2016-04-21

    In this work we investigate the interaction between plasmonic and excitonic resonances in hybrid MoSe2@Au nanostructures. The latter were fabricated by combining chemical vapor deposition of MoSe2 atomic layers, Au disk processing by nanosphere lithography and a soft lift-off/transfer technique. The samples were characterized by scanning electron and atomic force microscopy. Their optical properties were investigated experimentally using optical absorption, Raman scattering and photoluminescence spectroscopy. The work is focused on a resonant situation where the surface plasmon resonance is tuned to the excitonic transition. In that case, the near-field interaction between the surface plasmons and the confined excitons leads to interference between the plasmonic and excitonic resonances that manifests in the optical spectra as a transparency dip. The plasmonic-excitonic interaction regime is determined using quantitative analysis of the optical extinction spectra based on an analytical model supported by numerical simulations. We found that the plasmonic-excitonic resonances do interfere thus leading to a typical Fano lineshape of the optical extinction. The near-field nature of the plasmonic-excitonic interaction is pointed out experimentally from the dependence of the optical absorption on the number of monolayer stacks on the Au nanodisks. The results presented in this work contribute to the development of new concepts in the field of hybrid plasmonics.

  6. Femtosecond THz Studies of Intra-Excitonic Transitions

    SciTech Connect

    Huber, Rupert; Schmid, Ben A.; Kaindl, Robert A.; Chemla, Daniel S.

    2007-10-02

    Few-cycle THz pulses are employed to resonantly access the internal fine structure of photogenerated excitons in semiconductors, on the femtosecond time scale. This technique allows us to gain novel insight into many-body effects of excitons and reveal key quantum optical processes. We discuss experiments that monitor the density-dependent re?normalization of the binding energy of a high-density exciton gas in GaAs/AlGaAs quantum wells close to the Mott transition. In a dilute ensemble of 3p excitons in Cu2O, stimulated THz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10-14 cm2. Simultaneous interband excitation of both exciton levels drives quantum beats, which cause efficient THz emission at the difference frequency. By extending this principle to various other exciton resonances, we develop a novel way of mapping the fine structure by two-dimensional THz emission spectroscopy.

  7. Magnons and Phonons Optically Driven out of Local Equilibrium in a Magnetic Insulator.

    PubMed

    An, Kyongmo; Olsson, Kevin S; Weathers, Annie; Sullivan, Sean; Chen, Xi; Li, Xiang; Marshall, Luke G; Ma, Xin; Klimovich, Nikita; Zhou, Jianshi; Shi, Li; Li, Xiaoqin

    2016-09-01

    The coupling and possible nonequilibrium between magnons and other energy carriers have been used to explain several recently discovered thermally driven spin transport and energy conversion phenomena. Here, we report experiments in which local nonequilibrium between magnons and phonons in a single crystalline bulk magnetic insulator, Y_{3}Fe_{5}O_{12}, has been created optically within a focused laser spot and probed directly via micro-Brillouin light scattering. Through analyzing the deviation in the magnon number density from the local equilibrium value, we obtain the diffusion length of thermal magnons. By explicitly establishing and observing local nonequilibrium between magnons and phonons, our studies represent an important step toward a quantitative understanding of various spin-heat coupling phenomena. PMID:27636490

  8. Magnons and Phonons Optically Driven out of Local Equilibrium in a Magnetic Insulator

    NASA Astrophysics Data System (ADS)

    An, Kyongmo; Olsson, Kevin S.; Weathers, Annie; Sullivan, Sean; Chen, Xi; Li, Xiang; Marshall, Luke G.; Ma, Xin; Klimovich, Nikita; Zhou, Jianshi; Shi, Li; Li, Xiaoqin

    2016-09-01

    The coupling and possible nonequilibrium between magnons and other energy carriers have been used to explain several recently discovered thermally driven spin transport and energy conversion phenomena. Here, we report experiments in which local nonequilibrium between magnons and phonons in a single crystalline bulk magnetic insulator, Y3Fe5O12 , has been created optically within a focused laser spot and probed directly via micro-Brillouin light scattering. Through analyzing the deviation in the magnon number density from the local equilibrium value, we obtain the diffusion length of thermal magnons. By explicitly establishing and observing local nonequilibrium between magnons and phonons, our studies represent an important step toward a quantitative understanding of various spin-heat coupling phenomena.

  9. Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency

    PubMed Central

    Gonzalez-Izquierdo, Bruno; King, Martin; Gray, Ross J.; Wilson, Robbie; Dance, Rachel J.; Powell, Haydn; Maclellan, David A.; McCreadie, John; Butler, Nicholas M. H.; Hawkes, Steve; Green, James S.; Murphy, Chris D.; Stockhausen, Luca C.; Carroll, David C.; Booth, Nicola; Scott, Graeme G.; Borghesi, Marco; Neely, David; McKenna, Paul

    2016-01-01

    Control of the collective response of plasma particles to intense laser light is intrinsic to relativistic optics, the development of compact laser-driven particle and radiation sources, as well as investigations of some laboratory astrophysics phenomena. We recently demonstrated that a relativistic plasma aperture produced in an ultra-thin foil at the focus of intense laser radiation can induce diffraction, enabling polarization-based control of the collective motion of plasma electrons. Here we show that under these conditions the electron dynamics are mapped into the beam of protons accelerated via strong charge-separation-induced electrostatic fields. It is demonstrated experimentally and numerically via 3D particle-in-cell simulations that the degree of ellipticity of the laser polarization strongly influences the spatial-intensity distribution of the beam of multi-MeV protons. The influence on both sheath-accelerated and radiation pressure-accelerated protons is investigated. This approach opens up a potential new route to control laser-driven ion sources. PMID:27624920

  10. Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency.

    PubMed

    Gonzalez-Izquierdo, Bruno; King, Martin; Gray, Ross J; Wilson, Robbie; Dance, Rachel J; Powell, Haydn; Maclellan, David A; McCreadie, John; Butler, Nicholas M H; Hawkes, Steve; Green, James S; Murphy, Chris D; Stockhausen, Luca C; Carroll, David C; Booth, Nicola; Scott, Graeme G; Borghesi, Marco; Neely, David; McKenna, Paul

    2016-01-01

    Control of the collective response of plasma particles to intense laser light is intrinsic to relativistic optics, the development of compact laser-driven particle and radiation sources, as well as investigations of some laboratory astrophysics phenomena. We recently demonstrated that a relativistic plasma aperture produced in an ultra-thin foil at the focus of intense laser radiation can induce diffraction, enabling polarization-based control of the collective motion of plasma electrons. Here we show that under these conditions the electron dynamics are mapped into the beam of protons accelerated via strong charge-separation-induced electrostatic fields. It is demonstrated experimentally and numerically via 3D particle-in-cell simulations that the degree of ellipticity of the laser polarization strongly influences the spatial-intensity distribution of the beam of multi-MeV protons. The influence on both sheath-accelerated and radiation pressure-accelerated protons is investigated. This approach opens up a potential new route to control laser-driven ion sources. PMID:27624920

  11. Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency

    NASA Astrophysics Data System (ADS)

    Gonzalez-Izquierdo, Bruno; King, Martin; Gray, Ross J.; Wilson, Robbie; Dance, Rachel J.; Powell, Haydn; MacLellan, David A.; McCreadie, John; Butler, Nicholas M. H.; Hawkes, Steve; Green, James S.; Murphy, Chris D.; Stockhausen, Luca C.; Carroll, David C.; Booth, Nicola; Scott, Graeme G.; Borghesi, Marco; Neely, David; McKenna, Paul

    2016-09-01

    Control of the collective response of plasma particles to intense laser light is intrinsic to relativistic optics, the development of compact laser-driven particle and radiation sources, as well as investigations of some laboratory astrophysics phenomena. We recently demonstrated that a relativistic plasma aperture produced in an ultra-thin foil at the focus of intense laser radiation can induce diffraction, enabling polarization-based control of the collective motion of plasma electrons. Here we show that under these conditions the electron dynamics are mapped into the beam of protons accelerated via strong charge-separation-induced electrostatic fields. It is demonstrated experimentally and numerically via 3D particle-in-cell simulations that the degree of ellipticity of the laser polarization strongly influences the spatial-intensity distribution of the beam of multi-MeV protons. The influence on both sheath-accelerated and radiation pressure-accelerated protons is investigated. This approach opens up a potential new route to control laser-driven ion sources.

  12. Valley excitons in two-dimensional semiconductors

    SciTech Connect

    Yu, Hongyi; Cui, Xiaodong; Xu, Xiaodong; Yao, Wang

    2014-12-30

    Monolayer group-VIB transition metal dichalcogenides have recently emerged as a new class of semiconductors in the two-dimensional limit. The attractive properties include: the visible range direct band gap ideal for exploring optoelectronic applications; the intriguing physics associated with spin and valley pseudospin of carriers which implies potentials for novel electronics based on these internal degrees of freedom; the exceptionally strong Coulomb interaction due to the two-dimensional geometry and the large effective masses. The physics of excitons, the bound states of electrons and holes, has been one of the most actively studied topics on these two-dimensional semiconductors, where the excitons exhibit remarkably new features due to the strong Coulomb binding, the valley degeneracy of the band edges, and the valley dependent optical selection rules for interband transitions. Here we give a brief overview of the experimental and theoretical findings on excitons in two-dimensional transition metal dichalcogenides, with focus on the novel properties associated with their valley degrees of freedom.

  13. Valley excitons in two-dimensional semiconductors

    DOE PAGES

    Yu, Hongyi; Cui, Xiaodong; Xu, Xiaodong; Yao, Wang

    2014-12-30

    Monolayer group-VIB transition metal dichalcogenides have recently emerged as a new class of semiconductors in the two-dimensional limit. The attractive properties include: the visible range direct band gap ideal for exploring optoelectronic applications; the intriguing physics associated with spin and valley pseudospin of carriers which implies potentials for novel electronics based on these internal degrees of freedom; the exceptionally strong Coulomb interaction due to the two-dimensional geometry and the large effective masses. The physics of excitons, the bound states of electrons and holes, has been one of the most actively studied topics on these two-dimensional semiconductors, where the excitons exhibitmore » remarkably new features due to the strong Coulomb binding, the valley degeneracy of the band edges, and the valley dependent optical selection rules for interband transitions. Here we give a brief overview of the experimental and theoretical findings on excitons in two-dimensional transition metal dichalcogenides, with focus on the novel properties associated with their valley degrees of freedom.« less

  14. Characterisation of optically driven microstructures for manipulating single DNA molecules under a fluorescence microscope.

    PubMed

    Terao, Kyohei; Masuda, Chihiro; Inukai, Ryo; Gel, Murat; Oana, Hidehiro; Washizu, Masao; Suzuki, Takaaki; Takao, Hidekuni; Shimokawa, Fusao; Oohira, Fumikazu

    2016-06-01

    Optical tweezers are powerful tools for manipulating single DNA molecules using fluorescence microscopy, particularly in nanotechnology-based DNA analysis. We previously proposed a manipulation technique using microstructures driven by optical tweezers that allows the handling of single giant DNA molecules of millimetre length that cannot be manipulated by conventional techniques. To further develop this technique, the authors characterised the microstructures quantitatively from the view point of fabrication and efficiency of DNA manipulation under a fluorescence microscope. The success rate and precision of the fabrications were evaluated. The results indicate that the microstructures are obtained in an aqueous solution with a precision ∼50 nm at concentrations in the order of 10(6) particles/ml. The visibility of these microstructures under a fluorescence microscope was also characterised, along with the elucidation of the fabrication parameters needed to fine tune visibility. Manipulating yeast chromosomal DNA molecules with the microstructures illustrated the relationship between the efficiency of manipulation and the geometrical shape of the microstructure. This report provides the guidelines for designing microstructures used in single DNA molecule analysis based on on-site DNA manipulation, and is expected to broaden the applications of this technique in the future.

  15. Temperature driven evolution of thermal, electrical, and optical properties of Ti–Al–N coatings

    PubMed Central

    Rachbauer, Richard; Gengler, Jamie J.; Voevodin, Andrey A.; Resch, Katharina; Mayrhofer, Paul H.

    2012-01-01

    Monolithic single phase cubic (c) Ti1−xAlxN thin films are used in various industrial applications due to their high thermal stability, which beneficially effects lifetime and performance of cutting and milling tools, but also find increasing utilization in electronic and optical devices. The present study elucidates the temperature-driven evolution of heat conductivity, electrical resistivity and optical reflectance from room temperature up to 1400 °C and links them to structural and chemical changes in Ti1−xAlxN coatings. It is shown that various decomposition phenomena, involving recovery and spinodal decomposition (known to account for the age hardening phenomenon in c-Ti1−xAlxN), as well as the cubic to wurtzite phase transformation of spinodally formed AlN-enriched domains, effectively increase the thermal conductivity of the coatings from ∼3.8 W m−1 K−1 by a factor of three, while the electrical resistivity is reduced by one order of magnitude. A change in the coating color from metallic grey after deposition to reddish-golden after annealing to 1400 °C is related to the film structure and discussed in terms of film reflectivity. PMID:23482424

  16. Exciton-photon correlations in bosonic condensates of exciton-polaritons.

    PubMed

    Kavokin, Alexey V; Sheremet, Alexandra S; Shelykh, Ivan A; Lagoudakis, Pavlos G; Rubo, Yuri G

    2015-07-08

    Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers.

  17. Exciton-phonon system on a star graph: A perturbative approach

    NASA Astrophysics Data System (ADS)

    Yalouz, Saad; Pouthier, Vincent

    2016-05-01

    Based on the operatorial formulation of the perturbation theory, the properties of an exciton coupled with optical phonons on a star graph are investigated. Within this method, the dynamics is governed by an effective Hamiltonian, which accounts for exciton-phonon entanglement. The exciton is dressed by a virtual phonon cloud whereas the phonons are clothed by virtual excitonic transitions. In spite of the coupling with the phonons, it is shown that the energy spectrum of the dressed exciton resembles that of a bare exciton. The only differences originate in a polaronic mechanism that favors an energy shift and a decay of the exciton hopping constant. By contrast, the motion of the exciton allows the phonons to propagate over the graph so that the dressed normal modes drastically differ from the localized modes associated to bare phonons. They define extended vibrations whose properties depend on the state occupied by the exciton that accompanies the phonons. It is shown that the phonon frequencies, either red shifted or blue shifted, are very sensitive to the model parameter in general, and to the size of the graph in particular.

  18. Radiative and nonradiative exciton energy transfer in monolayers of two-dimensional group-VI transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Manolatou, Christina; Wang, Haining; Chan, Weimin; Tiwari, Sandip; Rana, Farhan

    2016-04-01

    We present results on the rates of interlayer energy transfer between excitons in monolayers of two-dimensional group-VI transition metal dichalcogenides (TMDs). We consider both radiative (mediated by real photons) and nonradiative (mediated by virtual photons) mechanisms of energy transfer using a unified Green's function approach that takes into account modification of the exciton energy dispersions as a result of interactions. The large optical oscillator strengths associated with excitons in TMDs result in very fast energy transfer rates. The energy transfer times depend on the exciton momentum, exciton linewidth, and the interlayer separation and can range from values less than 100 femtoseconds to more than tens of picoseconds. Whereas inside the light cone the energy transfer rates of longitudinal and transverse excitons are comparable, outside the light cone the energy transfer rates of longitudinal excitons far exceed those of transverse excitons. Average energy transfer times for a thermal ensemble of longitudinal and transverse excitons is temperature dependent and can be smaller than a picosecond at room temperature for interlayer separations smaller than 10 nm. Energy transfer times of localized excitons range from values less than a picosecond to several tens of picoseconds. When the exciton scattering and dephasing rates are small, energy transfer dynamics exhibit coherent oscillations. Our results show that electromagnetic interlayer energy transfer can be an efficient mechanism for energy exchange between TMD monolayers.

  19. Exciton condensation in microcavities under three-dimensional quantization conditions

    SciTech Connect

    Kochereshko, V. P. Platonov, A. V.; Savvidis, P.; Kavokin, A. V.; Bleuse, J.; Mariette, H.

    2013-11-15

    The dependence of the spectra of the polarized photoluminescence of excitons in microcavities under conditions of three-dimensional quantization on the optical-excitation intensity is investigated. The cascade relaxation of polaritons between quantized states of a polariton Bose condensate is observed.

  20. Excitons in AgI-BASED-GLASSES and -

    NASA Astrophysics Data System (ADS)

    Fujishiro, Fumito; Mochizuki, Shosuke

    2007-01-01

    We summarize our recent optical studies on different pristine AgI films, different AgI-based glasses and different AgI-oxide fine particle composites. The exciton spectra of these specimens give useful information about the ionic and electronic structures at the AgI/glass and AgI/oxide particle interfaces.

  1. Excitons in atomically thin black phosphorus

    NASA Astrophysics Data System (ADS)

    Surrente, A.; Mitioglu, A. A.; Galkowski, K.; Tabis, W.; Maude, D. K.; Plochocka, P.

    2016-03-01

    Raman scattering and photoluminescence spectroscopy are used to investigate the optical properties of single layer black phosphorus obtained by mechanical exfoliation of bulk crystals under an argon atmosphere. The Raman spectroscopy, performed in situ on the same flake as the photoluminescence measurements, demonstrates the single layer character of the investigated samples. The emission spectra, dominated by excitonic effects, display the expected in-plane anisotropy. The emission energy depends on the type of substrate on which the flake is placed due to the different dielectric screening. Finally, the blueshift of the emission with increasing temperature is well described using a two-oscillator model for the temperature dependence of the band gap.

  2. Feasibility study of a nuclear exciton laser

    NASA Astrophysics Data System (ADS)

    ten Brinke, Nicolai; Schützhold, Ralf; Habs, Dietrich

    2013-05-01

    Nuclear excitons known from Mössbauer spectroscopy describe coherent excitations of a large number of nuclei—analogous to Dicke states (or Dicke super-radiance) in quantum optics. In this paper, we study the possibility of constructing a laser based on these coherent excitations. In contrast to the free-electron laser (in its usual design), such a device would be based on stimulated emission and thus might offer certain advantages, e.g., regarding energy-momentum accuracy. Unfortunately, inserting realistic parameters, the window of operability is probably not open (yet) to present-day technology; but our design should be feasible in the UV regime, for example.

  3. Recent developments in laser-driven and hollow-core fiber optic gyroscopes

    NASA Astrophysics Data System (ADS)

    Digonnet, M. J. F.; Chamoun, J. N.

    2016-05-01

    Although the fiber optic gyroscope (FOG) continues to be a commercial success, current research efforts are endeavoring to improve its precision and broaden its applicability to other markets, in particular the inertial navigation of aircraft. Significant steps in this direction are expected from the use of (1) laser light to interrogate the FOG instead of broadband light, and (2) a hollow-core fiber (HCF) in the sensing coil instead of a conventional solid-core fiber. The use of a laser greatly improves the FOG's scale-factor stability and eliminates the source excess noise, while an HCF virtually eliminates the Kerr-induced drift and significantly reduces the thermal and Faraday-induced drifts. In this paper we present theoretical evidence that in a FOG with a 1085-m coil interrogated with a laser, the two main sources of noise and drift resulting from the use of coherent light can be reduced below the aircraft-navigation requirement by using a laser with a very broad linewidth, in excess of 40 GHz. We validate this concept with a laser broadened with an external phase modulator driven with a pseudo-random bit sequence at 2.8 GHz. This FOG has a measured noise of 0.00073 deg/√h, which is 30% below the aircraft-navigation requirement. Its measured drift is 0.03 deg/h, the lowest reported for a laser-driven FOG and only a factor of 3 larger than the navigation-grade specification. To illustrate the potential benefits of a hollow-core fiber in the FOG, this review also summarizes the previously reported performance of an experimental FOG utilizing 235 m of HCF and interrogated with broadband light.

  4. Studies of low temperature photoluminescence spectra and excitonic valley polarization in monolayer MoTe2

    NASA Astrophysics Data System (ADS)

    Koirala, Sandhaya; Mouri, Shinichiro; Miyauchi, Yuhei; Matsuda, Kazunari; Kyoto University Team

    Recently, atomically thin layered transition-metal dichalcogenide (TMDs) in the form MX2 (M = Mo, W, X = S, Se, Te) have attracted much interest from the viewpoints of their fundamental physics and potential applications. The characteristic optical features of semiconducting TMDs arise from excitons confined in their atomically thin layers. Molybdenum ditelluride MoTe2 has attracted emerging research interest because of optical gap energy (lowest exciton transition) of 1.09 eV, and large spin-orbit coupling of 250 meV. Temperature-dependent photoluminescence (PL) and polarization-resolved PL measurement were performed for mechanically exfoliated monolayer MoTe2 from 4.4 to 300 K. At a low temperature, the PL spectra from MoTe2 show two sharp peaks for excitons and charged excitons (trions). The systematic temperature-dependent PL measurements revel that the homogeneous linewidth of the exciton peak broadens linearly as the temperature increased due to exciton-acoustic-phonon interactions. From polarization-resolved PL measurements, the valley polarization of above 40 % in the exciton state has been observed at low temperatures. In this meeting, we will discuss about exciton dephasing and valley polarization in monolayer MoTe2.

  5. Radiative coupling of A and B excitons in ZnO

    NASA Astrophysics Data System (ADS)

    Kinoshita, Takashi; Ishihara, Hajime

    2016-07-01

    Radiation-induced coupling between A and B excitons in ZnO is theoretically studied. Considering the center-of-mass motion of excitons in bulk and thin-film structures, we calculate the eigenmodes of an exciton-radiation coupled system and reveal the ratio of each excitonic component in the respective eigenmodes, which is determined from diagonalization of the self-consistent equation between the polarization and the Maxwell electric field. In particular, in a nano-to-bulk crossover size regime, the large interaction volume between multipole-type excitonic waves and radiation waves causes radiative coupling between excitons from different valence bands, which leads to an enhancement of the radiative correction. The results presented in this study are in striking contrast with the conventional view of the optical response of excitons in ZnO, where A and B excitons are independently assigned to their respective spectral structures. We demonstrate an alternative spectral assignment of nonlinear optical signals by focusing on the degenerate four-wave mixing.

  6. Influence of Exciton Lifetime on Charge Carrier Dynamics in an Organic Heterostructure

    SciTech Connect

    Agrawal, Kanika L.; Sykes, Matthew E.; An, Kwang Hyup; Frieberg, Bradley; Green, P. F.; Shtein, Max

    2013-03-18

    Interactions between charge carriers and excitons, as well as between excitons and optical cavity modes in organic optoelectronic devices are fundamental to their operational limits and chief in preventing the realization of certain phenomena, such as electrically pumped organic lasing. We uncovered a previously unreported phenomenon, wherein optical cavity-modulated exciton decay rate leads to a concomitant modulation in the electrical current of an archetypal NPD/Alq₃ organic light emitting device operated in forward bias. The magnitude of this variation is sensitive to the local dielectric environment of the device and is found to be as large as 15%.

  7. Excitonic fine structure and binding energies of excitonic complexes in single InAs quantum dashes

    NASA Astrophysics Data System (ADS)

    Mrowiński, P.; Zieliński, M.; Świderski, M.; Misiewicz, J.; Somers, A.; Reithmaier, J. P.; Höfling, S.; Sek, G.

    2016-09-01

    The fundamental electronic and optical properties of elongated InAs nanostructures embedded in quaternary InGaAlAs barrier are investigated by means of high-resolution optical spectroscopy and many-body atomistic tight-binding theory. These wire-like shaped, self-assembled nanostructures are known as quantum dashes and are typically formed during the molecular beam epitaxial growth on InP substrates. In this paper, we study properties of excitonic complexes confined in quantum dashes emitting in a broad spectral range from below 1.2 to 1.55 μm. We find peculiar trends for the biexciton and negative trion binding energies, with pronounced trion binding in smaller size quantum dashes. These experimental findings are then compared and qualitatively explained by atomistic theory. The theoretical analysis shows a fundamental role of correlation effects for the absolute values of excitonic binding energies. Eventually, we determine the bright exciton fine structure splitting (FSS), where both the experiment and theory predict a broad distribution of the splitting varying from below 50 to almost 180 μeV. We identify several key factors determining the FSS values in such nanostructures, including quantum dash size variation and composition fluctuations.

  8. X-ray emission from core excitons

    NASA Astrophysics Data System (ADS)

    Carson, R. D.; Schnatterly, S. E.

    1987-07-01

    We have observed soft x-ray emission from core excitons in several semiconductors and insulators and find that the exciton intensity is related to its binding energy. We propose an explanation for these excitons and this relationship using a Wannier model. The validity of the Wannier model is further tested by comparing our measured exciton binding energies with predicted values. We conclude that this model appears to be a good starting point in the understanding of core excitons.

  9. Momentum dependence of the excitons in pentacene

    SciTech Connect

    Roth, Friedrich; Schuster, Roman; Koenig, Andreas; Knupfer, Martin; Berger, Helmuth

    2012-05-28

    We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*, b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function.

  10. Crossed excitons in a semiconductor nanostructure of mixed dimensionality

    SciTech Connect

    Owschimikow, Nina Kolarczik, Mirco; Kaptan, Yücel I.; Grosse, Nicolai B.; Woggon, Ulrike

    2014-09-08

    Semiconductor systems of reduced dimensionality, e.g., quantum dots or quantum wells, display a characteristic spectrum of confined excitons. Combining several of these systems may lead to the formation of “crossed” excitons, and thus new equilibrium states and scattering channels. We derive gain excitation spectra from two-color pump-probe experiments on an In(Ga)As based quantum dot semiconductor optical amplifier by analyzing the amplitudes of the traces. This grants access to the quantum dot response, even in the presence of strong absorption by the surroundings at the excitation energy. The gain excitation spectra yield evidence of crossed quantum dot-bulk states.

  11. Excitonic complexes in single zinc-blende GaN/AlN quantum dots grown by droplet epitaxy

    SciTech Connect

    Sergent, S.; Kako, S.; Bürger, M.; Schupp, T.; As, D. J.; Arakawa, Y.

    2014-10-06

    We study by microphotoluminescence the optical properties of single zinc-blende GaN/AlN quantum dots grown by droplet epitaxy. We show evidences of both excitonic and multiexcitonic recombinations in individual quantum dots with radiative lifetimes shorter than 287 ± 8 ps. Owing to large band offsets and a large exciton binding energy, the excitonic recombinations of single zinc-blende GaN/AlN quantum dots can be observed up to 300 K.

  12. Multichannel emission spectrometer for high dynamic range optical pyrometry of shock-driven materials

    NASA Astrophysics Data System (ADS)

    Bassett, Will P.; Dlott, Dana D.

    2016-10-01

    An emission spectrometer (450-850 nm) using a high-throughput, high numerical aperture (N.A. = 0.3) prism spectrograph with stepped fiberoptic coupling, 32 fast photomultipliers and thirty-two 1.25 GHz digitizers is described. The spectrometer can capture single-shot events with a high dynamic range in amplitude and time (nanoseconds to milliseconds or longer). Methods to calibrate the spectrometer and verify its performance and accuracy are described. When a reference thermal source is used for calibration, the spectrometer can function as a fast optical pyrometer. Applications of the spectrometer are illustrated by using it to capture single-shot emission transients from energetic materials or reactive materials initiated by kmṡs-1 impacts with laser-driven flyer plates. A log (time) data analysis method is used to visualize multiple kinetic processes resulting from impact initiation of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) or a Zr/CuO nanolaminate thermite. Using a gray body algorithm to interpret the spectral radiance from shocked HMX, a time history of temperature and emissivity was obtained, which could be used to investigate HMX hot spot dynamics. Finally, two examples are presented showing how the spectrometer can avoid temperature determination errors in systems where thermal emission is accompanied by atomic or molecular emission lines.

  13. Visualization and measurement of capillary-driven blood flow using spectral domain optical coherence tomography.

    PubMed

    Cito, Salvatore; Ahn, Yeh-Chan; Pallares, Jordi; Duarte, Rodrigo Martinez; Chen, Zhongping; Madou, Marc; Katakis, Ioanis

    2012-09-01

    Capillary-driven flow (CD-flow) in microchannels plays an important role in many microfluidic devices. These devices, the most popular being those based in lateral flow, are becoming increasingly used in health care and diagnostic applications. CD-flow can passively pump biological fluids as blood, serum or plasma, in microchannels and it can enhance the wall mass transfer by exploiting the convective effects of the flow behind the meniscus. The flow behind the meniscus has not been experimentally identified up to now because of the lack of high-resolution, non-invasive, cross-sectional imaging means. In this study, spectral-domain Doppler optical coherence tomography is used to visualize and measure the flow behind the meniscus in CD-flows of water and blood. Microchannels of polydimethylsiloxane and glass with different cross-sections are considered. The predictions of the flow behind the meniscus of numerical simulations using the power-law model for non-Newtonian fluids are in reasonable agreement with the measurements using blood as working fluid. The extension of the Lucas-Washburn equation to non-Newtonian power-law fluids predicts well the velocity of the meniscus of the experiments using blood. PMID:23795150

  14. Excitons in semiconducting superlattices, quantum wells, and ternary alloys

    SciTech Connect

    Sturge, M.D. . Dept. of Physics); Nahory, R.E.; Tamargo, M.C. )

    1990-08-22

    It is now possible to fabricated semiconducting layered structures with precisely defined layer thicknesses of a few atomic diameters. Examples are the quantum well'' and the superlattice'' structures, in which semiconductors with different band gaps are interleaved. Microstructures'' can be produced from this material by patterning and etching them on a small ({approximately}10nm) scale. Their electronic properties are quite different from those of the constituents and offer interesting new possibilities both in device design and in basic physics. This proposal aims to improve our understanding of optically excited states ( excitons'' and electron-hole plasmas'') in these structures. Work will also continue on ternary alloys, primarily to establish if the alloy disorder produces a mobility edge for excitons, and on II-VI compounds, where the principal interest at present is in the nature of the exciton-phonon coupling.

  15. Charged two-exciton emission from a single semiconductor nanocrystal

    SciTech Connect

    Hu, Fengrui; Zhang, Qiang; Zhang, Chunfeng; Wang, Xiaoyong; Xiao, Min

    2015-03-30

    Here, we study the photoluminescence (PL) time trajectories of single CdSe/ZnS nanocrystals (NCs) as a function of the laser excitation power. At the low laser power, the PL intensity of a single NC switches between the “on” and “off” levels arising from the neutral and positively charged single excitons, respectively. With the increasing laser power, an intermediate “grey” level is formed due to the optical emission from a charged multiexciton state composed of two excitons and an extra electron. Both the inter-photon correlation and the PL decay measurements demonstrate that lifetime-indistinguishable photon pairs are emitted from this negatively charged two-exciton state.

  16. Multiple beats of weakly confined excitons with inverted selection rule

    NASA Astrophysics Data System (ADS)

    Yasuda, Hideki; Ishihara, Hajime

    2009-05-01

    The phenomenon of multiple beats (MBs) arising from nondipole-type excitons weakly confined in a thin film is theoretically elucidated using a nonlocal transient-response theory. Kojima previously demonstrated for a GaAs thin film that the degenerate four-wave mixing signals from the quantized levels of the center-of-mass motion of excitons exhibit complex interference between beats under femtosecond-order pulse incidence [Kojima , J. Phys. Soc. Jpn. 77, 044701 (2008)]. This leads to an ultrafast optical response on the order of femtoseconds. This effect occurs in a size region beyond the long-wavelength approximation regime due to the resonant enhancement of the internal field, wherein the usual dipole selection rule is violated. Our analysis of MBs employs a model of the nonlocal multilevel system that considers the spatial interplay between excitonic waves and the radiation field to elucidate the mechanism behind the observed ultrafast response.

  17. Onset of exciton-exciton annihilation in single-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Surrente, A.; Mitioglu, A. A.; Galkowski, K.; Klopotowski, L.; Tabis, W.; Vignolle, B.; Maude, D. K.; Plochocka, P.

    2016-08-01

    The exciton dynamics in monolayer black phosphorus is investigated over a very wide range of photoexcited exciton densities using time resolved photoluminescence. At low excitation densities, the exciton dynamics is successfully described in terms of a double exponential decay. With increasing exciton population, a fast, nonexponential component develops as exciton-exciton annihilation takes over as the dominant recombination mechanism under high excitation conditions. Our results identify an upper limit for the injection density, after which exciton-exciton annihilation reduces the quantum yield, which will significantly impact the performance of light emitting devices based on single-layer black phosphorus.

  18. Radiative recombination from dark excitons in nanocrystals: Activation mechanisms and polarization properties

    NASA Astrophysics Data System (ADS)

    Rodina, Anna V.; Efros, Alexander L.

    2016-04-01

    We analyze theoretically physical mechanisms responsible for the radiative recombination of the ground optically passive ("dark") exciton (DE), which dominates in photoluminescence (PL) of colloidal nanocrystals (NCs) at low temperatures. The DE becomes optically active due to its mixing with the bright excitons caused by an external magnetic field, dangling-bond spins or by acoustic and optical phonons. These activation mechanisms mix the DE with different bright excitons and, consequently, lead to different PL polarization properties, because they are determined by dipole orientations of the bright excitons, which the DE is coupled with. We show that the PL polarization properties of prolate and oblate shape NCs are different due to different activation mechanisms responsible for the DE recombination.

  19. Self-trapped exciton and core-valence luminescence in BaF{sub 2} nanoparticles

    SciTech Connect

    Vistovskyy, V. V. Zhyshkovych, A. V.; Chornodolskyy, Ya. M.; Voloshinovskii, A. S.; Myagkota, O. S.; Gloskovskii, A.; Gektin, A. V.; Vasil'ev, A. N.; Rodnyi, P. A.

    2013-11-21

    The influence of the BaF{sub 2} nanoparticle size on the intensity of the self-trapped exciton luminescence and the radiative core-valence transitions is studied by the luminescence spectroscopy methods using synchrotron radiation. The decrease of the self-trapped exciton emission intensity at energies of exciting photons in the range of optical exciton creation (hν ≤ E{sub g}) is less sensitive to the reduction of the nanoparticle sizes than in the case of band-to-band excitation, where excitons are formed by the recombination way. The intensity of the core-valence luminescence shows considerably weaker dependence on the nanoparticle sizes in comparison with the intensity of self-trapped exciton luminescence. The revealed regularities are explained by considering the relationship between nanoparticle size and photoelectron or photohole thermalization length as well as the size of electronic excitations.

  20. Frenkel-like Wannier-Mott excitons in few-layer Pb I2

    NASA Astrophysics Data System (ADS)

    Toulouse, Alexis S.; Isaacoff, Benjamin P.; Shi, Guangsha; Matuchová, Marie; Kioupakis, Emmanouil; Merlin, Roberto

    2015-04-01

    Optical measurements and first-principles calculations of the band structure and exciton states in direct-gap bulk and few-layer Pb I2 indicate that the n =1 exciton is Frenkel-like in nature in that its energy exhibits a weak dependence on thickness down to atomic-length scales. Results reveal large increases in the gap and exciton binding energy with a decreasing number of layers and a transition of the fundamental gap, which becomes indirect for one and two monolayers. Calculated values are in reasonable agreement with a particle-in-a-box model relying on the Wannier-Mott theory of exciton formation. General arguments and existing data suggest that the Frenkel-like character of the lowest exciton is a universal feature of wide-gap layered semiconductors whose effective masses and dielectric constants give bulk Bohr radii that are on the order of the layer spacing.

  1. Revealing the dark side of a bright exciton-polariton condensate.

    PubMed

    Ménard, J-M; Poellmann, C; Porer, M; Leierseder, U; Galopin, E; Lemaître, A; Amo, A; Bloch, J; Huber, R

    2014-01-01

    Condensation of bosons causes spectacular phenomena such as superfluidity or superconductivity. Understanding the nature of the condensed particles is crucial for active control of such quantum phases. Fascinating possibilities emerge from condensates of light-matter-coupled excitations, such as exciton-polaritons, photons hybridized with hydrogen-like bound electron-hole pairs. So far, only the photon component has been resolved, while even the mere existence of excitons in the condensed regime has been challenged. Here we trace the matter component of polariton condensates by monitoring intra-excitonic terahertz transitions. We study how a reservoir of optically dark excitons forms and feeds the degenerate state. Unlike atomic gases, the atom-like transition in excitons is dramatically renormalized on macroscopic ground state population. Our results establish fundamental differences between polariton condensation and photon lasing and open possibilities for coherent control of condensates. PMID:25115964

  2. Exciton annihilation and dephasing dynamics in semiconducting single-walled carbon nanotubes

    SciTech Connect

    Graham, Matt; Ma, Yingzhong; Green, Alexander A.; Hersam, Mark C.; Fleming, Graham

    2010-01-01

    Semiconducting single-walled carbon nanotubes (SWNTs) are one of the most intriguing nanomaterials due to their large aspect ratios, size tunable properties, and dominant many body interactions. While the dynamics of exciton population relaxation have been well characterized, optical dephasing processes have only been exam- ined indirectly through steady-state measurements such as single-molecule spectroscopy that can yield highly variable estimates of the homogeneous linewidth. To bring clarity to these conflicting estimates, a time-domain measurement of exciton dephasing at an ensemble level is necessary. Using two-pulse photon echo (2PE) spec- troscopy, comparatively long dephasing times approaching 200 fs are extracted for the (6,5) tube species at room temperature. In this contribution, we extend our previous study of 2PE and pump-probe spectroscopy to low temperatures to investigate inelastic exciton-exciton scattering. In contrast to the population kinetics observed upon excitation of the second transition-allowed excitonic state (E22 ), our one-color pump-probe data instead shows faster relaxation upon cooling to 60 K when the lowest transition-allowed state (E11 ) is directly excited for the (6,5) tube species. Analysis of the kinetics obtained suggests that the observed acceleration of kinetic decay at low temperature originates from an increasing rate of exciton-exciton annihilation. In order to directly probe exciton-exciton scattering processes, femtosecond 2PE signal is measured as a function of excitation fluence and temperature. Consistent with the observed enhancement of exciton-exciton scattering and annihilation at low temperatures, the dephasing rates show a correlated trend with the temperature dependence of the population lifetimes extracted from one-color pump-probe measurements.

  3. Significant Lowering Optical Loss of Electrodes via using Conjugated Polyelectrolytes Interlayer for Organic Laser in Electrically Driven Device Configuration

    PubMed Central

    Yi, Jianpeng; Niu, Qiaoli; Xu, Weidong; Hao, Lin; Yang, Lei; Chi, Lang; Fang, Yueting; Huang, Jinjin; Xia, Ruidong

    2016-01-01

    One of the challenges toward electrically driven organic lasers is the huge optical loss associated with the contact of electrodes and organic gain medium in device. We demonstrated a significant reduction of the optical loss by using our newly developed conjugated polyelectrolytes (CPE) PPFN+Br− as interlayer between gain medium and electrode. The optically pumped amplified spontaneous emission (ASE) was observed at very low threshold for PFO as optical gain medium and up to 37 nm thick CPE as interlayer in device configuration, c.f., a 5.7-fold ASE threshold reduction from pump energy 150 μJ/cm2 for ITO/PFO to 26.3 μJ/cm2 for ITO/PPFN+Br−/PFO. Furthermore, ASE narrowing displayed at pump energy up to 61.8 μJ/cm2 for device ITO/PEDOT:PSS/PFO/PPFN+Br−/Ag, while no ASE was observed for the reference devices without CPE interlayer at pump energy up to 240 μJ/cm2. The optically pumped lasing operation has also been achieved at threshold up to 45 μJ/cm2 for one-dimensional distributed feedback laser fabricated on ITO etched grating in devices with CPE interlayer, demonstrating a promising device configuration for addressing the challenge of electrically driven organic lasers. PMID:27165729

  4. Significant Lowering Optical Loss of Electrodes via using Conjugated Polyelectrolytes Interlayer for Organic Laser in Electrically Driven Device Configuration

    NASA Astrophysics Data System (ADS)

    Yi, Jianpeng; Niu, Qiaoli; Xu, Weidong; Hao, Lin; Yang, Lei; Chi, Lang; Fang, Yueting; Huang, Jinjin; Xia, Ruidong

    2016-05-01

    One of the challenges toward electrically driven organic lasers is the huge optical loss associated with the contact of electrodes and organic gain medium in device. We demonstrated a significant reduction of the optical loss by using our newly developed conjugated polyelectrolytes (CPE) PPFN+Br- as interlayer between gain medium and electrode. The optically pumped amplified spontaneous emission (ASE) was observed at very low threshold for PFO as optical gain medium and up to 37 nm thick CPE as interlayer in device configuration, c.f., a 5.7-fold ASE threshold reduction from pump energy 150 μJ/cm2 for ITO/PFO to 26.3 μJ/cm2 for ITO/PPFN+Br-/PFO. Furthermore, ASE narrowing displayed at pump energy up to 61.8 μJ/cm2 for device ITO/PEDOT:PSS/PFO/PPFN+Br-/Ag, while no ASE was observed for the reference devices without CPE interlayer at pump energy up to 240 μJ/cm2. The optically pumped lasing operation has also been achieved at threshold up to 45 μJ/cm2 for one-dimensional distributed feedback laser fabricated on ITO etched grating in devices with CPE interlayer, demonstrating a promising device configuration for addressing the challenge of electrically driven organic lasers.

  5. Radiation effects from first principles : the role of excitons in electronic-excited processes.

    SciTech Connect

    Wong, Bryan Matthew

    2009-09-01

    Electron-hole pairs, or excitons, are created within materials upon optical excitation or irradiation with X-rays/charged particles. The ability to control and predict the role of excitons in these energetically-induced processes would have a tremendous impact on understanding the effects of radiation on materials. In this report, the excitonic effects in large cycloparaphenylene carbon structures are investigated using various first-principles methods. These structures are particularly interesting since they allow a study of size-scaling properties of excitons in a prototypical semi-conducting material. In order to understand these properties, electron-hole transition density matrices and exciton binding energies were analyzed as a function of size. The transition density matrices allow a global view of electronic coherence during an electronic excitation, and the exciton binding energies give a quantitative measure of electron-hole interaction energies in these structures. Based on overall trends in exciton binding energies and their spatial delocalization, we find that excitonic effects play a vital role in understanding the unique photoinduced dynamics in these systems.

  6. Exciton size and binding energy limitations in one-dimensional organic materials

    SciTech Connect

    Kraner, S. Koerner, C.; Leo, K.; Scholz, R.; Plasser, F.

    2015-12-28

    In current organic photovoltaic devices, the loss in energy caused by the charge transfer step necessary for exciton dissociation leads to a low open circuit voltage, being one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy to a value of the order of thermal energy, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley-Queisser limit. We determine the size of the excitons for different organic molecules and polymers by time dependent density functional theory calculations. For optically relevant transitions, the exciton size saturates around 0.7 nm for one-dimensional molecules with a size longer than about 4 nm. For the ladder-type polymer poly(benzimidazobenzophenanthroline), we obtain an exciton binding energy of about 0.3 eV, serving as a lower limit of the exciton binding energy for the organic materials investigated. Furthermore, we show that charge transfer transitions increase the exciton size and thus identify possible routes towards a further decrease of the exciton binding energy.

  7. High-gain optical Cherenkov oscillator driven by low-voltage electron beam

    SciTech Connect

    Smetanin, I.V.; Oraevsky, A.N.

    1995-12-31

    A novel scheme of high-gain optical (from IR up to UV) Cherenkov-type oscillator driven by low-voltage high-current electron beam is proposed in the present report. In the scheme discussed the magnetized electron beam propagates above the surface of absorbing medium of complex dielectric susceptibility {epsilon}{omega} = {epsilon}{sub 1}({omega}) + i{epsilon}{sub 2}({omega}), {epsilon}{sub 2}>0. We have found that at frequencies {omega} that {beta}{sup 2}> 2{epsilon}{sub 1}/{vert_bar}{epsilon}{vert_bar}{sup 2} ({beta} = v/c, v is the electron velocity), an amplification of co-propagating slow surface electromagnetic wave is possible. In contrast to the conventional Cherenkov oscillators, the absorption condition {epsilon}2>0 is crucial for the gain, which is absent for transparent medium. The physics of this amplification effect is analogous to that of electron beam dissipative instability. The wavelength generated is determined here by dielectric properties of the surface, and does not depend strongly on electron energy. Thus it is possible to use rather compact low voltage ({le} 1MeV) high-current accelerators as drivers. Optimum oscillation conditions are found to be at frequencies near the resonance absorption lines of surface material (i.e. from IR up to UV). The gain up to {approximately}0.5cm{sup -1} in the near IR ({approximately}10THz, SrF{sub 2} absorption line) is possible for 250keV high current (density {approximately}10{sup 12}cm{sup -3}) electron beam.

  8. Excitons and exciton-phonon interactions in 2D MoS2 , WS2 and WSe2 studied by resonance Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Pimenta, Marcos; Del Corro, Elena; Carvalho, Bruno; Malard, Leandro; Alves, Juliana; Fantini, Cristiano; Terrones, Humberto; Elias, Ana Laura; Terrones, Mauricio

    The 2D materials exhibit a very strong exciton binding energy, and the exciton-phonon coupling plays an important role in their optical properties. Resonance Raman spectroscopy (RRS) is a very useful tool to provide information about excitons and their couplings with phonons. We will present in this work a RRS study of different samples of 2D transition metal dichalcogenides (MoS2, WS2 and WSe2) with one, two and three layers (1L, 2L, 3L) and bulk samples, using more than 30 different laser excitation lines covering the visible range. We have observed that all Raman features are enhanced by resonances with excitonic transitions. From the laser energy dependence of the Raman excitation profile (REP) we obtained the energies of the excitonic states and their dependence with the number of atomic layers.. In the case of MoS2, we observed that the electron-phonon coupling is symmetry dependent, and our results provide experimental evidence of the C exciton recently predicted theoretically. The RRS results WSe2 show that the Raman modes are enhanced by the excited excitonic states and we will present the dependence of the excited states energies on the number of layers.

  9. Exciton transport, charge extraction, and loss mechanisms in organic photovoltaics

    NASA Astrophysics Data System (ADS)

    Scully, Shawn Ryan

    Organic photovoltaics have attracted significant interest over the last decade due to their promise as clean low-cost alternatives to large-scale electric power generation such as coal-fired power, natural gas, and nuclear power. Many believe power conversion efficiency targets of 10-15% must be reached before commercialization is possible. Consequently, understanding the loss mechanisms which currently limit efficiencies to 4-5% is crucial to identify paths to reach higher efficiencies. In this work, we investigate the dominant loss mechanisms in some of the leading organic photovoltaic architectures. In the first class of architectures, which include planar heterojunctions and bulk heterojunctions with large domains, efficiencies are primarily limited by the distance photogenerated excitations (excitons) can be transported (termed the exciton diffusion length) to a heterojunction where the excitons may dissociate. We will discuss how to properly measure the exciton diffusion length focusing on the effects of optical interference and of energy transfer when using fullerenes as quenching layers and show how this explains the variety of diffusion lengths reported for the same material. After understanding that disorder and defects limit exciton diffusion lengths, we suggest some approaches to overcome this. We then extensively investigate the use of long-range resonant energy transfer to increase exciton harvesting. Using simulations and experiments as support, we discuss how energy transfer can be engineered into architectures to increase the distance excitons can be harvested. In an experimental model system, DOW Red/PTPTB, we will show how the distance excitons are harvested can be increased by almost an order of magnitude up to 27 nm from a heterojunction and give design rules and extensions of this concept for future architectures. After understanding exciton harvesting limitations we will look at other losses that are present in planar heterojunctions. One of

  10. Intrinsic homogeneous linewidth and broadening mechanisms of excitons in monolayer transition metal dichalcogenides

    PubMed Central

    Moody, Galan; Kavir Dass, Chandriker; Hao, Kai; Chen, Chang-Hsiao; Li, Lain-Jong; Singh, Akshay; Tran, Kha; Clark, Genevieve; Xu, Xiaodong; Berghäuser, Gunnar; Malic, Ermin; Knorr, Andreas; Li, Xiaoqin

    2015-01-01

    The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton–exciton and exciton–phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors. PMID:26382305

  11. Tuning excitons in monolayer and few-layer MoS2

    NASA Astrophysics Data System (ADS)

    Qiu, Diana Y.; da Jornada, Felipe H.; Louie, Steven G.

    2014-03-01

    Our recent ab initio GW-BSE calculations showed that monolayer MoS2 is a computationally challenging system, requiring a large number of empty bands and very fine k-point sampling to converge its quasiparticle band structure and optical properties. Careful convergence of a GW-BSE calculation reveals that MoS2 has a large number of bound excitons with varying k-space characteristics. Specifically, there are two series of excitons: a low-energy series with k-space wavefunctions localized at the K/K' valleys in the Brillouin zone and a higher energy series localized in a ring around the Γ point. There is very little hybridization between these two exciton series in monolayer MoS2, but changes in electronic structure and screening due to additional layers, strain, or doping can lead to changes in exciton binding energies, character, and hybridization. Thus, we have carried out ab initio GW-BSE calculations to study the excitonic properties of few-layer MoS2. We find that layering and straining MoS2 systematically changes the exciton binding energies, the peak positions and amount of absorbance in the optical spectrum, and the character and hybridization of the excitons near Γ. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231.

  12. Analytical method for determining quantum well exciton properties in a magnetic field

    NASA Astrophysics Data System (ADS)

    Stépnicki, Piotr; Piétka, Barbara; Morier-Genoud, François; Deveaud, Benoît; Matuszewski, Michał

    2015-05-01

    We develop an analytical approximate method for determining the Bohr radii of Wannier-Mott excitons in thin quantum wells under the influence of magnetic field perpendicular to the quantum well plane. Our hybrid variational-perturbative method allows us to obtain simple closed formulas for exciton binding energies and optical transition rates. We confirm the reliability of our method through exciton-polariton experiments realized in a GaAs/AlAs microcavity with an 8 nm InxGa1 -xAs quantum well and magnetic field strengths as high as 14 T.

  13. Electrical Control of Exciton-Enhanced Second-Harmonic Generation in Monolayer WSe2

    NASA Astrophysics Data System (ADS)

    Seyler, Kyle; Schaibley, John; Gong, Pu; Rivera, Pasqual; Jones, Aaron; Wu, Sanfeng; Yan, Jiaqiang; Mandrus, David; Yao, Wang; Xu, Xiaodong

    2015-03-01

    Nonlinear optical frequency conversion, in which optical fields interact with a nonlinear medium to produce new field frequencies, is ubiquitous in modern photonic systems. However, the nonlinear electric susceptibilities that give rise to such phenomena are often challenging to tune in a given material, and so far, dynamical control of optical nonlinearities remains confined to research labs. In this talk, we report a new mechanism to electrically control second-order optical nonlinearities in monolayer WSe2. We show that the intensity of second-harmonic generation (SHG) at its lowest exciton resonance is widely tunable through electrostatic doping in a field-effect transistor device. Such remarkable tunability arises from the strong exciton charging effects in monolayer semiconductors, which allow for exceptional control over the exciton and trion oscillator strengths. Our study paves the way for a new platform of chip-scale, electrically tunable nonlinear optical devices based on two-dimensional semiconductors.

  14. Exciton coupling in molecular crystals

    NASA Technical Reports Server (NTRS)

    Ake, R. L.

    1976-01-01

    The implications of perfect exciton coupling and molecular vibrations were investigated, as well as the effect they have on the lifetime of singlet and triplet excitons coupled in a limiting geometry. Crystalline bibenzyl, Cl4Hl4, provided a situation in which these mechanisms involving exciton coupling can be studied in the limit of perfect coupling between units due to the crystal's geometry. This geometry leads to a coupling between the two halves of the molecule resulting in a splitting of the molecular excited states. The study reported involves an experimental spectroscopic approach and begins with the purification of the bibenzyl. The principal experimental apparatus was an emission spectrometer. A closed cycle cryogenic system was used to vary the temperature of the sample between 20 K and 300 K. The desired results are the temperature-dependent emission spectra of the bibenzyl; in addition, the lifetimes and quantum yields measured at each temperature reveal the effect of competing radiationless processes.

  15. Talbot Effect for Exciton Polaritons

    NASA Astrophysics Data System (ADS)

    Gao, T.; Estrecho, E.; Li, G.; Egorov, O. A.; Ma, X.; Winkler, K.; Kamp, M.; Schneider, C.; Höfling, S.; Truscott, A. G.; Ostrovskaya, E. A.

    2016-08-01

    We demonstrate, experimentally and theoretically, a Talbot effect for hybrid light-matter waves—an exciton-polariton condensate formed in a semiconductor microcavity with embedded quantum wells. The characteristic "Talbot carpet" is produced by loading the exciton-polariton condensate into a microstructured one-dimensional periodic array of mesa traps, which creates an array of phase-locked sources for coherent polariton flow in the plane of the quantum wells. The spatial distribution of the Talbot fringes outside the mesas mimics the near-field diffraction of a monochromatic wave on a periodic amplitude and phase grating with the grating period comparable to the wavelength. Despite the lossy nature of the polariton system, the Talbot pattern persists for distances exceeding the size of the mesas by an order of magnitude. Thus, our experiment demonstrates efficient shaping of the two-dimensional flow of coherent exciton polaritons by a one-dimensional "flat lens."

  16. Talbot Effect for Exciton Polaritons.

    PubMed

    Gao, T; Estrecho, E; Li, G; Egorov, O A; Ma, X; Winkler, K; Kamp, M; Schneider, C; Höfling, S; Truscott, A G; Ostrovskaya, E A

    2016-08-26

    We demonstrate, experimentally and theoretically, a Talbot effect for hybrid light-matter waves-an exciton-polariton condensate formed in a semiconductor microcavity with embedded quantum wells. The characteristic "Talbot carpet" is produced by loading the exciton-polariton condensate into a microstructured one-dimensional periodic array of mesa traps, which creates an array of phase-locked sources for coherent polariton flow in the plane of the quantum wells. The spatial distribution of the Talbot fringes outside the mesas mimics the near-field diffraction of a monochromatic wave on a periodic amplitude and phase grating with the grating period comparable to the wavelength. Despite the lossy nature of the polariton system, the Talbot pattern persists for distances exceeding the size of the mesas by an order of magnitude. Thus, our experiment demonstrates efficient shaping of the two-dimensional flow of coherent exciton polaritons by a one-dimensional "flat lens." PMID:27610883

  17. Exciton-photon correlations in bosonic condensates of exciton-polaritons

    PubMed Central

    Kavokin, Alexey V.; Sheremet, Alexandra S.; Shelykh, Ivan A.; Lagoudakis, Pavlos G.; Rubo, Yuri G.

    2015-01-01

    Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers. PMID:26153979

  18. Ultrahigh resolution endoscopic spectral domain optical coherence tomography with a tiny rotary probe driven by a hollow ultrasonic motor

    NASA Astrophysics Data System (ADS)

    Zhang, Ning; Chen, Tianyuan; Huo, Tiancheng; Wang, Chengming; Zheng, Jing-gao; Zhou, Tieying; Xue, Ping

    2013-03-01

    This paper proposes a novel rotary endoscopic probe for spectral-domain optical coherence tomography (SD-OCT). The probe with a large N.A. objective lens is driven by an ultra-small hollow rectangular ultrasonic motor for circular scanning. Compared to the conventional driven techniques, the hollow ultrasonic motor enables the fiber to pass through its inside. Therefore the fiber, the objective lens and the motor are all at the same side. This enables 360 degree unobstructed imaging without any shadow resulted from power wire as in the conventional motor-driven endoscopic OCT. Moreover, it shortens the length of the rigid tip and enhances the flexibility of the probe. Meanwhile, the ultrasonic motor is robust, simple, quiet and of high torque, very suitable for OCT endoscopic probe. The side length of the motor is 0.7 mm with 5mm in length. The outer diameter of the probe is 1.5mm. A significant improvement in the lateral resolution is demonstrated due to the novel design of the objective lens. A right-angle lens is utilized instead of the traditional right-angle prism as the last optics close to the sample, leading to a reduction of the working distance and an enlargement of the N.A. of the objective lens. It is demonstrated that the endoscopic SD-OCT system achieves an axial resolution of ~7μm, a lateral resolution of ~6μm and a SNR of ~96dB.

  19. Observation of an Excitonic Quantum Coherence in CdSe Nanocrystals.

    PubMed

    Dong, Shuo; Trivedi, Dhara; Chakrabortty, Sabyasachi; Kobayashi, Takayoshi; Chan, Yinthai; Prezhdo, Oleg V; Loh, Zhi-Heng

    2015-10-14

    Recent observations of excitonic coherences within photosynthetic complexes suggest that quantum coherences could enhance biological light harvesting efficiencies. Here, we employ optical pump-probe spectroscopy with few-femtosecond pulses to observe an excitonic quantum coherence in CdSe nanocrystals, a prototypical artificial light harvesting system. This coherence, which encodes the high-speed migration of charge over nanometer length scales, is also found to markedly alter the displacement amplitudes of phonons, signaling dynamics in the non-Born-Oppenheimer regime.

  20. Charge-Transfer Excitations Steer the Davydov Splitting and Mediate Singlet Exciton Fission in Pentacene

    NASA Astrophysics Data System (ADS)

    Beljonne, D.; Yamagata, H.; Brédas, J. L.; Spano, F. C.; Olivier, Y.

    2013-05-01

    Quantum-chemical calculations are combined to a model Frenkel-Holstein Hamiltonian to assess the nature of the lowest electronic excitations in the pentacene crystal. We show that an admixture of charge-transfer excitations into the lowest singlet excited states form the origin of the Davydov splitting and mediate instantaneous singlet exciton fission by direct optical excitation of coherently coupled single and double exciton states, in agreement with recent experiments.

  1. Resonant surface plasmon-exciton interaction in hybrid MoSe2@Au nanostructures

    NASA Astrophysics Data System (ADS)

    Abid, I.; Bohloul, A.; Najmaei, S.; Avendano, C.; Liu, H.-L.; Péchou, R.; Mlayah, A.; Lou, J.

    2016-04-01

    In this work we investigate the interaction between plasmonic and excitonic resonances in hybrid MoSe2@Au nanostructures. The latter were fabricated by combining chemical vapor deposition of MoSe2 atomic layers, Au disk processing by nanosphere lithography and a soft lift-off/transfer technique. The samples were characterized by scanning electron and atomic force microscopy. Their optical properties were investigated experimentally using optical absorption, Raman scattering and photoluminescence spectroscopy. The work is focused on a resonant situation where the surface plasmon resonance is tuned to the excitonic transition. In that case, the near-field interaction between the surface plasmons and the confined excitons leads to interference between the plasmonic and excitonic resonances that manifests in the optical spectra as a transparency dip. The plasmonic-excitonic interaction regime is determined using quantitative analysis of the optical extinction spectra based on an analytical model supported by numerical simulations. We found that the plasmonic-excitonic resonances do interfere thus leading to a typical Fano lineshape of the optical extinction. The near-field nature of the plasmonic-excitonic interaction is pointed out experimentally from the dependence of the optical absorption on the number of monolayer stacks on the Au nanodisks. The results presented in this work contribute to the development of new concepts in the field of hybrid plasmonics.In this work we investigate the interaction between plasmonic and excitonic resonances in hybrid MoSe2@Au nanostructures. The latter were fabricated by combining chemical vapor deposition of MoSe2 atomic layers, Au disk processing by nanosphere lithography and a soft lift-off/transfer technique. The samples were characterized by scanning electron and atomic force microscopy. Their optical properties were investigated experimentally using optical absorption, Raman scattering and photoluminescence spectroscopy. The

  2. Entropy-based measures of in vivo cilia-driven microfluidic mixing derived from quantitative optical imaging

    NASA Astrophysics Data System (ADS)

    Chandrasekera, Kenny; Jonas, Stephan; Bhattacharya, Dipankan; Khokha, Mustafa; Choma, Michael A.

    2012-02-01

    Motile cilia are cellular organelles that project from different epithelial surfaces including respiratory epithelium. They generate directional fluid flow that removes harmful pathogens and particulate matter from the respiratory system. While it has been known that primary ciliary dyskinesia increases the risk of recurrent pulmonary infections, there is now heightened interest in understanding the role that cilia play in a wide-variety of respiratory diseases. Different optical imaging technologies are being investigated to visualize cilia-driven fluid flow, and quantitative image analysis is used to generate measures of ciliary performance. Here, we demonstrate the quantification of in vivo cilia-driven microfluidic mixing using spatial and temporal measures of Shannon information entropy. Using videomicroscopy, we imaged in vivo cilia-driven fluid flow generated by the epidermis of the Xenopus tropicalis embryo. Flow was seeded with either dyes or microparticles. Both spatial and temporal measures of entropy show significant levels of mixing, with maximum entropy measures of ~6.5 (out of a possible range of 0 to 8). Spatial entropy measures showed localization of mixing "hot-spots" and "cold-spots" and temporal measures showed mixing throughout.In sum, entropy-based measures of microfluidic mixing can characterize in vivo cilia-driven fluid flow and hold the potential for better characterization of ciliary dysfunction.

  3. Relative ordering between bright and dark excitons in single-walled carbon nanotubes.

    PubMed

    Zhou, Weihang; Nakamura, Daisuke; Liu, Huaping; Kataura, Hiromichi; Takeyama, Shojiro

    2014-11-11

    The ordering and relative energy splitting between bright and dark excitons are critical to the optical properties of single-walled carbon nanotubes (SWNTs), as they eventually determine the radiative and non-radiative recombination processes of generated carriers. In this work, we report systematic high-field magneto-optical study on the relative ordering between bright and dark excitons in SWNTs. We identified the relative energy position of the dark exciton unambiguously by brightening it in ultra-high magnetic field. The bright-dark excitonic ordering was found to depend not only on the tube structure, but also on the type of transitions. For the 1(st) sub-band transition, the bright exciton appears to be higher in energy than its dark counterpart for any chiral species and is robust against environmental effect. While for the 2(nd) sub-band, their relative ordering was found to be chirality-sensitive: the bright exciton can be either higher or lower than the dark one, depending on the specific nanotube structures. These findings provide new clues for engineering the optical and electronic properties of SWNTs.

  4. Spatially and temporally resolved exciton dynamics and transport in single nanostructures and assemblies

    NASA Astrophysics Data System (ADS)

    Huang, Libai

    2015-03-01

    The frontier in solar energy conversion now lies in learning how to integrate functional entities across multiple length scales to create optimal devices. To address this new frontier, I will discuss our recent efforts on elucidating multi-scale energy transfer, migration, and dissipation processes with simultaneous femtosecond temporal resolution and nanometer spatial resolution. We have developed ultrafast microscopy that combines ultrafast spectroscopy with optical microscopy to map exciton dynamics and transport with simultaneous ultrafast time resolution and diffraction-limited spatial resolution. We have employed pump-probe transient absorption microscopy to elucidate morphology and structure dependent exciton dynamics and transport in single nanostructures and molecular assemblies. More specifically, (1) We have applied transient absorption microscopy (TAM) to probe environmental and structure dependent exciton relaxation pathways in sing-walled carbon nanotubes (SWNTs) by mapping dynamics in individual pristine SWNTs with known structures. (2) We have systematically measured and modeled the optical properties of the Frenkel excitons in self-assembled porphyrin tubular aggregates that represent an analog to natural photosynthetic antennae. Using a combination of ultrafast optical microscopy and stochastic exciton modeling, we address exciton transport and relaxation pathways, especially those related to disorder.

  5. Bright, long-lived and coherent excitons in carbon nanotube quantum dots

    NASA Astrophysics Data System (ADS)

    Hofmann, Matthias S.; Glückert, Jan T.; Noé, Jonathan; Bourjau, Christian; Dehmel, Raphael; Högele, Alexander

    2013-07-01

    Carbon nanotubes exhibit a wealth of unique physical properties. By virtue of their exceptionally low mass and extreme stiffness they provide ultrahigh-quality mechanical resonances, promise long electron spin coherence times in a nuclear-spin free lattice for quantum information processing and spintronics, and feature unprecedented tunability of optical transitions for optoelectronic applications. Excitons in semiconducting single-walled carbon nanotubes could facilitate the upconversion of spin, mechanical or hybrid spin-mechanical degrees of freedom to optical frequencies for efficient manipulation and detection. However, successful implementation of such schemes with carbon nanotubes has been impeded by rapid exciton decoherence at non-radiative quenching sites, environmental dephasing and emission intermittence. Here we demonstrate that these limitations may be overcome by exciton localization in suspended carbon nanotubes. For excitons localized in nanotube quantum dots we found narrow optical lines free of spectral wandering, radiative exciton lifetimes and effectively suppressed blinking. Our findings identify the great potential of localized excitons for efficient and spectrally precise interfacing of photons, phonons and spins in novel carbon nanotube-based quantum devices.

  6. Strong Quantum Coherence between Fermi Liquid Mahan Excitons.

    PubMed

    Paul, J; Stevens, C E; Liu, C; Dey, P; McIntyre, C; Turkowski, V; Reno, J L; Hilton, D J; Karaiskaj, D

    2016-04-15

    In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called "Mahan excitons." The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the optical Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system. PMID:27127985

  7. Translational mass of an exciton

    NASA Astrophysics Data System (ADS)

    Cafolla, A. A.; Schnatterly, S. E.; Tarrio, C.

    1985-12-01

    From transmission electron-energy-loss measurements we show that the mass of an exciton M* is greater than the sum of the effective masses of the electron and hole me+mh. This result is consistent with a recent prediction by Mattis and Gallinar.

  8. Low noise and low drift in a laser-driven fiber optic gyroscope with a 1-km coil

    NASA Astrophysics Data System (ADS)

    Chamoun, J. N.; Evans, A.; Mosca, F. A.; Digonnet, M. J. F.

    2014-05-01

    We report an experimental fiber optic gyroscope (FOG) utilizing a 1085-m coil of 8-cm diameter driven with a laser of 10- MHz linewidth, with a record rotation-rate noise as low as 0.2 deg/h/√Hz and a drift below 0.038 deg/h. Simulations and comparison to the measured performance of a similar 150-m FOG show that the noise is limited approximately equally by coherent backscattering and polarization coupling in the sensing coil, and unaffected by the Kerr effect.

  9. Excitons and Valley Dynamics in MoS2, MoSe2 and WSe2 monolayers

    NASA Astrophysics Data System (ADS)

    Marie, Xavier

    2015-03-01

    We have investigated the optical and valley properties for both neutral and charged excitons in transition metal dichalcogenide monolayers (ML): MoS2, MoSe2 and WSe2. In WSe2 MLs, we have combined linear and non-linear optical spectroscopy (one and two-photon PLE, Second Harmonic Generation spectroscopy) to uncover the excited states of the neutral exciton. The clear identification of s and p exciton excited states combined with first principle calculations allows us to determine an exciton binding energy of the order of 600 meV. The deviation of the excited exciton spectrum from the standard Rydberg series will be discussed. Moreover we show that exciton valley coherence can be achieved following resonant one or two photon excitation. The neutral and charged exciton dynamics have been measured by time-resolved photoluminescence and pump-probe Kerr rotation dynamics. The neutral exciton valley polarization decays within about 6 ps, as a result of the intervalley coupling due the strong electron-hole Coulomb exchange interaction in bright excitons. The temperature dependence is well explained by the developed theory, taking into account the long-range exchange interaction. In contrast the valley polarization decay time for the charged exciton is much longer (~ 1ns). Finally we will compare the exciton dynamics in WSe2 mono and bi-layers We acknowledge partial funding from Programme Investissements d'Avenir ANR-11- IDEX-0002-02, reference ANR-10-LABX-0037-NEXT, ERC Grant No. 306719 and ANR MoS2ValleyControl.

  10. Low-temperature exciton absorption in InSe under pressure

    NASA Astrophysics Data System (ADS)

    Goi, A. R.; Cantarero, A.; Schwarz, U.; Syassen, K.; Chevy, A.

    1992-02-01

    We have investigated the effect of pressure on the lowest direct band-edge exciton of the layered semiconductor InSe by optical-absorption measurements at 10 K and for pressures up to 4 GPa. The Elliott-Toyozawa formalism is used to analyze the line shape of the exciton absorption spectra. In this way we determine the pressure dependence of the lowest direct band gap, the exciton binding energy, and the exciton linewidth. The band gap exhibits a pronounced nonlinear shift with pressure, which is a consequence of the strong anisotropy of intralayer and interlayer bonding. The exciton binding energy decreases with pressure, mainly due to the large increase of the low-frequency dielectric constant parallel to the c axis. The reversible broadening of the exciton line with pressure is related to phonon-assisted scattering of electrons into lower-lying indirect-gap states. From the pressure dependence of the exciton linewidth, we determine the magnitude of the related intervalley-deformation-potential constant.

  11. Effect of pressure on the low-temperature exciton absorption in GaAs

    NASA Astrophysics Data System (ADS)

    Goi, A. R.; Cantarero, A.; Syassen, K.; Cardona, M.

    1990-05-01

    We have measured low-temperature exciton optical-absorption spectra at the lowest direct band edge (E0) of GaAs as a function of pressure up to 9 GPa. Spectra are analyzed in terms of the Elliott model by taking into account the broadening of the exciton line. In this way, we determine the dependence on pressure of the E0 gap, the exciton binding energy scrR, and exciton linewidth at different temperatures. The pressure coefficient of the E0 fundamental gap [107(4) meV/GPa] is found to be independent of temperature. The exciton binding energy increases with pressure at a rate of d lnscrR/dP=0.083(3) GPa-1. The exciton lifetime becomes smaller for pressures above the crossover between Γ- and X-point conduction-band minima (P>4.2 GPa), a fact which is attributed to phonon-assisted intervalley scattering. From the pressure dependence of the exciton linewidth we determine an accurate value for the intervalley deformation-potential constant DΓX=4.8(3) eV/Å.

  12. Exciton-polariton state in nanocrystalline SiC films

    NASA Astrophysics Data System (ADS)

    Semenov, A. V.; Lopin, A. V.

    2016-05-01

    We studied the features of optical absorption in the films of nanocrystalline SiC (nc-SiC) obtained on the sapphire substrates by the method of direct ion deposition. The optical absorption spectra of the films with a thickness less than ~500 nm contain a maximum which position and intensity depend on the structure and thickness of the nc-SiC films. The most intense peak at 2.36 eV is observed in the nc-SiC film with predominant 3C-SiC polytype structure and a thickness of 392 nm. Proposed is a resonance absorption model based on excitation of exciton polaritons in a microcavity. In the latter, under the conditions of resonance, there occurs strong interaction between photon modes of light with λph=521 nm and exciton of the 3С polytype with an excitation energy of 2.36 eV that results in the formation of polariton. A mismatch of the frequencies of photon modes of the cavity and exciton explains the dependence of the maximum of the optical absorption on the film thickness.

  13. Semiconductor lasers driven by self-sustained chaotic electronic oscillators and applications to optical chaos cryptography.

    PubMed

    Kingni, Sifeu Takougang; Mbé, Jimmi Hervé Talla; Woafo, Paul

    2012-09-01

    In this work, we numerically study the dynamics of vertical cavity surface emitting laser (VCSEL) firstly when it is driven by Chua's oscillator, secondly in case where it is driven by a broad frequency spectral bandwidth chaotic oscillator developed by Nana et al. [Commun. Nonlinear Sci. Numer. Simul. 14, 2266 (2009)]. We demonstrated that the VCSEL generated robust chaotic dynamics compared to the ones found in VCSEL subject to a sinusoidally modulated current and therefore it is more suitable for chaos encryption techniques. The synchronization characteristics and the communication performances of unidirectional coupled VCSEL driven by the broad frequency spectral bandwidth chaotic oscillators are investigated numerically. The results show that high-quality synchronization and transmission of messages can be realized for suitable system parameters. Chaos shift keying method is successfully applied to encrypt a message at a high bitrate.

  14. Direct versus indirect band gap emission and exciton-exciton annihilation in atomically thin molybdenum ditelluride (MoTe2)

    NASA Astrophysics Data System (ADS)

    Froehlicher, Guillaume; Lorchat, Etienne; Berciaud, Stéphane

    2016-08-01

    We probe the room temperature photoluminescence of N -layer molybdenum ditelluride (MoTe2) in the continuous wave (cw) regime. The photoluminescence quantum yield of monolayer MoTe2 is three times larger than in bilayer MoTe2 and 40 times greater than in the bulk limit. Mono- and bilayer MoTe2 display almost symmetric emission lines at 1.10 and 1.07 eV, respectively, which predominantly arise from direct radiative recombination of the A exciton. In contrast, N ≥3 -layer MoTe2 exhibits a much reduced photoluminescence quantum yield and a broader, redshifted, and seemingly bimodal photoluminescence spectrum. The low- and high-energy contributions are attributed to emission from the indirect and direct optical band gaps, respectively. Bulk MoTe2 displays a broad emission line with a dominant contribution at 0.94 eV that is assigned to emission from the indirect optical band gap. As compared to related systems (such as MoS2,MoSe2,WS2, and WSe2), the smaller energy difference between the monolayer direct optical band gap and the bulk indirect optical band gap leads to a smoother increase of the photoluminescence quantum yield as N decreases. In addition, we study the evolution of the photoluminescence intensity in monolayer MoTe2 as a function of the exciton formation rate Wabs up to 3.6 ×1022cm-2s-1 . The line shape of the photoluminescence spectrum remains largely independent of Wabs, whereas the photoluminescence intensity grows sublinearly above Wabs˜1021cm-2s-1 . This behavior is assigned to exciton-exciton annihilation and is well captured by an elementary rate equation model.

  15. Exciton dynamics of C60-based single-photon emitters explored by Hanbury Brown–Twiss scanning tunnelling microscopy

    PubMed Central

    Merino, P.; Große, C.; Rosławska, A.; Kuhnke, K.; Kern, K.

    2015-01-01

    Exciton creation and annihilation by charges are crucial processes for technologies relying on charge-exciton-photon conversion. Improvement of organic light sources or dye-sensitized solar cells requires methods to address exciton dynamics at the molecular scale. Near-field techniques have been instrumental for this purpose; however, characterizing exciton recombination with molecular resolution remained a challenge. Here, we study exciton dynamics by using scanning tunnelling microscopy to inject current with sub-molecular precision and Hanbury Brown–Twiss interferometry to measure photon correlations in the far-field electroluminescence. Controlled injection allows us to generate excitons in solid C60 and let them interact with charges during their lifetime. We demonstrate electrically driven single-photon emission from localized structural defects and determine exciton lifetimes in the picosecond range. Monitoring lifetime shortening and luminescence saturation for increasing carrier injection rates provides access to charge-exciton annihilation dynamics. Our approach introduces a unique way to study single quasi-particle dynamics on the ultimate molecular scale. PMID:26416705

  16. Quantifying hyperoxia-mediated damage to mammalian respiratory cilia-driven fluid flow using particle tracking velocimetry optical coherence tomography

    PubMed Central

    Gamm, Ute A.; Huang, Brendan K.; Syed, Mansoor; Zhang, Xuchen; Bhandari, Vineet; Choma, Michael A.

    2015-01-01

    Abstract. Oxygen supplementation [hyperoxia, increased fraction of inspired oxygen (FiO2)] is an indispensable treatment in the intensive care unit for patients in respiratory failure. Like other treatments or drugs, hyperoxia has a risk-benefit profile that guides its clinical use. While hyperoxia is known to damage respiratory epithelium, it is unknown if damage can result in impaired capacity to generate cilia-driven fluid flow. Here, we demonstrate that quantifying cilia-driven fluid flow velocities in the sub-100 μm/s regime (sub-0.25 in./min regime) reveals hyperoxia-mediated damage to the capacity of ciliated respiratory mucosa to generate directional flow. Flow quantification was performed using particle tracking velocimetry optical coherence tomography (PTV-OCT) in ex vivo mouse trachea. The ability of PTV-OCT to detect biomedically relevant flow perturbations in the sub-100 μm/s regime was validated by quantifying temperature- and drug-mediated modulation of flow performance in ex vivo mouse trachea. Overall, PTV-OCT imaging of cilia-driven fluid flow in ex vivo mouse trachea is a powerful and straightforward approach for studying factors that modulate and damage mammalian respiratory ciliary physiology. PMID:26308164

  17. Quantifying hyperoxia-mediated damage to mammalian respiratory cilia-driven fluid flow using particle tracking velocimetry optical coherence tomography.

    PubMed

    Gamm, Ute A; Huang, Brendan K; Syed, Mansoor; Zhang, Xuchen; Bhandari, Vineet; Choma, Michael A

    2015-08-01

    Oxygen supplementation [hyperoxia, increased fraction of inspired oxygen (FiO 2 )] is an indispensable treatment in the intensive care unit for patients in respiratory failure. Like other treatments or drugs, hyperoxia has a risk-benefit profile that guides its clinical use. While hyperoxia is known to damage respiratory epithelium, it is unknown if damage can result in impaired capacity to generate cilia-driven fluid flow. Here, we demonstrate that quantifying cilia-driven fluid flow velocities in the sub-100 μm/s regime (sub-0.25 in./min regime) reveals hyperoxia-mediated damage to the capacity of ciliated respiratory mucosa to generate directional flow. Flow quantification was performed using particle tracking velocimetry optical coherence tomography (PTV-OCT) in ex vivo mouse trachea. The ability of PTV-OCT to detect biomedically relevant flow perturbations in the sub-100 μm/s regime was validated by quantifying temperature- and drug-mediated modulation of flow performance in ex vivo mouse trachea. Overall, PTV-OCT imaging of cilia-driven fluid flow in ex vivo mouse trachea is a powerful and straightforward approach for studying factors that modulate and damage mammalian respiratory ciliary physiology.

  18. Polycrystalline magnetic garnet films comprising weakly coupled crystallites for piezoelectrically-driven magneto-optic spatial light modulators

    SciTech Connect

    Mito, S.; Sakurai, H.; Takagi, H.; Inoue, M.; Baryshev, A. V.

    2012-04-01

    We have investigated the magnetization process of the polycrystalline magnetic garnet films in order to determine the most suitable composition of garnet films for piezoelectrically-driven magneto-optic spatial light modulators (MOSLMs). For experiment, the bismuth-dysprosium-aluminum-substituted yttrium iron (Bi{sub 1.3}Dy{sub 0.7}Y{sub 1.0}Fe{sub 3.1}Al{sub 1.9}O{sub 12}) garnet films were deposited by an RF magnetron sputter and annealed at 700 deg. C in air. The annealing time was varied in a range of several minutes to control the grain size. The saturation magnetization, the remanent magnetization and the composition of the fabricated garnet films slightly changed versus the annealing time. Experiments showed that the coercivity and the grain size increased at longer annealing; the coercivity was larger for films with bigger grains. This work shows that garnet films with smaller coercivity are most suitable for controlling the magnetization of garnet and, correspondingly, the magneto-optical rotation of MOSLM pixels driven by piezoelectrics.

  19. External pumping of hybrid nanostructures in microcavity with Frenkel and Wannier-Mott excitons

    NASA Astrophysics Data System (ADS)

    Dubovskiy, O. A.; Agranovich, V. M.

    2016-09-01

    The exciton-exciton interaction in hybrid nanostructures with resonating Frenkel and Wannier-Mott excitons was investigated in many publications. In microcavity the hybrid nanostructures can be exposed to different types of optical pumping, the most common one being pumping through one of the microcavity side. However, not investigated and thus never been discussed the hybrid excitons generation by pumping of confined quantum wells from the side of empty microcavity without nanostructures in a wave guided configuration. Here, we consider the hybrid excitations in cavity with organic and inorganic quantum wells and with different types of pumping from external source. The frequency dependence for intensity of excitations in hybrid structure is also investigated. The results may be used for search of most effective fluorescence and relaxation processes. The same approach may be used when both quantum wells are organic or inorganic.

  20. Multiple-exciton generation in lead selenide nanorod solar cells with external quantum efficiencies exceeding 120.

    PubMed

    Davis, Nathaniel J L K; Böhm, Marcus L; Tabachnyk, Maxim; Wisnivesky-Rocca-Rivarola, Florencia; Jellicoe, Tom C; Ducati, Caterina; Ehrler, Bruno; Greenham, Neil C

    2015-09-28

    Multiple-exciton generation-a process in which multiple charge-carrier pairs are generated from a single optical excitation-is a promising way to improve the photocurrent in photovoltaic devices and offers the potential to break the Shockley-Queisser limit. One-dimensional nanostructures, for example nanorods, have been shown spectroscopically to display increased multiple exciton generation efficiencies compared with their zero-dimensional analogues. Here we present solar cells fabricated from PbSe nanorods of three different bandgaps. All three devices showed external quantum efficiencies exceeding 100% and we report a maximum external quantum efficiency of 122% for cells consisting of the smallest bandgap nanorods. We estimate internal quantum efficiencies to exceed 150% at relatively low energies compared with other multiple exciton generation systems, and this demonstrates the potential for substantial improvements in device performance due to multiple exciton generation.

  1. Geometry strategy for engineering the recombination possibility of excitons in nanowires.

    PubMed

    Wang, Youwei; Zhang, Yubo; Zhu, Haiming; Liu, Jianjun; Lian, Tianquan; Zhang, Wenqing

    2016-04-01

    We proposed a geometry strategy to engineer the radiative recombination possibility and thus the lifetime of excitons in nanowires of some photovoltaic semiconductors by using theoretical analysis and first-principles calculations. We demonstrated that the shape can engineer the symmetry of the wave-functions of band-edge states and influence the radiative recombination possibility. The nanowires need to satisfy the following requirements to forbid the radiative recombination possibility of band-edge excitons: (i) wurtzite structure; (ii) pxy-characterized wave-function of VBM state and (iii) C3v-symmetry shape. The geometrical symmetry results in the pxy-characterized C3v-symmetry wave-function of VBM state and leads to forbidden radiative recombination of band-edge excitons. The geometry strategy offers a flexible proposal to prolong the exciton lifetime, leaving optical absorption impregnable. PMID:26980541

  2. Multiple-exciton generation in lead selenide nanorod solar cells with external quantum efficiencies exceeding 120.

    PubMed

    Davis, Nathaniel J L K; Böhm, Marcus L; Tabachnyk, Maxim; Wisnivesky-Rocca-Rivarola, Florencia; Jellicoe, Tom C; Ducati, Caterina; Ehrler, Bruno; Greenham, Neil C

    2015-01-01

    Multiple-exciton generation-a process in which multiple charge-carrier pairs are generated from a single optical excitation-is a promising way to improve the photocurrent in photovoltaic devices and offers the potential to break the Shockley-Queisser limit. One-dimensional nanostructures, for example nanorods, have been shown spectroscopically to display increased multiple exciton generation efficiencies compared with their zero-dimensional analogues. Here we present solar cells fabricated from PbSe nanorods of three different bandgaps. All three devices showed external quantum efficiencies exceeding 100% and we report a maximum external quantum efficiency of 122% for cells consisting of the smallest bandgap nanorods. We estimate internal quantum efficiencies to exceed 150% at relatively low energies compared with other multiple exciton generation systems, and this demonstrates the potential for substantial improvements in device performance due to multiple exciton generation. PMID:26411283

  3. Effect of periodic potential on exciton states in semiconductor carbon nanotubes

    DOE PAGES

    Roslyak, Oleksiy; Piryatinski, Andrei

    2016-05-28

    Here we develop a theoretical background to treat exciton states in semiconductor single-walled carbon nanotubes (SWCNTs) in the presence of a periodic potential induced by a surface acoustic wave (SAW) propagating along SWCNT. The formalism accounts for the electronic band splitting into the Floquet subbands induced by the Bragg scattering on the SAW potential. Optical transitions between the Floquet states and correlated electron–hole pairs (excitons) are numerically examined. Formation of new van Hove singularities within the edges of Floquet sub-bands and associated transfer of the exciton oscillator strengths resulting in the photoluminescence quenching are predicted. The simulations demonstrate the excitonmore » energy red Stark shift and reduction in the exciton binding energy. We provide comparison of our results with reported theoretical and experimental studies.« less

  4. Geometry strategy for engineering the recombination possibility of excitons in nanowires

    NASA Astrophysics Data System (ADS)

    Wang, Youwei; Zhang, Yubo; Zhu, Haiming; Liu, Jianjun; Lian, Tianquan; Zhang, Wenqing

    2016-03-01

    We proposed a geometry strategy to engineer the radiative recombination possibility and thus the lifetime of excitons in nanowires of some photovoltaic semiconductors by using theoretical analysis and first-principles calculations. We demonstrated that the shape can engineer the symmetry of the wave-functions of band-edge states and influence the radiative recombination possibility. The nanowires need to satisfy the following requirements to forbid the radiative recombination possibility of band-edge excitons: (i) wurtzite structure; (ii) pxy-characterized wave-function of VBM state and (iii) C3v-symmetry shape. The geometrical symmetry results in the pxy-characterized C3v-symmetry wave-function of VBM state and leads to forbidden radiative recombination of band-edge excitons. The geometry strategy offers a flexible proposal to prolong the exciton lifetime, leaving optical absorption impregnable.

  5. Multiple-exciton generation in lead selenide nanorod solar cells with external quantum efficiencies exceeding 120%

    PubMed Central

    Davis, Nathaniel J. L. K.; Böhm, Marcus L.; Tabachnyk, Maxim; Wisnivesky-Rocca-Rivarola, Florencia; Jellicoe, Tom C.; Ducati, Caterina; Ehrler, Bruno; Greenham, Neil C.

    2015-01-01

    Multiple-exciton generation—a process in which multiple charge-carrier pairs are generated from a single optical excitation—is a promising way to improve the photocurrent in photovoltaic devices and offers the potential to break the Shockley–Queisser limit. One-dimensional nanostructures, for example nanorods, have been shown spectroscopically to display increased multiple exciton generation efficiencies compared with their zero-dimensional analogues. Here we present solar cells fabricated from PbSe nanorods of three different bandgaps. All three devices showed external quantum efficiencies exceeding 100% and we report a maximum external quantum efficiency of 122% for cells consisting of the smallest bandgap nanorods. We estimate internal quantum efficiencies to exceed 150% at relatively low energies compared with other multiple exciton generation systems, and this demonstrates the potential for substantial improvements in device performance due to multiple exciton generation. PMID:26411283

  6. Excitonic resonances in thin films of WSe2: from monolayer to bulk material

    NASA Astrophysics Data System (ADS)

    Arora, Ashish; Koperski, Maciej; Nogajewski, Karol; Marcus, Jacques; Faugeras, Clément; Potemski, Marek

    2015-06-01

    We present optical spectroscopy (photoluminescence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono- to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interactions as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in a WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in the WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral excitons upon an increase of temperature is demonstrated.

  7. Consistency Properties of a Chaotic Semiconductor Laser Driven by Optical Feedback

    NASA Astrophysics Data System (ADS)

    Oliver, Neus; Jüngling, Thomas; Fischer, Ingo

    2015-03-01

    We experimentally study consistency properties of a semiconductor laser in response to a coherent optical drive originating from delayed feedback. The laser is connected to a short and a long optical fiber loop, switched such that only one is providing input to the laser at a time. This way, repeating the exact same optical drive twice, we find consistent or inconsistent responses depending on the pump parameter and we relate the kind of response to strong and weak chaos. Moreover, we are able to experimentally determine the sub-Lyapunov exponent, underlying the consistency properties.

  8. Two-exciton excited states of J-aggregates in the presence of exciton-exciton annihilation

    NASA Astrophysics Data System (ADS)

    Levinsky, B.; Fainberg, B. D.; Nesterov, L. A.; Rosanov, N. N.

    2016-07-01

    We study decay of two-exciton states of a J-aggregate that is collective in nature. We use mathematical formalism based on effective non-Hermitian Hamiltonian suggested in nuclear theory. We show that decay of two-exciton states is strongly affected by the interference processes in the exciton-exciton annihilation. Our evaluations of the imaginary part of the effective Hamiltonian show that it exceeds the spacing between real energies of the two-exciton states that gives rise to the transition to the regime of overlapping resonances supplying the system by the new collectivity - the possibility of coherent decay in the annihilation channel. The decay of two-exciton states varies from twice bimolecular decay rate to the much smaller values that is associated with population trapping. We have also considered the corresponding experiment in the framework of our approach, the picture of which appears to be more complex and richer than it was reasoned before.

  9. Fast optical frequency sweeping using voltage controlled oscillator driven single sideband modulation combined with injection locking.

    PubMed

    Wang, Jian; Chen, Dijun; Cai, Haiwen; Wei, Fang; Qu, Ronghui

    2015-03-23

    An ultrafast optical frequency sweeping technique for narrow linewidth lasers is reported. This technique exploits the large frequency modulation bandwidth of a wideband voltage controlled oscillator (VCO) and a high speed electro-optic dual parallel Mach-Zehnder modulator (DPMZM) which works on the state of carrier suppressed single sideband modulation(CS-SSB). Optical frequency sweeping of a narrow linewidth fiber laser with 3.85 GHz sweeping range and 80 GHz/μs tuning speed is demonstrated, which is an extremely high tuning speed for frequency sweeping of narrow linewidth lasers. In addition, injection locking technique is adopted to improve the sweeper's low optical power output and small side-mode suppression ratio (SMSR). PMID:25837048

  10. Steering, splitting, and cloning of an optical beam in a coherently driven Raman gain system

    NASA Astrophysics Data System (ADS)

    Verma, Onkar N.; Dey, Tarak N.

    2015-01-01

    We propose an all-optical antiwaveguide mechanism for steering, splitting, and cloning of an optical beam without diffraction. We use a spatially inhomogeneous pump beam to create an antiwaveguide structure in a Doppler broadened N -type four-level Raman gain medium for a copropagating weak probe beam. We show that a transverse modulated index of refraction and gain due to the spatially dependent pump beam hold the keys to steering, splitting, and cloning of an optical beam. We have also shown that an additional control field permits the propagation of an optical beam through an otherwise gain medium without diffraction and instability. We further discuss how finesse of the cloned images can be increased by changing the detuning of the control field. We arrive at similar results by using homogeneously broadened gain media at higher density.

  11. Second harmonic generation by charge-transfer excitons interacting with phonons

    SciTech Connect

    Reineker, P.; Yudson, V. I.

    2001-06-15

    Effects of exciton-phonon interaction on the nonlinear optical response of charge-transfer excitons (CTE) are studied in the framework of an exactly solvable model. It is found that the second order excitonic optical polarizability {beta} is modified due to the CTE-phonon interaction. For a nonresonant frequency range, where {beta} is relatively small, the change is not significant. On the contrary, in the vicinity of resonances (when the light frequency {omega}{approximately}{omega}{sub 0} or {omega}{approximately}{omega}{sub 0}/2, {omega}{sub 0} is the CTE transition frequency), the CTE-phonon interaction may remarkably diminish the value of {beta}. This should be taken into account when considering CTE systems in nonlinear optics.

  12. Exciton Seebeck effect in molecular systems

    SciTech Connect

    Yan, Yun-An; Cai, Shaohong

    2014-08-07

    We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.

  13. Vertically coupled dipolar exciton molecules

    NASA Astrophysics Data System (ADS)

    Cohen, Kobi; Khodas, Maxim; Laikhtman, Boris; Santos, Paulo V.; Rapaport, Ronen

    2016-06-01

    While the interaction potential between two dipoles residing in a single plane is repulsive, in a system of two vertically adjacent layers of dipoles it changes from repulsive interaction in the long range to attractive interaction in the short range. Here we show that for dipolar excitons in semiconductor heterostructures, such a potential may give rise to bound states if two such excitons are excited in two separate layers, leading to the formation of vertically coupled dipolar exciton molecules. Our calculations prove the existence of such bound states and predict their binding energy as a function of the layers separation as well as their thermal distributions. We show that these molecules should be observed in realistic systems such as semiconductor coupled quantum well structures and the more recent van der Waals bound heterostructures. Formation of such molecules can lead to new effects such as a collective dipolar drag between layers and new forms of multiparticle correlations, as well as to the study of dipolar molecular dynamics in a controlled system.

  14. Magnetic Brightening of Dark Excitons in Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Kono, Junichiro

    2007-03-01

    To gain insight into the internal energy structure and radiative properties of excitons in single-walled carbon nanotubes (SWNTs), we have studied photoluminescence (PL) from individualized HiPco and CoMoCAT samples as a function of magnetic field (B) and temperature (T). The PL intensity increased, or ``brightened,'' with B applied along the tube axis and the amount of brightening increased with decreasing T. These results are consistent with the existence of a dark state below the first bright state [1]. In the presence of time reversal symmetry, exchange-interaction-induced mixing between excitons in two equivalent valleys (the K and K' valleys) is expected to result in a set of exciton states, only one of which is optically active. This predicted bright state, however, is not the lowest in energy. Excitons would be trapped in the dark, lowest-energy state without a radiative recombination path. When a tube-threading B is applied, addition of an Aharonov-Bohm phase modifies the circumferential boundary conditions on the wave functions and lifts time reversal symmetry [2,3]. This symmetry breaking splits the K and K' valley transitions, lessening the intervalley mixing and causing the recovery of the unmixed direct K and K' excitons, which are both optically active. We have calculated PL spectra through B-dependent effective masses, populations of finite-k states, and acoustic phonon scattering, which quantitatively agree with the observations. These results demonstrate the existence of dark excitons, their influence on the PL quantum yield, and their elimination through symmetry manipulation by a B. This work was performed in collaboration with J. Shaver, S. Zaric, O. Portugall, V. Krstic, G. L. J. A. Rikken, X. Wei, S. A. Crooker, Y. Miyauchi, S. Maruyama, and V. Perebeinos and supported by the Robert A. Welch Foundation, the NSF, and EuroMagNET. [1] V. Perebeinos et al., Phys. Rev. Lett. 92, 257402 (2004); H. Zhao and S. Mazumdar, Phys. Rev. Lett. 93, 157402

  15. Charge-transfer excitons in DNA.

    PubMed

    Conwell, E M; McLaughlin, P M; Bloch, S M

    2008-02-21

    There have been a number of theoretical treatments of excitons in DNA, most neglecting both the intrachain and interchain wavefunction overlaps of the electron and hole, treating them as Frenkel excitons. Recently, the importance of the intrachain and interchain coupling has been highlighted. Experiments have shown that in (dA)n oligomers and in duplex (dA)n.(dT)n, to be abbreviated (A/T), where A is adenine and T is thymine, the exciton wavefunction is delocalized over several bases. In duplexes it is possible to have charge-transfer (CT) excitons. Theoretical calculations have suggested that CT excitons in DNA may have lower energy than single chain excitons. In all the calculations of excitons in DNA, the polarization of the surrounding water has been neglected. Calculations have shown, however, that polarization of the water by an excess electron or a hole in DNA lowers its energy by approximately 1/2 eV, causing it to become a polaron. It is therefore to be expected that polarization charge induced in the surrounding water has a significant effect on the properties of the exciton. In what follows, we present calculations of some properties CT excitons would have in an A/T duplex taking into account the wavefunction overlaps, the effect of the surrounding water, which results in the electron and hole becoming polarons, and the ions in the water. As expected, the CT exciton has lowest energy when the electron and hole polarons are directly opposite each other. By appropriate choice of the dielectric constant, we can obtain a CT exciton delocalized over the number of sites found in photoinduced absorption experiments. The absorption threshold that we then calculate for CT exciton creation in A/T is in reasonable agreement with the lowest singlet absorption deduced from available data. PMID:18232682

  16. Exciton-polariton mediated light propagation in anisotropic waveguides

    NASA Astrophysics Data System (ADS)

    Takeda, Hiroyuki; Sakoda, Kazuaki

    2012-11-01

    To analyze the exciton-polariton dispersion relation of highly anisotropic thiacyanine films and nanofibers, we formulated a plane-wave expansion method by which we could obtain the eigenfrequencies of polaritons as eigenvalues of a non-Hermitian and frequency-independent matrix. The group refractive index calculated from the slope of the dispersion curve agreed quite well with the Fabry-Perot interference patterns found in both the calculated and observed transmission spectra. We found that the dispersion relation of the anisotropic polariton was quite different from the isotropic case and depended strongly on the tilt angle of the optical transition dipole moment of the constituent molecules to the propagation direction. Material parameters such as the transverse and longitudinal exciton frequencies are also discussed.

  17. Direct measurement of exciton valley coherence in monolayer WSe2

    NASA Astrophysics Data System (ADS)

    Hao, Kai; Moody, Galan; Wu, Fengcheng; Dass, Chandriker Kavir; Xu, Lixiang; Chen, Chang-Hsiao; Sun, Liuyang; Li, Ming-Yang; Li, Lain-Jong; MacDonald, Allan H.; Li, Xiaoqin

    2016-07-01

    In crystals, energy band extrema in momentum space can be identified by a valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier, analogous to electronic charge or spin. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides. However, the valley coherence time--a crucial quantity for valley pseudospin manipulation--is difficult to directly probe. In this work, we use two-dimensional coherent spectroscopy to resonantly generate and detect valley coherence of excitons (Coulomb-bound electron-hole pairs) in monolayer WSe2 (refs ,). The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron-hole exchange interaction provides an important decoherence mechanism in addition to exciton population recombination. This work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications.

  18. Organic photovoltaics incorporating electron conducting exciton blocking layers

    NASA Astrophysics Data System (ADS)

    Lassiter, Brian E.; Wei, Guodan; Wang, Siyi; Zimmerman, Jeramy D.; Diev, Viacheslav V.; Thompson, Mark E.; Forrest, Stephen R.

    2011-06-01

    We demonstrate that 3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI) and 1,4,5,8-napthalene-tetracarboxylic-dianhydride (NTCDA) can function as electron conducting and exciton blocking layers when interposed between the acceptor layer and cathode. A low-resistance contact is provided by PTCBI, while NTCDA acts as an exciton blocking layer and optical spacer. Both materials serve as efficient electron conductors, leading to a fill factor as high as 0.70. By using an NTCDA/PTCBI compound blocking layer structure in a functionalized-squaraine/C60-based device, we obtain a spectrally corrected power conversion efficiency of 5.1±0.1% under 1 sun, AM 1.5G simulated solar illumination, an improvement of >25% compared to an analogous device using a conventional bathocuproine layer that has previously been shown to conduct electrons via damage-induced midgap states.

  19. On the output characteristics of a semiconductor optical amplifier driven by an ultrafast optical time division multiplexing pulse train

    NASA Astrophysics Data System (ADS)

    Zoiros, K. E.; Chasioti, R.; Koukourlis, C. S.; Houbavlis, T.

    2007-03-01

    A comprehensive theoretical analysis of a semiconductor optical amplifier (SOA) that is subject to an ultrafast optical time division multiplexing pulse stream is presented with the help of a simple but efficient model developed for this purpose. The model combines the necessary set of mathematical equations with the appropriate simplifying assumptions to describe in the time domain gain saturation and recovery for the case of multiple incoming pulses. In this manner, analytical expressions can be obtained for the power and chirp profile of the amplified pulses, essentially extending the work that has been performed for a single pulse only. This allows to identify the critical operational parameters and to investigate and evaluate their effect on these two output characteristics. The derived simulation curves are thoroughly studied to specify the limitations imposed on the SOA small signal gain and carrier lifetime as well as on the full-width at half-maximum (FWHM) and energy of the input pulses and, based on a series of logical arguments, to extract useful rules concerning their selection so as to achieve improved performance with respect to the practical applications of all-optical switching and pulse compression. The obtained results indicate that due to the continuous insertion of pulses, the requirements for the SOA small signal gain and the input pulse energy are stringent than those for the case of isolated pulse amplification. The combination of these two parameters determines also the regime in which the amplifier must be biased to operate in order to ensure distortionless pulse amplification and enhanced chirp for efficient pulse compression and it has been found that low saturation is necessary for the former case whilst heavy saturation for the latter. The scopes of the corresponding requirements for the carrier lifetime and the FWHM are also tight but to a less extent and can be simply satisfied with the available photonics technology. These results

  20. Excitability and optical pulse generation in semiconductor lasers driven by resonant tunneling diode photo-detectors.

    PubMed

    Romeira, Bruno; Javaloyes, Julien; Ironside, Charles N; Figueiredo, José M L; Balle, Salvador; Piro, Oreste

    2013-09-01

    We demonstrate, experimentally and theoretically, excitable nanosecond optical pulses in optoelectronic integrated circuits operating at telecommunication wavelengths (1550 nm) comprising a nanoscale double barrier quantum well resonant tunneling diode (RTD) photo-detector driving a laser diode (LD). When perturbed either electrically or optically by an input signal above a certain threshold, the optoelectronic circuit generates short electrical and optical excitable pulses mimicking the spiking behavior of biological neurons. Interestingly, the asymmetric nonlinear characteristic of the RTD-LD allows for two different regimes where one obtain either single pulses or a burst of multiple pulses. The high-speed excitable response capabilities are promising for neurally inspired information applications in photonics. PMID:24103966

  1. Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in a same device

    DOE PAGES

    Hu, Miao; Bi, Cheng; Yuan, Yongbo; Xiao, Zhengguo; Dong, Qingfeng; Shao, Yuchuan; Huang, Jinsong

    2015-01-15

    The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. Moreover, it is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system.

  2. Exciton induced photodesorption in rare gas solids

    NASA Astrophysics Data System (ADS)

    Hirayama, Takato; Arakawa, Ichiro

    2006-08-01

    This paper reviews our progress on the desorption induced by electronic transitions (DIET) in rare gas solids by selective excitation of valence excitons. Observation of metastable atoms desorbed by excitonic excitation gives us direct information on the exciton-induced desorption processes in rare gas solids. The validity of three desorption mechanisms, cavity ejection, excimer dissociation, and internal sputtering, is demonstrated by systematic measurements of kinetic energies and angular distributions of desorbed particles. The absolute yield of total and partial desorption was measured, which can lead us to the quantitative understanding of exciton-induced desorption processes.

  3. Ballistic spin transport in exciton gases

    NASA Astrophysics Data System (ADS)

    Kavokin, A. V.; Vladimirova, M.; Jouault, B.; Liew, T. C. H.; Leonard, J. R.; Butov, L. V.

    2013-11-01

    Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. The challenges for the realization of long-range electron spin transport include rapid spin relaxation due to electron scattering. Scattering and, in turn, spin relaxation can be effectively suppressed in excitonic devices where the spin currents are carried by electrically neutral bosonic quasiparticles: excitons or exciton-polaritons. They can form coherent quantum liquids that carry spins over macroscopic distances. The price to pay is a finite lifetime of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems supporting bosonic spin transport, in particular to indirect excitons in coupled quantum wells. We describe the effect of spin-orbit interaction for the electron and the hole on the exciton spin, account for the Zeeman effect induced by external magnetic fields and long-range and short-range exchange splittings of the exciton resonances. We also consider exciton transport in the nonlinear regime and discuss the definitions of the exciton spin current, polarization current, and spin conductivity.

  4. Testing of optical diagnostics for ion-beam-driven WDM experiments at NDCX-1

    SciTech Connect

    Ni, P.A.; Bieniosek, F.M.; Leitner, M.; Weber, C.; Waldron, W.L.

    2008-06-01

    We report on the testing of optical diagnostics developed for warm-dense-matter (WDM) experiments on the Neutralized Drift Compression Experiments (NDCX-1) at Lawrence Berkeley National Laboratory (LBNL). The diagnostics consist of a fast optical pyrometer, a streak camera spectrometer, and a VISAR.While the NDCX is in the last stage of commissioning for the target experiments, the diagnostics were tested elsewhere in an experiment where an intense laser pulse was used to generate the WDM state in metallic and carbon samples.

  5. Controlled exciton transfer between quantum dots with acoustic phonons taken into account

    SciTech Connect

    Golovinski, P. A.

    2015-09-15

    A system of excitons in two quantum dots coupled by the dipole–dipole interaction is investigated. The excitation transfer process controlled by the optical Stark effect at nonresonant frequencies is considered and the effect of the interaction between excitons and acoustic phonons in a medium on this process is taken into account. The system evolution is described using quantum Heisenberg equations. A truncated set of equations is obtained and the transfer dynamics is numerically simulated. High-efficiency picosecond switching of the excitation transfer by a laser pulse with a rectangular envelope is demonstrated. The dependence of picosecond switching on the quantum-dot parameters and optical-pulse length is presented.

  6. Exciton Brightening in Monolayer Phosphorene via Dimensionality Modification.

    PubMed

    Xu, Renjing; Yang, Jiong; Myint, Ye Win; Pei, Jiajie; Yan, Han; Wang, Fan; Lu, Yuerui

    2016-05-01

    Exciton brightening in monolayer phosphorene is achieved via the dimensionality modification of excitons from quasi-1D to 0D. The luminescence quantum yield of 0D-like excitons is >33.6 times larger than that of quasi-1D free excitons. 2D phosphorene with quasi-1D free excitons and 0D-like excitons provides a unique platform to investigate the fundamental phenomena in the ideal 2D-1D-0D hybrid system.

  7. Interaction-driven Lifshitz transition with dipolar fermions in optical lattices

    NASA Astrophysics Data System (ADS)

    van Loon, E. G. C. P.; Katsnelson, M. I.; Chomaz, L.; Lemeshko, M.

    2016-05-01

    Anisotropic dipole-dipole interactions between ultracold dipolar fermions break the symmetry of the Fermi surface and thereby deform it. Here we demonstrate that such a Fermi surface deformation induces a topological phase transition—the so-called Lifshitz transition—in the regime accessible to present-day experiments. We describe the impact of the Lifshitz transition on observable quantities such as the Fermi surface topology, the density-density correlation function, and the excitation spectrum of the system. The Lifshitz transition in ultracold atoms can be controlled by tuning the dipole orientation and, in contrast to the transition studied in crystalline solids, is completely interaction driven.

  8. Time- and Space-Resolved Optical Probing of Femtosecond-Laser-Driven Shock Waves in Aluminum

    SciTech Connect

    Evans, R.; Badger, A.D.; Fallies, F.; Mahdieh, M.; Hall, T.A.; Audebert, P.; Geindre, J.; Gauthier, J.; Mysyrowicz, A.; Grillon, G.; Antonetti, A.

    1996-10-01

    We present the first measurements of particle velocity histories at the interface between an aluminum sample shocked by a 120fs laser-driven pressure pulse and a fused silica window. Frequency-domain interferometry is used to provide space- and time-resolved measurements of the phase shift of a pair of probe pulses backscattered at the shocked interface. Pressures of 1{endash}3 Mbar are inferred from the simultaneous measurement of the particle and shock velocities along the aluminum Hugoniot curve for {approximately}10{sup 14} W/cm{sup 2} laser irradiances. {copyright} {ital 1996 The American Physical Society.}

  9. Heterogeneous integration of electrically driven microdisk based laser sources for optical interconnects and photonic ICs.

    PubMed

    Rojo Romeo, P; Van Campenhout, J; Regreny, P; Kazmierczak, A; Seassal, C; Letartre, X; Hollinger, G; Van Thourhout, D; Baets, R; Fedeli, J M; Di Cioccio, L

    2006-05-01

    A new approach for an electrically driven microlaser based on a microdisk transferred onto Silicon is proposed. The structure is based on a quaternary InGaAsP p-i-n junction including three InAsP quantum wells, on a thin membrane transferred onto silicon by molecular bonding. A p++/n++ tunnel junction is used as the p-type contact. The technological procedure is described and first experimental results show a laser emission in pulsed regime at room temperature, with a threshold current near 1.5 mA.

  10. Optical Spectroscopy Measurements of Shock Waves Driven by Intense Z-Pinch Radiation

    SciTech Connect

    Asay, J. Bernard, M.; Bailey, J.E.; Carlson, A.L.; Chandler, G.A.; Hall, C.A.; Hanson, D.; Johnston, R.; Lake, P.; Lawrence, J.

    1999-04-09

    Z-pinches created using the Z accelerator generate {approximately}220 TW, 1.7 MJ radiation pulses that heat large ({approximately}10 cm{sup 3}) hohlraums to 100-150 eV temperatures for times of order 10 nsec. We are performing experiments exploiting this intense radiation to drive shock waves for equation of state studies. The shock pressures are typically 1-10 Mbar with 10 nsec duration in 6-mm-diameter samples. In this paper we demonstrate the ability to perform optical spectroscopy measurements on shocked samples located in close proximity to the z-pinch. These experiments are particularly well suited to optical spectroscopy measurements because of the relatively large sample size and long duration. The optical emission is collected using fiber optics and recorded with a streaked spectrograph. Other diagnostics include VISAR and active shock breakout measurements of the shocked sample and a suite of diagnostics that characterize the radiation drive. Our near term goal is to use the spectral emission to obtain the temperature of the shocked material. Longer term objectives include the examination of deviations of the spectrum from blackbody, line emission from lower density regions, determination of kinetic processes in molecular systems, evaluation of phase transitions such as the onset of metalization in transparent materials, and characterization of the plasma formed when the shock exits the rear surface. An initial set of data illustrating both the potential and the challenge of these measurements is described.

  11. Cloud-Driven Changes in Aerosol Optical Properties - Final Technical Report

    SciTech Connect

    Ogren, John A.; Sheridan, Patrick S.; Andrews, Elisabeth

    2007-09-30

    The optical properties of aerosol particles are the controlling factors in determining direct aerosol radiative forcing. These optical properties depend on the chemical composition and size distribution of the aerosol particles, which can change due to various processes during the particles’ lifetime in the atmosphere. Over the course of this project we have studied how cloud processing of atmospheric aerosol changes the aerosol optical properties. A counterflow virtual impactor was used to separate cloud drops from interstitial aerosol and parallel aerosol systems were used to measure the optical properties of the interstitial and cloud-scavenged aerosol. Specifically, aerosol light scattering, back-scattering and absorption were measured and used to derive radiatively significant parameters such as aerosol single scattering albedo and backscatter fraction for cloud-scavenged and interstitial aerosol. This data allows us to demonstrate that the radiative properties of cloud-processed aerosol can be quite different than pre-cloud aerosol. These differences can be used to improve the parameterization of aerosol forcing in climate models.

  12. Features of exciton dynamics in molecular nanoclusters (J-aggregates): Exciton self-trapping (Review Article)

    NASA Astrophysics Data System (ADS)

    Malyukin, Yu. V.; Sorokin, A. V.; Semynozhenko, V. P.

    2016-06-01

    We present thoroughly analyzed experimental results that demonstrate the anomalous manifestation of the exciton self-trapping effect, which is already well-known in bulk crystals, in ordered molecular nanoclusters called J-aggregates. Weakly-coupled one-dimensional (1D) molecular chains are the main structural feature of J-aggregates, wherein the electron excitations are manifested as 1D Frenkel excitons. According to the continuum theory of Rashba-Toyozawa, J-aggregates can have only self-trapped excitons, because 1D excitons must adhere to barrier-free self-trapping at any exciton-phonon coupling constant g = ɛLR/2β, wherein ɛLR is the lattice relaxation energy, and 2β is the half-width of the exciton band. In contrast, very often only the luminescence of free, mobile excitons would manifest in experiments involving J-aggregates. Using the Urbach rule in order to analyze the low-frequency region of the low-temperature exciton absorption spectra has shown that J-aggregates can have both a weak (g < 1) and a strong (g > 1) exciton-phonon coupling. Moreover, it is experimentally demonstrated that under certain conditions, the J-aggregate excited state can have both free and self-trapped excitons, i.e., we establish the existence of a self-trapping barrier for 1D Frenkel excitons. We demonstrate and analyze the reasons behind the anomalous existence of both free and self-trapped excitons in J-aggregates, and demonstrate how exciton-self trapping efficiency can be managed in J-aggregates by varying the values of g, which is fundamentally impossible in bulk crystals. We discuss how the exciton-self trapping phenomenon can be used as an alternate interpretation of the wide band emission of some J-aggregates, which has thus far been explained by the strongly localized exciton model.

  13. Steady-state bistability and long-range order in optically driven Rydberg gases in the anti-blockade regime

    NASA Astrophysics Data System (ADS)

    Letscher, Fabian; Linzner, Dominik; Fleischhauer, Michael

    2016-05-01

    Motivated by recent experiments, we study spatial and temporal correlations of Rydberg excitations of optically driven ultra-cold atoms in the anti-blockade regime. In particular, we discuss the influence of dissipation on the excitation dynamics of a linear chain of atoms, described by the dissipative, transverse-field Ising model. Using t-DMRG simulations of the density matrix we identify parameter regimes with diverging correlation lengths in the coherent regime of weak dissipation. Correlation lengths remain short-ranged in the incoherent regime of strong dissipation, where classical rate equations can be employed. We discuss the different physical mechanisms determining the many-body dynamics in the two regimes and compare theoretical predictions with recent experimental results. In particular we discuss the formation of excitation cluster in the incoherent regime and explain the observed slow-down of the relaxation process due to cluster formation.

  14. Analysis of Self-Terminated Pressure-Driven Quantum Point Contacts with Ultrafast Optical Pulses

    NASA Astrophysics Data System (ADS)

    Soltani, Fatemeh; Wlasenko, Alex; Steeves, Geoff

    2009-05-01

    A self-terminated electrochemical method was used to fabricate atomic-scale contacts between two Au electrodes in a microfluidic channel. The conductance of the contacts varies in a stepwise fashion. The mechanism works by a pressure-driven flow parallel with a pair of Au electrodes with a 100 μm gap in an electrolyte of HCl. Without applied flow, dendrite growth and dense branching morphology were typically observed at the cathode. The addition of applied pressure-driven flow resulted in a densely packed gold structure that filled the channel. The electrochemical fabrication approach introduces large variance in the formation and location of individual junctions. Understanding and controlling this process will enable the precise positioning of reproducible geometries into nano-electronic devices. To investigate the high speed behaviour of a QPC, it can be integrated with a transmission line structure patterned on a photoconductive GaAs substrate. The nonlinear conductance of the QPC (due to the finite density of states of the conductors) can be examined and compared with recent theoretical studies. Samples are fabricated in situ using an electrochemical procedure to produce QPCs along the transmission line structure. This method may provide insight into Terahertz Optoelectronic devices and ultrafast communication systems.

  15. Piezoelectrically driven translatory optical MEMS actuator with 7mm apertures and large displacements

    NASA Astrophysics Data System (ADS)

    Quenzer, H.-J.; Gu-Stoppel, S.; Stoppel, F.; Janes, J.; Hofmann, U.; Benecke, W.

    2015-02-01

    The design and manufacturing of a piezoelectrically driven translatory MEMS actuator is presented, which features a 7 mm aperture and four thin-film PZT actuators achieving large displacements. The actuator performs piston mode oscillation in resonance which can serve for Fourier Transform Infrared Spectroscopy (FTIR). Thereby vertical displacements in piston mode of up to ± 800 μm at 163 Hz and 25 V driving sinusoidal voltage has been achieved under ambient conditions. Due to the low frequencies and the low driving voltages only low power consumption is required. The effect of residual gas friction and internal friction on the piezo-driven MEMS actuator is analyzed by measuring Qvalues associated with the piston mode. Laser Doppler Vibrometry (LDV) was also used to detect and analyses the parasitic effects especially tilting which superimposes the vertical movement of the mirror. The deviation from the pure vertical piston mode was found to 1.3 μm along the x and 3 μm in the y-axis.

  16. Josephson effects in condensates of excitons and exciton polaritons

    NASA Astrophysics Data System (ADS)

    Shelykh, I. A.; Solnyshkov, D. D.; Pavlovic, G.; Malpuech, G.

    2008-07-01

    We analyze theoretically the phenomena related to the Josephson effect for exciton and polariton condensates, taking into account their specific spin degrees of freedom. We distinguish between two types of Josephson effects: the extrinsic effect, related to the coherent tunneling of particles with the same spin between two spatially separated potential traps, and the intrinsic effect, related to the “tunneling” between different spinor components of the condensate within the same trap. We show that the Josephson effect in the nonlinear regime can lead to nontrivial polarization dynamics and produce spontaneous separation of the condensates with opposite polarization in real space.

  17. Josephson effects in condensates of excitons and exciton polaritons

    SciTech Connect

    Shelykh, I. A.; Solnyshkov, D. D.; Pavlovic, G.; Malpuech, G.

    2008-07-15

    We analyze theoretically the phenomena related to the Josephson effect for exciton and polariton condensates, taking into account their specific spin degrees of freedom. We distinguish between two types of Josephson effects: the extrinsic effect, related to the coherent tunneling of particles with the same spin between two spatially separated potential traps, and the intrinsic effect, related to the 'tunneling' between different spinor components of the condensate within the same trap. We show that the Josephson effect in the nonlinear regime can lead to nontrivial polarization dynamics and produce spontaneous separation of the condensates with opposite polarization in real space.

  18. Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy

    DOE PAGES

    Klots, A. R.; Newaz, A. K. M.; Wang, Bin; Prasai, D.; Krzyzanowska, H.; Lin, Junhao; Caudel, D.; Ghimire, N. J.; Yan, J.; Ivanov, B. L.; et al

    2014-10-16

    The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions ofmore » peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. In conclusion, the analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.« less

  19. Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy

    SciTech Connect

    Klots, A. R.; Newaz, A. K. M.; Wang, Bin; Prasai, D.; Krzyzanowska, H.; Lin, Junhao; Caudel, D.; Ghimire, N. J.; Yan, J.; Ivanov, B. L.; Velizhanin, K. A.; Burger, A.; Mandrus, D. G.; Tolk, N. H.; Pantelides, S. T.; Bolotin, K. I.

    2014-10-16

    The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. In conclusion, the analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

  20. Three dimensional optical twisters-driven helically stacked multi-layered microrotors

    NASA Astrophysics Data System (ADS)

    Xavier, Jolly; Dasgupta, Raktim; Ahlawat, Sunita; Joseph, Joby; Kumar Gupta, Pradeep

    2012-03-01

    We demonstrate tunable helically stacked multi-layered microrotors realized in vortex-embedded three dimensional (3D) optical twister patterns. Intensity-tunable annular irradiance profiles with higher order vortex are generated as well as simultaneously unfolded by phase-engineered multiple plane wave interference. In the individually tunable 3D helical bright arms of these unfolded vortex structures, 2 μm silica beads are optically trapped as spiraling multilayered handles of multi-armed microrotors. Further, multiple rows of such microrotors are parallelly actuated with controllable sense of rotation. We also present our observation on helical 3D stacking of micro-particles in these longitudinally gyrating multi-armed rotor traps.

  1. Directly driven source of multi-gigahertz, sub-picosecond optical pulses

    DOEpatents

    Messerly, Michael J.; Dawson, Jay W.; Barty, Christopher P.J.; Gibson, David J.; Prantil, Matthew A.; Cormier, Eric

    2015-10-20

    A robust, compact optical pulse train source is described, with the capability of generating sub-picosecond micro-pulse sequences, which can be periodic as well as non-periodic, and at repetition rates tunable over decades of baseline frequencies, from MHz to multi-GHz regimes. The micro-pulses can be precisely controlled and formatted to be in the range of many ps in duration to as short as several fs in duration. The system output can be comprised of a continuous wave train of optical micro-pulses or can be programmed to provide gated bursts of macro-pulses, with each macro-pulse consisting of a specific number of micro-pulses or a single pulse picked from the higher frequency train at a repetition rate lower than the baseline frequency. These pulses could then be amplified in energy anywhere from the nJ to MJ range.

  2. Holographic optical manipulation of motor-driven membranous structures in living NG-108 cells

    NASA Astrophysics Data System (ADS)

    Farré, Arnau; López-Quesada, Carol; Andilla, Jordi; Martín-Badosa, Estela; Montes-Usategui, Mario

    2010-08-01

    Optical tweezer experiments have partially unveiled the mechanical properties of processive motor proteins while driving polystyrene or silica microbeads in vitro. However, the set of forces underlying the more complex transport mechanisms in living samples remains poorly understood. Several studies have shown that optical tweezers are capable of trapping vesicles and organelles in the cytoplasm of living cells, which can be used as handles to mechanically interact with engaged (active) motors, or other components regulating transport. This may ultimately enable the exploration of the mechanics of this trafficking mechanism in vivo. These cell manipulation experiments have been carried out using different strategies to achieve dynamic beam steering capable of trapping these subcellular structures. We report here the first trapping and manipulation, to our knowledge, of such small motor-propelled cargos in living cells using holographic technology.

  3. Optically-driven red blood cell rotor in linearly polarized laser tweezers

    NASA Astrophysics Data System (ADS)

    Khan, Manas; Mohanty, Samarendra K.; Sood, A. K.

    2005-11-01

    We have constructed a dual trap optical tweezers set-up around an inverted microscope where both the traps can be independently controlled and manipulated in all the three dimensions. Here we report our observations on rotation of red blood cells (RBCs) in a linearly polarized optical trap. Red blood cells deform and become twisted in hypertonic phosphate buffer saline and when trapped, experience an unbalanced radiation pressure force. The torque generated from the unbalanced force causes the trapped RBC to rotate. Addition of Ca^{++} ions in the solution, keeping the osmolarity same, makes the cell membranes stiffer and the cells deform less. Thus the speed of rotation of the red blood cells can be controlled, as less deformation and in turn less asymmetry in shape produces less torque under the radiation pressure resulting in slower rotation at the same laser power.

  4. Observation of non-Hermitian degeneracies in a chaotic exciton-polariton billiard

    NASA Astrophysics Data System (ADS)

    Gao, T.; Estrecho, E.; Bliokh, K. Y.; Liew, T. C. H.; Fraser, M. D.; Brodbeck, S.; Kamp, M.; Schneider, C.; Höfling, S.; Yamamoto, Y.; Nori, F.; Kivshar, Y. S.; Truscott, A. G.; Dall, R. G.; Ostrovskaya, E. A.

    2015-10-01

    Exciton-polaritons are hybrid light-matter quasiparticles formed by strongly interacting photons and excitons (electron-hole pairs) in semiconductor microcavities. They have emerged as a robust solid-state platform for next-generation optoelectronic applications as well as for fundamental studies of quantum many-body physics. Importantly, exciton-polaritons are a profoundly open (that is, non-Hermitian) quantum system, which requires constant pumping of energy and continuously decays, releasing coherent radiation. Thus, the exciton-polaritons always exist in a balanced potential landscape of gain and loss. However, the inherent non-Hermitian nature of this potential has so far been largely ignored in exciton-polariton physics. Here we demonstrate that non-Hermiticity dramatically modifies the structure of modes and spectral degeneracies in exciton-polariton systems, and, therefore, will affect their quantum transport, localization and dynamical properties. Using a spatially structured optical pump, we create a chaotic exciton-polariton billiard--a two-dimensional area enclosed by a curved potential barrier. Eigenmodes of this billiard exhibit multiple non-Hermitian spectral degeneracies, known as exceptional points. Such points can cause remarkable wave phenomena, such as unidirectional transport, anomalous lasing/absorption and chiral modes. By varying parameters of the billiard, we observe crossing and anti-crossing of energy levels and reveal the non-trivial topological modal structure exclusive to non-Hermitian systems. We also observe mode switching and a topological Berry phase for a parameter loop encircling the exceptional point. Our findings pave the way to studies of non-Hermitian quantum dynamics of exciton-polaritons, which may uncover novel operating principles for polariton-based devices.

  5. Observation of non-Hermitian degeneracies in a chaotic exciton-polariton billiard.

    PubMed

    Gao, T; Estrecho, E; Bliokh, K Y; Liew, T C H; Fraser, M D; Brodbeck, S; Kamp, M; Schneider, C; Höfling, S; Yamamoto, Y; Nori, F; Kivshar, Y S; Truscott, A G; Dall, R G; Ostrovskaya, E A

    2015-10-22

    Exciton-polaritons are hybrid light-matter quasiparticles formed by strongly interacting photons and excitons (electron-hole pairs) in semiconductor microcavities. They have emerged as a robust solid-state platform for next-generation optoelectronic applications as well as for fundamental studies of quantum many-body physics. Importantly, exciton-polaritons are a profoundly open (that is, non-Hermitian) quantum system, which requires constant pumping of energy and continuously decays, releasing coherent radiation. Thus, the exciton-polaritons always exist in a balanced potential landscape of gain and loss. However, the inherent non-Hermitian nature of this potential has so far been largely ignored in exciton-polariton physics. Here we demonstrate that non-Hermiticity dramatically modifies the structure of modes and spectral degeneracies in exciton-polariton systems, and, therefore, will affect their quantum transport, localization and dynamical properties. Using a spatially structured optical pump, we create a chaotic exciton-polariton billiard--a two-dimensional area enclosed by a curved potential barrier. Eigenmodes of this billiard exhibit multiple non-Hermitian spectral degeneracies, known as exceptional points. Such points can cause remarkable wave phenomena, such as unidirectional transport, anomalous lasing/absorption and chiral modes. By varying parameters of the billiard, we observe crossing and anti-crossing of energy levels and reveal the non-trivial topological modal structure exclusive to non-Hermitian systems. We also observe mode switching and a topological Berry phase for a parameter loop encircling the exceptional point. Our findings pave the way to studies of non-Hermitian quantum dynamics of exciton-polaritons, which may uncover novel operating principles for polariton-based devices. PMID:26458102

  6. Noise-driven optical absorption coefficients of impurity doped quantum dots

    NASA Astrophysics Data System (ADS)

    Ganguly, Jayanta; Saha, Surajit; Pal, Suvajit; Ghosh, Manas

    2016-01-01

    We make an extensive investigation of linear, third-order nonlinear, and total optical absorption coefficients (ACs) of impurity doped quantum dots (QDs) in presence and absence of noise. The noise invoked in the present study is a Gaussian white noise. The quantum dot is doped with repulsive Gaussian impurity. Noise has been introduced to the system additively and multiplicatively. A perpendicular magnetic field acts as a source of confinement and a static external electric field has been applied. The AC profiles have been studied as a function of incident photon energy when several important parameters such as optical intensity, electric field strength, magnetic field strength, confinement energy, dopant location, relaxation time, Al concentration, dopant potential, and noise strength take on different values. In addition, the role of mode of application of noise (additive/multiplicative) on the AC profiles has also been analyzed meticulously. The AC profiles often consist of a number of interesting observations such as one photon resonance enhancement, shift of AC peak position, variation of AC peak intensity, and bleaching of AC peak. However, presence of noise alters the features of AC profiles and leads to some interesting manifestations. Multiplicative noise brings about more complexity in the AC profiles than its additive counterpart. The observations indeed illuminate several useful aspects in the study of linear and nonlinear optical properties of doped QD systems, specially in presence of noise. The findings are expected to be quite relevant from a technological perspective.

  7. Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence.

    PubMed

    Hallinan, G; Littlefair, S P; Cotter, G; Bourke, S; Harding, L K; Pineda, J S; Butler, R P; Golden, A; Basri, G; Doyle, J G; Kao, M M; Berdyugina, S V; Kuznetsov, A; Rupen, M P; Antonova, A

    2015-07-30

    Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons, as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere. Here we report simultaneous radio and optical spectroscopic observations of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs. PMID:26223623

  8. Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence.

    PubMed

    Hallinan, G; Littlefair, S P; Cotter, G; Bourke, S; Harding, L K; Pineda, J S; Butler, R P; Golden, A; Basri, G; Doyle, J G; Kao, M M; Berdyugina, S V; Kuznetsov, A; Rupen, M P; Antonova, A

    2015-07-30

    Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons, as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere. Here we report simultaneous radio and optical spectroscopic observations of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs.

  9. Role of many-body effects in the coherent dynamics of excitons in low-temperature-grown GaAs

    SciTech Connect

    Webber, D.; Hacquebard, L.; Hall, K. C.; Liu, X.; Dobrowolska, M.; Furdyna, J. K.

    2015-10-05

    Femtosecond four-wave mixing experiments on low-temperature-grown (LT-) GaAs indicate a polarization-dependent nonlinear optical response at the exciton, which we attribute to Coulomb-mediated coupling between excitons and electron-hole pairs simultaneously excited by the broad-bandwidth laser pulses. Strong suppression of the exciton response through screening by carriers injected by a third pump pulse was observed, an effect that is transient due to rapid carrier trapping. Our findings highlight the need to account for the complex interplay of disorder and many-body effects in the design of ultrafast optoelectronic devices using this material.

  10. Strong exciton-localized plasmon coupling in a-Ge24Se76/AuNP heterostructure

    NASA Astrophysics Data System (ADS)

    Sharma, Rituraj; Khan, Pritam; Aneesh, J.; Sangunni, K. S.; Csarnovics, I.; Kokenyesi, S.; Jain, H.; Adarsh, K. V.

    2016-10-01

    Metal nanoparticle-semiconductor interfaces are sites of complex light-matter interactions, in particular, the exciton-plasmon coupling which plays a key role in the optical response of such heterostructures. There exists a pathway of photoinduced charge transfer from the semiconductor to the metal, which can be used to controllably vary the driving forces at the interface that leads to tunable optoelectronic properties. In this letter, we report the observation of a dramatic suppression of plasmonic as well as excitonic absorption in a-Ge24Se76/gold nanoparticle heterostructures by trapped charges. Suppression of the excitonic absorption is strongly correlated with the plasmon wavelength.

  11. Observation of excitonic super-radiance in quantum well structures and its application for laser cooling of solids

    NASA Astrophysics Data System (ADS)

    Hassani Nia, Iman; Weinberg, David; Wheaton, Skylar; Weiss, Emily A.; Mohseni, Hooman

    2016-03-01

    Excitons, bound electron-hole pairs, possess distinct physical properties from free electrons and holes that can be employed to improve the performance of optoelectronic devices. In particular, the signatures of excitons are enhanced optical absorption and radiative emission. These characteristics could be of major benefit for the laser cooling of semiconductors, a process which has stringent requirements on the parasitic absorption of incident radiation and the internal quantum efficiency. Here we experimentally demonstrate the dominant ultrafast excitonic super-radiance of our quantum well structure from 78 K up to room temperature. The experimental results are followed by our detailed discussions about the advantages and limitations of this method.

  12. Bound exciton and free exciton states in GaSe thin slab

    PubMed Central

    Wei, Chengrong; Chen, Xi; Li, Dian; Su, Huimin; He, Hongtao; Dai, Jun-Feng

    2016-01-01

    The photoluminescence (PL) and absorption experiments have been performed in GaSe slab with incident light polarized perpendicular to c-axis of sample at 10 K. An obvious energy difference of about 34 meV between exciton absorption peak and PL peak (the highest energy peak) is observed. By studying the temperature dependence of PL and absorption spectra, we attribute it to energy difference between free exciton and bound exciton states, where main exciton absorption peak comes from free exciton absorption, and PL peak is attributed to recombination of bound exciton at 10 K. This strong bound exciton effect is stable up to 50 K. Moreover, the temperature dependence of integrated PL intensity and PL lifetime reveals that a non-radiative process, with activation energy extracted as 0.5 meV, dominates PL emission. PMID:27654064

  13. Optically driven translational and rotational motions of microrod particles in a nematic liquid crystal

    PubMed Central

    Eremin, Alexey; Hirankittiwong, Pemika; Chattham, Nattaporn; Nádasi, Hajnalka; Stannarius, Ralf; Limtrakul, Jumras; Haba, Osamu; Yonetake, Koichiro; Takezoe, Hideo

    2015-01-01

    A small amount of azo-dendrimer molecules dissolved in a liquid crystal enables translational and rotational motions of microrods in a liquid crystal matrix under unpolarized UV light irradiation. This motion is initiated by a light-induced trans-to-cis conformational change of the dendrimer adsorbed at the rod surface and the associated director reorientation. The bending direction of the cis conformers is not random but is selectively chosen due to the curved local director field in the vicinity of the dendrimer-coated surface. Different types of director distortions occur around the rods, depending on their orientations with respect to the nematic director field. This leads to different types of motions driven by the torques exerted on the particles by the director reorientations. PMID:25624507

  14. Optically driven translational and rotational motions of microrod particles in a nematic liquid crystal.

    PubMed

    Eremin, Alexey; Hirankittiwong, Pemika; Chattham, Nattaporn; Nádasi, Hajnalka; Stannarius, Ralf; Limtrakul, Jumras; Haba, Osamu; Yonetake, Koichiro; Takezoe, Hideo

    2015-02-10

    A small amount of azo-dendrimer molecules dissolved in a liquid crystal enables translational and rotational motions of microrods in a liquid crystal matrix under unpolarized UV light irradiation. This motion is initiated by a light-induced trans-to-cis conformational change of the dendrimer adsorbed at the rod surface and the associated director reorientation. The bending direction of the cis conformers is not random but is selectively chosen due to the curved local director field in the vicinity of the dendrimer-coated surface. Different types of director distortions occur around the rods, depending on their orientations with respect to the nematic director field. This leads to different types of motions driven by the torques exerted on the particles by the director reorientations. PMID:25624507

  15. Population Pulsation Resonances of Excitons in Monolayer MoSe2 with Sub-1 μ eV Linewidths

    NASA Astrophysics Data System (ADS)

    Schaibley, John R.; Karin, Todd; Yu, Hongyi; Ross, Jason S.; Rivera, Pasqual; Jones, Aaron M.; Scott, Marie E.; Yan, Jiaqiang; Mandrus, D. G.; Yao, Wang; Fu, Kai-Mei; Xu, Xiaodong

    2015-04-01

    Monolayer transition metal dichalcogenides, a new class of atomically thin semiconductors, possess optically coupled 2D valley excitons. The nature of exciton relaxation in these systems is currently poorly understood. Here, we investigate exciton relaxation in monolayer MoSe2 using polarization-resolved coherent nonlinear optical spectroscopy with high spectral resolution. We report strikingly narrow population pulsation resonances with two different characteristic linewidths of 1 and <0.2 μ eV at low temperature. These linewidths are more than 3 orders of magnitude narrower than the photoluminescence and absorption linewidth, and indicate that a component of the exciton relaxation dynamics occurs on time scales longer than 1 ns. The ultranarrow resonance (<0.2 μ eV ) emerges with increasing excitation intensity, and implies the existence of a long-lived state whose lifetime exceeds 6 ns.

  16. Population Pulsation Resonances of Excitons in Monolayer MoSe 2 with Sub- 1 μ eV Linewidths

    DOE PAGES

    Schaibley, John R.; Karin, Todd; Yu, Hongyi; Ross, Jason S.; Rivera, Pasqual; Jones, Aaron M.; Scott, Marie E.; Yan, Jiaqiang; Mandrus, D. G.; Yao, Wang; et al

    2015-04-01

    Monolayer transition metal dichalcogenides, a new class of atomically thin semiconductors, possess optically coupled 2D valley excitons. The nature of exciton relaxation in these systems is currently poorly understood. Here, we investigate exciton relaxation in monolayer MoSe₂ using polarization-resolved coherent nonlinear optical spectroscopy with high spectral resolution. We report strikingly narrow population pulsation resonances with two different characteristic linewidths of 1 and <0.2 μeV at low temperature. These linewidths are more than 3 orders of magnitude narrower than the photoluminescence and absorption linewidth, and indicate that a component of the exciton relaxation dynamics occurs on time scales longer than 1more » ns. The ultranarrow resonance (<0.2 μeV) emerges with increasing excitation intensity, and implies the existence of a long-lived state whose lifetime exceeds 6 ns.« less

  17. Tailoring double Fano profiles with plasmon-assisted quantum interference in hybrid exciton-plasmon system

    SciTech Connect

    Zhao, Dongxing; Wu, Jiarui; Gu, Ying Gong, Qihuang

    2014-09-15

    We propose tailoring of the double Fano profiles via plasmon-assisted quantum interference in a hybrid exciton-plasmon system. Tailoring is performed by the interference between two exciton channels interacting with a common localized surface plasmon. Using an applied field of low intensity, the absorption spectrum of the hybrid system reveals a double Fano lineshape with four peaks. For relatively large field intensity, a broad flat window in the absorption spectrum appears which results from the destructive interference between excitons. Because of strong constructive interference, this window vanishes as intensity is further increased. We have designed a nanometer bandpass optical filter for visible light based on tailoring of the optical spectrum. This study provides a platform for quantum interference that may have potential applications in ultracompact tunable quantum devices.

  18. DNA-mediated excitonic upconversion FRET switching

    SciTech Connect

    Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.

    2015-11-17

    Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.

  19. DNA-mediated excitonic upconversion FRET switching

    DOE PAGES

    Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.

    2015-11-17

    Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffersmore » from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.« less

  20. Exciton luminescence of 8-azasteroid microcrystals

    NASA Astrophysics Data System (ADS)

    Akhrem, A. A.; Borisevich, N. A.; Gulyakevich, O. V.; Knyukshto, V. N.; Mikhal'Chuk, A. L.; Tikhomirov, S. A.; Tolstorozhev, G. B.

    1999-05-01

    Luminescence of microcrystals of 2,3-methoxy-8-azagon-1,3,5(10),13-tetraene-12,17-dion of the class of molecules of biologically active steroids is detected at room temperature (293 K). It represents fast fluorescence of free and self-localized excitons and prolonged phosphorescence of triplet excitons.

  1. DNA-mediated excitonic upconversion FRET switching

    NASA Astrophysics Data System (ADS)

    Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.

    2015-11-01

    Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy upconversion via upconversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based upconversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an upconversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy upconversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy upconversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.

  2. Interaction-Driven Topological Insulator in Fermionic Cold Atoms on an Optical Lattice: A Design with a Density Functional Formalism.

    PubMed

    Kitamura, Sota; Tsuji, Naoto; Aoki, Hideo

    2015-07-24

    We design an interaction-driven topological insulator for fermionic cold atoms in an optical lattice; that is, we pose the question of whether we can realize in a continuous space a spontaneous symmetry breaking induced by the interatom interaction into a topological Chern insulator. Such a state, sometimes called a "topological Mott insulator," has yet to be realized in solid-state systems, since this requires, in the tight-binding model, large off-site interactions on top of a small on-site interaction. Here, we overcome the difficulty by introducing a spin-dependent potential, where a spin-selective occupation of fermions in A and B sublattices makes the on-site interaction Pauli forbidden, while a sizeable intersite interaction is achieved by a shallow optical potential with a large overlap between neighboring Wannier orbitals. This puts the system away from the tight-binding model, so that we adopt density functional theory for cold atoms, here extended to accommodate noncollinear spin structures emerging in the topological regime, to quantitatively demonstrate the phase transition to the topological Mott insulator.

  3. 20Hz membrane potential oscillations are driven by synaptic inputs in collision-detecting neurons in the frog optic tectum.

    PubMed

    Baranauskas, Gytis; Svirskiene, Natasa; Svirskis, Gytis

    2012-10-24

    Although the firing patterns of collision-detecting neurons have been described in detail in several species, the mechanisms generating responses in these neurons to visual objects on a collision course remain largely unknown. This is partly due to the limited number of intracellular recordings from such neurons, particularly in vertebrate species. By employing patch recordings in a novel integrated frog eye-tectum preparation we tested the hypothesis that OFF retinal ganglion cells were driving the responses to visual objects on a collision course in the frog optic tectum neurons. We found that the majority (22/26) of neurons in layer 6 responding to visual stimuli fitted the definition of η class collision-detectors: they readily responded to a looming stimulus imitating collision but not a receding stimulus (spike count difference ∼10 times) and the spike firing rate peaked after the stimulus visual angle reached a threshold value of ∼20-45°. In the majority of these neurons (15/22) a slow frequency oscillation (f=∼20Hz) of the neuronal membrane potential could be detected in the responses to a simulated collision stimulus, as well as to turning off the lights. Since OFF retinal ganglion cells could produce such oscillations, our observations are in agreement with the hypothesis that 'collision' responses in the frog optic tectum neurons are driven by synaptic inputs from OFF retinal ganglion cells.

  4. Temperature-driven band inversion in Pb0.77Sn0.23Se: Optical and Hall effect studies

    DOE PAGES

    Anand, Naween; Buvaev, Sanal; Hebard, A. F.; Tanner, D. B.; Chen, Zhiguo; Li, Zhiqiang; Choudhary, Kamal; Sinnott, S. B.; Gu, Genda; Martin, C.

    2014-12-23

    Optical and Hall-effect measurements have been performed on single crystals of Pb₀.₇₇Sn₀.₂₃Se, a IV-VI mixed chalcogenide. The temperature dependent (10–300 K) reflectance was measured over 40–7000 cm⁻¹ (5–870 meV) with an extension to 15,500 cm⁻¹ (1.92 eV) at room temperature. The reflectance was fit to the Drude-Lorentz model using a single Drude component and several Lorentz oscillators. The optical properties at the measured temperatures were estimated via Kramers-Kronig analysis as well as by the Drude-Lorentz fit. The carriers were p-type with the carrier density determined by Hall measurements. A signature of valence intraband transition is found in the low-energy opticalmore » spectra. It is found that the valence-conduction band transition energy as well as the free carrier effective mass reach minimum values at 100 K, suggesting temperature-driven band inversion in the material. Thus, density function theory calculation for the electronic band structure also make similar predictions.« less

  5. Broadband Absorbing Exciton-Plasmon Metafluids with Narrow Transparency Windows.

    PubMed

    Yang, Jihua; Kramer, Nicolaas J; Schramke, Katelyn S; Wheeler, Lance M; Besteiro, Lucas V; Hogan, Christopher J; Govorov, Alexander O; Kortshagen, Uwe R

    2016-02-10

    Optical metafluids that consist of colloidal solutions of plasmonic and/or excitonic nanomaterials may play important roles as functional working fluids or as means for producing solid metamaterial coatings. The concept of a metafluid employed here is based on the picture that a single ballistic photon, propagating through the metafluid, interacts with a large collection of specifically designed optically active nanocrystals. We demonstrate water-based metafluids that act as broadband electromagnetic absorbers in a spectral range of 200-3300 nm and feature a tunable narrow (∼100 nm) transparency window in the visible-to-near-infrared region. To define this transparency window, we employ plasmonic gold nanorods. We utilize excitonic boron-doped silicon nanocrystals as opaque optical absorbers ("optical wall") in the UV and blue-green range of the spectrum. Water itself acts as an opaque "wall" in the near-infrared to infrared. We explore the limits of the concept of a "simple" metafluid by computationally testing and validating the effective medium approach based on the Beer-Lambert law. According to our simulations and experiments, particle aggregation and the associated decay of the window effect are one example of the failure of the simple metafluid concept due to strong interparticle interactions. PMID:26808215

  6. Phase conjugation and adiabatic mode conversion in a driven optical parametric oscillator with orbital angular momentum

    SciTech Connect

    Coutinho dos Santos, B.; Souza, C. E. R.; Dechoum, K.; Khoury, A. Z.

    2007-11-15

    We developed a theoretical model for the spatial mode dynamics of an optical parametric oscillator under injection of orbital angular momentum. This process is interpreted in terms of a Poincare representation of first order spatial modes. The spatial properties of the down-converted fields can be easily understood from their symmetries in this geometric representation. By considering an adiabatic mode conversion of the injected signal, we calculate the evolution of the down-converted beams. A phase conjugation effect is predicted which is a consequence of the symmetry in the Poincare sphere. We also propose an experiment to measure this effect.

  7. Injected-charge-driven increase in electro-optic effect of KTN crystals

    SciTech Connect

    Toyoda, Seiji; Imai, Tadayuki; Miyazu, Jun; Okabe, Yuichi; Ueno, Masahiro; Kobayashi, Junya

    2014-05-15

    We report a significant increase in the electro-optic (EO) effect of KTa{sub x}Nb{sub 1-x}O{sub 3} (KTN) crystals that we achieved by injecting carriers into them. The dielectric constant of KTN was increased approximately twofold by carrier injection. The EO beam scanning performance was effectively improved by the increase in the EO effect resulting from the increased dielectric constant. The estimated densities of the trapped electrons were as small as 5.8 × 10{sup 20}m{sup -3}. The very small quantity of injected electrons greatly affected the dielectric constant and EO effect of the KTN crystals.

  8. Lightwave-driven quasiparticle collisions on a subcycle timescale.

    PubMed

    Langer, F; Hohenleutner, M; Schmid, C P; Poellmann, C; Nagler, P; Korn, T; Schüller, C; Sherwin, M S; Huttner, U; Steiner, J T; Koch, S W; Kira, M; Huber, R

    2016-05-12

    Ever since Ernest Rutherford scattered α-particles from gold foils, collision experiments have revealed insights into atoms, nuclei and elementary particles. In solids, many-body correlations lead to characteristic resonances--called quasiparticles--such as excitons, dropletons, polarons and Cooper pairs. The structure and dynamics of quasiparticles are important because they define macroscopic phenomena such as Mott insulating states, spontaneous spin- and charge-order, and high-temperature superconductivity. However, the extremely short lifetimes of these entities make practical implementations of a suitable collider challenging. Here we exploit lightwave-driven charge transport, the foundation of attosecond science, to explore ultrafast quasiparticle collisions directly in the time domain: a femtosecond optical pulse creates excitonic electron-hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes. The underlying dynamics of the wave packets, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands of the optical excitation. A full quantum theory explains our observations microscopically. This approach enables collision experiments with various complex quasiparticles and suggests a promising new way of generating sub-femtosecond pulses. PMID:27172045

  9. Lightwave-driven quasiparticle collisions on a subcycle timescale

    NASA Astrophysics Data System (ADS)

    Langer, F.; Hohenleutner, M.; Schmid, C. P.; Poellmann, C.; Nagler, P.; Korn, T.; Schüller, C.; Sherwin, M. S.; Huttner, U.; Steiner, J. T.; Koch, S. W.; Kira, M.; Huber, R.

    2016-05-01

    Ever since Ernest Rutherford scattered α-particles from gold foils, collision experiments have revealed insights into atoms, nuclei and elementary particles. In solids, many-body correlations lead to characteristic resonances—called quasiparticles—such as excitons, dropletons, polarons and Cooper pairs. The structure and dynamics of quasiparticles are important because they define macroscopic phenomena such as Mott insulating states, spontaneous spin- and charge-order, and high-temperature superconductivity. However, the extremely short lifetimes of these entities make practical implementations of a suitable collider challenging. Here we exploit lightwave-driven charge transport, the foundation of attosecond science, to explore ultrafast quasiparticle collisions directly in the time domain: a femtosecond optical pulse creates excitonic electron-hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes. The underlying dynamics of the wave packets, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands of the optical excitation. A full quantum theory explains our observations microscopically. This approach enables collision experiments with various complex quasiparticles and suggests a promising new way of generating sub-femtosecond pulses.

  10. Light driven optofluidic switch developed in a ZnO-overlaid microstructured optical fiber.

    PubMed

    Konidakis, Ioannis; Konstantaki, Maria; Tsibidis, George D; Pissadakis, Stavros

    2015-11-30

    A great challenge of Optofluidics remains the control of the fluidic properties of a photonic circuit by solely utilizing light. In this study, the development of a ZnO nanolayered microstructured optical fiber (MOF) Fabry-Perot interferometer is demonstrated, along with its fully reversible optofluidic switching behaviour. The actuation and switching principle is entirely based on the employment of light sources, i.e. UV 248 nm and green 532 nm lasers, while using modest irradiation doses. The synthesized ZnO within the MOF capillaries acts as a light triggered wettability transducer, allowing the controlled water filling and draining of the MOF Fabry-Perot cavity. The progression of the optofluidic cycle is monitored in situ with optical microscopy, while Fabry-Perot reflection spectra are monitored in real time to probe temporal infiltration behaviour. Finally, a first insight on the light triggered switching mechanism, employing photoluminescence and spectrophotometric measurements is presented. Results appear highly promising towards the design of smart in-fiber optofluidic light switching devices, suitable for actuating and sensing applications. PMID:26698774

  11. Ultrafast Spectral Diffusion of the First Subband Exciton in Single-Wall Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Schilling, Daniel; Hertel, Tobias

    2013-03-01

    The width of optical transitions in semiconductors is determined by homogeneous and inhomogeneous contributions. Here, we report on the determination of homogeneous linewidths for the first exciton subband transition and the dynamics of spectral diffusion in single-wall carbon nanotubes (SWNTs) using one- and two-dimensional time resolved spectral hole burning spectroscopy. Our investigation of highly purified semiconducting (6,5)-SWNTs suggests that room temperature homogeneous linewidths are on the order of 4 meV and are rapidly broadened by an ultrafast sub-ps spectral diffusion process. These findings are supported by our off-resonant excitation experiments where we observe sub-ps population transfer reflecting the thermal distribution of energy levels around the first subband exciton transition. The results of temperature-dependent spectral hole burning experiments between 17 K and 293 K suggest that homogeneous linewidths are due to exciton interaction with low energy optical phonons, most likely of the radial breathing mode type. In contrast, we find that inhomogeneous broadening is determined by an electronic degree of freedom such as ultrafast intra-tube exciton diffusion which is characteristic and unique for excitons in these one-dimensional semiconductors.

  12. Emission energy, exciton dynamics and lasing properties of buckled CdS nanoribbons

    PubMed Central

    Wang, Qi; Sun, Liaoxin; Lu, Jian; Ren, Ming-Liang; Zhang, Tianning; Huang, Yan; Zhou, Xiaohao; Sun, Yan; Zhang, Bo; Chen, Changqing; Shen, Xuechu; Agarwal, Ritesh; Lu, Wei

    2016-01-01

    We report the modulation of emission energy, exciton dynamics and lasing properties in a single buckled CdS nanoribbon (NR) by strain-engineering. Inspired by ordered structure fabrication on elastomeric polymer, we develop a new method to fabricate uniform buckled NRs supported on polydimethylsiloxane (PDMS). Wavy structure, of which compressive and tensile strain periodically varied along the CdS NR, leads to a position-dependent emission energy shift as large as 14 nm in photoluminescence (PL) mapping. Both micro-PL and micro-reflectance reveal the spectral characteristics of broad emission of buckled NR, which can be understood by the discrepancy of strain-induced energy shift of A- and B-exciton of CdS. Furthermore, the dynamics of excitons under tensile strain are also investigated; we find that the B-exciton have much shorter lifetime than that of redshifted A-exciton. In addition, we also present the lasing of buckled CdS NRs, in which the strain-dominated mode selection in multi-mode laser and negligible mode shifts in single-mode laser are clearly observed. Our results show that the strained NRs may serve as new functional optical elements for flexible light emitter or on-chip all-optical devices. PMID:27210303

  13. Emission energy, exciton dynamics and lasing properties of buckled CdS nanoribbons

    NASA Astrophysics Data System (ADS)

    Wang, Qi; Sun, Liaoxin; Lu, Jian; Ren, Ming-Liang; Zhang, Tianning; Huang, Yan; Zhou, Xiaohao; Sun, Yan; Zhang, Bo; Chen, Changqing; Shen, Xuechu; Agarwal, Ritesh; Lu, Wei

    2016-05-01

    We report the modulation of emission energy, exciton dynamics and lasing properties in a single buckled CdS nanoribbon (NR) by strain-engineering. Inspired by ordered structure fabrication on elastomeric polymer, we develop a new method to fabricate uniform buckled NRs supported on polydimethylsiloxane (PDMS). Wavy structure, of which compressive and tensile strain periodically varied along the CdS NR, leads to a position-dependent emission energy shift as large as 14 nm in photoluminescence (PL) mapping. Both micro-PL and micro-reflectance reveal the spectral characteristics of broad emission of buckled NR, which can be understood by the discrepancy of strain-induced energy shift of A- and B-exciton of CdS. Furthermore, the dynamics of excitons under tensile strain are also investigated; we find that the B-exciton have much shorter lifetime than that of redshifted A-exciton. In addition, we also present the lasing of buckled CdS NRs, in which the strain-dominated mode selection in multi-mode laser and negligible mode shifts in single-mode laser are clearly observed. Our results show that the strained NRs may serve as new functional optical elements for flexible light emitter or on-chip all-optical devices.

  14. Emission energy, exciton dynamics and lasing properties of buckled CdS nanoribbons.

    PubMed

    Wang, Qi; Sun, Liaoxin; Lu, Jian; Ren, Ming-Liang; Zhang, Tianning; Huang, Yan; Zhou, Xiaohao; Sun, Yan; Zhang, Bo; Chen, Changqing; Shen, Xuechu; Agarwal, Ritesh; Lu, Wei

    2016-01-01

    We report the modulation of emission energy, exciton dynamics and lasing properties in a single buckled CdS nanoribbon (NR) by strain-engineering. Inspired by ordered structure fabrication on elastomeric polymer, we develop a new method to fabricate uniform buckled NRs supported on polydimethylsiloxane (PDMS). Wavy structure, of which compressive and tensile strain periodically varied along the CdS NR, leads to a position-dependent emission energy shift as large as 14 nm in photoluminescence (PL) mapping. Both micro-PL and micro-reflectance reveal the spectral characteristics of broad emission of buckled NR, which can be understood by the discrepancy of strain-induced energy shift of A- and B-exciton of CdS. Furthermore, the dynamics of excitons under tensile strain are also investigated; we find that the B-exciton have much shorter lifetime than that of redshifted A-exciton. In addition, we also present the lasing of buckled CdS NRs, in which the strain-dominated mode selection in multi-mode laser and negligible mode shifts in single-mode laser are clearly observed. Our results show that the strained NRs may serve as new functional optical elements for flexible light emitter or on-chip all-optical devices. PMID:27210303

  15. Single-pulse driven, large-aperture 2×1 array plasma-electrodes optical switch for SG-II upgrading facility

    NASA Astrophysics Data System (ADS)

    Zhang, Jun; Wu, Dengsheng; Zheng, Jiangang; Zheng, Kuixing; Zhu, Qihua; Zhang, Xiongjun

    2014-12-01

    We demonstrate the design and performance of an optical switch that has been constructed for the SG-II upgrading facility. The device is a longitudinal, potassium di-hydrogen phosphate (KDP), 360 mm×360 mm aperture, and 2×1 array electro-optical switch driven by a 20 kV output switching-voltage pulse generator through two plasma electrodes produced at the rise edge of the switching-voltage pulse. The results show that the temporal responses and the spatial performance of the optical switch fulfill the operation requirements of the SG-II upgrading facility.

  16. Dynamical Critical Phenomena in Driven-Dissipative Systems

    NASA Astrophysics Data System (ADS)

    Sieberer, L. M.; Huber, S. D.; Altman, E.; Diehl, S.

    2013-05-01

    We explore the nature of the Bose condensation transition in driven open quantum systems, such as exciton-polariton condensates. Using a functional renormalization group approach formulated in the Keldysh framework, we characterize the dynamical critical behavior that governs decoherence and an effective thermalization of the low frequency dynamics. We identify a critical exponent special to the driven system, showing that it defines a new dynamical universality class. Hence critical points in driven systems lie beyond the standard classification of equilibrium dynamical phase transitions. We show how the new critical exponent can be probed in experiments with driven cold atomic systems and exciton-polariton condensates.

  17. Tiny endoscopic optical coherence tomography probe driven by a miniaturized hollow ultrasonic motor

    NASA Astrophysics Data System (ADS)

    Chen, Tianyuan; Zhang, Ning; Huo, Tiancheng; Wang, Chengming; Zheng, Jing-gao; Zhou, Tieying; Xue, Ping

    2013-08-01

    We present an endoscopic probe for optical coherence tomography (OCT) equipped with a miniaturized hollow ultrasonic motor that rotates the objective lens and provides an internal channel for the fiber to pass through, enabling 360 deg unobstructed circumferential scanning. This probe has an outer diameter of 1.5 mm, which is ultra-small for motorized probes with an unobstructed view in distal scanning endoscopic OCT. Instead of a mirror or prism, a customized aspheric right-angle lens is utilized, leading to an enlargement of the numerical aperture and thus high transverse resolution. Spectral-domain OCT imaging of bio-tissue and a phantom are demonstrated with resolution of 7.5 μm(axial)×6.6 μm(lateral) and sensitivity of 96 dB.

  18. Electro-optical properties of low viscosity driven holographic polymer dispersed liquid crystals

    NASA Astrophysics Data System (ADS)

    Moon, K. R.; Bae, S. Y.; Kim, B. K.

    2015-04-01

    Relative diffraction efficiency (RDE), operating voltage, and response times are most important performance characteristics of holographic polymer dispersed liquid crystals (HPDLC). Two types of triallyl isocyanurate (TI) having different structures were incorporated into the conventional transmission grating of HPDLC. Premix viscosity decreased by 13-18% with up to 3% TI, beyond which it increased. TI eliminated induction period and augmented initial grating formation rate at all contents. Saturation RDE increased over 200% while threshold voltage and rise time decreased to about half and 2/3, respectively up to 3% TI, beyond which the tendencies were reversed. Among the two TIs, low viscosity monomer (TA) showed high RDE, while high miscibility monomer (TE) low characteristic voltages and short response times. It is concluded that grating formation is largely favored by low viscosity, while interface tensions and electro-optical performances by miscibility at similar viscosities.

  19. Atomistic model for excitons: Capturing Strongly Bound Excitons in Monolayer Transition-Metal Dichalcogenides

    NASA Astrophysics Data System (ADS)

    Tseng, Frank; Simsek, Ergun; Gunlycke, Daniel

    2015-03-01

    Monolayer transition-metal dichalcogenides form a direct bandgap predicted in the visible regime making them attractive host materials for various electronic and optoelectronic applications. Due to a weak dielectric screening in these materials, strongly bound electron-hole pairs or excitons have binding energies up to at least several hundred meV's. While the conventional wisdom is to think of excitons as hydrogen-like quasi-particles, we show that the hydrogen model breaks down for these experimentally observed strongly bound, room-temperature excitons. To capture these non-hydrogen-like photo-excitations, we introduce an atomistic model for excitons that predicts both bright excitons and dark excitons, and their broken degeneracy in these two-dimensional materials. For strongly bound exciton states, the lattice potential significantly distorts the envelope wave functions, which affects predicted exciton peak energies. The combination of large binding energies and non-degeneracy of exciton states in monolayer transition metal dichalogendies may furthermore be exploited in room temperature applications where prolonged exciton lifetimes are necessary. This work has been funded by the Office of Naval Research (ONR), directly and through the Naval Research Laboratory (NRL). F.T and E.S acknowledge support from NRL through the NRC Research Associateship Program and ONR Summer Faculty Program, respectively.

  20. Line-driven ablation of circumstellar discs - I. Optically thin decretion discs of classical Oe/Be stars

    NASA Astrophysics Data System (ADS)

    Kee, Nathaniel Dylan; Owocki, Stanley; Sundqvist, J. O.

    2016-05-01

    discussion of future extensions to study line-driven ablation of denser, optically thick, accretion discs of pre-main-sequence massive stars.

  1. Line-driven ablation of circumstellar discs – I. Optically thin decretion discs of classical Oe/Be stars

    PubMed Central

    Kee, Nathaniel Dylan; Owocki, Stanley; Sundqvist, J. O.

    2016-01-01

    discussion of future extensions to study line-driven ablation of denser, optically thick, accretion discs of pre-main-sequence massive stars. PMID:27346978

  2. Oblique half-solitons and their generation in exciton-polariton condensates

    SciTech Connect

    Flayac, H.; Solnyshkov, D. D.; Malpuech, G.

    2011-05-15

    We describe oblique half-solitons, a new type of topological defects in a two-dimensional spinor Bose-Einstein condensate. A realistic protocol based on the optical spin Hall effect is proposed toward their generation within an exciton-polariton system.

  3. Tunable Magnetic Alignment between Trapped Exciton-Polariton Condensates

    NASA Astrophysics Data System (ADS)

    Ohadi, H.; del Valle-Inclan Redondo, Y.; Dreismann, A.; Rubo, Y. G.; Pinsker, F.; Tsintzos, S. I.; Hatzopoulos, Z.; Savvidis, P. G.; Baumberg, J. J.

    2016-03-01

    Tunable spin correlations are found to arise between two neighboring trapped exciton-polariton condensates which spin polarize spontaneously. We observe a crossover from an antiferromagnetic to a ferromagnetic pair state by reducing the coupling barrier in real time using control of the imprinted pattern of pump light. Fast optical switching of both condensates is then achieved by resonantly but weakly triggering only a single condensate. These effects can be explained as the competition between spin bifurcations and spin-preserving Josephson coupling between the two condensates, and open the way to polariton Bose-Hubbard ladders.

  4. Correlated fluctuations in the exciton dynamics and spectroscopy of DNA

    NASA Astrophysics Data System (ADS)

    Dijkstra, Arend G.; Tanimura, Yoshitaka

    2010-05-01

    The absorption of ultraviolet light creates excitations in DNA, which subsequently start moving in the helix. Their fate is important for an understanding of photodamage, and is determined by the interplay of electronic couplings between bases and the structure of the DNA environment. We model the effect of dynamical fluctuations in the environment and study correlation, which is present when multiple base pairs interact with the same mode in the environment. We find that the correlations strongly affect the exciton dynamics, and show how they are observed in the decay of the anisotropy as a function of coherence and population time in a nonlinear optical experiment.

  5. Tunable Magnetic Alignment between Trapped Exciton-Polariton Condensates.

    PubMed

    Ohadi, H; Del Valle-Inclan Redondo, Y; Dreismann, A; Rubo, Y G; Pinsker, F; Tsintzos, S I; Hatzopoulos, Z; Savvidis, P G; Baumberg, J J

    2016-03-11

    Tunable spin correlations are found to arise between two neighboring trapped exciton-polariton condensates which spin polarize spontaneously. We observe a crossover from an antiferromagnetic to a ferromagnetic pair state by reducing the coupling barrier in real time using control of the imprinted pattern of pump light. Fast optical switching of both condensates is then achieved by resonantly but weakly triggering only a single condensate. These effects can be explained as the competition between spin bifurcations and spin-preserving Josephson coupling between the two condensates, and open the way to polariton Bose-Hubbard ladders.

  6. Exciton recombination dynamics in single ZnO tetrapods

    SciTech Connect

    Fernandes-Silva, Lígia C.; Martín, Maria D.; Meulen, Herko P. van der; Calleja, José M.; Viña, Luis; Klopotowski, Lukasz

    2013-12-04

    We present the optical properties of individual ZnO tetrapods as a function of excitation power and temperature by time-integrated and time-resolved spectroscopy. At 10K, we identify the different excitonic transitions by both their characteristic energy and their excitation power dependence. When we increase the tetrapod temperature we observe that the emission intensity decrease and occur a red shift of the emission energies. Our time-resolved studies confirm the predominance of the radiative recombination at low temperatures (< 45 K). Increasing the temperature opens up the non-radiative channels, which are evidenced by a much faster decay time.

  7. Survival Probabilities in Coherent Exciton Transfer with Trapping

    SciTech Connect

    Muelken, Oliver; Blumen, Alexander; Amthor, Thomas; Giese, Christian; Reetz-Lamour, Markus; Weidemueller, Matthias

    2007-08-31

    In the quest for signatures of coherent transport we consider exciton trapping in the continuous-time quantum walk framework. The survival probability displays different decay domains, related to distinct regions of the spectrum of the Hamiltonian. For linear systems and at intermediate times the decay obeys a power law, in contrast with the corresponding exponential decay found in incoherent continuous-time random walk situations. To differentiate between the coherent and incoherent mechanisms, we present an experimental protocol based on a frozen Rydberg gas structured by optical dipole traps.

  8. Chirality inversion in the bilirubin molecular exciton.

    PubMed

    Boiadjiev, S E; Lightner, D A

    2001-05-15

    The bichromophoric pigment bilirubin acts as a molecular exciton in its UV-visible and circular dichroism (CD) spectroscopy. In both polar and nonpolar solvents, an optically active analog, (beta R,beta 'R)-dimethylmesobilirubin-XIII alpha (1), exhibits intense bisignate CD Cotton effects in the region of its long wavelength UV-vis absorption near 400 nm: Delta epsilon(434)(max) + 337, Delta epsilon(389)(max) - 186 (CHCl(3)), and Delta epsilon(431)(max) + 285, Delta epsilon(386)(max) - 177 (CH(3)OH). However, introduction of an amine into a CHCl(3) solution of 1 causes the Cotton effect signs to become inverted, e.g., after addition of NH(3), Delta epsilon(433)(max) - 345, Delta epsilon(389)(max) + 243, and after addition of ethylene diamine, Delta epsilon(435)(max) - 420, Delta epsilon(390)(max) + 299. The sign inversions imply inversion of molecular chirality of the bilirubin and the phenomenon appears to be general for amines, including alpha,omega-diamines. 1,8-Diaminooctane was found to be more effective than longer or shorter chain analogs in producing CD sign inversion.

  9. Excitons and Davydov splitting in sexithiophene from first-principles many-body Green’s function theory

    SciTech Connect

    Leng, Xia; Yin, Huabing; Liang, Dongmei; Ma, Yuchen

    2015-09-21

    Organic semiconductors have promising and broad applications in optoelectronics. Understanding their electronic excited states is important to help us control their spectroscopic properties and performance of devices. There have been a large amount of experimental investigations on spectroscopies of organic semiconductors, but theoretical calculation from first principles on this respect is still limited. Here, we use density functional theory (DFT) and many-body Green’s function theory, which includes the GW method and Bethe-Salpeter equation, to study the electronic excited-state properties and spectroscopies of one prototypical organic semiconductor, sexithiophene. The exciton energies of sexithiophene in both the gas and bulk crystalline phases are very sensitive to the exchange-correlation functionals used in DFT for ground-state structure relaxation. We investigated the influence of dynamical screening in the electron-hole interaction on exciton energies, which is found to be very pronounced for triplet excitons and has to be taken into account in first principles calculations. In the sexithiophene single crystal, the energy of the lowest triplet exciton is close to half the energy of the lowest singlet one. While lower-energy singlet and triplet excitons are intramolecular Frenkel excitons, higher-energy excitons are of intermolecular charge-transfer type. The calculated optical absorption spectra and Davydov splitting are in good agreement with experiments.

  10. Effect of heteroboundary spreading on the properties of exciton states in Zn(Cd)Se/ZnMgSSe quantum wells

    SciTech Connect

    Adiyatullin, A. F. Belykh, V. V.; Kozlovsky, V. I.; Krivobok, V. S. Martovitsky, V. P.; Nikolaev, S. N.

    2012-11-15

    Exciton states in Zn(Cd)Se/ZnMgSSe quantum wells with different diffusion spreading of interfaces are studied by optical spectroscopy methods. It is shown that the emission spectrum of quantum wells at low temperatures is determined by free excitons and bound excitons on neutral donors. The nonlinear dependence of the stationary photoluminescence intensity on the excitation power density and the biexponential luminescence decay are explained by the neutralization of charged defects upon photoexcitation of heterostructures. With the stationary illumination on, durable (about 40 min) reversible changes in the reflection coefficient near the exciton resonances of quantum wells are observed. It is shown that, along with the shift of exciton levels, the spreading of heteroboundaries leads to three effects: an increase in the excitonphonon interaction, an increase in the energy shift between the emission lines of free and bound excitons, and a decrease in the decay time of exciton luminescence in a broad temperature range. The main reasons for these effects are discussed.

  11. Optical signatures of electric-field-driven magnetic phase transitions in graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Basak, Tista; Shukla, Alok

    2016-06-01

    Experimental challenges in identifying various types of magnetic ordering in graphene quantum dots (QDs) pose a major hurdle in the application of these nanostructures for spintronic devices. Based upon phase diagrams obtained by employing the π -electron Pariser-Parr-Pople (PPP) model Hamiltonian, we demonstrate that the magnetic states undergo phase transition under the influence of an external electric field. Our calculations of the electroabsorption spectra of these QDs indicate that the spectrum in question carries strong signatures of their magnetic state (FM vs AFM), thus suggesting the possibility of an all-optical characterization of their magnetic nature. Further, the gaps for the up and the down spins are the same in the absence of an external electric field, both for the antiferromagnetic (AFM) and the ferromagnetic (FM) states of QDs. But, once the QDs are exposed to a suitably directed external electric field, gaps for different spins split and exhibit distinct variations with respect to the strength of the field. The nature of variation exhibited by the energy gaps corresponding to the up and down spins is different for the AFM and FM configurations of QDs. This selective manipulation of the spin-polarized gap splitting by an electric field in finite graphene nanostructures can open up new frontiers in the design of graphene-based spintronic devices.

  12. A self-stabilized coherent phonon source driven by optical forces

    PubMed Central

    Navarro-Urrios, D.; Capuj, N. E.; Gomis-Bresco, J.; Alzina, F.; Pitanti, A.; Griol, A.; Martínez, A.; Sotomayor Torres, C. M.

    2015-01-01

    We report a novel injection scheme that allows for “phonon lasing” in a one-dimensional opto-mechanical photonic crystal, in a sideband unresolved regime and with cooperativity values as low as 10−2. It extracts energy from a cw infrared laser source and is based on the triggering of a thermo-optical/free-carrier-dispersion self-pulsing limit-cycle, which anharmonically modulates the radiation pressure force. The large amplitude of the coherent mechanical motion acts as a feedback that stabilizes and entrains the self-pulsing oscillations to simple fractions of the mechanical frequency. A manifold of frequency-entrained regions with two different mechanical modes (at 54 and 122 MHz) are observed as a result of the wide tuneability of the natural frequency of the self-pulsing. The system operates at ambient conditions of pressure and temperature in a silicon platform, which enables its exploitation in sensing, intra-chip metrology or time-keeping applications. PMID:26503448

  13. A self-stabilized coherent phonon source driven by optical forces

    NASA Astrophysics Data System (ADS)

    Navarro-Urrios, D.; Capuj, N. E.; Gomis-Bresco, J.; Alzina, F.; Pitanti, A.; Griol, A.; Martínez, A.; Sotomayor Torres, C. M.

    2015-10-01

    We report a novel injection scheme that allows for “phonon lasing” in a one-dimensional opto-mechanical photonic crystal, in a sideband unresolved regime and with cooperativity values as low as 10-2. It extracts energy from a cw infrared laser source and is based on the triggering of a thermo-optical/free-carrier-dispersion self-pulsing limit-cycle, which anharmonically modulates the radiation pressure force. The large amplitude of the coherent mechanical motion acts as a feedback that stabilizes and entrains the self-pulsing oscillations to simple fractions of the mechanical frequency. A manifold of frequency-entrained regions with two different mechanical modes (at 54 and 122 MHz) are observed as a result of the wide tuneability of the natural frequency of the self-pulsing. The system operates at ambient conditions of pressure and temperature in a silicon platform, which enables its exploitation in sensing, intra-chip metrology or time-keeping applications.

  14. Electrical and Optical Measurements in an RF-Driven Micro-Discharge Source

    NASA Astrophysics Data System (ADS)

    Mahony, C. M. O.; Gans, T.; Graham, W. G.; Maguire, P. D.; Petrovic, Z. Lj.

    2007-10-01

    Microdischarge properties are distinctly different to those of larger sources, leading to potential applications such as: high density tailored plasmas, local heating, fast material processing and scale up to large area sources. Hollow cathode operation is unlikely in micro-hollow cathode devices of diameter <= 100 μm^[1] because short mean free paths inhibit pendular electron motion. Thus diameters as small as 10 μm may be required for HC operation, a critical stability challenge. We report radio frequency operation in micro-hollow cathode structures for diameters as small as 25μm. The sources are operated in argon and helium at pressures of 20 to 600 Torr and ignite readily at ˜20 W, operating stably at powers <10W. Measurements of breakdown characteristics, rf current and voltage and optical emission were recorded. A number of operating modes have been observed in these sub-100μm dimensions and OES of argon and helium discharges indicates there is less sputtering with helium. Positive dc bias has been observed in the cathode potential under rf operation, similar to that reported by Guo & Hong^[2] at a diameter of 300 μm. [1] Kushner, J. Phys. D: Appl. Phys. 38 (2005) 1633 [2]Guo & Hong, Jpn. J. Appl. Phys. 42 (2003) 6598

  15. Thermal radiation from optically driven Kerr (χ{sup (3)}) photonic cavities

    SciTech Connect

    Khandekar, Chinmay; Rodriguez, Alejandro W.; Lin, Zin

    2015-04-13

    We describe thermal radiation from nonlinear (χ{sup (3)}) photonic cavities coupled to external channels and subject to incident monochromatic light. Our work extends related work on nonlinear mechanical oscillators to the problem of thermal radiation, demonstrating that bistability can enhance thermal radiation by orders of magnitude and result in strong lineshape alternations, including “super-narrow spectral peaks” occurring at the onset of kinetic phase transitions. We show that when the cavities are designed to exhibit perfect linear emissivity (rate matching), such thermally activated transitions can be exploited to dramatically tune the output power and radiative properties of the cavity, leading to a kind of Kerr-mediated thermo-optic effect. Finally, we demonstrate that in certain parameter regimes, the output radiation exhibits Stokes and anti-Stokes side peaks whose relative magnitudes can be altered by tuning the internal temperature of the cavity relative to its surroundings, a consequence of strong correlations and interference between the emitted and reflected radiation.

  16. Orbital diamagnetic susceptibility in excitonic condensation phase

    NASA Astrophysics Data System (ADS)

    Sugimoto, Koudai; Ohta, Yukinori

    2016-08-01

    We study the orbital diamagnetic susceptibility in excitonic condensation phase using the mean-field approximation for a two-band model defined on a square lattice. We find that, in semiconductors, the excitonic condensation acquires a finite diamagnetic susceptibility due to spontaneous hybridization between the valence and the conduction bands, whereas in semimetals, the diamagnetic susceptibility in the normal phase is suppressed by the excitonic condensation. We also study the orbital diamagnetic and Pauli paramagnetic susceptibilities of Ta2NiSe5 using a two-dimensional three-band model and find that the calculated temperature dependence of the magnetic susceptibility is in qualitative agreement with experiment.

  17. Optically pumped ultraviolet and infrared lasers driven by exploding metal films and wires

    SciTech Connect

    Jones, C.R.; Ware, K.D.

    1983-01-01

    The 342-nm molecular iodine and 1315-nm atomic iodine lasers have been optically pumped by intense light from exploding-metal-film and exploding-wire discharges. Brightness temperatures for the exploding-film discharges were approx. 25,000 K and for the wire discharges were approx. 30,000 K. For the I/sub 2/ laser the 3.5-cm-diameter by 40-cm-long pumped volume lies adjacent to the wire or film of the same length. Pressures of 1 to 6 torr I/sub 2/ and 1 to 3 atm SF, CF/sub 4/, or Ar were used in the stainless-steel cell. Using 20-..mu..F capacitance charged to 40 kV, a 0.25-mm tungsten wire, 3-torr I/sub 2/, and a 2-atm SF/sub 6/, an energy of 2 J was obtained from the laser in a pulse of 8-..mu..s duration. The specific output energy was 7 J/l. Substitution of a cylindrical Al film for the wire, under otherwise similar conditions, led to a X10 output energies and efficiencies were obtained with similar input energy. An output pulse of 12 J and 12-..mu..s duration was measured for a specific output energy of 18 J/l. A laser energy of 110 J in a 20-us-long pulse has been measured from atomic iodine using a wire discharge along the axis of a larger cell. The active volume available was 20 cm in diameter and 80 cm in length. Input energy was 32 kJ. In similar measurements using a cylindrical Al film for discharge initiation, the measured output energy was 40 J.

  18. Ultracold fermions in a one-dimensional bipartite optical lattice: Metal-insulator transitions driven by shaking

    NASA Astrophysics Data System (ADS)

    Di Liberto, M.; Malpetti, D.; Japaridze, G. I.; Morais Smith, C.

    2014-08-01

    We theoretically investigate the behavior of a system of fermionic atoms loaded in a bipartite one-dimensional optical lattice that is under the action of an external time-periodic driving force. By using Floquet theory, an effective model is derived. The bare hopping coefficients are renormalized by zeroth-order Bessel functions of the first kind with different arguments for the nearest-neighbor and next-nearest-neighbor hopping. The insulating behavior characterizing the system at half filling in the absence of driving is dynamically suppressed, and for particular values of the driving parameter the system becomes either a standard metal or an unconventional metal with four Fermi points. The existence of the four-Fermi-point metal relies on the fact that, as a consequence of the shaking procedure, the next-nearest-neighbor hopping coefficients become significant compared to the nearest-neighbor ones. We use the bosonization technique to investigate the effect of on-site Hubbard interactions on the four-Fermi-point metal-insulator phase transition. Attractive interactions are expected to enlarge the regime of parameters where the unconventional metallic phase arises, whereas repulsive interactions reduce it. This metallic phase is known to be a Luther-Emery liquid (spin-gapped metal) for both repulsive and attractive interactions, contrary to the usual Hubbard model, which exhibits a Mott-insulator phase for repulsive interactions. Ultracold fermions in driven one-dimensional bipartite optical lattices provide an interesting platform for the realization of this long-studied four-Fermi-point unconventional metal.

  19. Auger recombination of dark excitons in WS2 and WSe2 monolayers

    NASA Astrophysics Data System (ADS)

    Danovich, Mark; Zólyomi, Viktor; Fal’ko, Vladimir I.; Aleiner, Igor L.

    2016-09-01

    We propose a novel phonon assisted Auger process unique to the electronic band structure of monolayer transition metal dichalcogenides (TMDCs), which dominates the radiative recombination of ground state excitons in tungsten based TMDCs. Using experimental and density functional theory computed values for the exciton energies, spin–orbit splittings, optical matrix element, and the Auger matrix elements, we find that the Auger process begins to dominate at carrier densities as low as {10}9-10 {{cm}}-2, thus providing a plausible explanation for the low quantum efficiencies reported for these materials.

  20. Excitons in solids with time-dependent density-functional theory: the bootstrap kernel and beyond

    NASA Astrophysics Data System (ADS)

    Byun, Young-Moo; Yang, Zeng-Hui; Ullrich, Carsten

    Time-dependent density-functional theory (TDDFT) is an efficient method to describe the optical properties of solids. Lately, a series of bootstrap-type exchange-correlation (xc) kernels have been reported to produce accurate excitons in solids, but different bootstrap-type kernels exist in the literature, with mixed results. In this presentation, we reveal the origin of the confusion and show a new empirical TDDFT xc kernel to compute excitonic properties of semiconductors and insulators efficiently and accurately. Our method can be used for high-throughput screening calculations and large unit cell calculations. Work supported by NSF Grant DMR-1408904.

  1. Type II InAs/GaAsSb quantum dots: Highly tunable exciton geometry and topology

    SciTech Connect

    Llorens, J. M.; Wewior, L.; Cardozo de Oliveira, E. R.; Alén, B.; Ulloa, J. M.; Utrilla, A. D.; Guzmán, A.; Hierro, A.

    2015-11-02

    External control over the electron and hole wavefunctions geometry and topology is investigated in a p-i-n diode embedding a dot-in-a-well InAs/GaAsSb quantum structure with type II band alignment. We find highly tunable exciton dipole moments and largely decoupled exciton recombination and ionization dynamics. We also predicted a bias regime where the hole wavefunction topology changes continuously from quantum dot-like to quantum ring-like as a function of the external bias. All these properties have great potential in advanced electro-optical applications and in the investigation of fundamental spin-orbit phenomena.

  2. Plasmonic-exciton coupling in synthesized metal/semiconductor hybrid nanocomposites

    SciTech Connect

    Gadalla, A.; Hamad, D. A.; Mohamed, M. B.

    2015-12-31

    A new method has been developed to grow plasmonic semiconductor nanocomposites of Au/CdSe and Ag/CdSe. Their chemical composition and crystal structure are determined by X-ray diffraction. The collective optical properties of the prepared semiconductor nanohybrid have been measured using spectrophotometer techniques and compared to those of the individual components. The electron transfer processes from CdSe to the gold are faster than that of the silver. Au/CdSe has a strong plasmonic-excitonic coupling, but Ag/CdSe has a weak plasmonic-excitonic coupling.

  3. Auger recombination of dark excitons in WS2 and WSe2 monolayers

    NASA Astrophysics Data System (ADS)

    Danovich, Mark; Zólyomi, Viktor; Fal'ko, Vladimir I.; Aleiner, Igor L.

    2016-09-01

    We propose a novel phonon assisted Auger process unique to the electronic band structure of monolayer transition metal dichalcogenides (TMDCs), which dominates the radiative recombination of ground state excitons in tungsten based TMDCs. Using experimental and density functional theory computed values for the exciton energies, spin-orbit splittings, optical matrix element, and the Auger matrix elements, we find that the Auger process begins to dominate at carrier densities as low as {10}9-10 {{cm}}-2, thus providing a plausible explanation for the low quantum efficiencies reported for these materials.

  4. Center-of-mass and internal motion of excitons in quantum wires

    NASA Astrophysics Data System (ADS)

    Glutsch, S.; Bechstedt, F.

    The properties of excitons, which are optically excited in single or coupled quantum wires, are studied within the effective-mass approximation. The two-particle wave functions and energies obey a Schrödinger equation with screened Coulomb interaction of electron and hole and their corresponding wire confinement potentials. This equation is approximately separated into an equation for the center-of-mass motion and another one more or less for the internal motion of the electron hole pairs. This allows a representation of absorption and luminescence spectra near a quantum-well exciton transition by a generalized Elliott formula.

  5. Multiple Exciton Generation Solar Cells

    SciTech Connect

    Luther, J. M.; Semonin, O. E.; Beard, M. C.; Gao, J.; Nozik, A. J.

    2012-01-01

    Heat loss is the major factor limiting traditional single junction solar cells to a theoretical efficiency of 32%. Multiple Exciton Generation (MEG) enables efficient use of the solar spectrum yielding a theoretical power conversion efficiency of 44% in solar cells under 1-sun conditions. Quantum-confined semiconductors have demonstrated the ability to generate multiple carriers but present-day materials deliver efficiencies far below the SQ limit of 32%. Semiconductor quantum dots of PbSe and PbS provide an active testbed for developing high-efficiency, inexpensive solar cells benefitting from quantum confinement effects. Here, we will present recent work of solar cells employing MEG to yield external quantum efficiencies exceeding 100%.

  6. A Survey on Next-Generation Mixed Line Rate (MLR) and Energy-Driven Wavelength-Division Multiplexed (WDM) Optical Networks

    NASA Astrophysics Data System (ADS)

    Iyer, Sridhar

    2015-06-01

    With the ever-increasing traffic demands, infrastructure of the current 10 Gbps optical network needs to be enhanced. Further, since the energy crisis is gaining increasing concerns, new research topics need to be devised and technological solutions for energy conservation need to be investigated. In all-optical mixed line rate (MLR) network, feasibility of a lightpath is determined by the physical layer impairment (PLI) accumulation. Contrary to PLI-aware routing and wavelength assignment (PLIA-RWA) algorithm applicable for a 10 Gbps wavelength-division multiplexed (WDM) network, a new Routing, Wavelength, Modulation format assignment (RWMFA) algorithm is required for the MLR optical network. With the rapid growth of energy consumption in Information and Communication Technologies (ICT), recently, lot of attention is being devoted toward "green" ICT solutions. This article presents a review of different RWMFA (PLIA-RWA) algorithms for MLR networks, and surveys the most relevant research activities aimed at minimizing energy consumption in optical networks. In essence, this article presents a comprehensive and timely survey on a growing field of research, as it covers most aspects of MLR and energy-driven optical networks. Hence, the author aims at providing a comprehensive reference for the growing base of researchers who will work on MLR and energy-driven optical networks in the upcoming years. Finally, the article also identifies several open problems for future research.

  7. Measuring Exciton Migration in Conjugated Polymer Films with Ultrafast Time Resolved Stimulated Emission Depletion Microscopy

    NASA Astrophysics Data System (ADS)

    Penwell, Samuel

    Conjugated polymers are highly tunable organic semiconductors, which can be solution processed to form thin films, making them prime candidates for organic photovoltaic devices. One of the most important parameters in a conjugated polymer solar cell is the exciton diffusion length, which depends on intermolecular couplings, and is typically on the order of 10 nm. This mean exciton migration can vary dramatically between films and within a single film due to heterogeneities in morphology on length scales of 10's to 100's nm. To study the variability of exciton diffusion and morphology within individual conjugated polymer films, we are adapting stimulated emission depletion (STED) microscopy. STED is typically used in biology with sparse well-engineered fluorescent labels or on NV-centers in diamond. I will, however, describe how we have demonstrated the extension of STED to conjugated polymer films and nanoparticles of MEH-PPV and CN-PPV, despite the presence of two photon absorption, by taking care to first understand the material's photophysical properties. We then further adapt this approach, by introducing a second ultrafast STED pulse at a variable delay. Excitons that migrate away from the initial subdiffraction excitation volume during the ps-ns time delay, are preferentially quenched by the second STED pulse, while those that remain in the initial volume survive. The resulting effect of the second STED pulse is modulated by the degree of migration over the ultrafast time delay, thus providing a new method to study exciton migration. Since this technique utilizes subdiffraction optical excitation and detection volumes with ultrafast time resolution, it provides a means of spatially and temporally resolving measurements of exciton migration on the native length and time scales. In this way, we will obtain a spatiotemporal map of exciton distributions and migration that will help to correlate the energetic landscape to film morphology at the nanoscale.

  8. Optical emission spectroscopy at the large RF driven negative ion test facility ELISE: Instrumental setup and first results

    SciTech Connect

    Wünderlich, D.; Fantz, U.; Franzen, P.; Riedl, R.; Bonomo, F.

    2013-09-15

    One of the main topics to be investigated at the recently launched large (A{sub source}= 1.0 × 0.9 m{sup 2}) ITER relevant RF driven negative ion test facility ELISE (Extraction from a Large Ion Source Experiment) is the connection between the homogeneity of the plasma parameters close to the extraction system and the homogeneity of the extracted negative hydrogen ion beam. While several diagnostics techniques are available for measuring the beam homogeneity, the plasma parameters are determined by optical emission spectroscopy (OES) solely. First OES measurements close to the extraction system show that without magnetic filter field the vertical profile of the plasma emission is more or less symmetric, with maxima of the emission representing the projection of the plasma generation volumes, and a distinct minimum in between. The profile changes with the strength of the magnetic filter field but under all circumstances the plasma emission in ELISE is much more homogeneous compared to the smaller IPP prototype sources. Planned after this successful demonstration of the ELISE OES system is to combine OES with tomography in order to determine locally resolved values for the plasma parameters.

  9. Exciton transfer in close-packed arrays of quantum dots

    NASA Astrophysics Data System (ADS)

    Satanin, Arkady; Hu, Fan; Cosby, Ron; Joe, Yong

    2004-03-01

    Colloidal semiconductor quantum dots (QD), such as close-packed CdSe and InP offer a promising route to new kind of artificial solids which provide media for potential novel optoelectronic applications. Optical properties of densely packed QD have attracted much attention recently [1,2]. The long-range energy transfer was investigated in pure (mono "atomic" QD arrays) and mixed (binary QD arrays) close-packed CdSe solids. It was pointed out that the data for luminescence may be fit by the Förster's decay law. Meanwhile, it is well-known that the Förster's law has been established for diluted systems. Thereupon, an intriguing question arises about the application of Förster approach to the dense media. We present here a theory for exciton transfer in pure and mixed close-packed QD arrays. Our approach is based on the master equation for the calculation of the density of excitons in two-dimensional lattices. The transfer rate between two dots has been calculated from a microscopic theory by using a spherical model of QD. The random parameters have been used to describe the dot-size variations. For mixed QD's a random binary model has been used. We have found an exact solution of the master equation for a perfect lattice, which gives a non-Förster's decay law. The master equation has been investigated numerically on 2D-disordered lattices. It was established that in the formation of the decaying law, the random walk of excitons on a disordered lattice plays a similar role as the exciton's random-abandon of molecules in solutions. It has been demonstrated that with an increase of the disorder the time-dependence of exciton density is more and more similar to the Förster's decaying function. We discuss the correspondence between our numerical results and experimental data [1]. 1. C. R. Kagan, C. B. Murray, and M. G. Bawendi, Phys. Rev. Lett. 76, 1517 (1996). 2. S. A.Crooker, J. A.Hollingsworth, S. Tretiak, and V. I. Klimov, Phys. Rev. Lett. 89, 186802 (1996

  10. Atypical Exciton-Phonon Interactions in WS2 and WSe2 Monolayers Revealed by Resonance Raman Spectroscopy.

    PubMed

    Del Corro, E; Botello-Méndez, A; Gillet, Y; Elias, A L; Terrones, H; Feng, S; Fantini, C; Rhodes, Daniel; Pradhan, N; Balicas, L; Gonze, X; Charlier, J-C; Terrones, M; Pimenta, M A

    2016-04-13

    Resonant Raman spectroscopy is a powerful tool for providing information about excitons and exciton-phonon coupling in two-dimensional materials. We present here resonant Raman experiments of single-layered WS2 and WSe2 using more than 25 laser lines. The Raman excitation profiles of both materials show unexpected differences. All Raman features of WS2 monolayers are enhanced by the first-optical excitations (with an asymmetric response for the spin-orbit related XA and XB excitons), whereas Raman bands of WSe2 are not enhanced at XA/B energies. Such an intriguing phenomenon is addressed by DFT calculations and by solving the Bethe-Salpeter equation. These two materials are very similar. They prefer the same crystal arrangement, and their electronic structure is akin, with comparable spin-orbit coupling. However, we reveal that WS2 and WSe2 exhibit quite different exciton-phonon interactions. In this sense, we demonstrate that the interaction between XC and XA excitons with phonons explains the different Raman responses of WS2 and WSe2, and the absence of Raman enhancement for the WSe2 modes at XA/B energies. These results reveal unusual exciton-phonon interactions and open new avenues for understanding the two-dimensional materials physics, where weak interactions play a key role coupling different degrees of freedom (spin, optic, and electronic). PMID:26998817

  11. Optical properties of iron oxides

    NASA Astrophysics Data System (ADS)

    Musfeldt, Janice

    2012-02-01

    Magnetoelectric coupling in materials like multiferroics, dilute magnetic semiconductors, and topological insulators has attracted a great deal of attention, although most work has been done in the static limit. Optical spectroscopy offers a way to investigate the dynamics of charge-spin coupling, an area where there has been much less effort. Using these techniques, we discovered that charge fluctuation in LuFe2O4, the prototypical charge ordered multiferroic, has an onset well below the charge ordering transition, supporting the ``order by fluctuation'' mechanism for the development of charge order superstructure. Bragg splitting and large magneto-optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field. At the same time, dramatic splitting of the LuO2 layer phonon mode is attributed to charge-rich/poor proximity effects, and its temperature dependence reveals the antipolar nature of the W layer pattern. Using optical techniques, we also discovered that α-Fe2O3, a chemically-similar parent compound and one of the world's oldest and most iconic antiferromagnetic materials, appears more red in applied magnetic field than in zero field conditions. This effect is driven by a field-induced reorientation of magnetic order. The oscillator strength lost in the color band is partially transferred to the magnon side band, a process that also reveals a new exciton pattern induced by the modified exchange coupling. Analysis of the exciton pattern exposes C2/c monoclinic symmetry in the high field phase of hematite. Taken together, these findings advance our understanding of iron-based materials under extreme conditions. [4pt] Collaborators include: X. S. Xu, P. Chen, Q. -C. Sun, T. V. Brinzari (Tennessee); S. McGill (NHMFL); J. De Groot, M. Angst, R. P. Hermann (Julich); A. D. Christianson, B. C. Sales, D. Mandrus (ORNL); A. P. Litvinchuk (Houston); J. -W. Kim (Ames); Z. Islam (Argonne); N. Lee, S. -W. Cheong

  12. Exciton spectra in GaAs/Ga1-xAlxAs quantum wells in an externally applied electric field

    NASA Astrophysics Data System (ADS)

    Zhu, Bangfen

    1988-12-01

    A theory on the exciton spectra in quantum wells in the presence of an external electric field is presented. The theory emphasizes the usually ignored aspect, namely, that the different exciton spinor components correspond to different in-plane angular momenta and only a single spinor component contributes to the optical transition, which in conjunction with the hybridization of the heavy and light holes will affect the exciton binding energies and oscillator strengths drastically. Numerical calculations based on the theory explain the contradictory behavior of the h12a peak observed by Collins et al., which is actually the 2p state of the light-hole-conduction-band (LH1-CB1) exciton.

  13. Field-modulation spectroscopy of pentacene thin films using field-effect devices: Reconsideration of the excitonic structure

    NASA Astrophysics Data System (ADS)

    Haas, Simon; Matsui, Hiroyuki; Hasegawa, Tatsuo

    2010-10-01

    We report pure electric-field effects on the excitonic absorbance of pentacene thin films as measured by unipolar field-effect devices that allowed us to separate the charge accumulation effects. The field-modulated spectra between 1.8 and 2.6 eV can be well fitted with the first derivative curve of Frenkel exciton absorption and its vibronic progression, and at higher energy a field-induced feature appears at around 2.95 eV. The results are in sharp contrast to the electroabsorption spectra reported by Sebastian in previous studies [Chem. Phys. 61, 125 (1981)10.1016/0301-0104(81)85055-0], and leads us to reconsider the excitonic structure including the location of charge-transfer excitons. Nonlinear π -electronic response is discussed based on second-order electro-optic (Kerr) spectra.

  14. Exciton Hierarchies in Gapped Carbon Nanotubes

    SciTech Connect

    Konik, R.M.

    2011-04-01

    We present evidence that the strong electron-electron (e-e) interactions in gapped carbon nanotubes lead to finite hierarchies of excitons within a given nanotube subband. We study these hierarchies by employing a field theoretic reduction of the gapped carbon nanotube permitting e-e interactions to be treated exactly. We analyze this reduction by employing a Wilsonian-like numerical renormalization group. We are so able to determine the gap ratios of the one-photon excitons as a function of the effective strength of interactions. We also determine within the same subband the gaps of the two-photon excitons, the single particle gaps, as well as a subset of the dark excitons. The strong e-e interactions in addition lead to strongly renormalized dispersion relations where the consequences of spin-charge separation can be readily observed.

  15. Exciton hierarchies in gapped carbon nanotubes.

    PubMed

    Konik, Robert M

    2011-04-01

    We present evidence that the strong electron-electron (e-e) interactions in gapped carbon nanotubes lead to finite hierarchies of excitons within a given nanotube subband. We study these hierarchies by employing a field theoretic reduction of the gapped carbon nanotube permitting e-e interactions to be treated exactly. We analyze this reduction by employing a Wilsonian-like numerical renormalization group. We are so able to determine the gap ratios of the one-photon excitons as a function of the effective strength of interactions. We also determine within the same subband the gaps of the two-photon excitons, the single particle gaps, as well as a subset of the dark excitons. The strong e-e interactions in addition lead to strongly renormalized dispersion relations where the consequences of spin-charge separation can be readily observed.

  16. Hybrid interlayer excitons with tunable dispersion relation

    NASA Astrophysics Data System (ADS)

    Skinner, Brian

    When two semiconducting materials are layered on top of each other, interlayer excitons can be formed by the Coulomb attraction of an electron in one layer to a hole in the opposite layer. The resulting exciton is a composite boson with a dispersion relation that is a hybrid between the dispersion relations of the electron and the hole separately. In this talk I show how such hybridization is particularly interesting when one layer has a ``Mexican hat''-shaped dispersion relation and the other has a conventional parabolic dispersion. In this case the interlayer exciton can have a range of qualitatively different dispersion relations, which can be continuously altered by an external field. This tunability in principle allows one to continuously tune a collection of interlayer excitons between different quantum many-body phases, including Bose-Einstein condensate, Wigner crystal, and fermion-like ``moat band'' phases.

  17. An exciton-polariton laser based on biologically produced fluorescent protein.

    PubMed

    Dietrich, Christof P; Steude, Anja; Tropf, Laura; Schubert, Marcel; Kronenberg, Nils M; Ostermann, Kai; Höfling, Sven; Gather, Malte C

    2016-08-01

    Under adequate conditions, cavity polaritons form a macroscopic coherent quantum state, known as polariton condensate. Compared to Wannier-Mott excitons in inorganic semiconductors, the localized Frenkel excitons in organic emitter materials show weaker interaction with each other but stronger coupling to light, which recently enabled the first realization of a polariton condensate at room temperature. However, this required ultrafast optical pumping, which limits the applications of organic polariton condensates. We demonstrate room temperature polariton condensates of cavity polaritons in simple laminated microcavities filled with biologically produced enhanced green fluorescent protein (eGFP). The unique molecular structure of eGFP prevents exciton annihilation even at high excitation densities, thus facilitating polariton condensation under conventional nanosecond pumping. Condensation is clearly evidenced by a distinct threshold, an interaction-induced blueshift of the condensate, long-range coherence, and the presence of a second threshold at higher excitation density that is associated with the onset of photon lasing. PMID:27551686

  18. An exciton-polariton laser based on biologically produced fluorescent protein

    PubMed Central

    Dietrich, Christof P.; Steude, Anja; Tropf, Laura; Schubert, Marcel; Kronenberg, Nils M.; Ostermann, Kai; Höfling, Sven; Gather, Malte C.

    2016-01-01

    Under adequate conditions, cavity polaritons form a macroscopic coherent quantum state, known as polariton condensate. Compared to Wannier-Mott excitons in inorganic semiconductors, the localized Frenkel excitons in organic emitter materials show weaker interaction with each other but stronger coupling to light, which recently enabled the first realization of a polariton condensate at room temperature. However, this required ultrafast optical pumping, which limits the applications of organic polariton condensates. We demonstrate room temperature polariton condensates of cavity polaritons in simple laminated microcavities filled with biologically produced enhanced green fluorescent protein (eGFP). The unique molecular structure of eGFP prevents exciton annihilation even at high excitation densities, thus facilitating polariton condensation under conventional nanosecond pumping. Condensation is clearly evidenced by a distinct threshold, an interaction-induced blueshift of the condensate, long-range coherence, and the presence of a second threshold at higher excitation density that is associated with the onset of photon lasing. PMID:27551686

  19. Nanoscale transport of surface excitons at the interface between ZnO and a molecular monolayer

    NASA Astrophysics Data System (ADS)

    Friede, Sebastian; Kuehn, Sergei; Sadofev, Sergey; Blumstengel, Sylke; Henneberger, Fritz; Elsaesser, Thomas

    2015-03-01

    Excitons play a key role for the optoelectronic properties of hybrid systems. We apply steady-state and time-resolved near-field scanning optical microscopy with a 100-nm spatial resolution to study the photoluminescence (PL) of surface excitons (SX) in a 20-nm-thick ZnO film capped with a monolayer of stearic acid molecules. The PL spectra exhibit emission from SX, donor-bound (DX), and—at sample temperatures T >20 K —free (FX) excitons. The 4 meV broad smooth envelope of SX emission at T <10 K points to an inhomogeneous distribution of SX transition energies and spectral diffusion caused by diffusive SX transport on a 50-nm scale with a diffusion coefficient of DSX(T <10 K ) =0.30 cm 2/s .

  20. One-dimensional ZnO exciton polaritons with negligible thermal broadening at room temperature

    NASA Astrophysics Data System (ADS)

    Trichet, A.; Sun, L.; Pavlovic, G.; Gippius, N. A.; Malpuech, G.; Xie, W.; Chen, Z.; Richard, M.; Dang, Le Si

    2011-01-01

    Single ZnO microwires are investigated by angle-resolved photoluminescence spectroscopy. We show that confined optical modes similar to whispering gallery modes can strongly interact with excitons to form one-dimensional exciton polaritons at room temperature, with normal mode splitting exceeding 200 meV. With such a splitting, which is much larger than LO phonon energy, a strong quenching of the polariton-phonon interaction is achieved, even at room temperature and for large excitonic fractions. Thus, a record figure of merit of 50 for the ratio of the Rabi splitting to the polariton full width at half maximum is achieved as a consequence of negligible thermal contribution to dephasing.

  1. Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor

    NASA Astrophysics Data System (ADS)

    Bradley, Aaron; Ugeda, Miguel M.; Shi, Su-Fei; da Jornada, Felipe H.; Zhang, Yi; Qiu, Diana Y.; Ruan, Wei; Mo, Sung-Kwan; Hussain, Zahid; Shen, Zhi-Xun; Wang, Feng; Louie, Steven G.; Crommie, Michael F.

    2015-03-01

    Reduced screening in 2D has been predicted to result in dramatically enhanced Coulomb interactions that should cause giant bandgap renormalization and exotic excitonic effects in single-layer TMD semiconductors. Here we present a direct experimental observation of extraordinarily high exciton binding energy and bandgap renormalization in a single-layer of a semiconducting MoSe2, grown on bilayer graphene, using high-resolution scanning tunneling spectroscopy and photoluminescence spectroscopy. We have measured both the quasiparticle electronic bandgap and the optical transitions, obtaining an exciton binding energy of 0.55 eV - a value orders of magnitude larger than in conventional 3D semiconductors. We have also studied the influence of external dielectric screening by repeating measurements on MoSe2/HOPG. These results are important for room-temperature optoelectronic devices involving 2D TMDs, as well as more complex layered heterostructures.

  2. Couple molecular excitons to surface plasmon polaritons in an organic-dye-doped nanostructured cavity

    NASA Astrophysics Data System (ADS)

    Zhang, Kun; Shi, Wen-Bo; Wang, Di; Xu, Yue; Peng, Ru-Wen; Fan, Ren-Hao; Wang, Qian-Jin; Wang, Mu

    2016-05-01

    In this work, we demonstrate experimentally the hybrid coupling among molecular excitons, surface plasmon polaritons (SPPs), and Fabry-Perot (FP) mode in a nanostructured cavity, where a J-aggregates doped PVA (polyvinyl alcohol) layer is inserted between a silver grating and a thick silver film. By tuning the thickness of the doped PVA layer, the FP cavity mode efficiently couples with the molecular excitons, forming two nearly dispersion-free modes. The dispersive SPPs interact with these two modes while increasing the incident angle, leading to the formation of three hybrid polariton bands. By retrieving the mixing fractions of the polariton band components from the measured angular reflection spectra, we find all these three bands result from the strong coupling among SPPs, FP mode, and excitons. This work may inspire related studies on hybrid light-matter interactions, and achieve potential applications on multimode polariton lasers and optical spectroscopy.

  3. Excitonic spectra and energy band structure of ZnAl2Se4 crystals

    NASA Astrophysics Data System (ADS)

    Syrbu, N. N.; Zalamai, V. V.; Tiron, A. V.; Tiginyanu, I. M.

    2015-11-01

    Absorption, reflection and wavelength modulated reflection spectra were investigated in ZnAl2Se4 crystals. The energy positions of ground and excited states for three excitonic series (A, B and C) were determined. The main parameters of excitons and more precise values of energy intervals V1(Γ7)-C1(Γ6), V2(Γ6)-C1(Γ6), and V3(Γ7)-C1(Γ6) were estimated. Values of splitting due to crystal field and spin-orbital interaction were calculated. Effective masses of electrons (mC1∗) and holes (mV1∗, mV2∗, mV3∗) were estimated. Reflection spectra contours in excitonic region were calculated using dispersion equations. Optical functions for E > Eg from measured reflection spectra were assigned on the base of Kramers-Kronig relations.

  4. Excitonic effects in two-dimensional semiconductors: Path integral Monte Carlo approach

    SciTech Connect

    Velizhanin, Kirill A.; Saxena, Avadh

    2015-11-01

    The most striking features of novel two-dimensional semiconductors (e.g., transition metal dichalcogenide monolayers or phosphorene) is a strong Coulomb interaction between charge carriers resulting in large excitonic effects. In particular, this leads to the formation of multicarrier bound states upon photoexcitation (e.g., excitons, trions, and biexcitons), which could remain stable at near-room temperatures and contribute significantly to the optical properties of such materials. In our work we have used the path integral Monte Carlo methodology to numerically study properties of multicarrier bound states in two-dimensional semiconductors. Specifically, we have accurately investigated and tabulated the dependence of single-exciton, trion, and biexciton binding energies on the strength of dielectric screening, including the limiting cases of very strong and very weak screening. Our results of this work are potentially useful in the analysis of experimental data and benchmarking of theoretical and computational models.

  5. Cavity-less on-chip optomechanics using excitonic transitions in semiconductor heterostructures

    PubMed Central

    Okamoto, Hajime; Watanabe, Takayuki; Ohta, Ryuichi; Onomitsu, Koji; Gotoh, Hideki; Sogawa, Tetsuomi; Yamaguchi, Hiroshi

    2015-01-01

    The hybridization of semiconductor optoelectronic devices and nanomechanical resonators provides a new class of optomechanical systems in which mechanical motion can be coupled to light without any optical cavities. Such cavity-less optomechanical systems interconnect photons, phonons and electrons (holes) in a highly integrable platform, opening up the development of functional integrated nanomechanical devices. Here we report on a semiconductor modulation-doped heterostructure–cantilever hybrid system, which realizes efficient cavity-less optomechanical transduction through excitons. The opto-piezoelectric backaction from the bound electron–hole pairs enables us to probe excitonic transition simply with a sub-nanowatt power of light, realizing high-sensitivity optomechanical spectroscopy. Detuning the photon energy from the exciton resonance results in self-feedback cooling and amplification of the thermomechanical motion. This cavity-less on-chip coupling enables highly tunable and addressable control of nanomechanical resonators, allowing high-speed programmable manipulation of nanomechanical devices and sensor arrays. PMID:26477487

  6. Strong exciton-photon coupling in organic single crystal microcavity with high molecular orientation

    NASA Astrophysics Data System (ADS)

    Goto, Kaname; Yamashita, Kenichi; Yanagi, Hisao; Yamao, Takeshi; Hotta, Shu

    2016-08-01

    Strong exciton-photon coupling has been observed in a highly oriented organic single crystal microcavity. This microcavity consists of a thiophene/phenylene co-oligomer (TPCO) single crystal laminated on a high-reflection distributed Bragg reflector. In the TPCO crystal, molecular transition dipole was strongly polarized along a certain horizontal directions with respect to the main crystal plane. This dipole polarization causes significantly large anisotropies in the exciton transition and optical constants. Especially the anisotropic exciton transition was found to provide the strong enhancement in the coupling with the cavity mode, which was demonstrated by a Rabi splitting energy as large as ˜100 meV even in the "half-vertical cavity surface emitting lasing" microcavity structure.

  7. Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots.

    PubMed

    Rabouw, Freddy T; Kamp, Marko; van Dijk-Moes, Relinde J A; Gamelin, Daniel R; Koenderink, A Femius; Meijerink, Andries; Vanmaekelbergh, Daniël

    2015-11-11

    The efficiency and stability of emission from semiconductor nanocrystal quantum dots (QDs) is negatively affected by "blinking" on the single-nanocrystal level, that is, random alternation of bright and dark periods. The time scales of these fluctuations can be as long as many seconds, orders of magnitude longer than typical lifetimes of exciton states in QDs. In this work, we investigate photoluminescence from QDs delayed over microseconds to milliseconds. Our results prove the existence of long-lived charge-separated states in QDs. We study the properties of delayed emission as a direct way to learn about charge carrier separation and recovery of the exciton state. A new microscopic model is developed to connect delayed emission to exciton recombination and blinking from which we conclude that bright periods in blinking are in fact not characterized by uninterrupted optical cycling as often assumed.

  8. Excitonic effects in two-dimensional semiconductors: Path integral Monte Carlo approach

    DOE PAGES

    Velizhanin, Kirill A.; Saxena, Avadh

    2015-11-01

    The most striking features of novel two-dimensional semiconductors (e.g., transition metal dichalcogenide monolayers or phosphorene) is a strong Coulomb interaction between charge carriers resulting in large excitonic effects. In particular, this leads to the formation of multicarrier bound states upon photoexcitation (e.g., excitons, trions, and biexcitons), which could remain stable at near-room temperatures and contribute significantly to the optical properties of such materials. In our work we have used the path integral Monte Carlo methodology to numerically study properties of multicarrier bound states in two-dimensional semiconductors. Specifically, we have accurately investigated and tabulated the dependence of single-exciton, trion, and biexcitonmore » binding energies on the strength of dielectric screening, including the limiting cases of very strong and very weak screening. Our results of this work are potentially useful in the analysis of experimental data and benchmarking of theoretical and computational models.« less

  9. Finite-difference time-domain analysis for the dynamics and diffraction of exciton-polaritons.

    PubMed

    Chen, Minfeng; Chang, Yia-Chung; Hsieh, Wen-Feng

    2015-10-01

    We adopted a finite-difference time-domain (FDTD) scheme to simulate the dynamics and diffraction of exciton-polaritons, governed by the coupling of polarization waves with electromagnetic waves. The polarization wave, an approximate solution to the Schrödinger's equation at low frequencies, essentially captures the exciton behavior. Numerical stability of the scheme is analyzed and simple examples are provided to prove its validity. The system considered is both temporally and spatially dispersive, for which the FDTD analysis has attracted less attention in the literature. Here, we demonstrate that the FDTD scheme could be useful for studying the optical response of the exciton-polariton and its dynamics. The diffraction of a polariton wave from a polaritonic grating is also considered, and many sharp resonances are found, which manifest the interference effect of polariton waves. This illustrates that the measurement of transmittance or reflectance near polariton resonance can reveal subwavelength features in semiconductors, which are sensitive to polariton scattering.

  10. External quantum efficiency above 100% in a singlet-exciton-fission-based organic photovoltaic cell.

    PubMed

    Congreve, Daniel N; Lee, Jiye; Thompson, Nicholas J; Hontz, Eric; Yost, Shane R; Reusswig, Philip D; Bahlke, Matthias E; Reineke, Sebastian; Van Voorhis, Troy; Baldo, Marc A

    2013-04-19

    Singlet exciton fission transforms a molecular singlet excited state into two triplet states, each with half the energy of the original singlet. In solar cells, it could potentially double the photocurrent from high-energy photons. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in a portion of the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, we show a peak external quantum efficiency of (109 ± 1)% at wavelength λ = 670 nanometers for a 15-nanometer-thick pentacene film. The corresponding internal quantum efficiency is (160 ± 10)%. Analysis of the magnetic field effect on photocurrent suggests that the triplet yield approaches 200% for pentacene films thicker than 5 nanometers. PMID:23599489

  11. Excitonic effects in two-dimensional semiconductors: Path integral Monte Carlo approach

    SciTech Connect

    Velizhanin, Kirill A.; Saxena, Avadh

    2015-11-11

    The most striking features of novel two-dimensional semiconductors (e.g., transition metal dichalcogenide monolayers or phosphorene) is a strong Coulomb interaction between charge carriers resulting in large excitonic effects. In particular, this leads to the formation of multicarrier bound states upon photoexcitation (e.g., excitons, trions, and biexcitons), which could remain stable at near-room temperatures and contribute significantly to the optical properties of such materials. In our work we have used the path integral Monte Carlo methodology to numerically study properties of multicarrier bound states in two-dimensional semiconductors. Specifically, we have accurately investigated and tabulated the dependence of single-exciton, trion, and biexciton binding energies on the strength of dielectric screening, including the limiting cases of very strong and very weak screening. Our results of this work are potentially useful in the analysis of experimental data and benchmarking of theoretical and computational models.

  12. Cavity-less on-chip optomechanics using excitonic transitions in semiconductor heterostructures.

    PubMed

    Okamoto, Hajime; Watanabe, Takayuki; Ohta, Ryuichi; Onomitsu, Koji; Gotoh, Hideki; Sogawa, Tetsuomi; Yamaguchi, Hiroshi

    2015-01-01

    The hybridization of semiconductor optoelectronic devices and nanomechanical resonators provides a new class of optomechanical systems in which mechanical motion can be coupled to light without any optical cavities. Such cavity-less optomechanical systems interconnect photons, phonons and electrons (holes) in a highly integrable platform, opening up the development of functional integrated nanomechanical devices. Here we report on a semiconductor modulation-doped heterostructure-cantilever hybrid system, which realizes efficient cavity-less optomechanical transduction through excitons. The opto-piezoelectric backaction from the bound electron-hole pairs enables us to probe excitonic transition simply with a sub-nanowatt power of light, realizing high-sensitivity optomechanical spectroscopy. Detuning the photon energy from the exciton resonance results in self-feedback cooling and amplification of the thermomechanical motion. This cavity-less on-chip coupling enables highly tunable and addressable control of nanomechanical resonators, allowing high-speed programmable manipulation of nanomechanical devices and sensor arrays. PMID:26477487

  13. Efficient Excitonic Photoluminescence in Direct and Indirect Band Gap Monolayer MoS2.

    PubMed

    Steinhoff, A; Kim, J-H; Jahnke, F; Rösner, M; Kim, D-S; Lee, C; Han, G H; Jeong, M S; Wehling, T O; Gies, C

    2015-10-14

    We discuss the photoluminescence (PL) of semiconducting transition metal dichalcogenides on the basis of experiments and a microscopic theory. The latter connects ab initio calculations of the single-particle states and Coulomb matrix elements with a many-body description of optical emission spectra. For monolayer MoS2, we study the PL efficiency at the excitonic A and B transitions in terms of carrier populations in the band structure and provide a quantitative comparison to an (In)GaAs quantum well-structure. Suppression and enhancement of PL under biaxial strain is quantified in terms of changes in the local extrema of the conduction and valence bands. The large exciton binding energy in MoS2 enables two distinctly different excitation methods: above-band gap excitation and quasi-resonant excitation of excitonic resonances below the single-particle band gap. The latter case creates a nonequilibrium distribution of carriers predominantly in the K-valleys, which leads to strong emission from the A-exciton transition and a visible B-peak even if the band gap is indirect. For above-band gap excitation, we predict a strongly reduced emission intensity at comparable carrier densities and the absence of B-exciton emission. The results agree well with PL measurements performed on monolayer MoS2 at excitation wavelengths of 405 nm (above) and 532 nm (below the band gap). PMID:26322814

  14. Characterizing and tuning excitons in monolayer and few-layer MoS 2

    NASA Astrophysics Data System (ADS)

    Qiu, Diana Y.; da Jornada, Felipe H.; Louie, Steven G.

    2015-03-01

    We use the GW-BSE method to study excitons arising from transitions in different regions of momentum space in mono- and few-layer MoS2 and consider mechanisms to fundamentally change the features and character of the optical spectra. Our calculations show that sharp spatial variations in dielectric screening make 2D systems, such as MoS2 , computationally challenging, requiring very fine k-space sampling to resolve the structure of excitonic wave functions and converge binding energies. In highly converged calculations, we identify a series of excitons arising from transitions at the K/K' valleys in the Brillouin zone, a higher energy series arising from transitions in the valley of a Mexican hat potential centered at the Γ point, and transitions at the indirect gap from Γ to Λ in few-layer MoS2 . As layer number changes, these states, which have varying character, momentum-space structure and real-space locations, are affected differently by changes in confinement and hybridization. By tuning layer number and strain, we find that we not only can tune the excitation energies but can also change the relative energies of the various excitonic series, allowing for movement of the lowest energy exciton between different regions of the Brillouin zone This work was supported by NSF Grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231.

  15. Measuring Exciton Diffusion in Conjugated Polymer Films with Super-resolution Microscopy

    NASA Astrophysics Data System (ADS)

    Penwell, Samuel; Ginsberg, Lucas; Noriega Manez, Rodrigo; Ginsberg, Naomi

    2015-03-01

    Conjugated polymers are highly tunable organic semiconductors, which can be solution processed to form thin films, making them prime candidates for organic photovoltaic devices. One of the most important parameters in a conjugated polymer solar cell is the exciton diffusion length, which depends on intermolecular couplings, and is typically on the order of 10 nm. This mean exciton migration can vary dramatically between films and within a single film due to heterogeneities in morphology on length scales of 10's to 100's nm. To study the variability of exciton diffusion and morphology within individual conjugated polymer films, we are adapting stimulated emission depletion microscopy. STED is typically used in biology with well-engineered fluorescent labels or on NV-centers in diamond. I will, however, describe how we have demonstrated STED in conjugated polymer films of MEH-PPV and CN-PPV by taking care to first understand the film's photophysical properties. This new approach provides a way to study exciton diffusion by utilizing subdiffraction optical excitation volumes. In this way, we will obtain a spatiotemporal map of exciton distributions that will help to correlate the energetic landscape to film morphology at the nanoscale. This research is supported in part by the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under Contract No. DE-AC05-06.

  16. Excitonic transitions in ZnO/MgZnO quantum well heterostructures

    NASA Astrophysics Data System (ADS)

    Coli, Giuliano; Bajaj, K. K.

    2001-03-01

    We present a calculation of the excitonic transition energies in ZnO/MgZnO quantum well heterostructures, including the effects of the exciton-optical phonon interaction. The results of our calculations clearly show that the use of the static screened Coulomb potential to describe the electron-hole interaction and of the polaron masses for the electron and the hole leads to a poor agreement with the available experimental data[1]. On the other hand, including the exciton-phonon interaction in the calculation of the exciton binding energies, leads to the values of the excitonic transition energies which agree very well with the recently published experimental data[1]. A critical discussion of the choice of the various physical parameters used in ZnO is also presented. This leads us to suggest a value for the heavy-hole band mass of 0.78m0 and a conduction to valence band offset ratio in the range 60/40-70/30. [1] T. Makino, C. H. Chia, N. T. Tuan, H.D. Sun, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura and H. Koinuma, Appl. Phys. Lett. 77, 975 (2000)

  17. Efficient Exciton Diffusion and Resonance-Energy Transfer in Multilayered Organic Epitaxial Nanofibers

    PubMed Central

    2015-01-01

    Multilayered epitaxial nanofibers are exemplary model systems for the study of exciton dynamics and lasing in organic materials because of their well-defined morphology, high luminescence efficiencies, and color tunability. We use temperature-dependent continuous wave and picosecond photoluminescence (PL) spectroscopy to quantify exciton diffusion and resonance-energy transfer (RET) processes in multilayered nanofibers consisting of alternating layers of para-hexaphenyl (p6P) and α-sexithiophene (6T) serving as exciton donor and acceptor material, respectively. The high probability for RET processes is confirmed by quantum chemical calculations. The activation energy for exciton diffusion in p6P is determined to be as low as 19 meV, proving p6P epitaxial layers also as a very suitable donor material system. The small activation energy for exciton diffusion of the p6P donor material, the inferred high p6P-to-6T resonance-energy-transfer efficiency, and the observed weak PL temperature dependence of the 6T acceptor material together result in an exceptionally high optical emission performance of this all-organic material system, thus making it well suited, for example, for organic light-emitting devices. PMID:26191119

  18. Nonanalyticity, Valley Quantum Phases, and Massless Excitons in Monolayer Transition Metal Dichalcogenides

    NASA Astrophysics Data System (ADS)

    Qiu, Diana Y.; Cao, Ting; Louie, Steven G.

    Exciton dispersion as a function of center-of-mass momentum Q is essential to the understanding of exciton dynamics, relaxation, and condensation. We use the ab initio GW-Bethe-Salpeter equation(GW-BSE) method to calculate the dispersion of excitons in monolayer MoS2 and find a nonanalytic lightlike dispersion. This behavior arises from the interplay of an unusual | Q | -term in both the intra- and intervalley exchange of the electron-hole interaction, which concurrently gives rise to a valley quantum phase of winding number two. We have derived a simple, effective Hamiltonian and analytic solutions, which quantitatively describe this physics, and we predict that signatures of this unusual dispersion can be measured with a linearly polarized optical beam tilted away from normal incidence. The existence of a nonanalytic exciton dispersion can be generalized to other 2D semiconductors with excitons whose amplitudes are localized in a small region of the Brillioun zone. This work was supported by NSF Grant No. DMR15-1508412 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NERSC and XSEDE.

  19. Exciton dynamics in a site-controlled quantum dot coupled to a photonic crystal cavity

    SciTech Connect

    Jarlov, C. Lyasota, A.; Ferrier, L.; Gallo, P.; Dwir, B.; Rudra, A.; Kapon, E.

    2015-11-09

    Exciton and cavity mode (CM) dynamics in site-controlled pyramidal quantum dots (QDs), integrated with linear photonic crystal membrane cavities, are investigated for a range of temperatures and photo-excitation power levels. The absence of spurious multi-excitonic effects, normally observed in similar structures based on self-assembled QDs, permits the observation of effects intrinsic to two-level systems embedded in a solid state matrix and interacting with optical cavity modes. The coupled exciton and CM dynamics follow the same trend, indicating that the CM is fed only by the exciton transition. The Purcell reduction of the QD and CM decay times is reproduced well by a theoretical model that includes exciton linewidth broadening and temperature dependent non-radiative processes, from which we extract a Purcell factor of 17 ± 5. For excitation powers above QD saturation, we show the influence of quantum wire barrier states at short delay time, and demonstrate the absence of multiexcitonic background emission.

  20. Exciton properties of selected aromatic hydrocarbon systems

    NASA Astrophysics Data System (ADS)

    Roth, Friedrich; Mahns, Benjamin; Hampel, Silke; Nohr, Markus; Berger, Helmuth; Büchner, Bernd; Knupfer, Martin

    2013-02-01

    We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the excitation spectra of the two hydrocarbon families significantly differ. Moreover, close inspection of the data indicates that there is an increasing importance of charge-transfer excitons at lowest excitation energy with increasing length of the molecules.

  1. Ultrafast exciton dynamics at molecular surfaces

    NASA Astrophysics Data System (ADS)

    Monahan, Nicholas R.

    Further improvements to device performance are necessary to make solar energy conversion a compelling alternative to fossil fuels. Singlet exciton fission and charge separation are two processes that can heavily influence the power conversion efficiency of a solar cell. During exciton fission one singlet excitation converts into two triplet excitons, potentially doubling the photocurrent generated by higher energy photons. There is significant discord over the singlet fission mechanism and of particular interest is whether the process involves a multiexciton intermediate state. I used time-resolved two-photon photoemission to investigate singlet fission in hexacene thin films, a model system with strong electronic coupling. My results indicate that a multiexciton state forms within 40 fs of photoexcitation and loses singlet character on a 280 fs timescale, creating two triplet excitons. This is concordant with the transient absorption spectra of hexacene single crystals and definitively proves that exciton fission in hexacene proceeds through a multiexciton state. This state is likely common to all strongly-coupled systems and my results suggest that a reassessment of the generally-accepted singlet fission mechanism is required. Charge separation is the process of splitting neutral excitons into carriers that occurs at donor-acceptor heterojunctions in organic solar cells. Although this process is essential for device functionality, there are few compelling explanations for why it is highly efficient in certain organic photovoltaic systems. To investigate the charge separation process, I used the model system of charge transfer excitons at hexacene surfaces and time-resolved two-photon photoemission. Charge transfer excitons with sufficient energy spontaneously delocalize, growing from about 14 nm to over 50 nm within 200 fs. Entropy drives this delocalization, as the density of states within the Coulomb potential increases significantly with energy. This charge

  2. Anisotropic exciton Stark shift in black phosphorus

    NASA Astrophysics Data System (ADS)

    Chaves, A.; Low, Tony; Avouris, P.; ćakır, D.; Peeters, F. M.

    2015-04-01

    We calculate the excitonic spectrum of few-layer black phosphorus by direct diagonalization of the effective mass Hamiltonian in the presence of an applied in-plane electric field. The strong attractive interaction between electrons and holes in this system allows one to investigate the Stark effect up to very high ionizing fields, including also the excited states. Our results show that the band anisotropy in black phosphorus becomes evident in the direction-dependent field-induced polarizability of the exciton.

  3. Theory of Orbital Susceptibility on Excitonic Insulator

    NASA Astrophysics Data System (ADS)

    Matsuura, Hiroyasu; Ogata, Masao

    2016-09-01

    We study the temperature dependence of the orbital susceptibility of an excitonic insulator on the basis of a two-band model. It is shown that a drastic change (an anomalous enhancement) in susceptibility as a function of temperature occurs owing to the occurrence of additional orbital susceptibility due to the excitonic gap. We calculate explicitly the temperature dependence of orbital susceptibility for a model of Ta2NiSe5, and show that the result is consistent with experimental results.

  4. Exciton transport by surface acoustic waves

    NASA Astrophysics Data System (ADS)

    Rudolph, J.; Hey, R.; Santos, P. V.

    2007-05-01

    Long-range acoustic transport of excitons in GaAs quantum wells (QWs) is demonstrated. The mobile strain field of a surface acoustic wave creates a dynamic lateral type I modulation of the conduction and valence bands in a double-quantum-well (DQW) structure. This mobile potential modulation transports long-living indirect excitons in the DQW over several hundreds of μm.

  5. Exciton-polariton wakefields in semiconductor microcavities

    NASA Astrophysics Data System (ADS)

    Terças, H.; Mendonça, J. T.

    2016-02-01

    We consider the excitation of polariton wakefields due to a propagating light pulse in a semiconductor microcavity. We show that two kinds of wakes are possible, depending on the constituents fraction (either exciton or photon) of the polariton wavefunction. The nature of the wakefields (pure excitonic or polaritonic) can be controlled by changing the speed of propagation of the external pump. This process could be used as a diagnostic for the internal parameters of the microcavity.

  6. Exciton quasicondensation in one-dimensional systems

    NASA Astrophysics Data System (ADS)

    Werman, Yochai; Berg, Erez

    2015-06-01

    Two Luttinger liquids, with an equal density and opposite sign of charge carriers, may exhibit enhanced excitonic correlations. We term such a system an exciton quasicondensate, with a possible realization being two parallel oppositely doped quantum wires, coupled by repulsive Coulomb interactions. We show that this quasiexciton condensate can be stabilized in an extended range of parameters, in both spinless and spinful systems. We calculate the interwire tunneling current-voltage characteristic, and find that a negative differential conductance is a signature of excitonic correlations. For spinful electrons, the excitonic regime is shown to be distinct from the usual quasi-long-range ordered Wigner crystal phase characterized by power-law density wave correlations. The two phases can be clearly distinguished through their interwire tunneling current-voltage characteristics. In the quasiexciton condensate regime the tunneling conductivity diverges at low temperatures and voltages, whereas in the Wigner crystal it is strongly suppressed. Both the Wigner crystal and the excitonic regime are characterized by a divergent Coulomb drag at low temperature. Finally, metallic carbon nanotubes are considered as a special case of such a one-dimensional setup, and it is shown that exciton condensation is favorable due to the additional valley degree of freedom.

  7. Surface Exciton-Plasmons in Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Bondarev, Igor; Tatur, Kevin; Woods, Lilia

    2008-03-01

    We study theoretically the interactions of excitonic states with surface electromagnetic modes of a single-walled carbon nanotube. We use our previously developed Green's function formalism to quantize an electromagnetic field in the presence of quasi-1D absorbing bodies [1]. We show that these interactions result in the exciton-plasmon coupling that is significant in its strength due to the presence of weakly-dispersive low-energy (˜0.5-2eV) interband surface plasmon modes [2] and large exciton excitation energies ˜1eV in small-diameter nanotubes [3]. We estimate the exciton-plasmon Rabi splitting to be ˜0.01-0.1eV which is close to that observed in organic semiconductors [4] and much larger than that reported for hybrid semiconductor-metal nanoparticle molecules [5]. We calculate the exciton absorption lineshape and demonstrate a clear line splitting effect as the exciton energy is tuned to the closest interband surface plasmon resonance. [1] I.V.Bondarev and Ph.Lambin, Phys. Rev. B72, 035451 (2005). [2] T.Pichler, et al., Phys. Rev. Lett. 80, 4729 (1998). [3] D.Spataru, et al., Phys. Rev. Lett. 95, 247402 (2005). [4] J.Belessa, et al., Phys. Rev. Lett. 93, 036404 (2004). [5] W.Zhang, A.O.Govorov, G.W.Bryant, Phys. Rev. Lett. 97, 146804 (2006).

  8. Exploring ultrafast dynamics of excitons and multiexcitons in "giant" nanocrystal quantum dots

    NASA Astrophysics Data System (ADS)

    Sampat, Siddharth

    In this work, we have performed extensive time resolved photoluminescence (PL) studies to further the understanding of charge dynamics in semiconductor nanocrystal quantum dots (QDs). Recent developments in QD synthesis have introduced a new set of QD known as "giant" quantum dots (gQDs) that consist of a CdSe core coated with up to 19 monolayers of a CdS shell. The thick shell layer is grown using a SILAR method resulting in a defect free, alloyed CdSe/CdS interface. This has been attributed to gQDs exhibiting excellent optical properties such as high excitonic quantum yield (QY), prolonged photostability and inhibition of flourescence intermittency ("blinking"), which is regularly observed in conventional QDs. In gQDs, however, owing to unique fabrication methods and material selection, the Auger process is strongly suppressed resulting in efficient radiative recombination of photogenerated excitons as well as high PL QY of charged excitonic and multiexcitonic species. We perform extensive single gQDs studies that establish the role played by gQD shell thickness and core size in governing their optical properties. It is found that both the core and shell dimensions can be tuned in order to achieve the smallest gQDs with the highest vii Auger suppression resulting in photostable dots with high QYs. Next, we perform a study of multiexcitonic species in gQDs that are encapsulated in an insulating SiO2shell. These silica-coated gQDs exhibit strong PL from charged excitons, biexcitons as well as triexcitons. This observation has led to an accurate description of excitonic and multiexcitonic behavior which is modeled using a statistical scaling approach. As a demonstration of the practical applicability of gQDs, energy transfer of excitons as well as multiexcitons to different substrates is studied. Finally, a back gated silicon nanomembrane FET device is discussed that exhibits a large photocurrent increase when sensitized with QDs.

  9. Enhanced magneto-plasmonic effect in Au/Co/Au multilayers caused by exciton-plasmon strong coupling

    NASA Astrophysics Data System (ADS)

    Hamidi, S. M.; Ghaebi, O.

    2016-09-01

    In this paper, we have investigated magneto optical Kerr rotation using the strong coupling of exciton-plasmon. For this purpose, we have demonstrated strong coupling phenomenon using reflectometry measurements. These measurements revealed the formation of two split polaritonic extrema in reflectometry as a function of wavelength. Then we have shown exciton-plasmon coupling in dispersion diagram which presented an anti-crossing between the polaritonic branches. To assure the readers of strong coupling, we have shown an enhanced magneto-optical Kerr rotation by comparing the reflectometry results of strong coupling of surface Plasmon polariton of Au/Co/Au multilayer and R6G excitons with surface Plasmon polariton magneto-optical kerr effect experimental setup.

  10. Excitons in semiconducting superlattices, quantum wells, and ternary alloys

    SciTech Connect

    Sturge, M.D. ); Nahory, R.E.; Tamargo, M.C. )

    1991-08-15

    It is now possible to fabricate semiconducting layered structures with precisely defined layer thicknesses down to one monolayer (two atomic diameters). An example is the superlattice'' (SL) structure, in which two semiconductors with different band gaps are interleaved. The electronic properties of the SL are quite different from those of the constitutents and offer interesting new possibilities both in device design and in basic physics. This proposal aims to improve our understanding of optically excited states ( excitons'' and electron-hole plasmas'') in a particular class of these structures: the so-called Type 2 indirect'' SL's in which the electron and hole created by optical excitation are separated both in real and in momoentum space. Time-resolved tunable laser spectroscopy, with and without external perturbations such as magnetic field, electric field, and uniaxial stress, are used principally to study the following phenomena. 1. Exciton states in SLs with only a few atomic layers per period, for which the familiar effective mass model'' of semiconductor states breaks down. 2. The electron-hole plasma which forms when the excitation density is high. This plasma may be in a liquid state at low temperatures. In the short period superlattices are our primary concern, electrons and holes are spatially separated, leading to internal electric fields which might be expected to have a pronounced effect on the plasma properties.

  11. Excitonic entanglement of protected states in quantum dot molecules

    NASA Astrophysics Data System (ADS)

    Borges, H. S.; Sanz, L.; Alcalde, A. M.

    2016-09-01

    The entanglement of an optically generated electron-hole pair in artificial quantum dot molecules is calculated considering the effects of decoherence by interaction with environment. Since the system evolves into mixed states and due to the complexity of energy level structure, we use the negativity as entanglement quantifier, which is well defined in D ⊗D‧ composite vector spaces. By a numerical analysis of the non-unitary dynamics of the exciton states, we establish the feasibility of producing protected entangled superposition by an appropriate tuning of bias electric field, F. A stationary state with a high value of negativity (high degree of entanglement) is obtained by fine tuning of F close to a resonant condition between indirect excitons. We also found that when the optical excitation is approximately equal to the electron tunneling coupling, Ω /Te ∼ 1, the entanglement reaches a maximum value. In front of the experimental feasibility of the specific condition mentioned before, our proposal becomes an useful strategy to find robust entangled states in condensed matter systems.

  12. Microscopic theories of excitons and their dynamics

    NASA Astrophysics Data System (ADS)

    Berkelbach, Timothy C.

    This thesis describes the development and application of microscopically-defined theories of excitons in a wide range of semiconducting materials. In Part I, I consider the topic of singlet exciton fission, an organic photophysical process which generates two spin-triplet excitons from one photoexcited spin-singlet exciton. I construct a theoretical framework that couples a realistic treatment of the static electronic structure with finite-temperature quantum relaxation techniques. This framework is applied separately, but consistently, to the problems of singlet fission in pentacene dimers, crystalline pentacene, and crystalline hexacene. Through this program, I am able to rationalize observed behaviors and make non-trivial predictions, some of which have been confirmed by experiment. In Part II, I present theoretical developments on the properties of neutral excitons and charged excitons (trions) in atomically thin transition metal dichalcogenides. This work includes an examination of material trends in exciton binding energies via an effective mass approach. I also present an experimental and theoretical collaboration, which links the unconventional disposition of excitons in the Rydberg series to the peculiar screening properties of atomically thin materials. The light-matter coupling in these materials is examined within low-energy models and is shown to give rise to bright and dark exciton states, which can be qualitatively labeled in analogy with the hydrogen series. In Part III, I explore theories of relaxation dynamics in condensed phase environments, with a focus on methodology development. This work is aimed towards biological processes, including resonant energy transfer in chromophore complexes and electron transfer in donor-bridge-acceptor systems. Specifically, I present a collaborative development of a numerically efficient but highly accurate hybrid approach to reduced dynamics, which exploits a partitioning of environmental degrees of freedom into

  13. Utilization of optical tracking to validate a software-driven isocentric approach to robotic couch movements for proton radiotherapy

    SciTech Connect

    Hsi, Wen C. E-mail: Wenchien.hsi@sphic.org.cn; Zeidan, Omar A.; Law, Aaron; Schreuder, Andreas N.

    2014-08-15

    Purpose: An optical tracking and positioning system (OTPS) was developed to validate the software-driven isocentric (SDI) approach to control the six-degrees-of-freedom movement of a robotic couch. Methods: The SDI approach to movements rotating around a predefined isocenter, referred to as a GeoIso, instead of a mechanical pivot point was developed by the robot automation industry. With robotic couch-sag corrections for weight load in a traditional SDI approach, movements could be accurately executed for a GeoIso located within a 500 mm cubic volume on the couch for treatments. The accuracy of SDI movement was investigated using the OTPS. The GeoIso was assumed to align with the proton beam isocenter (RadIso) for gantry at the reference angle. However, the misalignment between GeoIso and RadIso was quantitatively investigated by measuring the displacements at various couch angles for a target placed at the RadIso at an initial couch angle. When circular target displacements occur on a plane, a relative isocenter shift (RIS) correction could be applied in the SDI movement to minimize target displacements. Target displacements at a fixed gantry angle without and with RIS correction were measured for 12 robotic couches. Target displacements for various gantry angles were performed on three couches in gantry rooms to study the gantry-induced RadIso shift. The RIS correction can also be applied for the RadIso shift. A new SDI approach incorporating the RIS correction with the couch sag is described in this study. In parallel, the accuracy of SDI translation movements for various weight loads of patients on the couch was investigated during positioning of patients for proton prostate treatments. Results: For a fixed gantry angle, measured target displacements without RIS correction for couch rotations in the horizontal plane varied from 4 to 20 mm. However, measured displacements perpendicular to couch rotation plane were about 2 mm for all couches. Extracted

  14. Optical remote sensing of penetration into the lower thermosphere of neutral wind and composition perturbations driven by magnetospheric forcing

    NASA Astrophysics Data System (ADS)

    Conde, M. G.; Anderson, C.; Hecht, J. H.

    2011-12-01

    Numerous observations of thermospheric neutral winds at altitudes of 240 km and higher clearly show wind structures occurring at auroral latitudes in response to magnetospheric forcing. It is also known from observations that magnetospheric forcing is not a major driver of winds down at mesopause heights and below. Because it is difficult to measure winds in the intervening "transition region" between these height regimes, very little is known about how deeply the magnetospherically driven neutral wind structures penetrate into the lower thermosphere, what factors affect this penetration, and what consequences it may have for transport of chemical species. Here we will show neutral wind maps obtained at F-region and E-region heights in the auroral zone using Fabry-Perot Doppler spectroscopy of the 630 nm and 558 nm optical emissions. Although thermospheric neutral winds are smoothed by viscosity and inertia, observed responses to magnetospheric forcing still include wind responses on time scales as short as 10 minutes or less, and on length scales shorter than 100 km horizontally and 5 km vertically. The data also show that the degree of penetration of magnetospheric forcing into the lower thermospheric wind field is highly variable from day to day. Signatures of magnetospheric forcing are sometimes seen at altitudes as low as 120 km, whereas at other times the E-region does not seem to respond at all. Possible links will be explored between this variability and the day to day differences seen in the column integrated thermospheric [O]/[N2] ratio over Alaska.

  15. Exciton transport and dissociation at organic interfaces

    NASA Astrophysics Data System (ADS)

    Beljonne, David

    2011-03-01

    This paper focuses on modeling studies of exciton transport and dissociation at organic interfaces and includes three parts: 1) Experiments have shown that the values of exciton diffusion length LD in conjugated polymers (CPs) are rather low, in the range of 5-10 nm, apparently regardless of their chemical structure and solid-state packing. In contrast, larger LD values have been reported in molecular materials that are chemically more well-defined than CPs. Here we demonstrate that energetic disorder alone reduces the exciton diffusion length more than one order of magnitude, from values typically encountered in molecules (> 50nm) to values actually measured in CPs (<10nm). 2) A number of organic crystals show anisotropic excitonic couplings, with weak interlayer interactions between molecules that are more strongly coupled within the layers. The resulting energy carriers are intra-layer 2D excitons that diffuse along the interlayer direction. We model this analytically for infinite layers and using quantum-chemical calculations of the electronic couplings for anthracene clusters. We show that the exciton hopping rates and diffusion lengths depend in a subtle manner on the size and shape of the interacting aggregates, temperature and the presence of energetic disorder. 3) The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organic-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules. We will discuss how the quadrupoles on the pentacene molecules produce direct electrostatic interactions with charge carriers and how

  16. Temperature Evolution of Excitonic Absorptions in Cd(1-x)Zn(x)Te Materials

    NASA Technical Reports Server (NTRS)

    Quijada, Manuel A.; Henry, Ross

    2007-01-01

    The studies consist of measuring the frequency dependent transmittance (T) and reflectance (R) above and below the optical band-gap in the UV/Visible and infrared frequency ranges for Cd(l-x),Zn(x),Te materials for x=0 and x=0.04. Measurements were also done in the temperature range from 5 to 300 K. The results show that the optical gap near 1.49 eV at 300 K increases to 1.62 eV at 5 K. Finally, we observe sharp absorption peaks near this gap energy at low temperatures. The close proximity of these peaks to the optical transition threshold suggests that they originate from the creation of bound electron-hole pairs or excitons. The decay of these excitonic absorptions may contribute to a photoluminescence and transient background response of these back-illuminated HgCdTe CCD detectors.

  17. Structure vs. excitonic transitions in self-assembled porphyrin nanotubes and their effect on light absorption and scattering.

    PubMed

    Arteaga, Oriol; Canillas, Adolf; El-Hachemi, Zoubir; Crusats, Joaquim; Ribó, Josep M

    2015-12-28

    The optical properties of diprotonated meso-tetrakis(4-sulphonatophenyl)porphyrin (TPPS(4)) J-aggregates of elongated thin particles (nanotubes in solution and ribbons when deposited on solid interfaces) are studied by different polarimetric techniques. The selective light extinction in these structures, which depends on the alignment of the nanoparticle with respect to the polarization of light, is contributed by excitonic absorption bands and by resonance light scattering. The optical response as a function of the polarization of light is complex because, although the quasi-one-dimensional structure confines the local fields along the nanotube axis, there are two orthogonal excitonic bands, of H- and J-character, that can work in favor of or against the field confinement. Results suggest that resonance light scattering is the dominant effect in solid state preparations, i.e. in collective groups (bundles) of ribbons but in diluted solutions, i.e. with isolated nanotubes, the absorption at the excitonic transitions remains dominant and linear dichroism spectra can be a direct probe of the exciton orientations. Therefore, by analyzing scattering and absorption data we can determine the alignment of the excitonic bands within the nanoparticle, i.e. of the orientation of the basic 2D porphyrin architecture in the nanoparticle. This is a necessary first step for understanding the directions of energy transport, charge polarization and non-linear optical properties in these materials.

  18. Exciton and charge carrier dynamics in few-layer WS2

    NASA Astrophysics Data System (ADS)

    Vega-Mayoral, Victor; Vella, Daniele; Borzda, Tetiana; Prijatelj, Matej; Tempra, Iacopo; Pogna, Eva A. A.; Dal Conte, Stefano; Topolovsek, Peter; Vujicic, Natasa; Cerullo, Giulio; Mihailovic, Dragan; Gadermaier, Christoph

    2016-03-01

    Semiconducting transition metal dichalcogenides (TMDs) have been applied as the active layer in photodetectors and solar cells, displaying substantial charge photogeneration yields. However, their large exciton binding energy, which increases with decreasing thickness (number of layers), as well as the strong resonance peaks in the absorption spectra suggest that excitons are the primary photoexcited states. Detailed time-domain studies of the photoexcitation dynamics in TMDs exist mostly for MoS2. Here, we use femtosecond optical spectroscopy to study the exciton and charge dynamics following impulsive photoexcitation in few-layer WS2. We confirm excitons as the primary photoexcitation species and find that they dissociate into charge pairs with a time constant of about 1.3 ps. The better separation of the spectral features compared to MoS2 allows us to resolve a previously undetected process: these charges diffuse through the samples and get trapped at defects, such as flake edges or grain boundaries, causing an appreciable change of their transient absorption spectra. This finding opens the way to further studies of traps in TMD samples with different defect contents.Semiconducting transition metal dichalcogenides (TMDs) have been applied as the active layer in photodetectors and solar cells, displaying substantial charge photogeneration yields. However, their large exciton binding energy, which increases with decreasing thickness (number of layers), as well as the strong resonance peaks in the absorption spectra suggest that excitons are the primary photoexcited states. Detailed time-domain studies of the photoexcitation dynamics in TMDs exist mostly for MoS2. Here, we use femtosecond optical spectroscopy to study the exciton and charge dynamics following impulsive photoexcitation in few-layer WS2. We confirm excitons as the primary photoexcitation species and find that they dissociate into charge pairs with a time constant of about 1.3 ps. The better

  19. Excitons in semiconducting superlattices, quantum wells, and ternary alloys

    SciTech Connect

    Sturge, M.D. . Dept. of Physics); Nahory, R.E.; Tamargo, M.C. )

    1992-06-01

    Semiconducting layered structures can now be fabricated with precisely defined layer thicknesses down to one monolayer. An example is the superlattice'' (SL) structure, in which to semiconductors with different band gaps are interleaved. The electronic and optical properties of the SL are quite different from those of the constitutents and offer interesting new possibilities both in device design and in basic physics. This proposal aims to improve our understanding of optically excited states in SL's, particularly in the so-called Type 2 indirect'' SL's in which in electron and hole created by optical excitation are separated both in real and in momentum space. We study these structures by time-resolved tunable laser spectroscopy, with and without external perturbations such as magnetic field, electric field, and uniaxial stress. In SLs with only a few atomic layers per period the familiar effective mass model'' of semiconductor states breaks down. We have made precise optical experiments on well-characterized material to test current first principles'' calculations of the band structure. Our work under this grant has shown that the material we are using is of sufficiently high quality to test the theoretical predictions. Comparison of theory and experiment provides a new and sensitive probe of the interface quality on a fine scale. Statistical analysis of the temperature dependence of the exciton decay dynamics provides complementary information. From a careful study of the exciton spectra of the recently discovered mixed type 1- type 2 CdTe/CdZnTe SLs we have obtained the band offset at the CdTe/CdZnTe interface to unprecedented accuracy.

  20. Giant optical enhancement of strain gradient in ferroelectric BiFeO3 thin films and its physical origin

    SciTech Connect

    Li, Yuelin; Adamo, C.; Chen, Pice; Evans, Paul G.; Nakhmanson, Serge M.; Parker, William; Rowland, Clare E.; Schaller, Richard D.; Schlom, Darrell G.; Walko, Donald A.; Wen, Haidan; Zhang, Qingteng

    2015-11-20

    Through mapping of the spatiotemporal strain profile in ferroelectric BiFeO3 epitaxial thin films, we report an optically initiated dynamic enhancement of the strain gradient of 105–106 m-1 that lasts up to a few ns depending on the film thickness. Correlating with transient optical absorption measurements, the enhancement of the strain gradient is attributed to a piezoelectric effect driven by a transient screening field mediated by excitons. In conclusion, these findings not only demonstrate a new possible way of controlling the flexoelectric effect, but also reveal the important role of exciton dynamics in photostriction and photovoltaic effects in ferroelectrics.

  1. Excitons in a quasi-one-dimensional quantum nanorod under a strong electric field

    SciTech Connect

    Lyo, S. K.

    2014-03-21

    The response of an exciton in the ground and first excited states to a strong DC electric field is studied in a quasi-one-dimensional nano quantum well (i.e., nanorod) bounded by high symmetric barriers by studying the energy, the oscillator strength, the root-mean-square (RMS) average of the electron-hole (e-h) separation, and the average positions of the electron and the hole. The interplaying effect between the barrier confinement, e-h attraction, and the field-induced e-h separation for exciton binding is examined. We find that, for a long nanorod, the exciton energy, as well as, the oscillator strength drops abruptly as a function of the field near the exciton-dissociation field while the RMS average of the e-h separation rises rapidly. For shorter rods, the transition is more gradual due to the combined effect of the confinement and the long-range e-h interaction. A strong field is shown to transform the optically-inactive first excited state into an optically-active state in the field range between the dissociation field of the ground state and that of the first excited level. We also find that, in the ground state, the (lighter) electron is dragged by the (heavier) hole below the dissociation field. The dependence of the above mentioned properties on the rod length is also investigated for varying fields. The results are compared with those obtained for the rods with parabolic confinement.

  2. Biexciton formation and exciton coherent coupling in layered GaSe

    NASA Astrophysics Data System (ADS)

    Dey, P.; Paul, J.; Moody, G.; Stevens, C. E.; Glikin, N.; Kovalyuk, Z. D.; Kudrynskyi, Z. R.; Romero, A. H.; Cantarero, A.; Hilton, D. J.; Karaiskaj, D.

    2015-06-01

    Nonlinear two-dimensional Fourier transform (2DFT) and linear absorption spectroscopy are used to study the electronic structure and optical properties of excitons in the layered semiconductor GaSe. At the 1s exciton resonance, two peaks are identified in the absorption spectra, which are assigned to splitting of the exciton ground state into the triplet and singlet states. 2DFT spectra acquired for co-linear polarization of the excitation pulses feature an additional peak originating from coherent energy transfer between the singlet and triplet. At cross-linear polarization of the excitation pulses, the 2DFT spectra expose a new peak likely originating from bound biexcitons. The polarization dependent 2DFT spectra are well reproduced by simulations using the optical Bloch equations for a four level system, where many-body effects are included phenomenologically. Although biexciton effects are thought to be strong in this material, only moderate contributions from bound biexciton creation can be observed. The biexciton binding energy of ˜2 meV was estimated from the separation of the peaks in the 2DFT spectra. Temperature dependent absorption and 2DFT measurements, combined with "ab initio" theoretical calculations of the phonon spectra, indicate strong interaction with the A1 ' phonon mode. Excitation density dependent 2DFT measurements reveal excitation induced dephasing and provide a lower limit for the homogeneous linewidth of the excitons in the present GaSe crystal.

  3. Double strong exciton-plasmon coupling in gold nanoshells infiltrated with fluorophores

    SciTech Connect

    De Luca, A. Dhama, R.; Rashed, A. R.; Coutant, C.; Ravaine, S.; Barois, P.; Infusino, M.; Strangi, G.

    2014-03-10

    We report on the broadband resonant energy transfer processes observed in dye doped gold nanoshells, consisting of spherical particles with a dielectric core (SiO{sub 2}) covered by a thin gold shell. The silica core has been doped with rhodamine B molecules in order to harness a coherent plasmon-exciton coupling between chromophores and plasmonic shell. This plasmon-exciton interplay depends on the relative spectral position of their bands. Here, we present a simultaneous double strong coupling plasmon-exciton and exciton-plasmon. Indeed, experimental observations reveal of a transmittance enhancement as function of the gain in a wide range of optical wavelengths (about 100 nm), while scattering cross sections remains almost unmodified. These results are accompanied by an overall reduction of chromophore fluorescence lifetimes that are a clear evidence of nonradiative energy transfer processes. The increasing of transmission in the range of 630–750 nm is associated with a striking enhancement of the extinction cross-section in the 510–630 nm spectral region. In this range, the system assumes super-absorbing features. This double behavior, as well as the broadband response of the presented system, represents a promising step to enable a wide range of electromagnetic properties and fascinating applications of plasmonic nanoshells as building blocks for advanced optical materials.

  4. Saturation behaviour of colloidal PbSe quantum dot exciton emission coupled into silicon photonic circuits.

    PubMed

    Foell, Charles A; Schelew, Ellen; Qiao, Haijun; Abel, Keith A; Hughes, Stephen; van Veggel, Frank C J M; Young, Jeff F

    2012-05-01

    We report coupling of the excitonic photon emission from photoexcited PbSe colloidal quantum dots (QDs) into an optical circuit that was fabricated in a silicon-on-insulator wafer using a CMOS-compatible process. The coupling between excitons and sub-μm sized silicon channel waveguides was mediated by a photonic crystal microcavity. The intensity of the coupled light saturates rapidly with the optical excitation power. The saturation behaviour was quantitatively studied using an isolated photonic crystal cavity with PbSe QDs site-selectively located at the cavity mode antinode position. Saturation occurs when a few μW of continuous wave HeNe pump power excites the QDs with a Gaussian spot size of 2 μm. By comparing the results with a master equation analysis that rigorously accounts for the complex dielectric environment of the QD excitons, the saturation is attributed to ground state depletion due to a non-radiative exciton decay channel with a trap state lifetime ~ 3 μs.

  5. Saturation behaviour of colloidal PbSe quantum dot exciton emission coupled into silicon photonic circuits.

    PubMed

    Foell, Charles A; Schelew, Ellen; Qiao, Haijun; Abel, Keith A; Hughes, Stephen; van Veggel, Frank C J M; Young, Jeff F

    2012-05-01

    We report coupling of the excitonic photon emission from photoexcited PbSe colloidal quantum dots (QDs) into an optical circuit that was fabricated in a silicon-on-insulator wafer using a CMOS-compatible process. The coupling between excitons and sub-μm sized silicon channel waveguides was mediated by a photonic crystal microcavity. The intensity of the coupled light saturates rapidly with the optical excitation power. The saturation behaviour was quantitatively studied using an isolated photonic crystal cavity with PbSe QDs site-selectively located at the cavity mode antinode position. Saturation occurs when a few μW of continuous wave HeNe pump power excites the QDs with a Gaussian spot size of 2 μm. By comparing the results with a master equation analysis that rigorously accounts for the complex dielectric environment of the QD excitons, the saturation is attributed to ground state depletion due to a non-radiative exciton decay channel with a trap state lifetime ~ 3 μs. PMID:22565670

  6. Effect of Crystal Packing on the Excitonic Properties of Rubrene Polymorphs

    NASA Astrophysics Data System (ADS)

    Wang, Xiaopeng; Garcia, Taylor; Monaco, Stephen; Schatschneider, Bohdan; Marom, Noa

    Singlet fission, the down-conversion of one singlet exciton into two triplet excitons, has been recently observed in molecular crystals of rubrene. The orthorhombic form of rubrene is the most stable in ambient conditions. However, rubrene has two additional known polymorphs, a triclinic form and a monoclinic form. To investigate the relative stability of the three polymorphs under different temperature and pressure conditions we use dispersion-inclusive density functional theory (DFT) with the pairwise Tkatchenko-Scheffler (TS) method and the many-body dispersion (MBD) method. Many-body perturbation theory is then employed to study the effect of crystal structure on the electronic and excitonic properties. Band structures are calculated within the GW approximation, where G is the one-particle Green's function and W is the screened Coulomb interaction, and optical properties are calculated by solving the Bethe-Salpeter equation (BSE). We find that crystal packing affects the band gaps, band dispersion, optical gaps, singlet-triplet gaps, and exciton localization in the three polymorphs of rubrene. Singlet fission efficiency may thus be improved by modulating the crystal packing.

  7. Biexciton formation and exciton coherent coupling in layered GaSe

    SciTech Connect

    Dey, P.; Paul, J.; Stevens, C. E.; Glikin, N.; Karaiskaj, D.; Moody, G.; Kovalyuk, Z. D.; Kudrynskyi, Z. R.; Romero, A. H.; Cantarero, A.; Hilton, D. J.

    2015-06-07

    Nonlinear two-dimensional Fourier transform (2DFT) and linear absorption spectroscopy are used to study the electronic structure and optical properties of excitons in the layered semiconductor GaSe. At the 1s exciton resonance, two peaks are identified in the absorption spectra, which are assigned to splitting of the exciton ground state into the triplet and singlet states. 2DFT spectra acquired for co-linear polarization of the excitation pulses feature an additional peak originating from coherent energy transfer between the singlet and triplet. At cross-linear polarization of the excitation pulses, the 2DFT spectra expose a new peak likely originating from bound biexcitons. The polarization dependent 2DFT spectra are well reproduced by simulations using the optical Bloch equations for a four level system, where many-body effects are included phenomenologically. Although biexciton effects are thought to be strong in this material, only moderate contributions from bound biexciton creation can be observed. The biexciton binding energy of ∼2 meV was estimated from the separation of the peaks in the 2DFT spectra. Temperature dependent absorption and 2DFT measurements, combined with “ab initio” theoretical calculations of the phonon spectra, indicate strong interaction with the A{sub 1}{sup ′} phonon mode. Excitation density dependent 2DFT measurements reveal excitation induced dephasing and provide a lower limit for the homogeneous linewidth of the excitons in the present GaSe crystal.

  8. Vapor-transport growth of high optical quality WSe{sub 2} monolayers

    SciTech Connect

    Clark, Genevieve; Wu, Sanfeng; Rivera, Pasqual; Finney, Joseph; Nguyen, Paul; Cobden, David H.; Xu, Xiaodong

    2014-10-01

    Monolayer transition metal dichalcogenides are atomically thin direct-gap semiconductors that show a variety of novel electronic and optical properties with an optically accessible valley degree of freedom. While they are ideal materials for developing optical-driven valleytronics, the restrictions of exfoliated samples have limited exploration of their potential. Here, we present a physical vapor transport growth method for triangular WSe{sub 2} sheets of up to 30 μm in edge length on insulating SiO{sub 2} substrates. Characterization using atomic force microscopy and optical microscopy reveals that they are uniform, monolayer crystals. Low temperature photoluminescence shows well resolved and electrically tunable excitonic features similar to those in exfoliated samples, with substantial valley polarization and valley coherence. The monolayers grown using this method are therefore of high enough optical quality for routine use in the investigation of optoelectronics and valleytronics.

  9. An Analytical Solution for Exciton Generation, Reaction, and Diffusion in Nanotube and Nanowire-Based Solar Cells.

    PubMed

    Bellisario, Darin O; Paulson, Joel A; Braatz, Richard D; Strano, Michael S

    2016-07-21

    Excitonic solar cells based on aligned or unaligned networks of nanotubes or nanowires offer advantages with respect of optical absorption, and control of excition and electrical carrier transport; however, there is a lack of predictive models of the optimal orientation and packing density of such devices to maximize efficiency. Here-in, we develop a concise analytical framework that describes the orientation and density trade-off on exciton collection computed from a deterministic model of a carbon nanotube (CNT) photovoltaic device under steady-state operation that incorporates single- and aggregate-nanotube photophysics published earlier (Energy Environ Sci, 2014, 7, 3769). We show that the maximal film efficiency is determined by a parameter grouping, α, representing the product of the network density and the effective exciton diffusion length, reflecting a cooperativity between the rate of exciton generation and the rate of exciton transport. This allows for a simple, master plot of EQE versus film thickness, parametric in α allowing for optimal design. This analysis extends to any excitonic solar cell with anisotropic transport elements, including polymer, nanowire, quantum dot, and nanocarbon photovoltaics. PMID:27357970

  10. Dynamical phase transitions and pattern formation induced by a pulse pumping of excitons to a system near a thermodynamic instability

    NASA Astrophysics Data System (ADS)

    Brazovskii, Serguei; Kirova, Natasha

    2016-08-01

    We suggest a phenomenological theory of dynamical phase transitions and the subsequent spaciotemporal evolution induced by a short optical pulse in a system which is already prone to a thermodynamic instability. We address the case of pumping to excitons whose density contributes additively to the thermodynamic order parameter like for charge-transfer excitons in electronic charge-ordering transitions. To describe both thermodynamic and dynamical effects on equal footing, we adopt for the phase transition a view of the "excitonic insulator" (EI) and suggest a formation of the macroscopic quantum state for the pumped excitons. The double nature of the ensemble of excitons leads to an intricate time evolution: the dynamical transition between number-preserved and phase-locked regimes, macroscopic quantum oscillations from interference between the Bose condensate of excitons, and the ground state of the EI. Modeling for an extended sample shows also stratification in domains of low and high densities which evolve through local dynamical phase transitions and a sequence of domain merges.

  11. Plasmon-Exciton Coupling Using DNA Templates.

    PubMed

    Roller, Eva-Maria; Argyropoulos, Christos; Högele, Alexander; Liedl, Tim; Pilo-Pais, Mauricio

    2016-09-14

    Coherent energy exchange between plasmons and excitons is a phenomenon that arises in the strong coupling regime resulting in distinct hybrid states. The DNA-origami technique provides an ideal framework to custom-tune plasmon-exciton nanostructures. By employing this well controlled self-assembly process, we realized hybrid states by precisely positioning metallic nanoparticles in a defined spatial arrangement with fixed nanometer-sized interparticle spacing. Varying the nanoparticle diameter between 30 nm and 60 nm while keeping their separation distance constant allowed us to precisely adjust the plasmon resonance of the structure to accurately match the energy frequency of a J-aggregate exciton. With this system we obtained strong plasmon-exciton coupling and studied far-field scattering at the single-structure level. The individual structures displayed normal mode splitting up to 170 meV. The plasmon tunability and the strong field confinement attained with nanodimers on DNA-origami renders an ideal tool to bottom-up assembly plasmon-exciton systems operating at room temperature. PMID:27531635

  12. Plasmon-Exciton Coupling Using DNA Templates.

    PubMed

    Roller, Eva-Maria; Argyropoulos, Christos; Högele, Alexander; Liedl, Tim; Pilo-Pais, Mauricio

    2016-09-14

    Coherent energy exchange between plasmons and excitons is a phenomenon that arises in the strong coupling regime resulting in distinct hybrid states. The DNA-origami technique provides an ideal framework to custom-tune plasmon-exciton nanostructures. By employing this well controlled self-assembly process, we realized hybrid states by precisely positioning metallic nanoparticles in a defined spatial arrangement with fixed nanometer-sized interparticle spacing. Varying the nanoparticle diameter between 30 nm and 60 nm while keeping their separation distance constant allowed us to precisely adjust the plasmon resonance of the structure to accurately match the energy frequency of a J-aggregate exciton. With this system we obtained strong plasmon-exciton coupling and studied far-field scattering at the single-structure level. The individual structures displayed normal mode splitting up to 170 meV. The plasmon tunability and the strong field confinement attained with nanodimers on DNA-origami renders an ideal tool to bottom-up assembly plasmon-exciton systems operating at room temperature.

  13. Photoinduced gap closure in an excitonic insulator

    NASA Astrophysics Data System (ADS)

    Golež, Denis; Werner, Philipp; Eckstein, Martin

    2016-07-01

    We study the dynamical phase transition out of an excitonic insulator phase after photoexcitation using a time-dependent extension of the self-consistent GW method. We connect the evolution of the photoemission spectra to the dynamics of the excitonic order parameter and identify two dynamical phase transition points marked by a slowdown in the relaxation: one critical point is connected with the trapping in a nonthermal state with reduced exciton density and the second corresponds to the thermal phase transition. The transfer of kinetic energy from the photoexcited carriers to the exciton condensate is shown to be the main mechanism for the gap melting. We analyze the low energy dynamics of screening, which strongly depends on the presence of the excitonic gap, and argue that it is difficult to interpret the static component of the screened interaction as the effective interaction of some low energy model. Instead we propose a phenomenological measure for the effective interaction which indicates that screening has minor effects on the low energy dynamics.

  14. Interaction-induced hopping phase in driven-dissipative coupled photonic microcavities.

    PubMed

    Rodriguez, S R K; Amo, A; Sagnes, I; Le Gratiet, L; Galopin, E; Lemaître, A; Bloch, J

    2016-01-01

    The Bose-Hubbard model (BHM) describes bosons hopping across sites and interacting on-site. Inspired by the success of BHM simulators with atoms in optical lattices, proposals for implementing the BHM with photons in coupled nonlinear cavities have recently emerged. Two coupled semiconductor microcavities constitute a model system where the hopping, interaction and decay of exciton polaritons-mixed light-matter quasiparticles-can be engineered in combination with site-selective coherent driving to implement the driven-dissipative two-site optical BHM. Here we explore the interplay of interference and nonlinearity in this system, in a regime where three distinct density profiles can be observed under identical driving conditions. We demonstrate how the phase acquired by polaritons hopping between cavities can be controlled through polariton-polariton interactions. Our results open new perspectives for synthesizing density-dependent gauge fields using polaritons in two-dimensional multicavity systems. PMID:27307038

  15. Interaction-induced hopping phase in driven-dissipative coupled photonic microcavities

    PubMed Central

    Rodriguez, S. R. K.; Amo, A.; Sagnes, I.; Le Gratiet, L.; Galopin, E.; Lemaître, A.; Bloch, J.

    2016-01-01

    The Bose-Hubbard model (BHM) describes bosons hopping across sites and interacting on-site. Inspired by the success of BHM simulators with atoms in optical lattices, proposals for implementing the BHM with photons in coupled nonlinear cavities have recently emerged. Two coupled semiconductor microcavities constitute a model system where the hopping, interaction and decay of exciton polaritons—mixed light-matter quasiparticles—can be engineered in combination with site-selective coherent driving to implement the driven-dissipative two-site optical BHM. Here we explore the interplay of interference and nonlinearity in this system, in a regime where three distinct density profiles can be observed under identical driving conditions. We demonstrate how the phase acquired by polaritons hopping between cavities can be controlled through polariton-polariton interactions. Our results open new perspectives for synthesizing density-dependent gauge fields using polaritons in two-dimensional multicavity systems. PMID:27307038

  16. Interaction-induced hopping phase in driven-dissipative coupled photonic microcavities.

    PubMed

    Rodriguez, S R K; Amo, A; Sagnes, I; Le Gratiet, L; Galopin, E; Lemaître, A; Bloch, J

    2016-06-16

    The Bose-Hubbard model (BHM) describes bosons hopping across sites and interacting on-site. Inspired by the success of BHM simulators with atoms in optical lattices, proposals for implementing the BHM with photons in coupled nonlinear cavities have recently emerged. Two coupled semiconductor microcavities constitute a model system where the hopping, interaction and decay of exciton polaritons-mixed light-matter quasiparticles-can be engineered in combination with site-selective coherent driving to implement the driven-dissipative two-site optical BHM. Here we explore the interplay of interference and nonlinearity in this system, in a regime where three distinct density profiles can be observed under identical driving conditions. We demonstrate how the phase acquired by polaritons hopping between cavities can be controlled through polariton-polariton interactions. Our results open new perspectives for synthesizing density-dependent gauge fields using polaritons in two-dimensional multicavity systems.

  17. Exciton-Exciton Annihilation in Copper-Phthalocyanine Single-Crystal Nanowires

    SciTech Connect

    Ma, Yingzhong; Xiao, Kai; Shaw, Robert W

    2012-01-01

    Femtosecond one-color pump-probe spectroscopy was applied to study exciton dynamics in single-crystal copper-phthalocyanine (CuPc) nanowires grown on an opaque silicon substrate. The transient reflectance kinetics measured at different pump fluences exhibit a remarkable intensity-dependent decay behavior which accelerates significantly with increasing pump pulse intensity. All the kinetic decays can be satisfactorily described using a bi-exponential decay function with lifetimes of 22 and 204 ps, and corresponding relative amplitudes depending on the pump intensity. The accelerated decay behavior observed at high pump intensities arises from a nonlinear exciton-exciton annihilation process. While this phenomenon has been found previously in crystalline metallophthalocyanine (MPc) polymorphs such as colloidal particles and thin films, the results obtained using the CuPc nanowires are markedly distinct, namely, much longer decay times and a linear intensity dependence of the initial peak amplitudes. Despite these differences, detailed data analysis further shows that, as found for other metal-phthalocyanine polymorphs, exciton-exciton annihilation in the CuPc nanowires is one-dimensional (1D) diffusion-limited, which possibly involves intra-chain exciton diffusion along 1D molecular stacks. The significantly long-lived excitons of CuPc nanowires in comparison to those of other crystalline polymorphs make them particularly suitable for photovoltaic applications.

  18. Microscopic theory of two-dimensional spatially-indirect-exciton condensates and exciton-polariton condensates

    NASA Astrophysics Data System (ADS)

    Xue, Fei; Wu, Feng Cheng; MacDonald, Allan

    BEC of excitons and polaritons have drawn attention in recent years because of the demonstration of their ability to host macroscopic quantum phenomena and because of their promise for applications. We study the case of a system containing two TMD monolayers that are separated and surrounded by h-BN. Under appropriate conditions this system is expected to support a spatially indirect thermal equilibrium exciton condensate. We combine a microscopic mean-field calculation and a weakly interacting boson model to explore the bilayer exciton condensates phase diagram. By varying the layer separation and exciton density, we find a phase transition occurs between states containing one and two condensate flavors. We also use a microscopic time-dependent mean-field theory to address condensate collective mode spectra and quantum fluctuations. Next we study the case of exciton-polariton formed by strong coupling between quantum well excitons and confined photon modes when the system is placed in a vertical microcavity. We build a microscopic mean-field theory starting from electrons and holes, and account for their coupling to coherent light field. We compare our model with the normal weakly interacting boson model that starts from weakly interacting excitons that are coupled to photons. This work was supported by the SRC and NIST under the Nanoelectronic Research Initiative (NRI) and SWAN, by the Welch Foundation under Grant No. F1473, and by the ARO Grant No. 26-3508-81.

  19. Probing Bose-Einstein condensation of excitons with electromagnetic radiation.

    PubMed

    Johnsen, K; Kavoulakis, G M

    2001-01-29

    We examine the absorption spectrum of electromagnetic radiation from excitons, where an exciton in the 1s state absorbs a photon and makes a transition to the 2p state. We demonstrate that the absorption spectrum depends strongly on the quantum degeneracy of the exciton gas, and that it will generally manifest many-body effects. Based on our results we propose that absorption of infrared radiation could resolve recent contradictory experimental results on excitons in Cu(2)O.

  20. Site-dependence of van der Waals interaction explains exciton spectra of double-walled tubular J-aggregates.

    PubMed

    Megow, Jörg; Röhr, Merle I S; Schmidt am Busch, Marcel; Renger, Thomas; Mitrić, Roland; Kirstein, Stefan; Rabe, Jürgen P; May, Volkhard

    2015-03-14

    The simulation of the optical properties of supramolecular aggregates requires the development of methods, which are able to treat a large number of coupled chromophores interacting with the environment. Since it is currently not possible to treat large systems by quantum chemistry, the Frenkel exciton model is a valuable alternative. In this work we show how the Frenkel exciton model can be extended in order to explain the excitonic spectra of a specific double-walled tubular dye aggregate explicitly taking into account dispersive energy shifts of ground and excited states due to van der Waals interaction with all surrounding molecules. The experimentally observed splitting is well explained by the site-dependent energy shift of molecules placed at the inner or outer side of the double-walled tube, respectively. Therefore we can conclude that inclusion of the site-dependent dispersive effect in the theoretical description of optical properties of nanoscaled dye aggregates is mandatory. PMID:25620460