Showing papers on "Exciton published in 1989"

Excitonic and nonlinear-optical properties of dielectric quantum-well structures

Abstract: Excitonic and nonlinear-optical properties of dielectric quantum-well (DQW) structures are investigated theoretically. A DQW is a quantum well sandwiched by barrier materials with a smaller dielectric constant and a larger band gap than the well material. The fundamental physics determining the excitonic properties in a DQW, i.e., exciton binding energy, exciton oscillator strength, and nonlinear-optical response, are clarified. The most important mechanisms for enhancing the excitonic properties are quantum-confinement effect, mass-confinement effect, and dielectric-confinement effect. Quantum confinement increases the spatial overlap between an electron and a hole as a result of the potential well confinement, and it enhances oscillator strength. Mass confinement is based on the penetration of the carrier wave function into barrier layers with a heavier effective mass than the well layer. It increases the exciton reduced mass and hence the exciton binding energy. Dielectric confinement arises from the reduction of the effective dielectric constant of the whole system due to the penetration of the electric field into the barrier medium having a smaller dielectric constant than the well and enhances the Coulomb interaction between the electron and hole. On the basis of these analyses, the general guiding principles are established for designing DQW structures with optimum excitonic properties. Various practical examples of DQW's are examined with respect to the lattice-constant matching, the difference in the dielectric constant, and the difference in the carrier effective masses. ZnSe is found to be one of the most promising candidates for the barrier material of the GaAs DQW.

Biexciton states in semiconductor quantum dots and their nonlinear optical properties

Abstract: Biexciton states in semiconductor quantum dots (spherical microcrystallites) are investigated variationally, and the biexciton binding energy and the oscillator strength are calculated as a function of the quantum-dot radius, the electron-to-hole mass ratio, and the dielectric constant ratio of the semiconductor to the surrounding medium. The most important mechanisms for enhancing the biexciton binding energy and the oscillator strength are clarified. One is the quantum confinement effect, which increases the spatial overlap between carriers, leading to enhanced Coulomb interaction. Another is the dielectric confinement effect due to the dielectric constant discontinuity at the interface between a semiconductor microcrystallite and the surrounding medium. This effect arises from the penetration of electric force lines through the surrounding medium with a relatively small dielectric constant and leads to an enhancement of the Coulomb interaction. It is found that the frequency dispersion of the third-order nonlinear susceptibility ${\ensuremath{\chi}}^{(3)}$ shows an out-of-phase behavior at the one- and two-photon resonances, which is characteristic of the exciton and biexciton transitions. For typical materials which are promising for observation of the biexciton state in microcrystallites, the values of the biexciton binding energy, the third-order nonlinear susceptibility ${\ensuremath{\chi}}^{(3)}$, and the two-photon absorption coefficient ${K}_{2}$ of the biexciton state are predicted theoretically.

Linear- and nonlinear-optical properties of semiconductor clusters

Abstract: CdS and PbS clusters with sizes ranging from a few angstroms to 150 A can be synthesized in polymers. The dependence of the band gap on the cluster size deviates from the prediction of a simple particle-in-a-box model owing to the breakdown of the effective-mass approximation. Instead, the dependence can be described by a simple tight-binding cluster model. The optical nonlinearity, expressed as α2/α0, of 50-A CdS clusters in Nafion film is determined to be −6.1 × 10−7 cm2 W−1 at 480 nm. The nonlinearity originates from the bleaching of the excitonic absorption owing to the presence of trapped carriers on the cluster surfaces. By passivating the CdS surfaces with ammonia, we have shown that the nonlinearity can be controlled by surface chemistry. We have determined that the presence of one trapped electron–hole pair can bleach the excitonic absorption of the whole CdS cluster. This efficient bleaching can be understood by using a model that considers the shifting of the exciton resonance and weakening of its oscillator strength in the presence of a trapped electron or hole. We also discuss two new classes of material: superclusters in zeolites and surface-capped clusters. Both represent our first steps toward the systematic synthesis of clusters of controlled surfaces and sizes.

Strain effects and band offsets in GaAs/InGaAs strained layered quantum structures

Abstract: Strained single quantum wells composed of GaAs/InGaAs/GaAs were grown by molecular beam epitaxy and characterized at room temperature by photoreflectance and at 6 and 77 K by photoluminescence spectroscopy. For the InGaAs/GaAs heterojunction, utilizing a band offset ratio of 85:15 (conduction band:valence band) for the intrinsic (nonstrained) interface and a contribution of the hydrostatic compression to the valence band movement corresponding to the pressure sensitivity of the spin orbit band, excellent agreement is found between calculated excitonic transition energies and those found by experiment at all temperatures studied. Our analysis indicates that material parameters and the combined strain components used to calculate band structure are not temperature dependent to our degree of sensitivity. An empirical equation, which differs slightly from that for bulk InGaAs crystals, describing the nonstrained band‐gap energy as a function of In fraction at 77 K is presented. The difference between band offset ratios for the intrinsic and strained heterojunction are found to be significant and the relative merits of each are discussed.

Quaternary quantum wells for electro‐optic intensity and phase modulation at 1.3 and 1.55 μm

Abstract: We demonstrate for the first time that large quantum‐confined Stark effect can be obtained in high quality InGaAsP/InP quantum well p‐i‐n heterostructures. The compositional flexibility of this material system is particularly suited for quantum well device applications in optical communications systems operating between 1.55 and 1.3 μm. We measure the magnitude of the shift of the ground‐state exciton transition with applied electric field and find that it is significantly enhanced over ternary wells. We have also determined the electro‐optic intensity and phase modulation response in these structures.

The Liquid State and Its Electrical Properties

Abstract: Theories of Liquid Structure.- Field Theoretic Models of Liquids.- The Structure of Simple Fluids.- Dynamic Processes in Liquids and Selected Topics Related to the Dynamics of Ions and Electrons in Liquids.- Ionic and Electronic Processes.- Theories of Electrolyte Solutions.- Theoretical Studies of Electrons in Fluids.- Geometrical Perspectives of a Solvated Electron.- Electron Localization and Femtosecond Nonlinear Optical Responses in Liquids.- Strong Field Effects and Molecular Dynamics Simulations.- Electron Kinetics in Nonpolar Liquids - Energy and Pressure Effects.- Photoconductivity, Conduction Electron Energies, and Excitons in Simple Fluids.- Electron Scattering and Mobility in Dielectric Liquids.- Hot Electron Mobility and Electron Attachment in Non-Polar Liquids.- Gas/Liquid Transition: Interphase Physics.- Calculations of Vo and the Energy Dispersion of Electrons in Rare Gas Liquids.- Interfacial Phenomena.- Electrical Aspects of Liquid/Vapor, Liquid/Liquid, and Liquid/Metal Interfaces.- Space Charge Effects in Dielectric Liquids..- Breakdown and Conduction.- An Overview of Electrical Processes Leading to Dielectric Breakdown of Liquids.- Electron Scattering and Dielectric Breakdown in Liquid and Solid Dielectrics.- Streamers in Liquids.- Electric Conduction in Dielectric Liquids.- Measurement of Electrical Breakdown in Liquids.- Appendices.- Appendix A: Abstracts of Poster Papers.- Appendix B: Organizing Committee.- Appendix C: Lecturers.- Appendix D: Participants.

Impact of sidewall recombination on the quantum efficiency of dry etched InGaAs/InP semiconductor wires

Abstract: We have investigated the lateral width (Lx) dependence of the quantum efficiency of the excitonic recombination in etched InGaAs/InP wires (40 nm≤Lx≤5 μm). The analysis of data obtained at different temperatures implies that the intensity decay observed for narrow wires is due to the formation of an optically inactive (‘‘dead’’) layer and due to surface recombination.

Optical spectroscopy of extrinsic recombinations in gallium selenide

Abstract: Luminescence at energy lower than the absorption edge has been investigated in crystals of GaSe, containing different degrees of lattice disorder, as a function of temperature, of photoexcitation intensity, and of excitation energy. At low excitation intensity, the extrinsic luminescence is composed of broad overlapping bands that present a blue shift when the temperature increases. Their shape and intensity is strongly dependent on the laser excitation frequency and on the degree of lattice disorder. The results are discussed in terms of a model involving the recombination of shallow donors and free electrons with deep acceptors. These two recombination mechanisms and those involving free and bound excitons are found to be competitive. Their relative importance is strongly dependent on the concentration of structural defects, on the temperature, on the excitation intensity, and on the excitation frequency.

Electroluminescence from sulfur impurities in a p‐n junction formed in epitaxial silicon

Abstract: We characterize the behavior of electroluminescence from a sulfur‐related impurity complex in a p‐n junction formed in epitaxial silicon. The spectrum of the electroluminescence matches that of previously reported photoluminescence from sulfur impurities and persists to ∼150 K. In our structure, we find that the electroluminescence exhibits an external quantum efficiency of 0.2–0.5%.

Dynamic Stark effect of exciton and continuum states in CdS

Abstract: The first observation of the optical Stark effect of electron-hole continuum states in addition to the bound-exciton states is reported under femtosecond excitation conditions chosen to minimize the generation of real carriers. The experimental results agree well with calculations using the generalized semiconductor Bloch equations. The theory shows that the commonly used adiabatic approximation is correct only for pulses which are longer than the coherence decay time. For shorter pulses coherent dynamic effects strongly influence the spectral changes.

Time-Resolved Excitonic Luminescence Processes in Poly(phenylenevinylene)

Abstract: Integrated luminescence spectra, time-resolved luminescence spectra, and luminescence decay times of stretch-oriented poly(phenylenevinylene) (PPV) are studied in the temperature range 300∼12 K. The reflectance spectrum at room temperature is also studied. The luminescence spectrum is composed of vibronic bands and a broad band, which are interpreted as caused by radiative annihilation of free and selftrapped excitons, respectively. Temperature dependence of the intensity of free-exciton luminescence and temperature dependence of the decay time are interpreted in terms of relaxation of excitons. The height of a potential barrier, which separates the free exciton state and the self-trapped state, is found to be 350 cm -1 . At 20 K, exciton relaxation toward the self-trapped state occurs by a quantum-mechanical tunneling process. The tunneling rate obtained is (90 ps) -1 .

Effects of Hartree, exchange, and correlation energy on intersubband transitions

Abstract: We calculate the effects of the Hartree and the exchange‐correlation potential on the subband levels of a doped GaAs/AlxGa1−xAs quantum well within the local density approximation. The intersubband transition energy appropriate to infrared detectors is calculated including both exciton and depolarization shifts. These effects are all known to be very important in the Si inversion layer and are shown here to be significant for the intersubband transitions in doped quantum wells. The effects of an applied electric field on these energy levels is explicitly included in the calculation. The Hartree potential is shown to effectively screen the electric field significantly reducing the Stark effect.

GaAs/AlGaAs quantum‐well intermixing using shallow ion implantation and rapid thermal annealing

Abstract: Low‐energy As+‐ion implantation followed by rapid thermal annealing (RTA) was utilized to modify exciton transition energies of GaAs/AlGaAs quantum wells (QW). A variety of structures were irradiated at an energy low enough that the disordered region was spatially separated from the QWs. After RTA, exciton energies showed large increases which were dependent on QW widths and the implantation fluence with no significant increases in peak linewidths. The observed energy shifts were interpreted as resulting from the modification of the shapes of the as‐grown QWs due to enhanced Ga and Al interdiffusion at heterointerfaces in irradiated areas. These results are consistent with the model of enhanced intermixing of Al and Ga atoms in depth of the material due to diffusion of vacancies generated near the surface.

One-phonon resonant Raman scattering: Fröhlich exciton-phonon interaction.

Abstract: An explicit expression for forbidden Raman scattering by one LO phonon with excitons as intermediate states is given. The theory can be applied at photon frequencies below and above the exciton energy. The matrix elements corresponding to transitions between different exciton states are calculated analytically. The different contributions to the squared Raman polarizability are compared; the most important one is found to be due to discrete-continuous transitions. It is shown in this case that the outgoing resonance in the Raman efficiency is always higher than the incoming one, a peculiarity seen experimentally in all III-V compound semiconductors. From the theoretical model a general criterion for the application of the free--electron-hole--pair theory is given in terms of the exciton Bohr radius. An analysis of the interference effect between the allowed and forbidden scattering is presented, and qualitative and quantitative differences with the free--electron-hole--pair theory are discussed. Absolute values of Raman polarizabilities are calculated and compared with recent measurements for GaP, yielding a good agreement without the use of any fit parameter.

Magnetic field effects on triplet exciton fission and fusion in a polydiacetylene

Abstract: We have studied the origin and decay dynamics of triplet excitons in the conjugated polymer poly(4BCMU) in its sol(yellow) and gel(red) phases. Wavelength and intensity dependencies of the triplet yield show that the triplet exciton cannot be produced by excitation into the singlet exciton edge but only from higher lying states. The observed lifetime of the triplet state, coupled with the magnetic field dependence of the triplet state production and decay, indicate that the triplet state is created by a fission process from the excited singlet. The time and magnetic field dependence of the triplet exciton decay indicate that the triplet exciton decay in the red phase occurs by diffusive bimolecular fusion, but in the yellow phase the triplet exciton decay is nondiffusive. We postulate that the unimolecular decay of the triplet exciton in the yellow phase results from exciton pinning by conformational disorder.

Two-photon absorption spectroscopy in GaAs quantum wells.

Abstract: Because the two-photon transition selection rules are different from those for linear absorption, levels which are not accessible In one-photon spectroscopies can be observed. Here we report an experimental investigation of two-photon absorption (TPA) spectroscopy in GaAs/AIGaAS quantum well structures. Experiments employed a waveguide geometry to enable the polarization of the incident light e either perpendicular or parallel to the confinement direction ẑ. Two samples with well thickness lz of 40 and 110 A were used. Distinct differences in TPA between the two polarization Configurations were observed and shown in Figs. 1 and 2.

Effect of Size Confinement on the Electronic States of CdS Cluster in a Germanium Oxide Matrix

Abstract: Change in the electronic state due to size confinement was studied for the CdS cluster embedded in GeO2 glass by photoacoustic, optical absorption and photoluminescence spectroscopies. The cluster size increases with increasing annealing temperature and time. The activation energy of ion diffusion during cluster growth is 0.45 eV. When the cluster size becomes less than about twice the exciton size, the energy bands become discrete. The energies of 1st and 2nd excited states increase with decreasing cluster size. The size dependence of the ist excited state determined experimentally coincides with the calculation. The energy of the surface state also shifts in parallel with the shift of the 1st excited state. When the cluster size decreases, the intensity of the edge emission decreases faster than that of the surface emission.

Optical nonlinearities and phase coherence in CdSe studied by transient four-wave mixing

Abstract: A picosecond time-resolved four-wave mixing technique is used to study the dephasing of nonlinear optical resonances in CdSe. At low temperature strong coherent, nonlinear signals at the free exciton resonance, at the biexciton resonance, and at the exciton-biexciton transition are observed. The observed correlation traces, i.e. the diffracted intensity versus the delay between the interfering light pulses, yield information on the dephasing process. Dephasing times for excitons as long as 40 ps, decreasing with increasing excitation density and temperature, are observed. A two-level model is applied to describe theoretically the dynamical behavior of the resonances, and to extract their homogeneous broadening, as well as their inhomogeneous broadening. >

Exciton versus Interband Absorption in Peierls Insulators

Abstract: The effect of long-range Coulomb interaction between an electron and a hole on the optical absorption spectrum of a one-dimensional Peierls insulator is studied theoretically. Energy levels and oscillator strengths of exciton states in the Peierls gap and the line shape of the continuous spectrum (interband transitions) above the gap are calculated by taking the interatomic nature of the optical transition matrix element into account. The effect turns out to be of great significance, in the sense that even a very weak interaction induces an anomalously large oscillator strength of the lowest exciton ( n =0) state.

Excitons in one-phonon resonant Raman scattering: Deformation-potential interaction

Abstract: A theory of one-phonon resonant Raman scattering in diamond and zinc-blende-type semiconductors which includes excitonic effects has been developed. The theory can be applied at frequencies below and above the band gap. We have considered the deformation-potential interaction for the electron--one-phonon coupling, and discrete and continuous exciton states have been taken as intermediate states in the process. The interband transitions between different valence bands (heavy and light holes and split-off bands) included in the calculation of the Raman tensor are characterized by excitonic states with different Bohr radii. General analytical expressions of the matrix elements corresponding to different transitions between excitonic states (discrete-discrete, discrete-continuous, and continuous-continuous) are reported as a function of the Bohr-radius ratio. A simplified expression of the Raman tensor obtained under the assumption of the same Bohr radii for both excitonic transitions is given. These results are used to calculate the absolute value of the Raman efficiency in the ${E}_{0}$ and ${E}_{0}$+${\ensuremath{\Delta}}_{0}$ absorption edge of III-V compound semiconductors. Comparison with the electron-hole uncorrelated theory and with the corresponding experimental data recently reported for GaP, GaAs, and InP explains these spectra and emphasizes the decisive role of excitons in the one-phonon resonant Raman scattering.

Coupled GaAs/AlGaAs quantum-well electroabsorption modulators for low-electric-field optical modulation

Abstract: Experimental and theoretical studies are presented for exciton transitions in p‐i‐n GaAs/AlGaAs multiple coupled‐quantum‐well structures where each quantum well consists of two identical wells with a thin barrier. Electroabsorption and photocurrent studies are carried out to identify how the excitonic peaks respond to transverse electric fields. With a careful choice of the dimensions of the coupled quantum well, it is seen that the lowest heavy‐hole exciton peak moves at a rate ∼2.5 faster than in a square well. Thus strong modulation is obtained at much lower electric fields. The nature of the higher‐energy transitions is also studied.

Room‐temperature exciton electroabsorption in partially intermixed GaAs/AlGaAs quantum well waveguides

Abstract: Perpendicular field electroabsorption is measured for the first time in GaAs/AlGaAs quantum well (QW) structures which have been modified via partial interdiffusion of the well and barrier layers. In waveguide samples containing two GaAs QWs, the impurity‐free vacancy diffusion process is shown to allow continuously variable permanent band‐edge energy shifts of at least 40 meV while still retaining clearly resolved heavy hole and light hole exciton absorption peaks at room temperature. Furthermore, the quantum‐confined Stark effect is shown to be preserved in the partially intermixed structures, greatly expanding the range of photon energies over which such behavior can be utilized in a single epitaxially grown sample. Transmission resonance calculations are used to model the observed enhanced electric‐field‐induced broadening of exciton absorption peaks in the partially intermixed QWs due to increased carrier tunneling through the graded and lowered potential barriers.