Showing papers on "Exciton published in 1988"
TL;DR: Recently discovered theories of heterogeneous reaction kinetics have dramatic consequences, such as fractal orders for elementary reactions, self-ordering and self-unmixing of reactants, and rate coefficients with temporal "memories".
Abstract: Classical reaction kinetics has been found to be unsatisfactory when the reactants are spatially constrained on the microscopic level by either walls, phase boundaries, or force fields. Recently discovered theories of heterogeneous reaction kinetics have dramatic consequences, such as fractal orders for elementary reactions, self-ordering and self-unmixing of reactants, and rate coefficients with temporal "memories." The new theories were needed to explain the results of experiments and supercomputer simulations of reactions that were confined to low dimensions or fractal dimensions or both. Among the practical examples of "fractal-like kinetics" are chemical reactions in pores of membranes, excitation trapping in molecular aggregates, exciton fusion in composite materials, and charge recombination in colloids and clouds.
777 citations
TL;DR: The nonlinear optical polarizability is shown to be greatly enhanced for an assembly of such microcrystallites as the exciton is quantized due to the confinement effect and the excitons in a single microCrystallite interact strongly enough to make theexcitons deviate from ideal harmonic oscillators.
Abstract: We analyze theoretically the oscillator strength and the third-order optical polarizability X 13, due to excitons in semiconductor microcrystallites. The nonlinear optical polarizability is shown to be greatly enhanced for an assembly of such microcrystallites as the exciton is quantized due to the confinement effect and the excitons in a single microcrystallite interact strongly enough to make the excitons deviate from ideal harmonic oscillators.
540 citations
TL;DR: The nonlinear optical phenomena are expected to have a fast response time of a picosecond in GaAs quantum wells and a subpicose Cond in CdS quantum wells through the short lifetime of excitons.
Abstract: An exciton has a macroscopic transition dipole moment because it is a coherent excitation over the whole crystal. The interaction of this exciton with a radiation field, which results in a polariton in a bulk crystal, brings about the rapid radiative decay of the exciton in low-dimensional systems due to breakdown of the translational symmetry. This large decay constant at the same time makes the excitons deviate from ideal bosons so that we have a large third-order optical susceptibility enhanced by the macroscopic transition dipole moment under near-resonant excitation. The nonlinear optical phenomena are expected to have a fast response time of a picosecond in GaAs quantum wells and a subpicosecond in CdS quantum wells through the short lifetime of excitons.
307 citations
TL;DR: In this paper, the influence of intense coherent laser fields on the electronic and optical properties of semiconductors has been investigated using nonequilibrium Green's function techniques and exploiting the analogies to superconducting and Bose-condensed systems.
Abstract: We consider the influence of intense coherent laser fields on the electronic and optical properties of semiconductors. Using nonequilibrium Green's-function techniques and exploiting the analogies to superconducting and Bose-condensed systems, we discuss the nature of the renormalizations and the collective excitations in the collisionless regime. Experimentally, this situation can be realized (i) under nonresonant excitation of virtual electron-hole pairs and (ii) under resonant excitation with ultrashort pulses. We explain the recently observed optical Stark effect as well as spectral hole burning and derive from first principles the longitudinal and transverse dielectric functions including exciton correlations.
271 citations
220 citations
TL;DR: In this paper, the third-order optical susceptibility of semiconductor microcrystallites is evaluated for different crystallite-size regimes ranging from weak quantum confinement, where only the center-of-mass motion of the electron-hole pairs is modified, all the way down to very small quantum dots, where the individual motion of electrons and holes is confined and the Coulomb attraction is unimportant.
Abstract: Optical nonlinearities in semiconductor microcrystallites are analyzed theoretically. The third-order optical susceptibility is evaluated for different crystallite-size regimes ranging from weak quantum confinement, where only the center-of-mass motion of the electron-hole pairs is modified, all the way down to very small quantum dots, where the individual motion of the electrons and holes is confined and the Coulomb attraction is unimportant. Large optical nonlinearities are computed for sufficiently narrow linewidths of the microcrystallites. It is predicted that the induced two-photon absorption resonance (biexciton resonance) shifts from below to above the exciton resonance when the crystallite radius is reduced from bulk to less than the exciton Bohr radius. The magnitude of the expected optical nonlinearities in the different confinement regimes is analyzed for various semiconductor materials.
167 citations
TL;DR: In this article, a many-body calculation of the electron-hole correlation enhancement is presented, which identifies this peak with the Mahan exciton, the result of the Coulomb interaction between the photoexcited hole in the valence band and the sea of electrons in the conduction band.
Abstract: The temperature-dependent optical absorption and luminescence spectra of GaAs/AlGaAs and InGaAs/InAlAs n-doped modulation-doped quantum wells is discussed with emphasis on the peak seen at the edge of the absorption spectra of these samples A many-body calculation of the electron-hole correlation enhancement is presented, which identifies this peak with the Mahan exciton-the result of the Coulomb interaction between the photoexcited hole in the valence band and the sea of electrons in the conduction band This calculation accounts for the strong dependence of the absorption edge peak on both the temperature and carrier concentration, in good qualitative agreement with experimental data and with previously published results The changes induced by the carriers on the subband structure through self-consistent calculations are also analyzed, and it is concluded that in these symmetric structures, the changes are small for achievable carrier densities >
156 citations
148 citations
TL;DR: In this article, the effect of laser-induced excitation on the luminescence and transient optical absorption induced by irradiation of crystalline SiO2 with an electron pulse have been studied.
Abstract: Linear polarisation, with respect to the z axis, and the effect of the subsequent laser-induced excitation on the luminescence and transient optical absorption induced by irradiation of crystalline SiO2 with an electron pulse have been studied. It is found that the luminescence spectrum consists of two bands peaked at 2.8 eV and at 2.5 eV and that the transition dipole moment of the former, which has been shown to be intrinsic, is nearly parallel to the z axis, while that of the latter is parallel to the x axis. In addition to the 5.2 eV transient optical absorption band, a satellite band at 4.2 eV is found to be induced by irradiation with an electron pulse. For both of these bands, the transition dipoles are found not to be parallel to any of the crystalline axes. Subsequent irradiation with a 4.0 or 5.6 eV laser pulse of a specimen irradiated with an electron pulse is found to eliminate both of these transient optical absorption bands and the 2.8 eV luminescence band. In view of previous work on optically detected magnetic resonance and volume changes induced by electron pulse irradiation, it is concluded that the 5.2 and 4.2 eV transient optical absorption bands and the 2.8 eV luminescence band are associated with self-trapped excitons. The existing models of self-trapped excitons are discussed on the basis of the present experimental results.
147 citations
TL;DR: In this article, a correlation between the microcrystal size and the blue shift of the exciton luminescence peak was established, and the quantum size effect on exciton was discussed.
Abstract: In alkali chlorides heavily doped with Cu+ ions, CuCl microcrystals of different sizes are grown under different heat treatments. Their size and shape are studied using small-angle X-ray scattering. A clear correlation is established between the microcrystal size and the blue shift of the exciton luminescence peak. The quantum size effect on the exciton is discussed. The exciton confinement, that is, the restriction of the exciton translational motion in one-dimensional quantum wells is found to be rather plausible for CuCl microcrystals in crystalline matrices.
Des microcristaux de CuCl de differentes tailles sont fabriques par differents traitements thermiques, dans des chlorures alcalins fortement dopes en ions Cu+. Leurs tailles et leur forme sont etudiees par diffusion aux rayons X a petit angle. Une relation bien determinee est mise en evidence entre la taille du microcristal et le deplacement vers le bleu du maximum de la raie de luminescence excitonique. L'effet de quantification dimensionnelle de l'exciton est discute. Le confinement de l'exciton, c'est a dire la restriction du deplacement de l'exciton dans un puit quantique a une dimension parait ětre le mecanisme le plus plausible dans des microcristaux de CuCl enfouis dans des matrices cristallines.
138 citations
TL;DR: In this paper, a simple photo-reflectance and electroreflectance setup is used in a laboratory to determine such important material parameters as alloy composition and carrier concentration in a very short time.
Abstract: Modulation spectroscopy is an optical characterization tool that can be of great utility to the materials scientist. We present here numerous examples where a simple photo-reflectance and electroreflectance setup is used in our laboratory to determine such important material parameters as alloy composition and carrier concentration in a very short time. For determining alloy composition in semiconductors, contactless room temperature photoreflectance is nearly as sensitive as low temperature photoluminescence. Examples will be given on how to determine: the effects of surface preparation and implant damage; alloy composition and carrier homogeneity for large area wafers to better than 1%; the segregation coefficient of isoelectronic impurities in bulk semiconductors; the sub-band energies in quantum well structures; and the presence and homogeneity of built-in electric fields in MODFET structures. Particular emphasis will be placed on band edge and exciton effects on the photoreflectance and on the criteria used to distinguish between them. Materials studied included Si doped GaAs, AlxGa1-xAs for variousx grown by OMVPE and MBE, bulk InP doped with iso-electronic As and Sb, and MODFET structures.
TL;DR: An effective-mass theory of the properties of excitons in isolatedGaAs in isolated GaAs and Al as quantum wells is presented and exciton mixing induced by the complicated valence-band structure is emphasized.
Abstract: An effective-mass theory of the properties of excitons in isolated GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum wells is presented. The phenomenon of exciton mixing induced by the complicated valence-band structure is emphasized. The effects of external perturbations such as electric and magnetic fields and uniaxial pressure normal to quantum wells grown in different crystal directions are calculated. Exciton mixing is found to cause many observable effects on transition energies and oscillator strengths.
TL;DR: In this article, a simple model of the excited state is discussed based on a coherent exciton extending over a certain domain, where the exciton moves over the aggregate, occasionally reaching the vicinity of an acceptor molecule.
Abstract: A monolayer of an oxacyanine dye organized in Scheibe aggregates is doped with a thiacyanine dye (e.g., one molecule of the guest per 104 molecules of the host). Strong energy transfer from the excited host to the guest takes place at room temperature, the efficiency decreasing with lowering temperature T. The rate of energy transfer is proportional to T. A simple model of the excited state is discussed based on a coherent exciton extending over a certain domain. The exciton moves over the aggregate, occasionally reaching the vicinity of an acceptor molecule. The domain size of the exciton is inversely proportional to the absolute temperature T. As a consequence, the lifetime of the exciton is proportional to T, and therefore the energy transfer efficiency is proportional to T.
TL;DR: Femtosecond measurements of coherent effects arising from exciton bleaching in bulk GaAs are reported and are shown to be a manifestation of the uncertainty relation in time-resolved spectroscopy.
Abstract: Femtosecond measurements of coherent effects arising from exciton bleaching in bulk GaAs are reported. This phenomenon, which is characterized by spectral oscillatory structures, is general and appears whenever the temporal resolution is shorter than the material coherence time. This is shown to be a manifestation of the uncertainty relation in time-resolved spectroscopy.
TL;DR: In this article, the authors describe fabrication and photoluminescence excitation of InGaAs/InP quantum wires with a lateral dimension of ∼350 A. The exciton energies agree with the theoretical predictions based on a new method of solving the two-dimensional effective mass Schrodinger equation.
Abstract: We describe fabrication and photoluminescence excitation of InGaAs/InP quantum wires with a lateral dimension of ∼350 A. Transverse confinement results in the splitting of the n=1 heavy hole‐electron transition. Three of these levels are observed in the excitation spectrum. The exciton energies agree with the theoretical predictions based on a new method of solving the two‐dimensional effective mass Schrodinger equation.
TL;DR: In this paper, the exciton binding energy increases with a smaller dielectric constant and a larger energy gap, so that the radiative decay rate and non-linear optical susceptibility are enhanced.
Abstract: Exciton effects are studied theoretically for a quantum well of a semiconductor sandwiched by barriers with a smaller dielectric constant and a larger energy gap. The exciton binding energy increases markedly so that the radiative decay rate of the exciton and the non-linear optical susceptibility are also shown to be enhanced.
TL;DR: In this paper, a GaAs/AlGaAs superlattices were partially intermixed via the impurity-free vacancy diffusion process, which allowed continuously variable energy shifts of at least 61 meV while still maintaining clearly resolved excitonic behavior.
Abstract: Substantial increases are observed in the energies of room‐temperature exciton transitions in GaAs/AlGaAs superlattices which have been partially intermixed via the impurity‐free vacancy diffusion process. Localized intermixing of the layered structure was accomplished by selective deposition of a SiO2 capping layer followed by rapid thermal annealing at temperatures between 850 and 950 °C for 15 s. In the samples studied, the above process allows continuously variable energy shifts of at least 61 meV while still maintaining clearly resolved excitonic behavior. Shifting and broadening of the exciton transitions are studied using room‐temperature photoluminescence and photocurrent spectroscopies. A transmission resonance calculation is used to determine the interdiffusion coefficient as a function of temperature from the measured energy shifts.
TL;DR: In this paper, the authors analyzed the excitonic properties in quantum-wire structures (QWS) by taking into account the band-mixing effects in the valence band of the structure and found that the effective mass value in the wire direction at the zone center of the lowest heavy-hole-like subband is as small as 0.027 m/sub 0/ for GaAs/AlGaAs QWS.
Abstract: The excitonic properties in quantum-wire structures (QWS) are analyzed by taking into account the band-mixing effects in the valence band of the structure. The effective mass value in the wire direction at the zone center of the lowest heavy-hole-like subband is found to be as small as 0.027 m/sub 0/ for GaAs/AlGaAs QWS. This reduced effective mass and the related nonparabolicity of the subband structure play a significant role in determining the exciton properties. Using these results, the maximum excitonic contribution to the refractive index value is estimated to be 0.59, i.e. 17.4% of the bulk value for a GaAs/Al/sub 0.4/Ga/sub 0.6/As QWS with a 50 A*50 AA cross section. This value is six times larger than that in the 50-AA quantum well. With an electric field of 8*10/sup 4/ V/cm perpendicular to the heterointerface, a maximum refractive index change 30% larger than this value is estimated. >
TL;DR: In this article, a variational theory is developed for excitons, especially accounting for the possibility of a small valence-band offset, and a band offset of 25 meV is deduced for the heavy-hole valence band, corresponding to a conduction-to-valence band offset ratio of about 14:1.
Abstract: The question of band offsets and details of exciton binding are investigated in the CdTe/(Cd,Mn)Te heterostructure in the quantum-well limit. Photoluminescence excitation spectroscopy in external magnetic fields is used to vary the quantum-well potential depths in this moderately strained (0.6% lattice mismatch) diluted magnetic semiconductor heterostructure. Large Zeeman splittings are observed at all the principal quantum-well transitions and at the barrier band gap for a structure of CdTe well thicknesses of 50 A\r{} and a Mn-ion concentration in the barrier layer of x=0.24. A variational theory is developed for excitons, especially accounting for the possibility of a small valence-band offset. Good agreement between theory and experiment is obtained and a band offset of 25 meV is deduced for the heavy-hole valence band, corresponding to a conduction- to valence-band offset ratio of about 14:1. This implies that the valence-band offset in a hypothetical strain-free case is virtually zero. The accuracy of the offset determination is believed to be better than 10 meV. Exciton binding energies are found to vary appreciably in the magnetic field; the zero-field value is approximately twice that for bulk CdTe.
TL;DR: In this article, the charge transfer excitation spectra of solid xenon doped with atomic halogens are presented, and their emission spectra are treated by a modified reflection approximation: reflection of the halogen-xenon radial distribution.
Abstract: The optically accessed charge transfer states of solid xenon doped with atomic halogens are excitonic in nature: an electron localized on the guest halogen atom and a delocalized hole centered on xenon atoms. These excitonic states are effectively self‐trapped such that luminescence is observed exclusively from the localized molecular charge transfer states: the triatomic xenon halide exciplexes. The latter relax radiatively. The emission spectra of Xe+2 I−, Xe+2 Br−,Xe+2 Cl−, and Xe+2 F− are centered at 390, 480, 573, and 775 nm, and their radiative lifetimes are 130, 185, 225, and 190 ns, respectively. The charge transfer excitation spectra of the atomic solids are presented. In the case of F doped solids, the vertical transitions correspond to the diatomic XeF (B←X) and (D←X) absorptions: fluorine is bound to xenon in the ground state. The heavier halogens isolate atomically. Their excitation spectra are treated by a modified reflection approximation: reflection of the halogen–xenon radial distribution...
TL;DR: The importance of dipolar interactions in a wide range of molecular crystal properties was highlighted in this article, showing that molecules respond via an effective polarizability to a local field arising from the applied field and the fields of the surrounding induced dipoles, determined self-consistently.
Abstract: The importance of dipolar interactions is highlighted in a wide range of molecular crystal properties. Molecules respond via an effective polarizability (appropriate to the crystal phase) to a local field arising from the applied field and the fields of the surrounding induced dipoles, determined self-consistently. An exact treatment is possible for perfect crystals and crystals with localized imperfections, yielding the inverse dielectric function characterizing the crystal response. From this, results follow for the crystal linear and nonlinear dielectric response, for Coulomb exciton energies, and for the intensities of lattice vibrational spectra. A direct extension yields the polarization energy associated with the induced dipoles. This provides insight into the energetics of excess charge carriers in perfect and imperfect crystals and of charge-transfer excitons, into electronic Stark spectra, and into aspects of lattice dynamics and phase transitions.
TL;DR: In this article, the confinement of excitons within a continuous two-dimensional GaAs-AlGaAs quantum well was demonstrated by partially etching a pattern through the upper AlGaAs barrier.
Abstract: We report evidence for lateral confinement of excitons within a continuous two‐dimensional GaAs‐AlGaAs quantum well. The confinement to ‘‘wires’’ within the well was produced by partially etching a pattern through the upper AlGaAs barrier. We propose a new mechanism, that of patterned strain, for lateral quantum confinement of carriers in semiconductor microstructures, to explain our results.
TL;DR: In this paper, the fundamental band gap and the exciton binding energies of polyacetylene have been evaluated with the aid of ab initio Hartree-Fock plus Mo/ller-Plesset perturbation theoretical calculations.
Abstract: The fundamental band gap and the exciton binding energies of t‐PA have been evaluated with the aid of ab initio Hartree–Fock plus Mo/ller–Plesset perturbation theoretical calculations. The irreducible self‐energy part has been expanded up to third order and the irreducible vertex part has been expanded up to second order. The results support one of several controversial assignments of the ∼2 eV peak in the experimental absorption spectrum of trans‐polyacetylene, namely that to interband transitions.
TL;DR: In this article, the Singlet-singlet-annihilation of the exciton domains of pseudoisocyanine (PIC) J-aggregates was proved by nonlinear spectroscopy.
TL;DR: In this article, the authors have grown ZnSe single crystals having various concentrations of Te atoms from a mixture of Te and Se solutions and observed an emission band having phonon structure and peaking at 2.67 eV in slightly Te-doped specimens.
TL;DR: Theory and experiments on steady-state and femtosecond time-resolved optical nonlinearities in semiconductors are reviewed in this article, where the spectral region around the fundamental absorption edge is considered.
Abstract: Theory and experiments on steady-state and femtosecond time-resolved optical nonlinearities in semiconductors are reviewed. A simple description of the physical processes underlying the nonlinearities is given. The discussion is focused on the spectral region around the fundamental absorption edge, and it covers coherent oscillations, the optical Stark effect as well as the bleaching of the exciton resonance with increasing excitation intensity, plasma screening and band-filling phenomena.
TL;DR: In this paper, the relationship between sample quality and exciton emission spectra is discussed, and the change in exciton photoluminescence power dependence with excitation wavelength indicates that the superlinear power dependence is due to the formation of excitons from photoexcited electrons and holes.
TL;DR: In this paper, the absorption coefficient for the electron heavy-hole exciton transition in a single quantum well sample is determined and a new understanding of the line shapes measured in photoreflectance experiments is provided.
Abstract: Reflectance experiments on GaAs/Ga1−xAlxAs single quantum well structures were performed at 4.2 K, with different thicknesses of the front GaAlAs barrier layer (100–1000 A). The observed exciton reflectance line shapes depend strongly on the thickness of the front barrier layer due to the interferences between the reflected waves from the front surface and the quantum well interfaces. Calculations of the reflectance line shapes show good agreement with the observations. The absorption coefficient for the electron heavy‐hole exciton transition in a single quantum well sample is determined. Our study also provides a new understanding of the line shapes measured in photoreflectance experiments.