Showing papers on "Exciton published in 1991"

Quantum crystallites and nonlinear optics

Abstract: This is a review and analysis of the optical properties of quantum crystallites, with principal emphasis on the electro-optic Stark effect and all optical third order nonlinearity. There are also introductory discussions on physical size regimes, crystallite synthesis, quantum confinement theory, and linear optical properties. The experiments describe CdSe crystallites, exhibiting strong confinement of electrons and holes, and CuCl crystallites, exhibiting weak confinement of the exciton center of mass. In the CdSe system, neither the Stark effect nor the third order nonlinearity is well understood. The Stark shifts appear to be smaller than calculated, and field inducted broadening also occurs. The third order nonlinearity is only modestly stronger than in bulk material, despite theoretical prediction. Unexpectedly large homogeneous widths, due to surface carrier trapping, in the nominally discrete crystallite excited states appear to be involved. The CuCl system shows far narrower spectroscopic homogeneous widths, and corresponds more closely to an ideal quantum dot in the weak confinement limit. CuCl also exhibits exciton superradiance at low temperature. Surface chemistry and crystallite encapsulation are critical in achieving the predicted large Stark and third order optical effects in II-VI and III-V crystallites.

Quantum Semiconductor Structures: Fundamentals and Applications

Abstract: Introduction. The Advent of Ultrathin, Well-Contained Semiconductor Heterostructures. A Prerequisite. The Mastering of Semiconductor Purity and Interfaces. The Electronic Properties of Thin Semiconductor Heterostructures. Quantum Well Energy Levels. Triangular Quantum Well Energy Levels. Two-Dimensional Density of States. Excitons and Shallow Impurities in Quantum Wells. Tunneling Structures, Coupled Quantum Wells, and Superlattices. Modulation Doping of Heterostructures. n-i-p-i Structures. Optical Properties of Thin Heterostructures. Optical Matrix Element. Selection Rules. Energy Levels, Band Discontinuities, and Layer Fluctuations. Low Temperature Luminescence. Carrier and Exciton Dynamics. Inelastic Light Scattering. Non-Linear and Electro-Optic Effects. Electrical Properties of Thin Heterostructures. Mobility in Parallel Transport. Hot Electron Effects in Parallel Transport. Perpendicular Transport. Quantum Transport. Applications of Quantized Semiconductor Heterostructures. Electronic Devices Based on Parallel Transport. Electronic Devices Based on Perpendicular Transport. Quantum Well Lasers. Towards 1D and 0D Physics and Devices. One- and Zero-Dimensional Systems. 1D and 0D Semiconductor Fabrication Techniques. Electrical Applications of 1D and 0D Structures. 1D and 0D Optical Devices. Selected Bibliography. References. Index.

Quantum well carrier sweep out: relation to electroabsorption and exciton saturation

Abstract: The authors studied the effects of changing the barrier design of GaAs-Al/sub x/Ga/sub 1-x/As quantum wells on the electroabsorption, exciton saturation, and carrier sweep-out times. Five samples with x values ranging from 0.2 to 0.4 and barrier thicknesses from 35 to 95 AA were studied. Within this range, the authors find that the electroabsorption is not very sensitive to the barrier thickness, but that the ionization field of the excitons approximately doubles for an increase of x from 0.2 to 0.4. The samples with high, thick barriers have lower internal quantum efficiencies than those with low, thin barriers. It was found that the exciton saturation intensity increases with increasing applied field, and decreasing barrier thickness or height. Time-resolved electroabsorption measurements confirm the variation in sweep-out rates between samples, and indicate that the escape mechanism at low field is probably a thermally-assisted tunneling process. >

Quantum confinement effects in semiconductor clusters

Abstract: The band gaps, band structure, and excited‐state (exciton) energies of CdS, GaAs, and GaP semiconductor clusters are calculated using pseudopotentials. In addition, the sensitivity of the exciton energies to the size, shape, crystal structure, and lattice constant of the unit cell are investigated. The calculated exciton energies of CdS clusters are in excellent agreement with experiment over a wide range of cluster sizes. Also, the exciton states of small CdS clusters are sensitive to whether their crystal structure is zinc blende or hexagonal. Such a sensitivity is absent in large CdS clusters. Furthermore, small GaAs clusters are shown to exhibit anomalous redshift of their absorption spectra, in sharp contrast to CdS and large GaAs clusters whose spectra always shift to blue with decreasing cluster size. Finally, the lowest‐energy non‐Franck–Condon transition in GaP clusters always shifts to blue with decreasing cluster size, whereas the higher‐energy Franck–Condon transition in small clusters exhibits the anomalous redshift. These novel findings reveal that (1) the optical spectroscopy of semiconductor clusters is strongly material and crystal structure dependent; (2) the spectroscopy of small clusters is dramatically different from those of large clusters and bulk; and (3) these effects cannot be explained, even qualitatively, using the effective‐mass approximation.

Subpicosecond spin relaxation dynamics of excitons and free carriers in GaAs quantum wells.

Abstract: We have obtained a coherent understanding of spin relaxation processes of electrons, holes, and excitons in quantum walls by investigating subpicosecond dynamics of luminescence polarization. We show that the spin behavior for electrons and holes in quasi-two-dimensional systems is distinct from that in bulk semiconductors and that many-body effects and formation process play an important role in exciton spin relaxation.

Excitons in anisotropic solids: The model of fractional-dimensional space

Abstract: Wannier-Mott excitons in anisotropic or confined systems are studied using the model of fractional-dimensional space. The excitons in an anisotropic solid are treated as ones in an isotropic fractional-dimensional space, where the dimension is determined by the degree of anisotropy. By solving the simple hydrogenic Schr\"odinger equation in the fractional-dimensional space, exciton wave functions, bound energies, and associated optical spectra are obtained as a function of spatial dimensionality. Dimensional behavior in binding energy, radial density, and angular momentum is discussed. The model provides a quantitative measure of anisotropy by a fractional dimension, as viewed from exciton dynamics, which can be determined experimentally from interband optical spectra. The results obtained here are also applicable to hydrogenic impurities in anisotropic solids.

Optical absorption and Sommerfeld factors of one-dimensional semiconductors: An exact treatment of excitonic effects.

Abstract: We investigate theoretically excitonic effects on the optical properties of one-dimensional (1D) semiconductors. In particular, absorption spectra near a band edge are exactly calculated within the effective-mass approximation for the 1D system with a direct allowed or forbidden gap. We employ two kinds of interaction potentials between an electron and a hole describing a modified Coulomb interaction and a short-range interaction, both of which are free from the well-known divergence problem of the 1D Coulomb system. The Sommerfeld factor, which is the absorption intensity ratio of the unbound (continuum) exciton to the free-electron-hole pair above the band edge, is found to be smaller than unity for the direct allowed transition, in striking contrast to the 3D and 2D cases. This peculiar feature is interpreted in terms of the anomalously strong concentration of the oscillator strength on the lowest discrete exciton state. On the other hand, for the direct forbidden transition, the Sommerfeld factor in the 1D system is larger than unity and shows similar behavior to those in the 3D and 2D cases. These properties hold irrespective of the interaction range of the electron-hole attractive potential. The feasibility of the model potentials is examined, and the Coulomb potential having a cusp-type cutoff is found to be the most effective to describe the potential in an actual semiconductor wire. A dielectric effect in the wire structure is shown to enhance these peculiar features of the 1D system.

Direct synthesis of corrugated superlattices on non-(100)-oriented surfaces.

Abstract: We report on the direct synthesis of superlattices with lateral corrugation of the interfaces on (211), (311), and (111) GaAs substrates by moelcular-beam epitaxy. Reflection electron diffraction directly shows the formation of arrays of macrosteps during epitaxial growth. High-resolution transmission electron microscopy confirms the transfer of the surface structure to the GaAs/AlAs interface which results in distinct energy shifts in the luminescence of GaAs/AlAs multilayer structures. The surace structure gives rise to lateral quantum-size effects which result in increased exciton continuum energies, in strong exciton-phonon interaction, and in pronounced optical anisotropy.

Dynamics of Frenkel excitons in disordered molecular aggregates

Abstract: This article reports on the optical dynamics in aggregates of pseudoisocyanine‐bromide and iodide. For PIC‐Br in an ethylene glycol/water glass, the results of resonance light scattering (RLS), time‐resolved emission, and photon echo decay measurements are discussed. Band structure calculations based on a linear‐chain model for the J aggregate have also been performed. The results show that the J band can be described as a disordered Frenkel exciton band in which superradiant states exist that extend over about 100 molecules. Numerical simulation studies of the J band, based on Anderson’s Hamiltonian with uncorrelated diagonal site energies, show that the ratio κ of the disorder parameter D over the nearest‐neighbor coupling parameter J12 is about 0.11. Using the frequency dependence of the ratio between the yields of vibrational fluorescence and Raman scattering as a probe, the dephasing process and derived parameters for the bath correlation function at three different temperatures have also been examin...

Evidence for exciton confinement in crystalline organic multiple quantum wells.

Abstract: Multiple-quantum-well structures based on two crystalline organic semiconductors, namely, 3,4,9,10 perylenetetracarboxylic dianhydride and 3,4,7,8 naphthalenetetracarboxylic dianhydride, have been grown by organic molecular-beam deposition. Both optical-absorption and time-resolved photoluminescence measurements reveal a significant effect on the binding energy and the radiative recombination probability of excitons due to localization of carriers. Variational calculations of the ground-state exciton energy in quantum wells have been done, and the results agree with the experimental data. This provides evidence for exciton confinement in organic quantum-well structures.

Analytical properties of polaron systems or: Do polaronic phase transitions exist or not?

Abstract: For more than 40 years it was thought that polaron- and exciton-phonon systems exhibited unexpected localization properties. Particular attention was paid to the so-called phonon-induced self-trapping transition, which, it was believed, should manifest itself as a point of nonanalyticity in the ground-state energy as a function of the electron-phonon coupling parameter. It will be demonstrated for a large class of (generalized Fr\"ohlich) models that no such transition exists. The dimensionality of space has no qualitative influence; insofar, an application of the authors' results to problems in lower dimensions (e.g., polarons in quantum wells) is straightforward. The same holds true if homogeneous external fields are involved; for example, a discontinuous mass stripping for magnetopolarons can be excluded. On the other hand, a phase-transition-like behavior will be found, if a polaron or exciton is exposed to a short-range potential, allowing a so-called pinning transition. The authors emphasize, however, that even in this case the transition is only modified, and not induced, by phonons.

Electroluminescence in organic thin films

Abstract: Mechanism of electroluminescence in organic dye films, mechanisms of carrier injection, carrier transport, carrier recombination, creation of molecular excitons, movement of molecular excitons, and emission from molecular excitons, is described. Using high performance electroluminescent devices, three attempts towards novel fields of photophysics and photochemistry of organic solids were performed. First, confinement of molecular excitons within a molecular-size area was reported. Second, emission from triplet excitons produced by electric excitation was observed. Third, the presence of quantum optical size effect of radiation field in organic thin films was demonstrated.

Excitonic and Raman properties of ZnSe/Zn1−xCdxSe strained‐layer quantum wells

Abstract: The optical properties of strained‐layer ZnSe/Zn0.86Cd0.14Se single quantum wells have been studied. The photoluminescence under direct and indirect excitation is investigated in detail. The temperature dependence of photoluminescence and resonant Raman scattering are investigated. Very strong 2LO‐phonon Raman scattering has been observed with Zn0.86Cd0.14Se quantum wells, where the scattered photon energy is in resonance with an exciton transition. Experimental exciton energies are compared with a finite‐square‐potential quantum‐well model including band nonparabolicity and the strain effect. Based on Hill’s theory [J. Phys. C 7, 521 (1974)] we have computed the band gap of Zn1−xCdxSe as a function of composition x.

Coherent propagation and quantum beats of quadrupole polaritons in Cu2O.

Abstract: Coherent propagation of quadrupole polaritons in ${\mathrm{Cu}}_{2}$O is demonstrated by time-resolved spectroscopy. This manifests itself in a strong distortion of the temporal shape of a picosecond optical pulse which is in resonance with the 1S exciton polariton. Oscillations with time-dependent period in the transmitted light intensity are quantitatively explained as resulting from quantum beats between the two branches of the exciton polariton. The analysis yields the homogeneous linewidth and oscillator strength.

Quantum size microcrystals grown using organometallic vapor phase epitaxy

Abstract: Needle‐shaped quantum size microcrystals as thin as 10 nm have been selectively grown by employing reduced pressure organometallic vapor phase epitaxy using trimethylgallium and arsine as source materials. The microcrystals grown within a SiO2 window area have their growth axes along the [111] direction. Transmission electron diffraction analysis shows that the crystal structure of microcrystals is consistent with the zinc‐blende structure of GaAs. The mechanism for growing the needle‐shaped crystals is similar to a vapor‐liquid‐solid (VLS) equilibrium phase growth model. From photoluminescence measurements at 4.2 K, it is found that the microcrystals show a very distinct spectra for free exciton and neutral acceptor‐bound exciton recombinations, meaning good crystal quality.

Exciton Green's-function approach to optical absorption in a quantum well with an applied electric field.

Abstract: An exciton Green's function is derived and used to calculate the polarization-dependent optical absorption in a semiconductor quantum well with an applied electric field. With use of the exciton (or Coulomb) Green's-function approach, the optical-absorption coefficient due to the bound and continuum states of excitons can be obtained simultaneously and this approach also takes into account the coupling between different subband pairs. This is in contrast with the conventional approach in which the 1s exciton bound state is calculated variationally and the continuum states are calculated simply using the Sommerfeld enhancement factor from the pure two-dimensional case without the correct quantum size effect. Also, the coupling between different subband pairs is usually neglected. We compare the numerical results of the Green's-function method with those of the commonly used variational method and find that the variational method overestimates the oscillator strength by 20% for the 1s bound state and by 50% for the continuum, although the 1s bound-state energy can be quite accurate. The numerical results using the exciton Green's function are compared with experimental data and found to be in very good agreement.

Optical properties of a two-dimensional electron gas: Evolution of spectra from excitons to Fermi-edge singularities.

Abstract: The evolution of the absorption and emission spectrum from an exciton to a Fermi-edge singularity as a function of a quasi-two-dimensional electron-gas density is examined. Band-gap renormalization, screening, shake up of the Fermi sea, and the effect of the finite hole mass are included. The real-time response of the Fermi sea to the creation and annihilation of the hole in the valence band is treated nonperturbatively. The time evolution of the self-energy and vertex corrections is shown to be governed by a set of nonlinear differential equations, which allows for a very efficient numerical solution. The effect of the finite hole mass is to wash out the Fermi-edge singularity in absorption.

Frequency and density dependent radiative recombination processes in III–V semiconductor quantum wells and superlattices

Abstract: In this paper we review the radiative recombination processes occurring in semiconductor quantum wells and superlattices under different excitation conditions. We consider processes whose radiative efficiency depends on the photogenerated density of elementary excitations and on the frequency of the exciting field, including luminescence induced by multiphoton absorption, exciton and biexciton radiative decay, luminescence arising from inelastic excitonic scattering, and electron-hole plasma recombination. Semiconductor quantum wells are ideal systems for the investigation of radiative recombination processes at different carrier densities owing to the peculiar wavefunction confinement which enhances the optical non-linearities and the bistable behaviour of the crystal. Radiative recombination processes induced by multi-photon absorption processes can be studied by exciting the crystal in the transparency region under an intense photon flux. The application of this non-linear spectroscopy gives d...

Interband absorption spectra and Sommerfeld factors of a one-dimensional electron-hole system.

Abstract: Optical absorption spectra are exactly calculated for direct interband transitions in a one-dimensional (1D) electron-hole system within the effective-mass approximation. We employ a modified Coulomb potential between an electron and a hole to avoid the well-known divergence problem in the 1D system. The Sommerfeld factor, which is the absorption intensity ratio of the unbound exciton to the free electron-hole pair above the band edge, is found for the first time to be less than unity for the direct allowed transition in striking contrast to the three- and two-dimensional cases. This feature can be understood in terms of anomalously strong concentration of oscillator strength on the lowest 1D exciton state.

Excitons, phonons, and interfaces in GaAs/AlAs quantum-well structures.

Abstract: We report photoluminescence and resonant-Raman-scattering studies of single GaAs/AlAs quantum-well structures. Splittings of the exciton peaks show that there is a large-scale island structure at the interfaces. Shifts in the absolute exciton energies of quantum wells grown at different substrate temperatures and also the form of the optical-phonon energies as a function of both mode index and quantum-well width indicate that there also exists a small-scale structure on the interfaces.

Excitonic effects in coupled quantum wells

Abstract: Electron-sublevel-anticrossing effects have been studied in coupled quantum wells where the exciton binding energy is comparable to the minimum sublevel splitting. The anticrossing was induced by applying an electric field to align the first and second sublevels of adjacent wells. In this situation the electron-hole Coulomb interaction has a strong effect on the splittings measured by optical techniques, because the optical spectra typically measure exciton energies rather than single-particle energies. The most striking effect is that the minimum splitting of the excitons associated with each of the split electron levels does not occur at the same field as for the minimum splitting of the bare-electron levels. One unexpected but readily observable consequence is that when the same electron-sublevel splitting is measured using two different pairs of intrawell and interwell exciton transitions, the field for minimum exciton splitting can differ by up to \ensuremath{\sim}10% from one pair of transitions to the other. We have constructed a variational model of the coupled excitons that explains these effects in terms of Coulomb mixing of the delocalized electron states. We have measured the exciton splittings directly by photocurrent spectroscopy in three GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well structures. The samples were similar in design except that the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As barrier thickness varied from 15 to 35 \AA{}. By fitting our variational model to the experimental anticrossing data, we have been able to deduce the actual bare-electron level splittings rather than the exciton splittings. Within the experimental accuracy, we find that the minimum splitting decreased exponentially with increasing barrier thickness, as would be expected for simple quantum-mechanical tunneling.

Exciton spectra of semiconductor clusters.

Abstract: The band gaps and exciton energies of CdS and GaP clusters are calculated for the first time using pseudopotentials. The calculated exciton energies of CdS over a wide range of cluster sizes are in excellent agreement with experiment. Furthermore, the exciton states of clusters with zinc-blende and hexagonal lattices are similar in large clusters, but differ dramatically in small clusters. Finally, the spectra of small GaP clusters shift to red, instead of to the blue, with decreasing cluster size. These effects provide novel ways of tuning the optical properties of clusters.

Resonant third-order optical nonlinearity of molecular aggregates with low-dimensional excitons

Abstract: The observation is reported of large resonant third-order optical nonlinearity from an organic system with low-dimensional excitons, the pseudoisocyanine J aggregates. The nonlinearity, expressed as α2/α0, is determined to be (2.6 ± 0.5) × 10−7 cm2/W, which compares favorably with those for CdSxSe1−x-doped glasses (1 × 10−7 cm2/W for Corning 3-69 glass) and polydiacetylene (7.4 × 10−8 cm2/W). The large nonlinearity originates from the efficient bleaching of the sharp exciton absorption (~180 cm−1 at room temperature). Also, induced absorption is observed at the high-energy side of the exciton peak, which contributes to the saturation of the nonlinearity at higher power. The mechanism of the exciton bleaching and the nature of the induced absorption are discussed. It is demonstrated that organic systems with low-dimensional excitons represent an interesting class of nonlinear optical material.

Wannier excitons in low-dimensional microstructures: Shape dependence of the quantum size effect.

Abstract: The shape dependence of the quantum size effect of the Wannier exciton is clarified by a simple variational calculation for a model of microcrystal with cylindrical shape. According to the change of the ratio of the radius of the section and the length of the cylinder to the effective Bohr radius of the exciton, the motional state of the exciton changes from three-dimensional to quasi-two-, one-, and zero-dimensional. It is shown that when there is a large anisotropy in the shape of the microstructure, the spatial extension of the exciton wave function along the relatively free coordinate shrinks from the bulk value, reflecting the low dimensionality due to the strong confinement along the axis perpendicular to it.

Many-body calculation of the surface-state energies for si(111)2 1

Abstract: The surface-state excitation energies for the Si(111)2{times}1 surface have been calculated in the {ital GW} approximation. The energy position and the dispersion of the occupied and empty surface states are in excellent agreement with photoemission and inverse-photoemission experiments. The calculated quasiparticle surface-state band gap, 0.62 eV, is 0.15 eV larger than the measured onset energy for electron-hole pair excitations. The possibility that excitonic effects are responsible for this difference is examined.

Absorption red shift in tio2 ultrafine particles with surfacial dipole layer

Abstract: It is found that the TiO2 ultrafine particles (UFP) coated with a layer of stearic acid radicals had a significant red shift of their absorption band edge and clear photoluminescence at room temperature (RT), which are in contrast with that of bulk TiO2 and naked TiO2 UFP. It was concluded that these results are related to the self‐trapped exciton absorption due to the interfacial dipole layer of stearic acid radicals on the TiO2 UFP surface.

Excitonic charge-density-wave instability of spatially separated electron-hole layers in strong magnetic fields.

Abstract: We use the Hartree-Fock approximation to investigate the ground state or a system consisting or spatially separated electron and hole layers in strong magnetic fields. When the layer separation is larger than a critical value a novel excitonic-density-wave state is round to have a lower energy than either a homogeneous exciton fluid or a double charge-density-wave state. The order parameters or the state satisfy a sum rule similar to that or a charge-density-wave state in a two-dimensional electron system. A possible connection between the new state and a recent experimental result is discussed