Critical Dependence of the Excitonic Absorption in Cuprous Oxide on Experimental Parameters

We study the modification of the exciton absorption in cuprous oxide by the presence of free carriers excited through above band gap excitation. Without this pumping, the absorption spectrum below the band gap consists of the yellow exciton series with principal quantum numbers up to more than n = 20, depending on the temperature, changing over to an about constant, only slowly varying absorption above the gap. Careful injection of free carriers, starting from densities well below 1 μm^–3, leads to a reduction of the band gap through correlation effects. The excitons in the Rydberg regime above n = 10 remain unaffected until the band gap approaches them. Then they lose oscillator strength and ultimately vanish upon crossing with the band gap.     

Electronic excitations and luminescence of SrMgF4 single crystals

The electronic and crystal structures of SrMgF₄ single crystals grown by the Bridgman method have been investigated. The undoped SrMgF₄ single crystals have been studied using low-temperature (T = 10 K) time-resolved fluorescence optical and vacuum ultraviolet spectroscopy under selective excitation by synchrotron radiation (3.7–36.0 eV). Based on the measured reflectivity spectra and calculated spectra of the optical constants, the following parameters of the electronic structure have been determined for the first time: the minimum energy of interband transitions E _g = 12.55 eV, the position of the first exciton peak E _n = 1 = 11.37 eV, the position of the maximum of the “exciton” luminescence excitation band at 10.7 eV, and the position of the fundamental absorption edge at 10.3 eV. It has been found that photoluminescence excitation occurs predominantly in the region of the low-energy fundamental absorption edge of the crystal and that, at energies above E _g , the energy transfer from the matrix to luminescence centers is inefficient. The exciton migration is the main excitation channel of photoluminescence bands at 2.6–3.3 and 3.3–4.2 eV. The direct photoexcitation is characteristic of photoluminescence from defects at 1.8–2.6 and 4.2–5.5 eV.     

Electron—hole liquid in CdS crystals as revealed by non-equilibrium transmission spectroscopy

Transmission measurements in the excitonic region on thin platelets under high N₂ laser excitation showed the disappearance of exciton absorption into a strong absorption continuum. The low energy edge of the latter — about 12 meV below the free exciton A_n=1 — coincides with the high energy edge of the gain spectrum of stimulated emission under similar excitations. These results favour the assumption of electron—hole drop formation with a ground state of the e-h-pair about 12 meV below A_n=1.     

Optical absorption due to paraexciton of Cu2O

In Cu₂O a new absorption line is observed at 97 cm^?1 below the n =1 of the yellow exciton (triply degenerate orthoexciton) under a strong magnetic field at 4.2 K. The line is assigned as a transition to a nondegenerate spin triplet state Γ⁺₂ (paraexciton). An analysis including the effects due to the n =1 of the green exciton yields 364 cm^?1 as the exchange energy, and 2.68 and ?1.02, or 1.02 and ?2.68 as the g-factors of the conduction and valence bands forming the yellow exciton.     

Excitation spectra of self-activated luminescence in ZnSe crystals under pressure

A narrow excitation band observed for S-A luminescence in ZnSe crystal is attributed to free exciton absorption. Some overlap with a higher characteristic band of low intensity is considered for low temperature spectra.This band shifts under pressure toward higher energies with a coefficient dE/dP = (7.5±0.3) meV Kb^-1 at 300 K and (7.4±0.5) meV Kb^-1 at 85 K.The pressure shift of the excitation due to edge absorption is (7.0± 0.5) meV Kb^-1.     

Shallow acceptor bound exciton and free exciton states in high-purity zinc telluride

We investigated excitons bound to shallow acceptors in high-purity ZnTe and measured excitation spectra of two-hole luminescence lines at 1.6 K using a tunable dye-laser. The electron-hole coupling in the bound exciton (BE) states appears to be very different for the various acceptors even for almost identical exciton localisation energies. Three different types of BE are reported. For the Li-acceptor BE we observe three sub-components separated by 0.22 and 0.17 meV and interpreted as J = $\text{1}\text{2}$, $\text{3}\text{2}$, $\text{5}\text{2}$ states. The Ag-acceptor BE exhibits a strong ground state and a weak excited state at 1.3 meV higher energy. For the as yet unidentified k-acceptor we observe a single BE level, degenerate with the Ag-acceptor BE ground state. Dips in the excitation spectra due to absorption into free exciton 1S, 2S, and 3S states yield an exciton Rydberg R₀ = 12.8±0.3 meV and a free exciton binding energy FE(1S) = 13.2±0.3 meV.     

Induced absorption and gain from high density excitons in CdS

Induced absorption and gain in CdS at 1.8 K has been investigated under excitation densities up to 10 MW cm^?2. The absorption and gain below the free exciton energy is governed by exciton interactions and optical conversion of excitons into excitonic molecules. At the highest density, induced transparency due to excitonic molecule recombinations is observed. E_M=5.1002 eV is determined.     

Dynamics of the phase transitions in the system of nonequilibrium charge carriers in quantum-dimensional Si1 − x Ge x /Si structures

The dynamics of the phase transition from an electron-hole plasma to an exciton gas is studied during pulsed excitation of heterostructures with Si_{1 ? x} Ge _x /Si quantum wells. The scenario of the phase transition is shown to depend radically on the germanium content in the Si_{1 ? x} Ge _x layer. The electron-hole system decomposes into a rarefied exciton and a dense plasma phases for quantum wells with a germanium content x = 3.5% in the time range 100–500 ns after an excitation pulse. In this case, the electron-hole plasma existing in quantum wells has all signs of an electron-hole liquid. A qualitatively different picture of the phase transition is observed for quantum wells with x = 9.5%, where no separation into phases with different electronic spectra is detected. The carrier recombination in the electron-hole plasma leads a gradual weakening of screening and the appearance of exciton states. For a germanium content of 5–7%, the scenario of the phase transition is complex: 20–250 ns after an excitation pulse, the properties of the electron-hole system are described in terms of a homogeneous electron-hole plasma, whereas its separation into an electron-hole liquid and an exciton gas is detected after 350 ns. It is shown that, for the electron-hole liquid to exist in quantum wells with x = 5–7% Ge, the exciton gas should have a substantially higher density than in quantum wells with x = 3.5% Ge. This finding agrees with a decrease in the depth of the local minimum of the electron-hole plasma energy with increasing germanium concentration in the SiGe layer. An increase in the density of the exciton gas coexisting with the electron-hole liquid is shown to enhance the role of multiparticle states, which are likely to be represented by trions T ⁺ and biexcitons, in the exciton gas.     

Spectroscopy of resonant excitation of exciton luminescence of GaSe–GaTe solid solutions

The luminescence excitation spectra of localized excitons in GaSe_0.85Te_0.15 solid solutions have been investigated at the temperature T = 2 K. It has been shown that the excitation spectra of excitons with the localization energy ε > 10 mV exhibit an additional maximum M _E located on the low-energy side of the maximum corresponding to the free exciton absorption band with n = 1. It has been found that the shift in the position of the maximum M _E in the excitation spectrum with respect to the energy of detected photons increases as the energy of detected photons decreases, i.e., with an increase in the localization energy of excitons. Under the resonant excitation of localized excitons by a monochromatic light from the region of the exciton emission band, in the exciton luminescence spectrum on the low-energy side from the excitation line, there is also a maximum of the luminescence (M _L ). The energy distance between the position of the excitation line and the position of the maximum in the luminescence spectrum increases with a decrease in the frequency of the excitation light. The possible mechanisms of the formation of the described structure of the luminescence excitation and exciton luminescence spectra of GaSe_0.85Te_0.15 have been considered. It has been concluded that the maximum M _E in the excitation spectrum and the maximum M _L in the luminescence spectrum are attributed to electronic–vibrational transitions with the creation and annihilation of localized excitons, respectively.     

Subnanosecond luminescence spectroscopy of Cd1−xMnxSe under high intensity optical excitation

Transient photoluminescence of Cd_1?xMn_xSe has been investigated for compositions x = 0, .05 and .10, in zero magnetic field, under high excitation intensity. The spectra yield information about the temporal evolution of inelastic scattering processes in a dense exciton gas, with a likely contribution by an electron-hole plasma. The short carrier lifetimes measured, less than 100 psec for x = .10, and a possible plasma expansion, might inhibit the generation of carrier densities sufficient for suggested exchange induced magnetic ordering.     

Excitons in monoclinic zinc diphosphide: Orthoexciton and polariton effects at n=1 resonance

Quantitative investigations of the hydrogen-like exciton B series in the absorption spectra of the β-ZnP₂ crystal for various wave vector directions and polarization states of radiation are conducted. It is shown that the B spectrum constitutes a single orthoexciton series with S-type envelope functions, and low-energy components in doublet lines belong to the S-type for lines in the series with n≥3. Polariton effects are clearly manifested at the B _n=1 exciton resonance, and Bouguer’s law is violated. The oscillator strength tensor components are determined for transitions to the exciton states of the B series, and the polariton parameters at the B _n=1 exciton resonance are calculated.     

Exciton-phonon interaction in mixed silver halides: Resonant Raman scattering vs photoluminescence

An investigation of resonant Raman scattering in mixed crystals of AgBr:Cl at 1.8 K shows that the zero-phonon and LO phonon-assisted exciton luminescence excited in the free indirect exciton absorption, exhibits an anomalous dependence on the exciton photon energy E_L. Close to the exciton gap, the bands show a Raman-like behaviour with their peaks at constant energetic distance from E_L. As E_L is tuned further into the absorption, the bands gradually develop into normal photoluminescence. The effect is explained by taking into account exciton relaxation via scattering by long-wavelength acoustic phonons, a process which is strongly energy dependent. In addition, resonant Raman scattering observed for excitation in the zero-phonon absorption suggests study for the first time of the mode behaviour of certain off-zone center phonons in this system.     

Scattering of an exciton polariton by impurity centers in GaAs

Scattering of an exciton polariton by impurity centers at low temperatures has not been investigated comprehensively in spite of its significant role in processes accompanying Bose–Einstein condensation of an exciton polariton. For studying the peculiarities of the interaction of an exciton polariton with impurity centers, we have studied the integrated absorption of the ground state (n ₀ = 1) of the exciton in GaAs in thin (micrometer-thick) wafers with an appreciable optical transmission. Comparative analysis of the transmission in the vicinity of the exciton resonance performed on 15 samples of crystalline GaAs wafers with different concentrations N of impurity has revealed an unexpected regularity. The value of N increases by almost five decimal orders of magnitude, while the normalized spectrally integrated absorption of light exhibits a slight increase, following the power dependence N ^m on the concentration, where m = 1/6. It has been shown that this dependence indicates the diffusion mechanism of propagation of the exciton polaritons through the bulk of the semiconductor, which is present along with the ballistic propagation of light through the sample.     

Exciton absorption in silicon at low electric fields

This paper reports for the first time high-resolution transverse EA spectra of Si in low electric fields at liquid-helium temperature. The data conclusively demonstrate that the observed effect is related to the discrete levels of the Wannier exciton and that for the n = 1 level it can be accounted for both qualitatively and quantitatively by means of a Stark approximation where field-induced broadening is phenomenologicaly introduced to allow for weak tunneling-like contributions. The exciton energy levels determined here are in good agreement with the results of wavelength-modulation spectroscopy and all the other parameters of the absorption process are consistent with previous absolute absorption experiments.     

Influence of electron-electron scattering on the luminescence quantum efficiency in solid argon

The quantum efficiency of the intrinsic luminescence of solid Ar has been investigated for excitation energies between 15 and 55 eV. The spectral dependence of the quantum efficiency yields a stepwise increase with increasing excitation energies. The threshold energies for the steps closely correspond to the sum of the gap energy and multiples of the energy of the n = 1 exciton. The results are discussed in the framework of electron-electron scattering and give evidence that excitation of free electronic polaron complexes occurs in solid rare gases at higher excitation energies.     

Resonant Raman scattering at the indirect exciton in silver bromide

By excitation in the indirect exciton absorption of AgBr at low temperatures selectively enhanced two-phonon Raman scattering is observed. Using different excitation wavelengths the resonance enhancement is found to be associated with the Г-L exciton as intermediate state for the resonant scattering process. The resonant phonons involved are pairs of LA and TA phonons with opposite wave vectors near L. Measurements in the temperature range 2 K ? T ? 40 K show a decrease of the scattering intensity with increasing temperature. The origin of this temperature dependence is due to lifetime broadening of the scattering state. Several features of the indirect exciton absorption of AgBr are discussed.     

Anisotropy of photoconductivity and nonlinear effects in GaSe monocrystals at high optical excitation

In this work, the photoelectric properties of gallium selenide (GaSe) monocrystals in the edge absorption region, with various configurations of current contacts, at low and high optical excitation levels are investigated. The photoconductivity spectrum behavior is determined by localized electronic and excitonic states along c-axis. It is shown that the localization of electronic and excitonic states in one-dimensional fluctuation potential along c-axis results to an anisotropy in photoconductivity spectrum at various current contacts configurations. At E⊥c the photoconductivity is observed in the hν < E_g and hν > E_g regions. In the case of hv < E_g, the maximum photoconductivity, in the impurity and exciton absorption region are observed at 1.975 eV and 2.102 eV, respectively. With rising of excitation energy level, suppression of photoconductivity in the exciton absorption region and increases in impurity absorption region is observed. At E||c contact configuration, the considerable photoconductivity is observed only in the impurity absorption region, which also increases with rising of excitation level. It is supposed that, suppression of photoconductivity in the exciton absorption region at high excitation levels is connected with exciton-exciton interaction, which results to a nonlinear light absorption. The results are compared with the absorption and photoluminescence measurements.     

ZnCdse-ZnSe多量子阱中的激子发光

在77-300K温度下研究了Zn_1-xCd_xSe-ZnSe多量子阱(MQWs)的光致发光特性.首次在77K,Ar离子激光器的457.9nm激发下,在Zn_0.68Cd_0.32Se-ZnseMQWs中观测到5个发光带,其中三个发光带被归因于不同的激子发射:即n=1重空穴(HH)激子;n=l轻-重空穴(LH)激子和n=IHH激子同时发射两个纵光学声子的复合发光,并且n=1HH激子发光可延续至室温.     

Reflectance spectrum of lithium hydride at the Li K-absorption edge

Reflectance spectra of LiH single crystals are measured at the Li⁺K- absorption edge for the first time. The ?₂ spectrum shows a prominent peak at 57.8 eV followed by several structures at the high energy side. We attribute the peak to a transition from the Li⁺ 1s to the n = 1 core exciton state associated with the p-like conduction band.     

Localized excitons and energy transfer in ZnxCd1−xS solid solutions

A new photoluminescence (PL) line I_L in Zn_xCd_1?xS solid solutions (x? 0.15) has been studied. Two main features of the I_L line, i.e. (1) concentration dependence of Stokes shift with respect to n = 1A absorption (reflection) peak (2) gradual high-energy shift at rising temperature, are due to localized exciton states (LES) produced by potential fluctuations and to spectral diffusion by phonon assisted tunneling between LES.