Directional dispersion of extraordinary phonons in layer compound InSe

The resonant Raman scattering of polar optical phonons has been measured in trigonal γ-InSe at the E'₁ exciton edge. The two-phonon scattering spectrum widely spreads over the frequency range of 50 cm^-1 with an extremely asymmetric lineshape. This spectrum has been interpreted in terms of the directional dispersion of extraordinary LO and TO phonons.     

Raman spectra of GaAs-AlxGa1-xAs superlattices

Systematic theoretical studies of Raman spectra of GaAs-Al_xGa_1-xAs superlattices are presented. The electronic states are described by an envelope-function method and the phonon modes are described in a microscopic rigid-ion model. Both resonant and nonresonant Raman scattering processes are considered. For resonant Raman scattering, the effects of discrete exciton states plus the continuum and the valence-band mixing are included via a k-space sampling method. Both the Fröhlich and deformation-potential mechanisms for electron-phonon coupling are considered. These two mechanisms are responsible for principal features in the z(x, x)z̄ and z(x, y)z̄ geometries, respectively. We find that the effects of exciton continuum states are quite important and the resonant Raman spectra so obtained are in much better agreement with experiment compared to those without including the exciton continuum states.     

Lineshape studies of the 1s yellow exciton in Cu2O by resonance Raman scattering

The enhancement of the cross section for Raman scattering has been measured near resonance with the 1S yellow exciton in Cu₂O $\text{(}\text{v}{}_{\mathrm{1SY}}\text{? 16,399 cm}{}^{-1}\text{)}$. The shape of the enhancement is shown to be Lorentzian to beyond 10 halfwidths from the peak. From the temperature dependence of the width of the enhancement profile, we extract separately the rates for exciton- acoustic phonon scattering, γ_sc(T), and for non-radiative decay, γ_nr, of this exciton state. We find γ_nr = 2.4 × 10¹⁰sec^-1 and γ_sc(T=5.4K) = 0.85 × 10¹⁰sec^-1.     

Resonant Brillouin scattering via free and bound excitons in CdSe

We report resonant Brillouin scattering results in CdSe. Enhancements in Brillouin scattering have been observed at both the I₂ bound exciton and the free exciton. As a result of the spatial dispersion of the exciton-polariton, the Brillouin frequencies vary with the polariton energy. From this variation of the Brillouin frequencies, we deduced the following parameters in CdSe: transverse exciton frequency =14713 cm^?1, splitting between longitudinal and transverse exciton frequencies = 4cm^?1 and exciton effective mass (perpendicular to the c-axis) =0.40 times free electron mass. The Brillouin linewidths were found to vary with polariton energies in qualitative agreement with the theory of Brenig, Zeyher and Birman.     

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.     

Magnetopolaritons in ZnTe

From an invariant expansion, we construct the exciton Hamiltonian for the Γ₆×Γ₈ excitons in theT _d -type material ZnTe represented by an 8×8 matrix including the influences of a finite wave vector and an external magnetic field. We diagonalize the Hamiltonian matrix to obtain the exciton states. Then the excitons are coupled to the electromagnetic radiation field thus giving the polariton states. The theoretical dispersion curves are fitted to the results of two-photon Raman scattering and reflection experiments in magnetic fields up to 22 T. From this fit we deduce precise values for the eigenergies, exciton masses,g-factors, and diamagnetic shifts.     

Raman scattering in monodomain RbCoF3

Polarized Raman spectra are reported for scattering from a single oriented antiferromagnetic domain in RbCoF₃. Monodomain samples were achieved by the application of a small [100] uniaxial stress. Spectra were recorded at 2 K using non-absorbed, orange laser light. The D_4h symmetry assignments for the principal features are: 34(Γ⁺₅), 335 (Γ⁺₁ and Γ⁺₁), 420 (Γ⁺₁), 950 (Γ⁺₃) and 1050 cm^-1 (Γ⁺₁). These results are not consistent with the prediction of previous models for magnetic exciton scattering.     

The kinetics of the sensitized flourescence of naphthalene single crystals doped with 2-methylnaphthalene and anthracene

The kinetics of the sensitized flourescence in naphthalene crystals doped with 2-methylnaphthalene (2-MN) and anthracene (A) was studied as a function of the temperature. These experiments yield singlet exciton diffusion constants of (2 ± 1) × 10^-4cm²s^-1 at 4.2 K and roughly 10^-5 cm² s^-1 at 300 K. The exciton trapping radii can be estimated to be of the order of 60 Å for A and 10 Å for 2-MN. At temperatures below 40 K a partially coherent exciton motion can be inferred, above 40 K the exciton motion appears to be purely incoherent.     

The location of the lowest conduction band minima in gallium phosphide from bound exciton luminescence

Two weak satellite series with acceptor-independent displacement energies of 4.2₃ meV and 7.8₃ meV have been observed in the luminescence of excitons bound to neutral acceptors in GaP. These satellites contain broad background luminescence. Rather well defined superimposed peaks show relative strength which increases dramatically with decrease in exciton binding energy. The most plausible mechanism for these satellites involves bound exciton recombination with emission of one phonon to conserve momentum in the indirect transition and one or two further phonons to conserve momentum in g-type inter-valley scattering processes. This model is consistent with all known properties of the satellites and is strongly supported by a quantum-mechanical line width calculation. The narrow components arise from a diffuse tail on the bound exciton wave-function which is enhanced by the electron-hole correlation. The existence of this g-scattering process shows that the conduction band minima in GaP lie at 0.953 K^max_〈100〉, not exactly at K^max_〈100〉 as believed hitherto. This revision has important consequences for several properties of n-type GaP.     

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.     

Raman scattering of light by 2LO-phonons

A theory of resonance Raman scattering of light by 2LO-phonons in semiconductor crystals is presented. Wannier excitons are considered as intermediate states. Analytic expressions are obtained that take into account contributions from different chains of intermediate states. The scattering cross section is shown to be weakly dependent on the wave vector imparted to the phonon system. The theoretical model permits the calculation of the scattering cross section for the energy of the exciting radiation photon below the level of exciton resonances.     

Exciton lifetime in quantum well with vicinal high-density excitons

Radiative lifetime of an exciton in a GaAs quantum well (QW) is controlled by high-density excitons, which restrict the exciton coherence through scattering. In order to circumvent the phase space filling effect of high-density excitons, we have prepared a QW structure in such a way that a reservoir for high-density excitons is separated from the QW. The lifetime increases (up to 30%) with the exciton density in the reservoir and saturates at 1×10¹⁷/cm³. The upper bound lifetime is determined by the excitonic relative motion.     

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.     

Energy Structure of an Individual Mn Acceptor in GaAs : Mn

The energy structure of the Mn acceptor, which is a complex of Mn²⁺ ion plus valence band hole, is investigated in the external magnetic field and under presence of an uniaxial stress has been studied. The spin-flip Raman spectra are studied under resonant excitation of exciton bound to the Mn acceptor. The gfactors of the ground F = 1 and the first excited F = 2 states are determined and selection rules for the optical transitions between the acceptor states are described. The value of the random field (stress or electric field) acting on manganese acceptor and the deformation potential for the exchange interaction constant of the Mn²⁺ + hole complex are obtained. A theoretical model is developed that takes into account the influence of random internal and uniaxial external stress and magnetic field. The proposed model describes well the lines of spin-flip Raman scattering of Mn acceptor.     

Photoluminescence and multiphonon resonant Raman scattering in Ni-and Co-doped Zn1−x MnxTe crystals

Low-temperature photoluminescence, exciton reflection, and multiphonon resonant Raman scattering spectra of Ni-and Co-doped Zn_1−x Mn_xTe crystals were investigated. Intense emission occurs in a broad spectral region (1100–17 000 cm⁻¹) in the crystals containing Ni atoms. It is caused by intracenter transitions involving Mn²⁺ ions and transitions between the conduction band and a level of the doubly charged acceptor. The features of the exciton photoluminescence and multiphonon resonant Raman scattering involving longitudinal-optical (LO) phonons at various temperatures are investigated. The insignificant efficiency of the localization of excitons on potential fluctuations in the Zn_1−x Mn_xTe:Co crystals is established. A temperature-induced increase in the intensity of the 5LO multiphonon resonant Raman scattering line due to the approach of the conditions for resonance between this line and the ground exciton state is observed in these crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 616–621 (April 1998)     

High density excitation effects on fluorescence from the crystals of aromatic hydrocarbons and charge-transfer complexes

High density excitation effects on fluorescence were studied for the crystals of fluoranthene, chrysene, benzo[g, h, i]perylene, pyrene and pyrene-d₁₀ and also for the crystal of 1,2,4,5-tetracyanobenzene (TCNB)-hexamethylbenzene (HMB) complex. Relative fluorescence intensity in the shorter wavelength region decreased with increasing excitation density (for chrysene, fluoranthene, and benzo[g, h, i]perylene), and the vibrational structure became diffuse at high density excitation (for fluoranthene and chrysene). The rate constants of the bimolecular quenching and exciton migration were obtained by analysis of the fluorescence decay curves at high density excitation for chrysene, pyrene, pyrene-d₁₀, and TCNB-HMB. The exciton hopping rates at 295 K were 2.4 × 10¹⁰ s^?1 in chrysene and 7.9 × 10⁸s^?1 in TCNB-HMB. The differences in the rate between chrysene and fluoranthene (2.2 × 10⁹s^?1) and between TCNB-HMB and TCNB-durene (4.2 × 10⁹s^?1) are discussed in terms of the coupling intensity between two neighboring oscillators accompanying the transition between ground and the lowest excited singlet state. The difference in the rate and activation energy of exciton migration between pyrene and pyrene-d₁₀ corresponded to a quasilocalized exciton model.     

Exciton binding energy in GaAs quantum wells deduced from magneto-optical absorption measurement

Magneto-optical absorption spectra due to exciton states and Landau-levels were measured in GaAs/AlAs multi-quantum-wells. By extrapolating the photon energies of the absorption peaks to zero magnetic field, the lowest state (1S) heavy hole exciton binding energy, E^B_$\text{h}$(1S), was obtained as a function of well size L_z in the range $\text{58}\text{A}\text{?}\text{?}\text{L}{}_{\mathrm{z}}\text{?252}\text{A}\text{?}$. The L_z dependence of E^B_h showed the change of the exciton character from three-dimensional to two-dimensional with decreasing L_z. The diamagnetic shift observed for the heavy hole exciton peak was larger than that for the light hole exciton peak, showing the anisotropic nature of the Luttinger-Kohn Hamiltonian. The diamagnetic shift of the heavy hole exciton peak became smaller as L_z was decreased, suggesting the enhancement of the two-dimensional exciton character.     

Exciton and impurity electroabsorption in GaSexTe1-x〈Sn〉 single crystals

The exciton and impurity electroabsorption has been studied in GaSe_xTe_1-x〈Sn〉 (0.7 ?χ?1) exhibiting the properties of residual conductivity (RC). It has been shown that the broadening of the negative exciton peak, the nature of the field dependence of the electroabsorption signal, and its anisotropy in the exciton and impurity region are associated with the presence of internal chaotic fields E_int ≈ 10³ ? 10⁴ V cm^?1 coming into play because of the fluctuating distribution of the tin impurity and the defects inherent in layer crystals. Based on investigations of the impurity electroabsorption (IEA), the depth of occurence (hv = 1.18 eV) and the concentration (N = 10¹⁶cm³) of impurity centres have been determined.     

Indirect exciton dispersion in III–V semiconductors: “Camel's back” in GaP

A new perturbation approach to exciton dispersion in indirect gap semiconductors is developed. For GaP and AlSb existence of the “camel's back” in exciton dispersion is confirmed, and a precise value of the “camel's back” parameter for X^c₁-minima in GaP is reported: E(X^c₁)?E_min(Δ^c₁)=3.5±0.3 meV. At the X-point the 21.44 and 19.48 meV exciton binding energies in GaP are obtained. The corresponding valley-anisotropy splitting is 1.96 meV.     

Multiple LO scattering in indium iodide

Resonant secondary emission is investigated in the energy range around the direct exciton in orthorhombic InI. Spectra at liquid nitrogen temperature exhibit multiple LO lines with characteristic intensity alternation as in InBr at 2K. Incident photon energy dependence of their scattering intensities is also studied. Pronounced outgoing resonance with 1s exciton is found in each LO line.