Indirect exciton luminescense and Raman scattering in CdI2

Intrinsic luminescence and Raman scattering in 4HCdI₂ have been investigated at 2 K. Weak emission bands observed near the absorption edge are attributed to the phonon-assistes indirect exciton luminescence. Several new Raman lines are observed under resonant excitation in addition to known lines. The symmetry of the phonon modes associated with the indirect transitions as well as with Raman scattering is discussed.     

Infrared absorption and Raman scattering on coupled plasmon-phonon modes in superlattices

We theoretically consider a superlattice formed by thin conducting layers spatially separated between insulating layers. The dispersion of two coupled phonon-plasmon modes of the system is analyzed by using the Maxwell equations, with the retardation effect included. Both transmission for the finite plate and the absorption for the semi-infinite superlattice in the infrared are calculated. Reflectance minima are determined by the longitudinal and transverse phonon frequencies in the insulating layers and by the density-state singularities of the coupled modes. We also evaluate the Raman cross section from the semi-infinite superlattice. The text was submitted by the authors in English.     

Raman scattering of Ge dot superlattices

Self-organised Ge dot superlattices grown by molecular beam epitaxy of Ge and Si layers utilizing Stranski-Krastanov growth mode were investigated by Raman spectroscopy. An average size of Ge quantum dots was obtained from transmission electron microscopy measurements. The strain and interdiffusion of Ge and Si atoms in Ge quantum dots were estimated from the analysis of frequency positions of optical phonons observed in the Raman spectra. Raman scattering by folded longitudinal acoustic phonons in the Ge dot superlattices was observed and explained using of elastic continuum theory. Received 25 January 2000     

Indirect exciton transitions in the two-photon absorption in semiconductors

The theory of indirect two-photon exciton transitions is developed from third-order time-dependent perturbation theory. The problem is treated by introducing three different band models; a four-band, a two-band and an intermediate (three-band) model. Selection rules and numerical estimates show that the three-band model is the most favourable for this type of process. An application for the case of GaP is performed.     

Polaron effects in exciton mediated resonant Raman scattering

1 LO forbidden resonant Raman scattering theory is reconsidered by including “polaron” effects in the intermediate exciton states. The Fröhlich Hamiltonian is used. Numerical calculations are carried out for TlCl and CdS: in both cases the polaron effects produce a large enhancement of the scattering cross section in the absorption region, improving the agreement between the simple hydrogenic model and the experimental results.     

Raman scattering and exciton luminescence at low temperatures

A theoretical study is made of the effect of Raman scattering of light in crystals on their exciton luminescence. The polariton concept is used to show that Raman scattering may play a determinative role in the low-temperature luminescence of excitons with a large oscillator strength. The luminescence spectrum calculated on a computer by the Monte Carlo method is in good agreement with experimental data. This enables a number of experimental facts concerning the luminescence of crystals at low temperatures to be ascribed to the influence of Raman scattering.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 100–103, January, 1973.     

Raman scattering from GaAsAlAs monolayer-controlled superlattices

Measurements of Raman scattering were performed on (GaAs)_n?(AlAs)_n monolayer-controlled superlattices with n = 1, 2, 3 and 4 grown by molecular beam epitaxy. The zone-folding effect for longitudinal acoustic phonons and frequency shift of longitudinal optic phonons were observed. The experimental results agree well with the calculated ones according to the elastic and linear chain models.     

Fano interference in intrasubband Raman scattering of semiconductor superlattices

Fano’s theory is used to study the interaction between a continuum of Raman-active electronic intrasubband transitions and a discrete LO-phonon state in a semiconductor superlattice (SL). An explicit analytical expression of the asymmetry parameter q has been derived, which indicates the possibility of the occurence of Fano interference in intrasubband Raman scattering in suitably designed SL structures. The analytical expression clearly shows that q not only depends strongly on the SL parameters, but also depends sensitively on the exciting wavelength. The corresponding suitable design and preparation of SL samples and the performance of Raman experiments have also been carefully carried out for the first time. The theoretical calculations and experimental measurements show good qualitative and quantitative agreement.     

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 spectroscopy of resonance exciton tunneling in GaAs/AlAs superlattices in electric fields

Optical-resonance-Raman scattering by acoustic phonons is used to study the effect of an electric field on the state of excitons in GaAs/AlAs superlattices. When the energy of the exciting photon coincides with the energy of an exciton bound to Wannier-Stark states of a heavy hole and electron with Δn=0,±1, the acoustic Raman scattering is enhanced. Oscillations in the intensity of the Raman spectrum in the electric field are explained by resonance delocalization of the exciton ground state as it interacts with Wannier-Stark states of neighboring quantum wells or with Wannier-Stark states of a higher electron miniband. Fiz. Tverd. Tela (St. Petersburg) 40, 827–829 (May 1998)     

Resonant Mandelstam-Brillouin light scattering and Raman scattering in semiconductors with intermediate exciton states belonging to discrete exciton bands and the continuous spectrum

The results of a theoretical and experimental investigation of resonant Mandelstam-Brillouin light scattering by thermal acoustic phonons with k=0 near the direct absorption edge (in the case of ZnSe crystals) are analyzed. The appearance of a new type of resonant increase in the intensity of Raman scattering by optical phonons with k≠0, which corresponds to resonance with the scattered light in the output channel, near the indirect absorption edge (in the case of semi-insulating GaP:N crystals) is also reported. The resonant gain reaches ∼4×10³ at frequencies corresponding to overtone scattering assisted by LO(X) and LO(L) phonons. Exciton states belonging to both discrete exciton bands and to the continuous spectrum are considered as the intermediate states involved in the scattering processes in calculations of the resonant scattering tensors. In addition, all the intraband transitions, as well as the interband transitions between the conduction band, the valence bands, and the spin-orbit split-off band are taken into account, and good agreement with the experimental results is obtained. Fiz. Tverd. Tela (St. Petersburg) 40, 938–940 (May 1998)     

Two-phonon resonant Raman scattering at the indirect exciton in silver chloride

Resonant Raman scattering in AgCl is reported for the first time. With excitation in the indirect absorption edge at 1.8K the observed scattering processes involve pairs of momentum-conserving TA(L) and LA(L) phonons with energies of 8.2 meV and 12.9 meV, respectively. The dependence of the scattered intensity on excitation energy suggests that, in contrast to AgBr, intravalley scattering by long wavelength acoustic phonons and intervalley scattering are negligibly small, the main relaxation mechanism of the free exciton being self-trapping. Some enhancement in the 2LO scattering intensity observed is probably due to resonance with the lowest direct exciton.