R. Levy

The dispersion of excitons, polaritons and biexcitons in direct-gap semiconductors

Abstract Excitons are the energetically lowest excitations of the electronic system in an ideal semiconductor at zero temperature. If the excitons couple to the electromagnetic field, a mixed state is formed, the quanta of which are called excitonic polaritons. Associates of two excitons, so-called biexcitons, have been observed in many semiconductors. Excitons are known for about forty years. During the first three decades, they have been investigated mainly by the classical spectroscopic methods, i.e., reflection, transmission and luminescence spectroscopy. In the last decade, several new techniques have been developed, which allow for a direct spectroscopy in momentum space. In this contribution, we review these novel techniques, both linear and nonlinear ones, and present results obtained for excitons, polaritons and biexcitons. The review is restricted to semiconductors which have their conduction band minimum and their valence band maximum at the same point of the Brillouin zone (direct-gap materials) and which have a band to band transition which is dipole allowed.

Optical bistability due to biexcitons in CuCl

Abstract Biexciton optical bistability is observed for the first time using CuCl as a nonlinear medium in a Fabry-Perot. The effect is obtained with a high intensity pulsed dye laser which excites the sample and modifies its refractive index near half the biexciton energy. The switching times of the device are shown to be inferior to 500 ps. The bistability is observed at pumped liquid helium and at liquid nitrogen temperatures.

Evidence of an induced polariton branch in CuCl by hyper-Raman scattering

Abstract We study the renormalization of the dispersion of excitonic polaritons, induced by a strong light source at frequency ωl. Since biexcitons with energy EBi show a giant oscillator strength, this renormalization effect is located around frequencies Ω such as h Ω + h ω l = E Bi . It gives rise to an induced polariton branch, the dispersion of which depends on the frequency of the exciting light source. It manifests itself through additional emission lines in hyper-Raman scattering processes.

Biexciton two-photon absorption in CuBr

Abstract The biexciton states in CuBr have been studied at K ≅ 0 by two-photon absorption, using two different laser sources. The energy and the symmetry of these states have been unambiguously assigned: EB(Γ1) = 5.9061 eV, EB(Γ5) = 5.9103 eV, EB(Γ3) = 5.9128 eV.

Energy and intensity dependence of renormalization effects in CuCl

Abstract We use the non-degenerate, non-collinear four-wave mixing as a spectrometric tool to study the energy and intensity dependence of different renormalization effects in CuCl.

Scattering processes involving bound excitons in highly excited ZnO

Abstract ZnO shows, under high excitations at low temperatures, two different extrinsic emission bands on the low energy side of bound exciton emission lines. The temperature dependence of one of these bands shows good agreement with theoretical predictions for a bound exciton-free electron scattering process. A comparison is made with results of other authors, where similar emission structures have been interpreted in terms of biexciton recombination.

A new emission line in highly excited ZnO at low temperature

Abstract A new emission line from ZnO has been observed under strong u.v.-excitation at 4.2 K, which shifts to longer wavelengths with increasing excitation. The experimental results are described by a model involving the recombination of a bound exciton which transfers part of its energy to a free electron.

Time-resolved investigations of induced absorption due to exciton-biexciton transitions in CuCl

Abstract The time-resolved induced absorption due to exciton-biexciton transition is observed in CuCl using an intense ultraviolet picosecond pulse excitation and a picosecond continuum as a probe beam. In the same experimental conditions, time-resolved luminescence is also recorded.

Optical characterization of pure ZnSe epilayers grown by metal organic vapour-phase epitaxy

Different ZnSe/GaAs epilayers prepared by low-pressure metal organic vapour-phase epitaxy under the conditions given by Nurmikko and Gunshor (1993 Physica B 185 16) are studied through their low-temperature transmission, reflection and photoluminescence spectra. Several samples are etched out from their GaAs substrate and the properties of as-grown surfaces and etched surfaces are compared. The etched samples seem to be free of in-plane strain and to behave like bulk ZnSe. The free-exciton excited state (n=2) is clearly observed in transmission, reflection and luminescence spectra. We also studied the thermal quenching of the photoluminescence in order to confirm the origin of some bound-exciton centres.

Time evolution of the nonlinear refractive index due to the induced exciton—biexciton transition in CuCl

Abstract The time evolution of the nonlinear index of refraction of CuCl at low temperature is studied through the spectral shift of interference fringes by a “pump and probe” technique. Time-delayed renormalization of the dispersion of polaritons in the spectral region of the exciton-biexciton transition is observed. These effects are related to the exciton and biexciton dynamics.