Saturation of the Band‐Edge Absorption of CdS_xSe_1−x Mixed Crystals at Room Temperature

Abstract

BYAt present non-linear optics of semiconductors i s rapidly developing, essentially because of its potential application for high-speed optical signal processing and laser design. In particular the room temperature realization of corresponding devices i s of special interest.Up to now saturable absorption as one of the basic principles for an optical non-linearity has been demonstrated for some narrow-gap materials a t room temperature / I , 2/. The findings were mainly attributed to a dynamic Burstein-Moss shift (band filling). Typically, very high excitation intensities of the o r d e r of some MW/cm were necessary to gain the effect. In what follows we report on the f i r s t room temperature observation of absorption bleaching for the wide-gap material CdSxSe l-x originating from a novel bandedge shift to higher energies already a t moderate excitation intensities.The samples investigated were platelet-like crystals grown from the vapour phase with the crystal axis 5 in the plane of the platelet. They were mounted on glass substrates for the measurements. The investigations were carried out by means of the excite-and-probe technique. Two dye lasers with a pulse duration of about 4 n s pumped by a N2 laser were used: one ( A = 480 nm) to excite the sample and the second (dye continuumA-510 to 550 nm) as probing source. The intensity of the probe beam was s m a l l e r than 100 W/cm . Because the electronic relaxation times are of the o r d e r of 100 p s /3/, a steady s t a t e of the c a r r i e r system should be adjusted in our experiments. This is an important difference in comparison w i t h the transient measurements mentioned above 11, 2/ where p s pulses were used. Both the wave vector of exciting and probing photons was always perpendicular to ?. The transmitted radiation of the broad-band dye laser was analysed by an OMA-2 system with a spectral 2 2 1) Neue Schonhauser Str. 20, DDR-1020 Berlin, GDR.

Electronic processes in small semiconductor particles

“…where R is the rate constant. The justification for the inclusion of recombination in the nanosecond group comes, firstly, from single-photon counting measurements of the room-temperature red-luminescence lifetime in this laboratory (Duonghong et al 1982) th a t indicated a very rapid decay (7 « 300 + 200 ps); and, secondly, from recent work on bulk single crystals by Puls & Henneberger (1984), who give an electronic relaxation time of the same order. An earlier investigation had reported a room tem perature red luminescence decay time in single crystals of ca.…”