Recently, there has been considerable interest to study changes in the refractive index of highly excited direct-gap semiconductors below the band edge. Interference spectroscopy on thin crystal platelets represents an appropriate method for corresponding experimental investigations. First experiments of this kind were reported by Miiller et al. /1/ for CdS. But the purpose of these measurements was mainly a correct determination of the optical gain.Detailed investigations on the refractive index of highly excited CdS were carried out by Klingshirn and co-workers /2/. The thickness of the crystal platelets used was, however, of the order of some microns. From this two difficulties arose:
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.
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