Picosecond Transient Optical Nonlinearities in CdS and CdSSe

Abstract

,Se,., crystals is studied around room temperature using a n excite-and-probe technique in the ps time region and taking Fabry-Perot interferences into account. Large nonlinear changes of the optical coefficients are obtained. The experimental spectra are in reasonable agreement with the theoretical ones assuming a heating of the excited carriers. The nonlinearities relax within about 1 ns.Nichtlineare Absorption in CdS und CdS,,,Se,,, wird unter Beriicksichtigung der Fabry-PerotInterferenzen mit der Zweistrahlmethode im ps-Zeitbereich in einem Temperaturintervall um 300 K untersucht. Grol3e nichtlineare Anderungen der optischen Koeffizienten werden registriert. Unter der Annahme einer erhohten Ladungstriigertemperatur stimmen die experimentellen Spektren mit den theoretischen anniihernd iiberein. Die Nichtlinearitiiten relaxieren innerhalb von ca. 1 ns. IritrodnctioriOptical nonlinearities of 11-VI semiconductor compounds a t room temperature have attracted much attention in recent time stimulated by a great physical interest and by potential applications for optical data processing. Large changes of the optical parameters in these materials at high excitation level [1, 21 are mainly attributed t,o screening of the excitonic bound states (Mott transition).We have measured transmission spectra in the excitonic spectral region of CdS and CdS,,,Se,,, crystals excited with ps laser pulses. The measurements on CdS are performed a t various temperatures near room temperature, and those 011 the mixed crystal are done at room temperature. Absorption coefficient and refractive index spectra are calculated and compared with t,heory. Measurement TechniqueThe monocrystal samples of CdS and CdS,,,Se,., (E,,, = 2.5 and 2.1 eV, respectively) are thin ( = 1 pm) vapour-phase-grown platelets with parallel faces along the c-axis. In Fig. 1

Mott Transition in CdS Revealed by Picosecond Nonlinear Absorption Measurements

“…3. Transmission spectra of a CdS sample (1) without excitation andswith time delays of (2) 20, (3) 13, (4) hw lev) -spectral region of CdS. Thus the response could be always regarded as linear with respect to the incident probe pulses.…”

Digital Circuits for All‐Optical Computing Based on Thin‐Film Semiconductor Interferometers

“…To investigate the optical nonlinear properties of all the three samples, we recorded their transmission spectra before and after excitation by Thus, along with the static shift, we have also observed a dynamic blue shift correlating with the decreasing size of the polycrystalline film grain and packing density. We assume that both this correlation and the picosecond relaxation times which are not characteristic of monocrystal samples [21] are due to the three-dimensional spatial confinement effects. Of course, due to the spread in sizes of individual grains the exciton absorption peak is strongly smeared and further search for ways of film structure regularization is required.…”

Femtosecond Studies of Room Temperature Optical Nonlinearities in Wide‐Gap II–VI Compounds

“…The result is a strong shrinkage of the energy gap below the exciton resonances abolishing the excitonic absorption (Mott transition). For free carrier densities clearly above the Mott density a n additional contribution of band filling (Burstein-Moss effect) can be found [7].…”

“…The dynamics of the optical nonlinearity have been studied on a CdS,,,Se,., mixed crystal [l], pure CdSe [2, 31, and CdS [7] using ps-excite-and-probe technique. Both for mixed and single crystals the recovery time of the absorption bleaching is of the order of the carrier band-to-band recombination time of some hundred ps.…”