PICOSECOND DYNAMICS OF ABSORPTION BLEACHING IN POLYCRYSTALLINE ZNCDTEFILMS

The spectral and temporal behavior of absorption bleaching and of self -diffraction in ZnxCd1-xTe polycrystalline films with a nominal compos ition x = 0.87 have been investigated. It has been found that the opti cal transmission increases 15 times at a wavelength of 575 nm for a la ser light intensity of 0.2 GW/cm2. In a pump and test experiment, the buildup and the relaxation times T1 of the bleaching were measured. Th eir values, between 10 and 40 and 20 and 45 ps, respectively, depend o n the wavelengths of the pump and the test beams. The saturation inten sity I(s) was also determined (between 1.5 and 6 GW/cm2). This absorpt ion saturation effect can be related to the high density of crystallit e grain surface states as well as to other imperfections in the polycr ystalline films studied. The high density of these states can explain the large linear absorption and the ultrafast relaxation of the photog enerated carriers. Then, in self-diffraction experiments, the phase re laxation time T2 = (10 +/- 5) ps of the localized surface states has b een determined. The long duration of T2 and the short energy lifetime T1 indicate that elastic scattering processes are highly suppressed in polycrystalline material when compared to monocrystals, while their c oherence time is increased.