Picosecond dynamics of absorption bleaching in polycrystalline ZnCdTe films

Abstract

The spectral and temporal behavior of absorption bleaching and of self-diffraction in ZnxCd1−xTe polycrystalline films with a nominal composition x=0.87 have been investigated. It has been found that the optical transmission increases 15 times at a wavelength of 575 nm for a laser 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. Their values, between 10 and 40 and 20 and 45 ps, respectively, depend on the wavelengths of the pump and the test beams. The saturation intensity Is was also determined (between 1.5 and 6 GW/cm2). This absorption saturation effect can be related to the high density of crystallite grain surface states as well as to other imperfections in the polycrystalline films studied. The high density of these states can explain the large linear absorption and the ultrafast relaxation of the photogenerated carriers. Then, in self-diffraction experiments, the phase relaxation time T2=(10±5) ps of the localized surface states has been 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 coherence time is increased.