Ultrafast dynamics of photorefractive gratings in undoped CdTe

Abstract

We recently developed a novel nondegenerate, polarization-rotation, transient-grating technique, which can be used to isolate and time resolve (with picosecond resolution) the dynamics of nonphotorefractive and photorefractive gratings in zincblende semiconductors. Here we use this technique to investigate the photorefractive grating in undoped CdTe. The grating was written by two 1 ps pulses at 960 nm generated by an amplified Styryl 13 dye laser system. At this wavelength, the space-charged field required for the photorefractive nonlinearity can only be formed between electronhole pairs, since free-carriers can only be generated by two-photon absorption in this material. A <5-ps pulse at 1.06 µm (generated by Raman-shifting a fraction of the 960-nm radiation in benzene) was used to read the photorefractive grating as a function of time delay. The measurements identified two contributions to the photorefractive effect: an initial transient response followed by a longer lived component that decays predominantly by ambipolar diffusion. The latter signal is therefore consistent with a photorefractive grating associated with the space-charged field between electron-hole pairs (i.e., the Dember field). Some possible origins of the initial transient signal, which indicates an enhanced photorefractive response on picosecond timescales, will be discussed.