Transient Grating Investigation of Photoexcitation and Transport in Glasses and Polar Crystals

Polar crystals have strong optical phonon modes. The movement of charge is strongly coupled with lattice phonons. In disordered solids like amorphous chalcogenide glass, charge carriers are trapped at lower energy sites. The effects lead to localization inhibited transport in both material systems where the carrier mobility is extremely low compared to the case in semiconductors. This work is dedicated to the study of excited state dynamics and transport in low mobility materials using transient gratings technique.;We reported the first isolated observation of non-dispersive transport in a-As2Se3 thin film. We observed a well defined transport process occurring at a time-scale of less than a microsecond and over a transport length of a few micrometers. We tentatively assign this fast process, corresponding to a mobility of 0.4±0.005 cm2V−1s −1, to the diffusion of photoexcited charge carriers with microsecond lifetime. We also study the carrier transport dynamics in chalcogenide based electro-optic crystal, Sn2P2S6. Using a short pulse pump and probe technique with picosecond time resolution we determined the mobility value of 2.5±0.8 cm2V−1s −1 caused by the diffusion of holes photoexcited with optical bandgap illumination along the crystallographic 1-axis of Sn 2P2S6. Such low mobility cannot be understood in the framework of the band transport model that has been so successful in explaining the transport processes in semiconductor materials. This inspired us to investigate the temperature response on the carrier transport in this crystal. The investigation led us to conclude that the transport process in this crystal is caused by the thermal activated jumps of photoexcited holes in the temperature range of 150K to 280K.;In addition, this work also successfully demonstrates the use of the short pulse photoexcitation technique for the characterization of Fe 2+ donor concentration in electro-optic Fe:KNbO3. We are able to directly detect the photoexcited carrier through the Bragg diffraction of a probe pulse from the index grating arising from the charge transport and with the spatial resolution (only limited to beam diameter) unmatched by any other technique.