Grating formation and its fixing in photorefractive ferroelectrics with ultra short optical pulses

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The purpose of this study is to investigate dynamics of space-charge fields and refractive index changes created in photorefractive ferroelectric materials by cw light and ultrashort optical pulses. In addition, fixing dynamics of these index gratings is also investigated. The main results of this study are described as follows :1. Dynamics of space-charge field created by picosecond pulses in photorefractive barium titanate is studied numerically. It is found that under single-shot-pulse excitations the dynamics strongly depens on pulse fluences. It is also found that dark buildup phenomena resulted from thermal excitations from shallow traps take place forhigh fluence pulses. For multiple pulse incidence the strength of the space-charge field increases with an increase of number of pulses. However, its rate is smaller than that for cw laser incidence. This trend was also confirmedexperimentally.2. Thermal fixing experiment using undoped barium titanate was performed. Role of ionic and hole gratings in temperature-dependent diffraction efficiency was elucidated. The activation energy of ionic species responsible for fixing holograms was also determined for the first time.