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

本研究旨在探讨连续光与超短光脉冲在光折变铁电材料中所产生的空间电荷场与折射率变化的动力学。此外，这些折射率光栅的固定动力学也进行了研究。本研究的主要结果如下：1.数值研究了皮秒脉冲在钛酸钡晶体中产生的空间电荷场的动力学。结果表明，在单脉冲激励下，系统的动力学行为强烈地依赖于脉冲注量。研究还发现，在高注量脉冲下，浅陷阱热激发产生的暗积累现象也会发生。对于多脉冲入射，空间电荷场的强度随着脉冲数的增加而增加。然而，它的速率小于连续激光入射。实验也证实了这一趋势.使用未掺杂的钛酸钡进行热固定实验。阐明了离子光栅和空穴光栅在衍射效率随温度变化中的作用。还首次确定了固定全息图的离子物种的激活能。

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.