Sub-picosecond region two dimensional parallel information processing in photorefractive crystals

我们计划并进行了以下实验：将承载时空信息的皮秒和亚皮秒光脉冲记录在空间上映射了时间演化的体全息图中，并从体全息图中读出。为此，我们发展了一种通过测量连续激光的衍射效率和相位随布拉格条件下角度失谐的变化来直接读出体全息图中记录的光栅的方法。通过解体全息图内部的耦合模方程，得到了光栅包络函数。利用3.5皮秒锁模倍频激光在光折变LiNbO_3晶体内部记录了信号光与参考光之间的干涉条纹，测量了LiNbO_3和BaTiO_3晶体对超短脉冲的光折变灵敏度和时间常数。我们发现它们的响应强烈地依赖于脉冲的峰值强度，但与脉冲宽度无关。特别是对于几十mW/cm~2的强入射光束，LiNbO_3的灵敏度提高了一个数量级。这是由于光伏效应的强度依赖性所致。我们还研究了半绝缘半导体禁带的光折变效应，得到了有关其电子态结构的信息。

We planned and carried out the following experiments : the pico-and subpico-second light pulses which bear the spatio-temporal information are recorded in and read out from volume holograms into which the temporal evolution is mapped spatially. For this aim, we developed the method to read out recorded gratings inside the volume hologram directly by measuring the diffraction efficiency and phase of cw laser beam as a function of angle detuning from the Bragg condition. The grating envelope function is obtained by solving the coupled mode equations inside the volume hologram. Then we recorded the interference fringes between the signal and reference beams, from the 3.5-picosecond duration mode-locked frequency-doubled Nd : YAG laster, inside a photorefractive LiNbO_3 crystal and could read out the information successfully.We measure the photorefractive sensitivities and time constants of LiNbO_3 and BaTiO_3 crystal for ultrashort pulses. We found that their responses strongly depended on the peak intensity of pulse, but they were independent of the pulse width. Especially, the sensitivity of LiNbO_3 increases an order of magnitude for intense input beams of several tens MW/cm^2. It is due to the intensity dependence of the photovoltaic effect. We also investigated the photorefractive effect of semi-insulating semiconductor GaP,and we obtained the information about its electrical state structure.