光折变晶体LiNbO_3:Fe中的特殊散射现象

用柱面透镜把经针孔滤波器扩束的激光束沿某一个方向聚焦成细丝状(或长条状),细丝状光束垂直入射到LiNbO_3:Fe晶体上,在远场位置观察散射光.我们发现光散射的方向与晶体的c轴方向不一致,不仅沿着晶体的c轴方向有散射光出现,沿光束的条形方向也出现很强的散射光.

Special scattering in photorefractive crystal LiNbO3:Fe

We observe special scattering light by using a simple experimental device and record the dynamic behavior with a camera. A laser beam from an Nd:YAG laser, which is expanded by the spatial filter (SF) and collimated by the lens L₁ (f₁=300 mm), is focused into a line light through a pair of cylindrical lenses L₂, L₃ (f₂=f₃=200 mm) and irradiates the LiNbO₃:Fe crystal. On condition that the directions of line beam (f) and c-axis of the crystal are both parallel to the horizontal direction, we observe experimentally that the scattering light spreads gradually in the horizontal direction in the far field as irradiation time goes on. Then the scattering light reaches a steady state after 10 min. The scattering light beam is composed of vertical filaments. When the line beam is horizontal and the c-axis is vertical, the scattering light composed of horizontal filaments firstly appears in the vertical direction. About 30 min later, the scattering light appears and spreads along the horizontal direction to the far field as irradiation time goes on. At this time, the scattering light is also composed of vertical filaments. That is to say, we observe the scattering light whose direction is inconsistent with the c axis of the crystal. We also give the corresponding theoretical explanation to the phenomenon. We suppose that the line beam consists of many close-set thread-like sub-beams, which are vertical to the direction of the line beam. When the line beam irradiates the photorefractive crystal, the sub-beams record the gratings in the crystal according to photorefractive nonlinear effect. The gratings diffract the input beam. The scattering light and the incident beam interfere with each other, thereby recording the new grating. At the same time, the new gratings also diffract the incident beam. It goes full circle. So energy transfers from incident beam to the scattering light beam. The direction of the scattering light beam spreads along the direction of the line beam.