自散焦向自聚焦转换中背景光作用的理论分析

Theoretical Analysis of Background Light Effect in Transition from Self-Defocusing to Self-Focusing

理论分析了背景光辐照在光折变晶体中从自散焦向自聚焦特性转化过程中的作用,得到了R>1是不同类型(Δn<0和Δn>0)晶体中这种转变的条件.实验观察到了铌酸锂晶体中这种转变的现象.并依据Glass常量的光伏打效应表征意义,提出了光伏孤子形成过程中载流子的竞争效应模型.基于此,分析了折射率变化为负的光生伏打晶体在背景光和信号光Glass常量比大于1条件下的载流子竞争效应,得到了与实验现象和已知理论分析相一致的结论.研究表明,背景光引起的载流子竞争效应是影响晶体自散焦向自聚焦特性转换的内在物理本质.

The influence of background illumination on the transition from self-defocusing to self-focusing characteristics in photorefractive crystal was analyzed theoretically and testified experimentally in lithium niobate crystal, which resulted in the concrete transiting conditions by R〉1 for crystal with different photorefractive property of △n〈0 and △n〉0. Based on the sense of Glass constant in photovoltaic effect,a theoretical model for carriers competition effect in photovoltaic solitons forming process was established, on which the relative spatial refractive perturbation in photovoltaic LN crystal was discussed, with a bigger Glass constant for background beam, and the same result as the in the experiment was obtained. So the conclusion that the carriers competition effect is just the intrinsic physical mechanism of the dynamical evolution from self-defocusing characteristics to self-focusing one was achieved eventually.