Anisotropy of nonlinear coupling of two counter-propagating waves in photorefractiveFe:KNbO3

Abstract

The dependence of self-pumped photorefractive two-beam coupling in $\mathrm{Fe}:{\mathrm{KNbO}}_{3}$ with crystal angle has been measured at $532\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for the $a$-$c$ and $b$-$c$ crystal planes using a combination of oil immersion and relay imaging. A significant deviation between existing theory and experiment is found for large crystal angles. We propose this is due to a highly anisotropic effective charge trap density in $\mathrm{Fe}:{\mathrm{KNbO}}_{3}$ of almost two orders of magnitude variation with crystal angle in the $a$-$c$ plane.