Absorption Gratings with Multiple Levels

Abstract

Absorption gratings can cause significant beam coupling in photorefractive crystals;1,2 in the case of BaTiO3, we found gains as large as 1 cm−1.3 This coupling is caused by the variation in absorption created by the spatially periodic distribution of empty and full trap sites throughout the crystal and it requires only one active level. There is also evidence that there are two or more photoactive levels in BaTiO3, as well as in other photorefractive crystals. In BaTiO3, these extra levels explain the sublinear intensity dependence of the photoconductivity,4,5 as well as light-induced absorption.6,7 Here we study the effects that multiple photo-active levels have on absorption grating coupling. We derive expressions for the two-beam absorption coupling gain vs. the magnitude of the grating wavevector k g and show that it does not vanish as kg tends to zero, in contrast to the single photo-active level case. This effect is verified experimentally and is used to evaluate the donor-to-acceptor density ratio in a BaTiO3 crystal.