Abstract

Photorefractive materials have found many applications in nonlinear optics including image processing, phase conjugation, laser beam combining, and interferometry. Glass et al.¹ proposed that uniform illumination of a photorefractive crystal could generate a dc voltage across the crystal. This photovoltaic field plays an important role in the theory of charge transport in photorefractive materials and in the performance of photorefractive devices. However, the photovoltaic field is notoriously difficult to measure using electrodes on the crystal faces, due to surface and space-charge effects. We present an all-optical technique to measure the magnitude, sign, and intensity dependence of the photovoltaic electric field in a photorefractive crystal. We use two-beam coupling in the crystal to transfer energy between two optical beams having a slight frequency shift δf(~1 Hz). In the absence of a spatially uniform electric field in the crystal, the magnitude of the energy coupling is symmetric in δf. The presence of an internal electric field causes a characteristic asymmetry in the coupling. We show that in barium titanate (BaTiO₃) the variation of the coupling strength with δf is asymmetric, and the asymmetry increases with optical intensity. A fit of the data to theory yields a saturating photovoltaic field along the direction of the +c axis whose saturated value is ~80 V/cm.

© 1986 Optical Society of America