Origins of the Photorefractive Phase Shift

Abstract

There is a spatial phase shift between a sinusoidal light intensity pattern and the resulting refractive index grating formed in a photorefractive crystal. For crystals where diffusion is the dominant mechanism for charge migration, this spatial shift is 1/4 of the grating spacing. Any deviation from this value implies some other transport mechanism, such as an applied or internally developed electric field, or the photogalvanic effect, in which optically excited charges preferentially move in one direction in the crystal. Due to its relevance to beam coupling, this spatial shift has been studied extensively using a variety of methods, most of them interferometric in nature1-5. However, interferometric methods only reveal the phase of the complex beam coupling coefficient, which is not necessarily the spatial phase shift of the electro-optically induced grating. Other effects, such as trap grating coupling, can affect the phase of the total coupling coefficient, and thereby prevent a measurement of the spatial phase shift.