Feedback-controlled two-wave coupling in reflection geometry: application to lithium niobate crystals subjected to extremely high external electric fields

Abstract

An active stabilization of photorefractive two-wave coupling by means of an electronic feedback loop has been used extensively during recent years in transmission geometry. It leads to 100% diffraction efficiency eta and also to periodic states instead of familiar steady states. We investigate the feedback operation in the case of reflection geometry, especially for iron-doped lithium niobate (LiNbO3:Fe) crystals. This includes formulation of the feedback equations, numerical analysis of the operation regimes for LiNbO3 crystals, and comparison between theory and experiment, which is performed in the range of applied electric fields from 0 to 650 kV cm(-1). The main findings are as follows: (i) the feedback does not lead to periodic states, (ii) it modifies the photorefractive response by introducing a frequency shift and maximizes eta, and (iii) there is a close relation between the enhancement of eta and the resonant excitation of space-charge waves predicted earlier in ferroelectric materials.