Dynamic holography with nonplane waves in sillenites

A three-dimensional model describing two-wave mixing in photorefractive crystals with finite beams of arbitrary shape is presented. The well-known longitudinal and transverse geometries are generalized by allowing an arbitrary orientation of the local grating vector. The coupled equations for the wave amplitudes are solved numerically in the small-angle approximation by introducing the light paths as characteristics. The influence of amplitude distributions and phase-front curvatures as well as the influence of geometrical arrangements and crystal properties (i.e. optical activity, externally applied voltage) on the energy exchange and the evolution of the polarization states is investigated. It is demonstrated that scanning the cross-section of the finite signal beam behind the crystal allows analysis of the coupling process inside the crystal which cannot be studied directly. The results of experiments with finite beams carried out on a BSO crystal are in good agreement with the numerical calculations.