Off-Bragg-angle light diffraction and structure of dynamic interband photorefractive gratings

Steady-state and time-resolved off-Bragg-angle diffraction experiments are used to determine the structure and the dynamics of photorefractive gratings induced by interband photoexcitation. In potassium niobate, we identify in such gratings basically a two-layer structure. Close to the surface, we find a space-charge electric field generated by a charge modulation stored directly in the bands. This grating component is typically 50 mu m thick, the amplitude of the refractive index modulation is larger than 10(-4), and the response time is a few mu s for resonant intensities of 100 mW cm(-2). This component is also robust under non-resonant illumination. Deeper in the crystal, a second holographic layer extends over a few hundreds of mu m, its amplitude is smaller, and its slower response time is in the ms range. The mutual phase shift between the grating components is also determined.