INTERBAND PHOTOREFRACTIVE EFFECTS IN KNBO3 INDUCED BY ULTRAVIOLET ILLUMINATION

A novel mechanism is used to record photorefractive gratings in dielec tric crystals. The photon energy of near-ultraviolet recording beams i s larger than the energy gap, and the absorbed photons excite electron s from the valence to the conduction band. These interband photorefrac tive gratings in undoped KNbO3 show a faster response than gratings re corded in the same material with visible light, i.e., below band-gap e xcitation. We measured a response time tau = 5 mus for an intensity I = 1 W/cm2 and a refractive-index change of 2 X 10(-5). In a transversa l readout geometry, large diffraction efficiencies of 60% in 7.5-mm-lo ng crystals are demonstrated. Interband photorefractive gratings are o nly weakly affected by visible light.