INTERBAND PHOTOREFRACTIVE EFFECTS IN KNBO3 INDUCED BY ULTRAVIOLET ILLUMINATION

A novel mechanism is used to record photorefractive gratings in dielectric crystals. The photon energy of near-ultraviolet recording beams is larger than the energy gap, and the absorbed photons excite electrons from the valence to the conduction band. These interband photorefractive gratings in undoped KNbO3 show a faster response than gratings recorded in the same material with visible light, i.e., below band-gap excitation. 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 transversal readout geometry, large diffraction efficiencies of 60% in 7.5-mm-long crystals are demonstrated. Interband photorefractive gratings are only weakly affected by visible light.