Interband Photorefractive Effects in KNbO3 crystals

1. Introduction Photorefractive gratings are most often produced by illumination of an electro-optic crystal with laser light below the band gap and the absorbed photons induce photoexcitation and redistribution of carriers within mid-gap trap centers [11. The photorefractive response time is longer or equal to the time needed to photoexcite a large enough number of earners to create the space-charge field [2], To increase the speed of response, beside using higher light intensities, also a large absorption constant a of the crystal may be helpful. By doping, a can be increased only in a limited range. In contrast, large absorption constants are observed without doping at the band edge of any photorefractive material. In this region, the absorbed photons can induce interband phototransitions of electrons. The creation of photorefractive gratings by photoexcitation over the band gap has been demonstrated recently in multiple quantum well (MQW) devices [3-6]. However, the thickness of such multilayer structures is limited because of the epitaxial growth process and reasonable diffraction efficiencies are obtained only making use of resonant nonlinear effects at the band-edge. This limits the read-out wavelength range to a width of 10-20 nm and the optical interaction length to few µm. The use of oxide crystals with large linear electro-optic coefficients instead of MQW’s overcomes this problem because the read-out can be done at any wavelength in the visible or infrared. Even though the thickness of the photorefractive grating depends on the absorption constant and on the light intensity of the writing beams, the interaction length L can be increased to the crystal size by propagating the non-absorbed readout beams parallel to the crystal surface.