Nanosecond Photorefractive Effects in KNbO3

Two laser beams interfering in a photorefractive crystal ionize light absorbing impurities and produce two initially overlapping concentration gratings. These two gratings consist of ionized donors that are immobile and of free charges that can move due to diffusion in a concentration gradient and drift in an external or internal electric field. Spatial separation between the two gratings produces a space-charge field which modulates the refractive indices of the crystal via the linear electro-optic effect. The photorefractive effect can be efficiently employed for two-wave and four-wave mixing already at the low light-intensities obtained with CW lasers [1]. The trade-off connected with low power optical beams is a decrease in response time. The photorefractive index change depends on the optical energy absorbed and not on the instantaneous optical intensity. In order to increase the speed of response beyond the limit given by the photogeneration rate one must use high intensity laser pulses. Photorefractive diffraction gratings with sub-microsecond response-times were already measured in LiNbO3[2], BaTiO3[3,4], KNbO3[5,6,15], and Bi12SiO20[7,8]. KNbO3 is known for its large linear electro-optic (EO) and second-order nonlinear optical coefficients. When chemically reduced the photorefractive response time decreases [9-11].