Anomalous behavior of two-beam coupling in barium titanate

Abstract

Recently Klein and Valley observed anomalous behavior in a BaTiO3 crystal showing a change in sign of the beam-coupling gain coefficient (Γ) as a function of grating period.1 Using a band-conduction model for the photorefractive effect, Strohkendl et al. later explained the behavior of nearly compensated BaTiO3 crystals in beam-coupling experiments and demonstrated agreement with all the data in Ref. 2. In our studies of two-beam coupling, we also observed a change in sign of Γ, and we examined the wavelength dependence of Γ using a single longitudinal mode argon-ion laser. For short wavelengths the coupling coefficient changes sign at small grating periods and, as the wavelength is increased, the grating period at the crossover point increases in proportion to the increase in wavelength. To our knowledge, this behavior has not been predicted or observed before. We also notice a possible change in the characteristic wave vector (k0) as the wavelength is changed. This may be caused by a change in the effective density of photorefractive charge carriers which may also explain the change in the coupling coefficient crossover point.