Abstract

We present experimental evidence indicating that in the paraelectric phase, slightly above the phase transition, photorefractive (PR) gratings are formed by spatial modulation in the low frequency dielectric constant. The PR effect is normally attributed to the formation of spatially modulated space charge, which induces a correlated modulation in the birefringence through the electrooptic effect. In the paraelectric phase the electrooptic effect is quadratic and is given by where Δn is the birefrongence, n_o is the refractive index, g is the quadratic electrooptic coefficient, and P is the polarization. Therefore, a spatial modulation in the birefringence induced by a modulation of the polarization is given by Assume that P is in the linear region, i. e. P = εE; where ε is the dielectric constant and E is the electric field in the crystal. Then, if E_sc(x) is the space charge formed in the PR process, the modulation of the birefringence is given by where E_o is the electric field in the crystal. This is the so called voltage controlled PR effect (1,2). Considering equ. [2] it can be seen that if a modulation in ε is formed, it will induce a modulation in the birefringence given by We have observed that slightly above the phase transition such a dielectric grating was formed in potassium lithium tantalate niobate (KLTN) doped with copper and vanadium. The experiments were conducted in a KLTN sample with a ferrelectric phase transition at T_c =170K. Prior to the experiment the crystal was cooled slowly from 200K to 160K at 0.5K/minute under a field to 1.8kV/cm, and was then warmed slowly to the experiment temperature.

© 1996 Optical Society of America