OPTICAL-ACTIVITY IN PHOTOREFRACTIVE BI12TIO20

The influence of optical activity on two-wave mixing (TWM) in photoref ractive BTO and BSO crystals in the absence of an applied field is stu died both theoretically and experimentally. For the conventinal orient ations of the grating vector, K parallel to [001] and K perpendicular to [001], the piezoelectric and photoelastic effects are either zero o r negligible. This makes an analytical treatment of the TWM problem po ssible. We obtain an analytical solution for the coupled wave equation s of TWM valid for arbitrary optical activity. This result is of speci al importance for BTO crystals. In these crystals under the condition of maximum energy transfer (\K\r(D) = 1, where r(D) is the Debye radiu s) neither the approximation of small optical activity nor the one of dominating optical activity is applicable and our analytical solution becomes essential. Our experimental setup uses beams with a trapezoida l overlap that allow us to study the thickness-dependence of the gain in a single measurement. Experimental and theoretical results for a BT O crystal are compared with those for a BSO crystal and are explained in the framework of the model used. (C) 1998 Elsevier Science B.V.