PHASE-MATCHING TECHNIQUES AND FREQUENCY-CONVERSION EFFICIENCY IN OPTICALLY-ACTIVE CRYSTALS

Many materials of potential interest for nonlinear optical application s exhibit optical activity and this is generally not taken into accoun t in the standard theoretical treatments of nonlinear optical effects. In this paper we examine the conditions under which optical activity significantly alters the nonlinear optical properties of a material an d show how this mechanism might be used in designing a nonlinear optic al device. First, an algorithm is developed to calculate the exact opt imum phase-matching angles and conversion efficiency for three-wave in teractions in optically active biaxial crystals. Corrections to the st andard theoretical expressions for birefringent phase-matching angles and conversion efficiency are obtained for application to biaxial crys tals with large natural birefringence. Second, a generalized quasi-pha se-matching scheme based on the presence of optical activity is formul ated for three-wave interactions in second-order nonlinear crystals. I t is shown that quasi-phase-matching via optical activity can be achie ved by way of an effective harmonic modulation of d(eff), the effectiv e second-order nonlinear coefficient. For both these effects, it is fo und that optical activity makes a negligible change in the nonlinear o ptical properties of a material such as KTiOPO4, but they may become i mportant for liquid crystals or polymers with high optical rotation an d small birefringence.