Relations between microscopic and macroscopic lowest-order optical nonlinearities of molecular crystals with one- or two-dimensional units

Efficiency of three-wave interactions in molecular crystals depends on the conjugation of the molecular unit, which in turn is a one- or two-dimensional property. This strong anisotropy reduces the number of non-negligible molecular lowest-order hyperpolarizability coefficients to four. The lowest-order macroscopic optical nonlinearity can be expressed, in the absence of significant intermolecular effects, as the tensorial sum of molecular hyperpolarizabilities. This analysis is applied to the 17 relevant noncentrosymmetric crystal point groups, generalizing a previous analysis of nonlinear-optical properties of methyl- (2, 4-dinitrophenyl) -aminopropanoate crystals. In several cases, the molecular unit anisotropy is shown to impose structural relations between coefficients of macroscopic nonlinearities, in addition to the usual relations resulting from the crystal point symmetry only. In such cases, nonlinear-optics experiments can be used for testing molecular anisotropy and molecular orientations within the unit cell in the absence of significant nonlinearity arising from intermolecular coupling. Similar relations can be derived between electro-optic coefficients, but limited to the case of weak contributions of intermolecular vibration to the electro-optic effect. We investigate for each point group the possibility of inferring hyperpolarizability coefficients from macroscopic nonlinear measurements, a complementary approach to that based on theoretical molecular calculations or electric-field-induced second-harmonic generation in solution. In the case of highly anisotropic one-dimensional charge-transfer systems (exemplified by p-nitroaniline), for each point group and a given molecular hyperpolarizability, the optimal orientation of the charge transfer axis, leading to the highest phase-matchable coefficient, is given. It is shown that crystal point groups 1, 2, m, and mm2 correspond to the highest possible value of this coefficient, while other crystal symmetry is less favorable. These considerations are applied to four available efficient molecular crystals and used either as a check of molecular orientations in a case of low crystalline symmetry or to estimate otherwise unavailable molecular nonlinear coefficients.