Theory of optically induced molecular reorientations and quantitative experiments on wave mixing and the self-focusing of light

We present a theoretical treatment of optical-field-induced molecular reorientations and quantitative experimental verifications of the associated nonlinear-optical processes in the nematic phase of liquid crystals. Explicit analytic expressions are obtained in the small-angle linearized approximation of the Euler-Lagrange equation describing the molecular reorientation. It is found that optical-field strengths much lower than the so-called optical Freedericksz transition threshold can create substantial molecular reorientations and generate easily observable nonlinear-optical effects such as self-focusing and degenerate four-wave mixings. The theoretically predicted dependences on the scattering geometry, the optical intensities, and the nematic thicknesses are in excellent agreement with experimental data. The relative contribution of thermal effect is also determined.