Optically induced molecular reorientation and third-order nonlinear optical processes in nematic liquid crystals

We report some quantitative studies of four-wave mixing in aligned nematic-liquid-crystal films. Using an unfocused cw laser of 200 mW power, at an estimated intensity of 5 W/${\mathrm{cm}}^2$, we have observed a conversion efficiency of ${10}^{-2\mathrm{}}$. The third-order nonlinear polarization that is responsible for this wave-mixing process is attributed to the optically induced molecular reorientation. A quantitative calculation of the wave-mixing efficiency and the intensity dependence is presented. It is shown that the experimental results are in good agreement with the theoretical predictions.