Optical cross modulation in nematic liquid-crystal films: application to $\chi ^{(3)}$ dispersion measurement on Rayleigh-line flank

Amplitude modulation is transferred from one laser beam to another in a planar-oriented mixture of nematic liquid crystals to determine the dispersion in the cubic nonlinearity $\chi ^{(3)}$ at a Rayleigh-line flank with a resolution of a few Hz. The anomalously large values are due in part to the collective behavior in the orientation ordering and in part to the anomalously large $\partial {\epsilon} / \partial T$ and dichroism. Measurements have been made on $x_{ijk}^{3}$, relaxation time, elastic moduli, and thermal diffusivity parallel and transverse to the liquid-crystal director.