Dynamic light scattering at optical-field-induced Freedericksz transition in nematic liquid crystal

The collective orientational fluctuations near optical-field-induced Freede ricksz transition in a homeotropically aligned cell of the nematic liquid c rystal 5CB were studied by dynamic light scattering. The beam size of the t ransition inducing Gaussian shape laser beam was considerably smaller than the thickness of the cell. A softening of the orientational fluctuation mod e corresponding to the transverse inhomogeneity of the optical field was de tected in a region which extended from about 70 % of the threshold intensit y up to the threshold intensity. Measurements above the threshold intensity showed that at large scattering angles the time response of the fluctuatio ns is very close to the usual exponential behaviour with a quadratic disper sion of the inverse relaxation time. At small scattering angles in addition a very gradual "tail" appears at long correlation times, signifying the in stability of the perturbed structure.