Abstract
Nonlinear optical propagation effects leading to pronounced changes in the transverse profiles of laser beams are of current interest in optical physics. Far-field interference rings, for example, have been observed in a number of systems, such as sodium vapor and semiconductors. In particular, as many as fifty to a hundred rings have been observed in nematic liquid crystal films because of the large optical nonlinearities of this medium. This phenomenon can be understood as the spatial analog of self-phase modulation of light as the laser beam traverses the sample. It is the phase shift impressed upon the laser beam profile by the nonlinear index of refraction that produces the rings. We have studied the effect of the dc electric field on optical nonlinearity and hence on propagation effects in a homeotropically aligned nematic liquid crystal, MBBA.