STABLE AZIMUTHAL STATIONARY STATE IN QUINTIC NONLINEAR-OPTICAL MEDIA

We analyze the structure of the first azimuthal stationary state for a nonlinear medium presenting simultaneously a cubic (focusing) and a q uintic (defocusing) dependence with the light intensity in the refract ive index. This solution takes the form of a dark vortex of light host ed in a compact light beam. The existence of these modes is guaranteed if the flux exceeds a certain minimum threshold and if the modes are extremely stable for fluxes larger than a critical value that we calcu lated. We verified the robust nature of this solution inducing interna l oscillations by an initial phase chirp. Using the variational method , we obtain an approximate picture for the beam internal dynamics. We also studied numerically the interactions between two near-vortex solu tions, finding that for a wide combination of beam parameters they sho w elastic collisions. (C) 1997 Optical Society of America.