Observations of continuous-wave self-deflection and spatial dark solitary waves in nonlinear media

Two novel nonlinear beam propagation phenomena have been observed for the first time: continuous-wave self-bending of a laser beam, and the formation of dark solitary stripes in the cross-section of a laser beam. Sodium vapor was used as a negative (self-defocusing type) nonlinear medium in both experiments. The cross-sectional intensity profile of the beam was monitored as the laser frequency was tuned through the {dollar}Dsb2{dollar} atomic resonance. Measurements of the self-deflection angle, which were as large as 5.9 mrad (or 8.4 times the diffraction angle), allowed us to determine for the first time that {dollar}nsb2{dollar} {dollar}simeq{dollar} {dollar}-{dollar}10{dollar}sp{lcub}-7{rcub}{dollar} cm{dollar}sp2{dollar}/W at {dollar}sim{dollar}200{dollar}spcirc{dollar}C (in vacuum). The nonlinear response is attributed to the change of population of the atomic transition, which can be approximated by an inhomogeneously broadened two-level system; however, we did not observe a saturation of the nonlinear refractive index, as predicted by numerical calculations based on this model. The observations that lead to the discovery of a rectangular grid of dark solitons was accomplished by focusing the image of a wire screen at the input face of a sodium vapor cell. An amazing transformation was observed in the far-field area as the laser frequency was tuned near (but below) the atomic resonance: the Fraunhofer diffraction pattern (which corresponds to linear diffraction) evolved into a pattern that resembled an image of the wire screen. Similar observations were also obtained when materials with other (negative) nonlinear mechanisms were used. At the output face of the nonlinear material, the intensity profile exhibited dark stripes that crossed each other at right angles. Numerical solutions of the nonlinear Schrodinger equation, used to model nonlinear beam propagation in these media, qualitatively reproduced the observed near and far-field intensity profiles, and provided additional evidence that the phenomenon is indeed attributed to the formation of dark solitons. There are a variety of possible applications for both dark spatial soliton and self-bending phenomena, particularly in the areas of nonlinear spectroscopy, optical signal processing, and radiation protection.