Abstract

Dark solitons [1] have provoked much interest [2-7] since they were first shown to be particular solutions of the two-dimensional (1+1)-D nonlinear Schrödinger equation (NSE) with a negative (self-defocusing type) nonlinear coefficient n₂ (see Eq. (1) below). So far, only (1+1)-D temporal dark solitons (i.e. intensity minimums propagating along a nonlinear fiber on a quasi-cw bright background) have been observed experimentally [4-7]. Here we report the first observations [8] of stable spatial structures (e. g., stripes, crosses and grids) in the transverse cross-section of a cw optical beam propagating in a material with a self-defocusing nonlinearity, with these structures having a strongly pronounced soliton nature — namely that of spatial dark solitons (SDS's). Although no (2+1)-D analytical solution for dark solitons in the NSE is known to date, our experimental and numerical data with various 2-D amplitude and phase masks provide strong evidence that the phenomenon observed by us is indeed due to spatial dark solitons. Furthermore, our results here on quasi-(1+1)-D propagation (see also [8,9]) have shown excellent agreement with the well known analytical results for (1+1)-D dark solitons [1]. In comparison to temporal dark solitons SDS's are easy to create and observe experimentally, requiring as little as a HeNe laser and some slightly absorbing fluid. Most recently, one of the authors (D.R.A.) with his coworkers have observed SDS's in pulse radiation in ZnSe crystals [10]. Various applications of SDS's can be envisioned, such as optical encoding, limiting, switching and computing, and nonlinear filtering.

© 1991 Optical Society of America