Fine structure in spatial self-phase modulation patterns: at a glance determination of the sign of optical nonlinearity in highly nonlinear films

Abstract

We present a fast and simple method to determine the sign of the nonlinear coefficient n2 in films with high nonlinearity. The method exploits the effect of spatial self-phase modulation and the role of wavefront curvature that leads to a specific far field pattern that is dependent on the sign of n2 and on the sample position. The method can be considered a 'visualization' of the Z-scan for highly nonlinear films. Application to nematic liquid crystals under different experimental conditions confirms the results expected from the theory.