Direct observation of self-focusing with subdiffraction limited resolution using near-field scanning optical microscope

The self-focusing effect in As2S3 glass has been studied using a near-field scanning optical microscope. Optical images of fine features in the self-focused beam were directly measured at the self-focus with approximately 100 nm spatial resolution. Because of the unusually large nonlinear refractive index at 690 nm, filaments with minimum size of 0.3 mum were observed in a 1.6 mW beam propagating through the 1.7 mum thin film. A qualitative analysis of our experimental results is presented. We show that nonparaxiality is responsible for arresting self-focusing, as predicted by recent theories.