Near-field optical study of self-focusing in thin film of amorphous silicon

Abstract

The third-order nonlinear optical properties of a material can be observed through optical bleaching and reverse saturation in absorption aspect, and self-focusing and self-defocusing in refraction side. Among these phenomena, optical bleaching and self-focusing phenomena have an effect of reducing the transmitted optical beam spot and might have an application to enhance the recording density of the optical storage media. In this work, third-order nonlinear optical properties of amorphous silicon (A-Si) film are investigated through open-aperture Z-scan method and near-field scanning optical microscopy (NSOM) technique. The A-Si film is deposited on the coring fusion glass and its thickness was 500nm. First, an open aperture Z-scan with He-Ne laser was used to find the imaginary characteristics of the third-order nonlinear optical susceptibility of a A-Si film. It showed a reverse saturation. Then NSOM technique is used to measure the intensity profile of the focused He-Ne laser beam. We observed a definite decrease in the beam size from 1 .45micro-m to 1 .2 micro-m as the intensity is increased from 5 kW/cm2 to 140 kW/cm2 even though A-Si has a behavior of reverse saturation. The similar self-focusing from 1.45 micro-m to 1.2 micro-m can be also observed with a film ofA-Si as thin as 100nm. Keywords: amorphous silicon film, self-focusing, z-scan, near-field scanning optical microscopy