Direct observation of self-focusing near the diffraction limit in polycrystalline silicon film

Abstract

We present direct observation of self-focusing near the diffraction limit by measuring the beam-spot size with a scanning fiber probe tip. We have used the polycrystalline silicon film, which exhibits a reverse-saturation (Im χ(3)≈8×10−3 esu) and self-focusing (Re χ(3)≈2×10−2 esu), as measured by the conventional z-scan method with He–Ne laser. It is observed that the beam radius of about its wavelength becomes smaller as the input laser intensity is increased, which indicates that the self-focusing effect dominates over the reverse saturation in the 300-nm-thick sample.