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

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 chi ((3))approximate to 8x10(-3) esu) and self-focusing (Re chi ((3))approximate to 2x10(-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. (C) 2001 American Institute of Physics.