A near-field z-scan, which directly measures the beam spot size in thin fil
ms, was done to examine the self-focusing effect near the diffraction limit
in polycrystalline silicon by using a scanning fiber probe-tip. The polycr
ystalline silicon showed both reverse-saturation (Im chi ((3)) approximate
to 8 X 10(-3) esu) and self-focusing (Re chi ((3)) approximate to 2 X 10(-2
) esu), as measured by using the conventional z-scan method. These results
were compared with the beam spot size directly determined from the near-hel
d z-scan measurements. The present input beam intensity dependence of the b
eam spot size revealed that the beam radius became smaller with increasing
input laser intensity, indicating that self-focusing effect in polycrystall
ine silicon was dominant over reverse saturation at high laser beam intensi
ty.