Concentric diffraction rings with bright center in the far field and dark c
enter in the near field of the probe beam were experimentally observed when
a strong pump laser beam (526 nm) and a weak probe laser beam (632.8 nm) c
ross under the nonlinear medium film (Polypyrrylarylenemethine/polyvinyl fi
lm). Based on Fresnel-Kirchhoff diffraction integral equation,this phenomen
on can be explained by the diffraction effect of the probe beam from the la
ser-induced phase-hole in the nonlinear medium caused by the pump beam. The
numeric calculations give good agreement with the experimental results.