LASER-INDUCED DIFFRACTION RINGS FROM AN ABSORBING SOLUTION

We observed multiple diffraction rings of a cw Ar+ laser beam from a n itrobenzene solution of saturable absorber BDN (bis-(1-dimethylaminodi thiobenzil)-nickel) caused by the spatial self-phase modulation at low incident optical intensities. We obtained 37 rings for a 200-mu m thi ck sample at an optical intensity of 38 W/mm(2). The refractive-index change Delta n and effective nonlinear refractive index n(2) were dete rmined from the number of observed rings and by the z-scan technique. We obtained large values of Delta n similar to 0.1 and n(2)=-2.9X10(-5 ) cm(2)/W. This large nonlinearity is attributed to a thermal effect r esulting from linear absorption.