Laser-Induced Diffraction Rings from an Absorbing Solution

We observed multiple diffraction rings of a cw Ar⁺ laser beam from a nitrobenzene solution of saturable absorber BDN (bis-(4-dimethylaminodithiobenzil)-nickel) caused by the spatial self-phase modulation at low incident optical intensities. We obtained 37 rings for a 200-μm thick sample at an optical intensity of 38 W/mm². The refractive-index change Δ;n and effective nonlinear refractive index n₂ were determined from the number of observed rings and by the z-scan technique. We obtained large values of Δ;n∼0.1 and n₂ = -2.9×;10^-5 cm²/W. This large nonlinearity is attributed to a thermal effect resulting from linear absorption.