Kazuhiko OGUSU, Yoshiaki KOHTANI and Heng SHAO

Department of Electrical and Electronic Engineering, Shizuoka University, 3-5-1, Johoku, Hamamatsu, 432 Japan

(Received February 8, 1996; Accepted June 10, 1996)

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 Δn 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 Δn~0.1 and n₂=-2.9×10^-5 cm²/W. This large nonlinearity is attributed to a thermal effect resulting from linear absorption.

Key words : spatial self-phase modulation, multiple diffraction rings, nonlinear refractive index, saturable absorbers, z-scan technique, thermal effects