Optical nonlinearities of bis(4-dimethylaminodithiobenzil)-nickel solution in the nanosecond regime

Nonlinear absorption and nonlinear refraction of the saturable absorber BDN (bis(4-dimethylaminodithiobenzil)-nickel) in a nitrobenzene solution are studied using a nonlinear transmission method and z-scan methods, respectively, under irradiation with 23 ns laser pulses at 1.0.47 mu m. The z-scan experiment shows that, in contrast to the previous work by Zhu and Garmire [IEEE J. Quantum Electron. 19 (1983) 1495], the main origin of the intensity-dependent refractive index change in the dye solution is the thermal effect even in the nanosecond regime. The buildup of the thermal lens is attributed to the photoacoustic effect and its rise time is proportional to the beam spot size. For a tightly focused beam, the rise time can be as fast as about ten nanoseconds. The fall time of the thermal lens is determined by thermal diffusion and is generally on the order of milliseconds. Moreover, nanosecond optical limiting and pulse narrowing have been demonstrated using strong self-defocusing induced by the photoacoustic effect.