Optical nonlinearity in a mode-locking dye: optical limiting and four wave mixing

We report the optical nonlinearity resonantly induced by picosecond laser excitation in Kodak 9740 mode-locking dye. The observed nonlinearity is due to the participation of high-lying singlet states. The specific processes studied are optical limiting and phase conjugation. We infer that the mechanism underlying the limiting behaviour is due to two-photon absorption, and obtain a value for the two-photon absorption coefficient β. Further, we report the intensity dependence of the phase conjugate signal and obtain the corresponding third-order susceptibility.