Time-resolved nonlinear refraction in femtosecond laser gain media.

We present time-resolved measurements of the electronic and the nuclear contributions to the third-order nonlinearities of Ti:sapphire and Cr:LiSGaF. The results suggest that nonlinear refraction in Ti:sapphire is resonantly enhanced relative to that of undoped sapphire. The implications of the nonlinear responses for short-pulse formation are discussed.