Technique for measuring time-dependent optical nonlinearities using continuous time-resolved Z-scan

A dual-beam (pulsed-pump, cw-probe) Z-scan technique (CWZ-scan) can simultaneously measure both fast and slow nonlinear mechanisms contributing to the nonlinear refractive index by recording cw-probe transmission, T(z,t), as a function of both time and z position. A similar approach was previously used as a sensitive measure of slow thermal variations in refractive index. This paper expand upon this work by demonstrating that CWZ-scan can be used to completely determine the nonlinear components of refraction n(I, t) and absorption α(I, t) as functions of intensity and time. This is accomplished by fitting the experimental transmission surface T(z, t) to the theory, which was simulated following the approach of Lee et.al. that includes effects of bound-electronic (Kerr) refraction, two-photon absorption (TPA), and free-carrier refraction (FCR) absorption (FCA), and recombination.