Discrimination of thermal and electronic refractive nonlinearities in gold nanocolloids by off-resonant high repetition polarized femtosecond z-scan

The nonlinear optical properties of gold nanocolloids in the off-resonant regime using the polarization-resolved Z-scan technique with high-repetition-rate femtosecond laser pulses are investigated in this study. Thermal and electronic refractive nonlinearities are distinguished by analyzing the closed-aperture Z-scan traces obtained for linear and circular polarization. The continuous-wave laser data reveal a dominant thermal nonlinearity, while experiments with high-repetition-rate femtosecond laser pulses show contributions from both thermal and electronic nonlinearities. We provide a quantitative evaluation of the thermal and electronic contributions by comparing experimental results, precisely determining the nonlinear refractive index. The results highlight the importance of thermal accumulation effects in high-repetition-rate laser pulses and offer valuable insights into the complex refractive nonlinearities of Au nanocolloid.