Retrieval of the photo-induced changes of the dielectric function of thin films in the pre-ablation regime

Abstract

The retrieval of the photoinduced changes in the dielectric function from time-resolved pump-probe spectroscopy of thin films is investigated. In addition to the Fabry-Perot effect on the probe beam, we consider modulated excitation across the sample thinkness due to interference effects on the pump. General expressions for calculating the standing wave intensity distribution for arbitrary pulse length to film thickness ratios are presented. Emphasis was put on films transparent at the pump wavelength where the excitation is through a nonlinear absorption process. A unique approach for the retrieval of the changes of the dielectric function from the measured changes of the probe reflection and transmission is discussed. The method is applicable to measurements with a white light continuum probe which is used to get spectral resolution in addition to time resolution. From the retrieved time evolution of the dielectric function one can indentify various relaxation processes; starting from hot electron thermalization to the dynamics of the formation of a new material state. The results help identify damage and incubation mechanisms.