Nonlinear photoconductivity in pump-probe spectroscopy experiments

Synopsis

In this thesis, we develop an analysis framework to derive the photoconductivity from pumpprobe spectroscopy experiments when the photoinduced response is nonlinear in fluence. In pump-probe experiments, an intense pump pulse is used to photoexcite a material, and a time-delayed probe pulse is used to monitor how its optical properties evolve in response to the photoexcitation. These experiments enable the investigation of the dynamics associated with the electronic, lattice, and spin degrees of freedom in solids at their fundamental time and energy scales. A standard practice in these experiments is to infer the photoinduced change in the response functions, such as the conductivity or permittivity, from the measured optical coefficients, such as the reflection or transmission coefficients. However, these optical coefficients are average responses integrated over the entire photoexcited volume, while the responses of interest are local quantities that can vary spatially or temporally within the medium. Therefore, we need a theoretical model to describe the photoexcitation depth profile to extract the local photoconductivity from the measurements. Common analysis techniques treat the photoexcited region as spatially uniform or decaying exponentially within the medium, but this assumption is valid only when the photoconductivity is linear in pump intensity. Although this assumption holds at low pump intensities, nonlinearities can emerge when the experiments are performed at high pump intensities and neglecting this nonlinearity can introduce systematic errors in the results of pump-probe experiments. To address this problem, we develop an analysis procedure to derive photoconductivity in pump-probe experiments that accounts for common optical nonlinearities. We highlight the significance of this analysis in pump-probe studies on K3C60, LBCO and YBCO that have been previously interpreted as evidence for photoinduced superconductivity. We show that several features in the reported response that were previously interpreted as evidence for photoinduced superconductivity are distorted due to optical saturation. Although our analysis focuses on studies of photoinduced superconductivity, the procedure we develop here is broadly applicable to pump-probe experiments, where fluence-dependent nonlinearities can occur.

For Zoom link info, please contact Lindiwe Coyne at physgrad@sfu.ca.