Accounting for Nonlinear Photoconductivity in Time-Resolved Terahertz Spectroscopy

We examine a procedure that is commonly used to determine the photoexcited conductivity from time-resolved terahertz spectroscopy measurements. We show that this procedure introduces large systematic errors when the photoconductivity has a nonlinear dependence on the excitation density, and we show how to account for this in the case of a saturable nonlinearity. We calculate the reflection amplitude of a medium with a saturable photoconductivity, discuss its general features, and show how the nonlinearity appears in a conventional analysis. We show how to derive the surface photoconductivity from measurements of the reflection amplitude as a function of incident pump fluence.