Dynamics of laser-induced optical switching in silicon and GaAs: impact of spatially resolved transport and density-dependent optical losses

A coupled transport-optical framework is developed to describe the time-resolved optical response of photoexcited semiconductor slabs subjected to nanosecond laser excitation. Carrier dynamics is formulated as a diffusion-recombination boundary-value problem in a finite geometry with two-sided surface recombination, yielding the spatiotemporal carrier distribution without invoking spatial averaging or effective lifetimes. The resulting depth-dependent density is mapped pointwise onto a transient dielectric function, and the optical response is obtained from an electromagnetic transfer-matrix treatment of probe-wave propagation in a stratified medium. To capture enhanced dissipation in the intermediate-density regime corresponding to the epsilon-near-zero condition, a density-dependent correction to the Drude relaxation rate is introduced at the level of the optical response. The predictive capability of the framework is demonstrated through its consistency with time-resolved reflectance, transmittance, and absorptance measurements for silicon and GaAs, representing benchmark indirect- and direct-gap semiconductors. The results show that resolving both spatial carrier redistribution and dynamic optical damping is essential for accurately describing laser-induced optical switching in finite semiconductor samples.