Transient optical properties of dielectrics and semiconductors excited by an ultrashort laser pulse

The transient permittivity of dielectrics and semiconductors excited by a powerful ultrashort laser pulse is introduced here in explicit form, which shows a decreasing contribution of valence electrons and an increasing contribution of free carriers with rising laser fluence. We describe the evolution of permittivity from the initial state up to transformation into plasma before ablation. A two orders of magnitude change in the electrons' collision rate during this transition is taken into account explicitly. The interplay between ionization nonlinearity and electron collisions dominates the transient optical properties of a swiftly excited material and results in unexpected minima and maxima in the permittivity and reflectivity. Our analysis of the transient permittivity of silica and silicon at 800 and 1300 nm reveals the differences in the femtosecond excitation of narrow- and wide-band-gap material, and also distinguishes a metal-like state of the ionized dielectric from the excited state in metals. The dependence of transient permittivity on electron density obtained can be directly mapped onto the fluence distribution in the time and space domains. We briefly discuss the possibility to measure these transient properties in pump-probe experiments.