Femtosecond Variable-Angle Spectro-Ellipsometry of a Rarefaction Wave Front

Abstract

Intense femtosecond (fs) irradiation of a solid surface creates transient (~ Mbar) internal pressure in the absorption region because the lattice is heated faster than thermal expansion can occur. Before the pressure is released (Δt ~ 1-2 ps) the heated surface remains optically sharp, so that time-resolved ellipsometric measurements can be analyzed with Fresnel’s equations, revealing the optical properties of novel pressurized fluids such as liquid carbon1,2 At later times (2 ps < Δt < 20 ps), a rarefaction wave front develops at the surface, as the laser induced strong compressional shock wave acts on the solid density plasma. The expansion dynamics can provide diagnostic information on initial pressure-temperature conditions. This expanding rarefaction wave front can also be probed by time-resolved ellipsometry measurements, provided they are analyzed by generalized Helmholtz wave equations3. We recently presented preliminary time-resolved reflectivity measurements of hydrodynamically expanding metal surfaces using a single probe wavelength (620 nm) and incidence angle (70°)4. In this paper, we report comprehensive measurements on free electron metals (Al, Ag), semiconductor (Si), and semimetal (C, graphite) targets using variable probe incidence angle, wavelength, and polarization. Such measurements permit our theoretical model of rarefaction wave structure, dielectric properties, and expansion dynamics to be defined much more quantitatively.