Ultrafast reflectivity dynamics of highly excited Si surfaces below the melting transition

Solid Si under intense femtosecond irradiation is investigated over a wide range of fluences below the melting transition by supercontinuum light (400--800 nm) transient reflectivity. By solving a system of time-dependent equations, the fast and slow components of the energy and carriers diffusion can be disentangled from the reflectivity data, providing considerable insight into the nonequilibrium phase-change dynamics. The study of the fluence values immediately preceding the dramatic melting transition can be useful for discriminating between the thermal and electronic origins of the disordering of the structure by looking at the modification of the solid-state properties of the Si surface.