Formation of absorbing heterogeneous plasma layer by femtosecond laser-induced melting and ablation of silicon

The experimental study of absorption in silicon in infrared and visible spectral ranges, where the photon energy is less or more than the bandgap width, is performed by means of the ultrafast interferometry technique. The exactly solvable model in the electromagnetic of heterogeneous lossy plasma layer was developed. The density of carriers, their frequency of collisions, absorbing depth of the probing waves, real and imaginary parts of dielectric function of nonuniform layer and their spatial gradients are determined from the reflectance data by means of this model subject to the pump fluence. The heterogeneity-induced effects are visualized due to comparison of obtained plasma parameters with those calculated in the framework of homogeneous plasma model It is shown that in the intensity range near thresholds of melting and ablation the absorption, occurring in both cases mainly within a thin (similar to 10 nm) absorbing layer (similarly to metals), is due to free carrier intraband absorption.