Evidence of counter-propagating solidification fronts in silicon upon femtosecond laser induced amorphization

We have investigated the melting and solidification dynamics in crystalline silicon upon single-pulse fs laser irradiation at a wide range of laser wavelengths (UV–mid-IR), employing real-time reflectivity measurements with ns resolution. Combining the experimental data with a simple optical model, we show that for fs excitation of Si(111) at 3000 nm wavelength a counter-propagating solidification front is initiated at the surface that competes with a solidification front originated at the maximum melt depth, propagating toward the material surface. This is the first demonstration of a surface-initiated solidification process upon fs laser excitation. The relative contribution of the two propagating interfaces has been determined as a function of pulse fluence. This process is not observed for (100) orientation, although for both orientations an amorphous surface layer with a fluence-dependent thickness is obtained. The presence of a surface-initiated solidification process is also observed for 1030 nm irradiation wavelength and might be present at other wavelengths. The results contribute to the understanding of the solidification dynamics in ultrafast laser annealing processes and form the basis for improved control of the final material state, considering its crystalline structure and transformation depth.