Dynamics of new phase formation in silicon during femtosecond laser ablation

We experimentally demonstrated (using micro-Raman spectroscopy and transmission electron microscopy) and through numerical modeling that when an intense (1013−1014 W/cm²) femtosecond (~100 fs) laser pulse impacts the surface of silicon with (111) orientation, new polymorphic phases Si-III and Si-XII are formed on the surface and in the volume, localized in lattice defects as well as at the periphery of the ablation crater. This localization of phases is caused by the multi-stage nature of laser-induced phase transitions in silicon, specifically, the phase transitions are initiated by a shock wave, resulting in a cascading transformation process on sub-nanosecond timescales: Si-I => Si-II => => Si-III/Si-XII. The phase transition Si-I => Si-II occurs at the front of the shock wave, while at the rear of the shock wave, a field of dynamic stresses arises in the material, allowing the phase transition Si-II => Si-III/Si-XII to occur. On sub-microsecond timescales, most of the new phases disappear as the material relaxes back to its original state.