Dynamics of the Femtosecond Mid-IR Laser Pulse Impact on a Bulk Silicon

Abstract

In this study, we reconstructed the dynamics of the impact of mid-IR-range (4.6 μm) femtosecond laser pulses on bulk silicon under tight focusing conditions (NA = 0.5). Our experimental results show that under this impact, the deposited energy density (DED) reaches approximately 4 kJ/cm3 (at an energy slightly above the plasma-formation threshold). Initially, the femtosecond pulse energy is absorbed by the laser-induced plasma, with a lifetime of approximately 160–320 fs (depending on the laser pulse energy). The energy transfer from the plasma to the atomic subsystem occurs on a sub-ps timescale, which generates a shock wave and excites coherent phonons on a sub-ps scale. The shift of atoms in the lattice at the front of the shock wave results in a cascade of phase transitions (Si-X => Si-VII => Si-VI => Si-XI => Si-II), leading to a change in the phonon spectra of silicon.