Modelling of the ultrafast dynamics and surface plasmon properties of silicon upon irradiation with mid-IR femtosecond laser pulses

We present a theoretical investigation of the ultrafast processes and dynamics of the produced excited carriers upon irradiation of silicon with femtosecond pulsed lasers in the mid-infrared (mid-IR) spectral region. The evolution of the carrier density and thermal response of the electron-hole and lattice subsystems are analyzed for various wavelengths lambda(L) in the range between 2.2 and 3.3 mu m, where the influence of two- and three-photon absorption mechanisms is explored. The role of induced Kerr effect is highlighted and it manifests a more pronounced influence at smaller wavelengths in the mid-IR range. Elaboration on the conditions that lead to surface plasmon (SP) excitation indicate the formation of weakly bound SP waves on the material surface. The lifetime of the excited SP is shown to rise upon increasing wavelength, yielding a larger one than that predicted for higher laser frequencies. The calculation of damage thresholds for various pulse durations tau(p), shows that they rise according to a power law (similar to tau(zeta(lambda L))(p)) where the increasing rate is determined by the exponent zeta(lambda(L)). Investigation of the multiphoton absorption rates and impact ionization contribution at different tau(p) manifests a lower damage for lambda(L) = 2.5 mu m compared to that for lambda(L) = 2.2 mu m for long tau(p).