Phase transitions in $a$-Si:H films on a glass irradiated by high-power femtosecond pulses: Manifestation of nonlinear and nonthermal effects

The crystallization of amorphous silicon films 90-nm thick irradiated by laser pulses (a wavelength of 800 nm and a pulse duration of 120 fs) is investigated using Raman scattering spectroscopy and electron microscopy. The absorption coefficient for 800-nm low-power probe radiation by an a-Si:H film is small but can increase owing to nonlinear effects for high-power pulses. According to the estimates, the energy absorbed in the film is insufficient for its heating and complete melting but is sufficient for the generation of free charge carriers with a density of about 10²² cm⁻³. The electron and phonon temperatures in this case are strongly different and silicon becomes unstable. Thus, the action of such a short laser pulse cannot be reduced only to the heating effects and subsequent phase transitions through the liquid or solid phase.