Evolution mechanism of transient optical properties of ultrafast laser-induced monocrystalline silicon

The evolution pattern of the transient optical properties on the surface of monocrystalline silicon materials under the action of lasers with different pulse widths and different energy densities in the sub-picosecond to picosecond range was studied. This research was based on a dual temperature equation, carrier number density model, that considered the latent heat of phase transition. The carrier temperature, lattice temperature, permittivity, and the number density of excited carriers during laser irradiation were calculated, energy transfers processes from photons to electrons and electrons to phonons was simulated. In the end, the variation results of refractive index and extinction coefficient of the monocrystalline silicon surface were obtained. This result helps to reveal the evolution mechanism of the transient optical properties of monocrystalline silicon materials under the irradiation of ultrashort pulse lasers in the sub-picosecond to picosecond pulse width range. Theoretical calculations show that if a single laser pulse cannot melt monocrystalline silicon, the effects of different laser energy densities and laser pulse widths on the minimum refractive index and extinction coefficient are minimal. In the laser energy density range from 0.3 J/cm² to 0.4 J/cm², the minimum refractive index change is less than 0.5% per 0.01 J/cm² change in energy density. Suppose a single laser pulse can melt monocrystalline silicon. In that case, different laser energy densities and pulse widths have different degrees of influence on the silicon surface's refractive index and extinction coefficient. This research results can provide some theoretical guidance for the processing and surface modification of monocrystalline silicon materials based on ultrashort pulse laser.