Periodic surface structure formation on silicon surfaces by irradiation of single and multiple femtosecond laser pulses

Laser-induced periodic surface structures (LIPSS) were fabricated on silicon substrates using a low number of ultrashort laser pulses with 266 fs pulse duration at a 1030 nm wavelength. The obtained structures were investigated with SEM and EDX techniques, then the obtained results were simulated by theoretical calculations. In the applied theoretical approach, the LIPSS formation is simulated by a semi-empirical model of interference of the incident laser beam with surface-scattered electromagnetic waves generated at a rough surface, which includes excitation of surface plasmon polaritons (SPP) and accounts for the transient intrapulse changes in optical properties due to photo-excitation of a dense plasma of electron-hole pairs. The threshold conditions for SPP excitation were shown to depend on the laser fluence, wavelength, polarization, and number of pulses. These thresholds were determined from first-principles calculations of the macroscopic dielectric function of the photoexcited material. The theoretical simulations demonstrate good agreement with the experimental observations obtained from SEM analysis. In particular, the optimal number of laser pulses required to achieve the highest LIPSS periodicity was determined to be two pulses from the theoretical model and three pulses from the experimental study. The corresponding LIPSS periods were found to be 0.922λ and 0.93λ, respectively. Overall, the applied theoretical framework shows strong consistency with experimental results and can be further used for predictive optimization of LIPSS formation conditions for different laser parameters.