First-principles calculation of multiphoton absorption cross section of α-quartz under femtosecond laser irradiation

Time-dependent density functional theory-based first-principles calculations have been used to study the ionization process and electron excitation. The results show that the number of excited electrons follows the power law σ_kI^k at peak intensities of I < 5 × 10¹³ W/cm², indicating that the multiphoton ionization plays a key role. The multiphoton absorption cross section of α-quartz σ_k is further calculated to be 3.54 × 10¹¹ cm⁻³ ps⁻¹ (cm²/TW)⁶. Using the plasma model, the theoretical results of the damage threshold fluences are consistent with the experimental data, which validates the calculated value of multiphoton absorption cross section. By employing the calculated cross section value in the plasma model, the damage threshold fluences are theoretically estimated, being consistent with the experimental data, which validates the calculated value of multiphoton absorption cross section. The preliminary multiscale model shows great potential in the simulation of laser processing.