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Femtosecond laser fabricated three-dimensional (3D) SiO 2 optical waveguide is not only a strong candidate to build integrated photonic chips but also a powerful multi-physics simulation platform. However, up to now, the ultrafast optical excitation microscopic dynamic process during laser-induced SiO 2 modification at the femtosecond timescale is still unclear. In particular, there is insufficient research on the time-dependent evolution of electronic excited states and the driving mechanism of lattice distortion. This work investigates the femtosecond laser stimulated electron excitation and the resultant lattice dynamics in α-quartz based on the real-time-dependent density functional theory (rt-TDDFT). It presents the time-evolution pattern of electron density distribution under photoexcitation, and captures the ultrafast transition of electrons from bonding states to antibonding states. It is found that the migration of oxygen atoms is more remarkable than that of silicon atoms during laser irradiation. The repulsive effect arising from such excited-state transition induces the collective elongation of Si-O bonds by 1.6%∼4.6%. The elongation of Si-O bonds reduces the overlap integral, leading to a decrease in the band gap of quartz during lattice evolution. Furthermore, the above phenomena can still occur at a laser power density as low as 1.13 × 10 13 W/cm 2 , which is close to the laser power density threshold for Type I optical waveguide fabrication. Within the theoretical framework of the time-dependent Schrödinger equation, this work elaborates the excited-state electronic dynamics mechanism of femtosecond laser-induced SiO 2 modification. Comparison of theoretical and experimental results demonstrates that the theoretical framework possesses quantitative guiding significance for laser processing experiments.
Zhao et al. (Mon,) studied this question.