Ultrafast electron dynamics of TiO2 and its ultrashort laser ablation mechanism: A modified model based on the electronic excitation and atomic bond rupture

This paper reports on a modified ultrashort laser ablation model of TiO₂ via electron dynamics. The ultrashort electron dynamics of TiO₂ under a time-dependent density functional theory plus Hubbard U (TDDFT + U) framework are investigated, and an electron excitation rate equation is built. Aiming at the time-consuming TDDFT + U method for computation of the optical property in the excited state, a new method of a combination of electron excitation and finite electron temperature via DFT is proposed. The real and imaginary parts of the permittivity of TiO₂ obtained by the as-proposed method are almost the same as the data with TDDFT + U. Additionally, a comparison of the theoretical and experimental differences in transmission verifies the accuracy of the theoretical permittivity of TiO₂ in the excited state. Then, a new criterion for determining the critical electron density via bond rupture is also proposed. The structure relaxation of TiO₂ in the excited state is performed, and the critical electron density turns out to be 2.15 × 10²⁸ m⁻³. Last, the theoretical ablation thresholds with the pulse duration ranging from 240 fs to 6 ps are in good agreement with the experimental data.