Electron dynamics in fused silica after strong field laser excitation detected by spectroscopic imaging pump-probe ellipsometry

Laser ablation of dielectrics by ultrashort pulsed laser radiation is induced by nonlinear photoexcitation, such as multiphoton and tunneling processes, and the subsequent impact excitation, due to electron-electron collisions. Previous theoretical and experimental studies demonstrate that these two phenomena strongly depend on the transient optical properties of the dielectric and the electron-electron collision times. To estimate these transient optical properties and the collision time, the excited electrons are usually considered as a nearly free-electron gas, being described by the Drude theory. All previous experimental studies support using the Drude theory by time-resolved measurements of the transient reflectance during and after irradiation. However, a comprehensive characterization of the transient complex refractive index, and, thus, a more complex view of the optical properties upon irradiation above the ablation threshold fluence was still missing. Therefore, this paper presents the time-, spectral-, and fluence-resolved complex refractive index of fused silica during the interaction with ultrashort pulsed laser radiation (800 nm, 40 fs, and 4.4 J/cm2) measured by spectroscopic imaging pump-probe ellipsometry. Comparing the state-of-the-art Drude model to the experimental data reveals that, although the Drude model replicates the transient reflectance of the material, it deviates significantly from the transient refractive index in the considered temporal range up to 200 fs after irradiation. Thus, the Drude model cannot correctly describe the transient optical properties, which leads to the assumption that the electrons cannot be considered as a nearly free-electron gas at this time. Alternatively, a Lorentz model, considering the excited electrons as bound, can replicate the measured data much better, which indicates that the electrons might be excited into localized states.