Wide-bandgap materials such as silicon dioxide (fused silica, α-quartz) can undergo strong excitation when exposed to high-power ultrashort laser pulses. This leads to a high transient electron density in the conduction band, causing distortion in the bands and resulting in a significant bandgap renormalization. Additionally, there is a spatial redistribution of the excited charges, leading to weakening of silica bonds and subsequent reorganization of the crystal structure, further contributing to the change in the bandgap. Through the use of Density Functional Theory, Time-Dependent Density Functional Theory, and GW approximation, the evolution of the bandgap is studied at different levels of excitation, revealing changes of up to several electronvolts on ultrashort timescales.
Fused silica is an indispensable material in emergent photonic applications due to its unique optical, mechanical, and thermal properties, especially when it is nano-structured by an ultrashort laser pulse. The precision of the laser-induced modifications relies heavily on the control of the electron excitations and transient optical properties during the laser pulse. In this work we explored the evolution of fused silica bandgap at high densities of excited electrons, using Finite-Temperature Fractional Occupation Density Functional Theory (FT-DFT). Using a molecular-level approach, a molecular cluster based on (SiO4)4- tetrahedra was shown to reproduce accurately the physical properties of amorphous silica. The proposed theoretical approach (FT-DFT) correctly describes electronic and spatial structure both at the ground state and photoexcitation-induced thermalized hot states. Under electron-matrix nonequilibrium conditions, a bandgap narrowing by 2 eV and more is shown. This is explained by a pure geometry relaxation driven by the electron redistribution during the strong laser-induced excitation. The reason for the bandgap decrease is atomic rearrangement resulting in weakening of the bonds. Such behaviour of the system under excitation has a significative impact on its stability even if changes in geometry are limited to 7.5% bond elongation before the loss of integrity of the system. According to experimental data this atomic rearrangement can be expected on the femtosecond timescale. Defect formation in fused silica due to bond breaking is finally expected to occur for electronic temperatures above 2.8 eV.
Ultrafast light coupling with metal surfaces shows strong potential for nanostructuring applications relying on the capacity to localize light energy on the nanoscale. Controlling light confinement requires to understand the transient variation of the optical response during ultrafast irradiation. The fundamental approach we propose based on ab initio calculations allows elucidating the influence of electron-phonon nonequilibrium on optical properties. This results from the investigation of the primary processes responsible for the optical change during laser-solid interaction. Calculations are carried out in the framework of the density functional theory associated to quantum molecular dynamics. Our results shed light on the intricate role of electronic structure modifications and possible optical transitions, driving the laser energy absorption into the material. The revealed key processes based on Fermi smearing on an evolving density of states are of paramount interest for controlling laser energy deposition, surface plasmon excitation and subsequent surface nanostructuring. The calculations predict the possibility of an ultrafast laser-driven plasmonic switch on a typically non-plasmonic material (W), confirmed by pump-probe ellipsometric measurements [1]. The consequence of our results is far reaching as they propose also a route for achieving the highest energy confinement under ultrashort laser irradiation.
[1] E. Bévillon, J.P. Colombier, V. Recoules, H. Zhang, C. Li, R. Stoian, “Ultrafast switching of surface plasmonic conditions in nonplasmonic metals”, Physical Review B 93 (16), 165416 (2016).
During propagation of high-power femtosecond laser pulse in air dynamic balance between Kerr self-focusing and laser
plasma defocusing results in high localization of the energy in a hundred microns region. Stable long laser filament
forms. For systems of atmospheric optics it is important to control high fluence of the filament. We investigated
numerically the influence of pulse duration and atmospheric aerosol particles scattering on energetic characteristics of
the filamented laser pulse. It was shown that in the conditions of pulse energy remaining the same increase of pulse
duration results in considerable increase of filament length and high-fluence energy transporting by pulse. It is
demonstrated that scattering on atmospheric aerosol particles causes high-fluence energy losses but doesn't prevent
further filament propagation.
The robustness and recoverability of the high-power femtosecond laser pulse filament in the presence of atmospheric
aerosol scattering layer was studied by means of computer simulation. The obtained patterns of fluence and electron
density in a laser filament demonstrate that these parameters acquire a stochastic character inside the aerosol layer and
recover on leaving it. Filament energy decreases with distance inside the layer because of the water particles scattering
and after the layer because of the amplitude-phase perturbations induced by aerosol particles. The equivalence of
nonlinear aerosol medium and linear damping medium with equal to disperse dissipations was investigated.
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