Electron dynamics in α -quartz induced by two-color 10-femtosecond laser pulses

Abstract

The time dependent density functional theory (TDDFT) is used to study the femtosecond laser induced excitation dynamics of electrons in bulk $\ensuremath{\alpha}$-quartz, and evaluate the laser energy deposition into this material. In order to properly distinguish the contributions of ionization (electron transitions from the valence band to the conduction band) and laser heating (electron transitions in the conduction band), two 10-femtosecond laser pulses exhibiting different wavelengths are used. Short wavelengths are expected to enhance the ionization rate, whereas longer wavelengths should be more suitable for excitation in the conduction band, thus providing a possible control of the whole electron dynamics. The influence of the pulse-to-pulse delay and intensities is studied. A significant enhancement of the interaction efficiency, in terms of excited electron density and their energy density, is observed for zero pulse-to-pulse delay. It is attributed to the opening of new ionization pathways involving various combinations of both photon energies ensuring the energy conservation, i.e., the sum of photon energies bridges the bandgap. This analysis is supported by a semi-analytical quantum model in the multiphoton absorption regime. The role and strength of direct interband transitions for the electron dynamics in the conduction band are highlighted. The associated laser energy deposition into the material is shown to be as efficient as collisional processes.