Ultrafast excitation of electrons in crystals: insights from nonequilibrium band structure calculations

Last decade, understanding of transient excitation of electrons in solids has brought important developments for several classes of materials, at the level of both fundamentals and applications. While laser-excitation of dielectrics induces measurable ultrafast currents in the PHz regime, employing few-cycle laser pulses with controlled carrier envelope phase enables a coherent control of the electron dynamics with reduced crystal damage probability. Notably, the possibility of transiently closing the band-gap of solids during their irradiation by linearly polarized ultrashort laser pulses was evidenced and attributed to the light-induced Zener tunneling. The corresponding ultrafast modification of the band structure induced by laser dressing of electronic states can be measured experimentally and analyzed theoretically using the Floquet formalism. Despite its simplicity and limitations, it is applicable to several classes of materials and enables to study the effects of light coupling with electrons in solids for a wide range of experimental conditions. In this work, after preparing the electronic band structures of two metals (Au and Mo), a semiconductor (Si) and a dielectric (α-SiO2) using the density functional theory (DFT), the effects of dressing by a polarized laser light on the corresponding electronic band structures were investigated using the Floquet formalism. While a selective excitation of the electrons can be achieved via a choice of laser wavelength and field strength, the Floquet simulations illustrate how the change in crystal orientation affects the electron dynamics in solids. Overall, the proposed approach outlines promising ways for selecting materials and laser parameters, via a computer-aided manner, broadening perspectives in ultrafast photonics.