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.
During the last decades, femtosecond laser irradiation of materials has led to the emergence of various applications based on functionalization of surfaces at the nano- and microscale. Via inducing a periodic modification on material surfaces (band gap modification, nanostructure formation, crystallization or amorphization), optical and mechanical properties can be tailored, thus turning femtosecond laser to a key technology for development of nanophotonics, bionanoengineering, and nanomechanics. Although modification of semiconductor surfaces with femtosecond laser pulses has been studied for more than two decades, the dynamics of coupling of intense laser light with excited matter remains incompletely understood. In particular, swift formation of a transient overdense electron-hole plasma dynamically modifies optical properties in the material surface layer and induces large gradients of hot charge carriers, resulting in ultrafast charge-transport phenomena. In this work, the dynamics of ultrafast laser excitation of a semiconductor material is studied theoretically on the example of silicon. A special attention is paid to the electron-hole pair dynamics, taking into account ambipolar diffusion effects. The results are compared with previously developed simulation models, and a discussion of the role of charge-carrier dynamics in localization of material modification is provided.
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