Effects of crystal symmetries in high harmonic generation from graphene

Abstract

[EN] High harmonic generation (HHG) stands nowadays as a well-established tool to produce highfrequency coherent radiation in the form of attosecond pulses. On the one hand, given the short period and wavelength of the radiated fields, it can be employed to probe matter at the microscopic scale. On the other hand, the emitted pulses themselves contain signatures of the electronic dynamics in the target, allowing for the so-called high-harmonic spectroscopy. HHG is often driven in gaseous targets, where this process has been thoroughly explored. In particular, in the last decade there has been a great interest in developing interaction schemes to tailor properties, such as the orbital angular momentum or the polarization, of the emitted harmonics. HHG can also be driven in solid and liquid targets, although these research areas have began to gain importance only very recently. Remarkably, considering crystalline solids as targets opens the door to exploring the signatures in the harmonic spectrum of the crystal symmetries and its interplay with the driving field’s symmetries. In this thesis work, we focus on the effects of the crystal symmetries in HHG from graphene. We explore two different interaction geometries, characterized by the incidence angle of the driver: grazing incidence and normal incidence. We demonstrate that, in the first scenario, the translation symmetry of the crystal plays a fundamental role, leading to a temporal matter Talbot effect in the electronic wavefunction, which can traced in the harmonic spectrum. We also show that HHG in this scenario is analogous to a Talbot-Lau interferometer in the subnanometer and sub-femtosecond spatial and temporal scales. On the other hand, in the case of normal incidence, we analyze the anisotropic response arising from the rotational symmetry of graphene and how it is affected by dephasing. We also explore the up-conversion of structured beams through high harmonic generation from graphene. In particular, we demonstrate that, in the case of linearly polarized vector beam drivers, the rotational symmetry of the crystal plays a fundamental role in the spatial structure of the harmonic field. Therefore, we propose topological spectroscopy, driven by linearly-polarized vector beams, as a tool to retrieve the nonlinear response of any anisotropic solid based on the topological structure of the harmonic far field. Finally, to illustrate the sensitivity of the topological features of the harmonic far field to changes in the crystal symmetry, we investigate HHG driven by linearly-polarized vector beams in strained graphene.