High-harmonic spectroscopy of two-dimensional materials

Abstract

Recent advancements in the generation of mid-infrared and terahertz laser pulses have enabled us to observe strong-field driven non-perturbative high-harmonic generation (HHG) from semiconductors, dielectrics, and semimetals. HHG has added another dimension to time-resolved ultrafast electron dynamics in materials with unprecedented temporal resolution. Present thesis discusses how HHG is an emerging method to probe static and dynamical properties in two-dimensional materials. In this thesis, two-dimensional materials with hexagonal symmetry are studied. We have demonstrated that the high-harmonic spectrum encodes the fingerprints of electronic band structure and interband coupling between different bands. Furthermore, by analysing gapped and gapless graphene, we show how electron dynamics in a semimetal and a semiconductor are different as the harmonic spectrum depends differently on the polarisation of the driving laser. To explore the role of defects in HHG, spin-polarised vacancy defects in hexagonal boron nitride are considered. It has been found that electron-electron interaction is crucial for electron dynamics in a defected solid. In all cases, we present how different symmetries of the lattice can be extracted from the harmonic spectrum. Finally, a light-driven method is proposed for observing valley-polarisation in pristine graphene, using a tailored laser pulse. Also, a recipe is discussed to write and read valley-selective electron excitations in materials with zero bandgap and zero Berry curvature.