High-harmonic spectroscopy of two-dimensional materials

TL;DR

This paper explores how high-harmonic generation (HHG) can be used as a powerful tool to probe electronic properties, defects, and valley polarization in two-dimensional materials with hexagonal symmetry, revealing insights into their band structure and electron dynamics.

Contribution

It demonstrates the potential of HHG to encode electronic band structure, analyze defect effects, and observe valley polarization in 2D materials, introducing new methods for ultrafast material characterization.

Findings

High-harmonic spectra encode electronic band structure fingerprints.

Electron dynamics differ between semimetals and semiconductors based on laser polarization.

Defects and electron-electron interactions significantly influence HHG signals.

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.