High-Harmonic Generation with a twist: all-optical characterization of magic-angle twisted bilayer graphene

TL;DR

This paper demonstrates that high harmonic generation spectroscopy can reveal the electronic properties and twist angle of magic-angle twisted bilayer graphene, enabling all-optical characterization of moiré materials.

Contribution

It introduces a novel use of high harmonic generation spectroscopy to probe the electronic structure of twisted bilayer graphene at the magic angle.

Findings

High harmonic signals decrease near the magic angle.

Band structure is imprinted onto the harmonic spectrum.

Harmonic generation can measure twist angle and electronic properties.

Abstract

If we stack up two layers of graphene while changing their respective orientation by some twisting angle, we end up with a system that has striking differences when compared to single-layer graphene. For a very specific value of this twist angle, known as magic angle, twisted bilayer graphene displays a unique phase diagram that cannot be found in other systems. Recently, high harmonic generation spectroscopy has been successfully applied to elucidate the electronic properties of quantum materials. The purpose of the present work is to exploit the nonlinear optical response of magic-angle twisted bilayer graphene to unveil its electronic properties. We show that the band structure of magic-angle twisted bilayer graphene is imprinted onto its high-harmonic spectrum. Specifically, we observe a drastic decrease of harmonic signal as we approach the magic angle. Our results show that high harmonic generation can be used as a spectroscopy tool for measuring the twist angle and also the electronic properties of twisted bilayer graphene, paving the way for an all-optical characterization of moir\'e materials.