High-harmonic generation with a twist: all-optical characterization of magic-angle twisted bilayer graphene

If we stack up two layers of graphene while changing their respective orientation by some twisting angle, we end up with a strikingly different system 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 moiré materials.