Nonlinear Optical Responses in a Twisted Bilayer Graphene―A Dynamical-Symmetry Viewpoint

Abstract

Twisted bilayer graphene (TBG), two sheets of graphene vertically stacked with an artificial twist angle, has recently attracted tremendous attention as the twist has turned out to induce various phenomena like superconductivity. Here, we theoretically study its nonlinear optical properties, showing that the TBG exhibits enriched high-harmonic generation that cannot occur in monolayer or conventional bilayers. We systematically elucidate the selection rule of the activate harmonic orders from the dynamical-symmetry viewpoint combined with the Floquet theory. These results imply nonlinear “optotwistronics,” or controlling optical properties of layered materials by artificial twists.