Optotwistronics of bilayer graphene

We present a study of the nonlinear optical response of twisted bilayer graphene. We discuss the contribution of the Berry phase to the nonlinearity when inversion symmetry is broken, thus underlining the interplay between band and real-space geometry, and nonlinear response. We also highlight an effect that is characteristic of extreme nonlinear optics, namely the generation of harmonics in disguise. This effect emerges in twisted bilayer graphene at relatively moderate field strengths because of the much reduced bandwidth. Our findings contribute to the understanding of the link between geometry and optical properties, as well as of the extreme nonlinear optical regime in twisted bilayer graphene.