  We present a study of the nonlinear optical response of twisted bilayer
graphene. We discuss the contribution of the Berry phase to the non-linearity
when inversion symmetry is broken, thus underlining the interplay between band
and real space geometry, and nonlinear response. We also highlight an effect
which is characteristic of extreme nonlinear optics: the generation of
harmonics in disguise. This effect emerges in twisted bilayer graphene at
relatively moderate fields strengths because of the much reduced band width.
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
