Intrinsic and extrinsic photogalvanic effects in twisted bilayer graphene

Photogalvanic effects have been widely used as a direct probe to access the geometrical properties of the electronic structure in many topological systems. In this work, we investigate the symmetry constraints imposed by particle-hole and point group symmetries on the non-linear photocurrents in twisted bilayer graphene. Under this analysis, we first show that the chiral lattice structure with D6 symmetry allows only for oblique incidence photocurrents which keep track of the magic angle band inversion. A detailed study of the extrinsic effects, due to substrates or gate potentials, is also presented to show that normal incidence components are constrained to vanish by the approximate particle-hole symmetry of the twisted bilayer Hamiltonian, while off-normal components are not. We conclude that a detailed comparison between intrinsic and extrinsic photocurrents discloses a wealth of information about the band structure and can also serve as a benchmark to constrain the symmetry breaking patterns of correlated states.