Influence of anisotropic dipole matrix element on optical response of AB-stacked graphene superlattice

When the polarization direction of the laser beam Ê lies on the graphene plane, the absorption spectrum A(ω)is isotropic and includes one sharp peak and some shoulders. As for Ê along the stacking direction, A(ω) is much weaker, and shows only one broadened peak. Because of the dipole matrix element M(cv), the optical excitations do not fully reflect the features of electronic structures [or the joint density of states (JDOS)]. M(cv) plays an important role in the relationship between A(ω) and JDOS. It is strongly dependent on Ê, showing an anisotropic property.