Optical Susceptibility Tuning in Twisted Bilayer Insulators

In hexagonal bilayer systems with different sublattices, twists can tune a crystal structure between two high symmetry stacked structures, one potentially having inversion symmetry. Inversion symmetry requires that the second-order optical susceptibility is zero, suppressing the nonlinear optical response. In cases like hexagonal Boron Nitride, since no such constraint is present in one of the two high symmetry stackings, twisting has the potential to present a natural platform for tuning non-linear response. Our work examines how the optical susceptibilities vary as the twist angle is tuned over the parameter space of shifting and twisting the layers with respect to one another. Removing the signal of the uncoupled bilayer, we discuss the change in response due to the twist induced chiral interlayer coupling. Our computations suggest additional ultraviolet resonances in the SHG signal and we discuss connections to the changes in the peak structure of the density of states due to the Moire potential and the underlying geometry.