We show that hexagonal boron nitride (hBN), a two-dimensional insulator supports tunable excitons in the near and middle ultraviolet if subjected to an external superlattice potential. Our calculations predict that as we increase the strength of the potential, the gap reduces, and the anisotropy of the dispersion is enhanced. Consequently, the binding-energies of the excitons decrease, leading to a red-shift of the excitonic levels. We also observe that the absorption is reduced when we change the polarization from along the periodicity of the potential to perpendicular to it, with the system acting as an optical polarizer. As we reduce the gap, the characteristic frequency range for which we can excite exciton-polaritons red-shifts as well. These modes behave quite differently from pristine hBN in extreme cases where the anisotropy of the system grows indefinitely. In this way, by tuning the potential, we can manipulate the excitonic and sub-gap optical properties of hBN.
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