Strongly Coupled Exciton–Hyperbolic-Phonon-Polariton Hybridized States in hBN-Encapsulated Biased Bilayer Graphene

Excitons in biased bilayer graphene are electrically tunable optical excitations residing in the mid-infrared (MIR) spectral range, where intrinsic optical transitions are typically scarce. Such a tunable material system with an excitonic response offers a rare platform for exploring light–matter interactions and optical hybridization of quasiparticles residing in the long wavelength spectrum. In this work, it is demonstrated that when the bilayer is encapsulated in hexagonal-boron-nitride (hBN)—a material supporting optical phonons and hyperbolic-phonon-polaritons (HPhPs) in the MIR—the excitons can be tuned into resonance with the HPhP modes. It is found that the overlap in energy and momentum of the two MIR quasi-particles facilitate the formation of multiple strongly coupled hybridized exciton-HPhP states. Using an electromagnetic transmission line model, the dispersion relations of the hybridized states are derived and showed that they are highly affected and can be manipulated by the symmetry of the system, determining the hybridization selection rules. These results establish a general tunable MIR platform for engineering strongly coupled quasiparticle states in biased graphene systems, opening new directions for studying and controlling light–matter interactions in the long-wavelength regime.