Tunable Plasmon–Phonon Polaritons in Layered Graphene–Hexagonal Boron Nitride Heterostructures

We use infrared spectroscopy to explore the hybridization of graphene plasmons and hexagonal boron nitride (hBN) phonons in their heterostructures with different compositions. We show that the degree of plasmon–phonon hybridization and the slowing of the light group velocity within the infrared transparency window due to the plasmon–phonon destructive interference are dominated by hBN phonon oscillating strength, which can be tuned by varying the hBN thickness in a layer-by-layer manner. However, the plasmon oscillating strength in metallic graphene governs the magnitude of infrared extinction, which exceeds 6% at around 7 μm in a graphene/hBN/graphene heterostructure due to the strong plasmon dipole–dipole coupling. Our work demonstrates that the infrared optical responses of graphene–hBN heterostructures can be engineered by controlling the coupling strength of plasmon–phonon hybridization and the overall plasmon oscillating strength simultaneously, thus opening the avenue for the light manipulation and detection in the mid-infrared regime based on such layered heterostructures.