Tunable polaritons from plasmon-phonon coupling in hyperbolic media

Using infrared nano-imaging and nano-spectroscopy, we report on the tunable hyperbolic response in heterostructures comprised of a monolayer graphene deposited on hexagonal boron nitride (G-hBN). Electrostatic gating of the top graphene layer allows for modification of the wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. When compared with the pristine hBN polaritons, the graphene modified ones exhibit a longer wavelength in the Type II hyperbolic region and shorter wavelength in the Type I region. Because of this modification, we achieve a 90{\%} increase of the propagation length for Type II polaritons in hBN. The physical origin of the modification is attributed to plasmon-phonon coupling in the G-hBN heterostructure. Our work provides a comprehensive study of plasmon-phonon coupling in a hyperbolic medium with the exploration of graphene's potential for modification of collective modes in other materials.