Amplitude Tuning and Propagation Modulation of Hyperbolic Plasmon Phonon Polaritons in Graphene-hBN Heterostructures

Subwavelength polaritons are an important topic in Nanophotonics of van der Waals materials. Here we present a study of the hybrid hyperbolic plasmon-phonon polaritons (HP3) in the graphene-hexagonal Boron Nitride (G-hBN) heterostructure using synchrotron infrared nanospectroscopy (SINS). By electric gating and by engineered micro-structured metasurfaces (air-Au patterned substrate), we establish control of the amplitude of HP3 modes. It is shown that the amplitude depends on the interaction between mid-infrared plasmons of graphene and Au with discrete hyperbolic phonon-polaritons (HPhP) of the hBN. Such effect is described by a Fano model (eq. 1). Regulation of polaritonic flow is demonstrated in the vicinity of air-Au in-plane heterointerface. In this region, there is an abrupt discontinuity of the dielectric permittivity, i.e. transition from air to Au, leading to momenta mismatch of polaritons. Hence, in-plane polaritonic waves travelling from air to Au present a higher reflection coefficient that waves propagating in the opposite directions (Au to air). The asymmetry in reflection/transmission features unprecedented HP3 anisotropic regulation of the flow of polariton waves [1]. The impact of our findings will be relevant for designing novel nanophotonic devices using polaritons as information carriers.