  Light-matter interaction in two-dimension photonic materials allows for
confinement and control of free-space radiation on sub-wavelength scales. Most
notably, the van der Waals heterostructure obtained by stacking graphene (G)
and hexagonal Boron Nitride (hBN) can provide for hybrid hyperbolic plasmon
phonon-polaritons (HP3). Here, we present a polariton diode effect and low-bias
control of HP3 modes confined in G-hBN. Using broadband infrared synchrotron
radiation coupled to a scattering-type near-field optical microscope, we launch
HP3 waves over both hBN Reststrahlen bands and observe the unidirectional
propagation of HP3 modes at in-plane heterointerfaces associated with the
transition between different substrate dielectrics. By electric gating we
further control the HP3 hybridization modifying the coupling between the
continuum graphene plasmons and the discrete hyperbolic phonon polaritons of
hBN as described by an extended Fano model. This is the first demonstration of
unidirectional control of polariton propagation, with break in
reflection/transmission symmetry for HP3 modes. G-hBN and related hyperbolic
metamaterial nanostructures can therefore provide the basis for novel logic
devices of on-chip nano-optics communication and computing.
