  A fundamental building block for nanophotonics is the ability to achieve
negative refraction of polaritons, because this could enable the demonstration
of many unique nanoscale applications such as deep-subwavelength imaging,
superlens, and novel guiding. However, to achieve negative refraction of highly
squeezed polaritons, such as plasmon polaritons in graphene and phonon
polaritons in boron nitride (BN) with their wavelengths squeezed by a factor
over 100, requires the ability to flip the sign of their group velocity at
will, which is challenging. Here we reveal that the strong coupling between
plasmon and phonon polaritons in graphene-BN heterostructures can be used to
flip the sign of the group velocity of the resulting hybrid
(plasmon-phonon-polariton) modes. We predict all-angle negative refraction
between plasmon and phonon polaritons, and even more surprisingly, between
hybrid graphene plasmons, and between hybrid phonon polaritons. Graphene-BN
heterostructures thus provide a versatile platform for the design of
nano-metasurfaces and nano-imaging elements.
