  Surface phonon polaritons (SPhPs) in polar dielectrics offer new
opportunities for infrared nanophotonics due to sub-diffraction confinement
with low optical losses. Though the polaritonic field confinement can be
significantly improved by modifying the dielectric environment, it is
challenging to break the fundamental limits in photon confinement and
propagation behavior of SPhP modes. In particular, as SPhPs inherently
propagate isotropically in these bulk polar dielectrics, how to collectively
realize ultra-large field confinement, in-plane hyperbolicity and
unidirectional propagation remains elusive. Here, we report an approach to
solve the aforementioned issues of bulk polar dielectric's SPhPs at one go by
constructing a heterostructural interface between biaxial van der Waals
material (e.g., MoO3) and bulk polar dielectric (e.g., SiC, AlN, and GaN). Due
to anisotropy-oriented mode couplings at the interface, the hybridized SPhPs
with a large confinement factor (>100) show in-plane hyperbolicity that has
been switched to the orthogonal direction as compared to that in natural MoO3.
More interestingly, this proof of concept allows steerable, angle-dependent and
unidirectional polariton excitation by suspending MoO3 on patterned SiC air
cavities. Our finding exemplifies a generalizable framework to manipulate the
flow of nano-light and engineer unusual polaritonic responses in many other
hybrid systems consisting of van der Waals materials and bulk polar
dielectrics.
