  In this work, the hyperbolic phonon polaritons (HPPs) in ultrathin hBN sheets
are numerically studied. The dispersion relation and distribution of electric
field are calculated to confirm the excitation of HPPs. Besides, the coupling
effect between HPPs of two ultrathin hBN sheets are investigated. When the
distance between two hBN sheets are smaller than the propagation length of the
HPPs in the air, the HPPs can be strongly coupled. Therefore, the photon
tunneling probability can be greatly enhanced. The split of the HPPs is similar
to that of the surface waves, and such phenomenon is well explained in this
work. We believe that this work will deepen our understanding of the HPPs in
ultrathin hyperbolic materials. In addition, the knowledge about the HPPs will
help us understand the near-field radiative heat transfer between hyperbolic
materials.
