Hybrid long-range hyperbolic phonon polariton waveguide using hexagonal boron nitride for mid-infrared subwavelength confinement.

Long propagation waveguides are critical for any photonic-on-chip applications. There has been an extensive investigation in using plasmon polaritons for near-infrared and optical networks, however, for mid- to long-wave IR applications phonon polaritons are required given that plasmonic polaritonic effects are negligible. In recent years, extensive research has been conducted on hexagonal boron nitride (h-BN), which has shown h-BN to have naturally occurring subwavelength, volumetrically confined hyperbolic phonon polaritons (HPhPs). This work presents numerical results for both long- and short-range phononic volumetric polariton modes in a slab of h-BN. A hybrid long-range phononic waveguide consisting of two identical dielectric cylinder wires symmetrically placed on each side of the h-BN slab is coupled to the long-range HPhP mode. Based on analytical coupled-mode theory and computational finite element analysis, we have investigated the modal characteristics of the hybrid long-range phonon polaritonic waveguide. Due to the strong coupling between the high index cylindrical-waveguide mode and the HPhPs in the h-BN thin film, subwavelength confinement can be achieved (modal area ranging from  10λo2 to  10λo2) while enabling long propagation distances (7λ-370λ).