PHONON POLARITON ENHANCED INFRARED WAVEGUIDES
The need for robust, reliable and sensitive terahertz (THz) sources and detectors has motivated the research into waveguiding structures. The increasing adoption of micro- and nanofabricated optical components addresses the critical challenge of guiding light with subwavelength confinement. Surface plasmon polariton (SPP) waveguides are among the most promising candidates for manipulating light on subwavelength scales and harbor many potential applications at visible and nearinfrared (NIR) wavelengths. However, the metal in the waveguide yields very weak or nonexistent response in the mid and long wave infrared range, since metal in this region is very lossy and has large magnitude real and imaginary permittivities. This is one of the major reasons for the rise in popularity of surface phonon polariton (SPhP) waveguides in recent research and micro- and nano-fabrication pursuits. Silicon carbide (SiC) is a good candidate in SPhP waveguides since it has negative dielectric permittivity in the long-wave infrared (LWIR) spectral region, indicative that coupling to surface phonon polaritons is realizable. Introducing surface phonon polaritons for waveguiding provides good modal confinement and enhanced propagation length. A hybrid waveguide structure at LWIR based on an eigenmode solver approach in Ansys HFSS is presented. The effect of a three layer configuration i.e., silicon wire on a benzocyclobutene (BCB) dielectric slab on SiC, and the effects of varying their dimensions on the modal field distribution and on the propagation length, is studied. 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) and surface phonon polaritons (SPhPs). First, we have designed a hybrid phononic waveguide containing a hyperbolic h-BN slab lying on a lossless dielectric substrate coupled with a cylinder dielectric waveguide in the air gap and the SPhP mode results in an extremely confined hybrid phononic mode. The coupling between photonic cylinder and phononic surface enhances the confined modal area up to 10ˉ³λ²₀ (λo is the free-space wavelength) and enables propagation distances up to more than two orders of magnitude above the operational wavelength. In order to improve the waveguide properties, the long range phonon polariton has been adopted in a new design. 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 results when two identical dielectric cylinder wires symmetrically placed on each side of the h-BN slab are 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. The strong coupling between the high index cylindrical waveguide mode and the HPhPs in a h-BN thin film, subwavelength modal area can be achieved which is approximately 10ˉ²λ²₀ while enabling propagation distances up to 370λo.