Exploiting and controlling lattice symmetry for infrared nanophotonics and thermal transport

The field of nanophotonics is based on the ability to confine light to sub-diffractional dimensions. In the infrared, this requires compression of the wavelength to length scales well below that of the free-space values. Two predominant forms of polaritons, the plasmon and phonon polariton, which are derived from light coupled with free carriers or polar optic phonons, respectively, are broadly applied in the mid- to long-wave infrared. Within anisotropic materials, these optical modes can be induced to propagate with different wavevectors along different axes, or even be restricted to propagate only along a single direction. Here, we discuss the influence of crystalline anisotropy in dictating the polaritonic dispersion, including highly directional hyperbolic shear polaritons in low-symmetry monoclinic and triclinic crystals, as well as the ability to control the wavelength and propagation direction of these modes using free-carrier injection and twist-optic concepts. Further, we highlight how such phonon polaritons can be employed as ultrafast and efficient carriers of thermal energy, providing alternative dissipation pathways for heat.