Exploiting and Controlling Lattice Symmetry for Infrared Nanophotonics and Ultrafast 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. While traditional dielectric materials do not exhibit indices of refraction high enough in non-dispersive media to realize such compression, the implementation of polaritons, quasi-particles comprised of oscillating charges and photons, enable such opportunities. 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. However, the short scattering lifetimes of free-carriers results in high losses and broad linewidths for the former, while the fast dispersion and narrow band of operation for the latter result in significant limitations for both forms. Through strong coupling strategies once can overcome some of these limitations by imparting benefits of both, as well as realizing emergent properties not existent in either individual material. 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 strong coupling and crystalline anisotropy in dictating the polaritonic dispersion, including recent observations from our group highlighting highly directional so-called hyperbolic shear polaritons in low-symmetry monoclinic and triclinic crystals, strong-coupling between optical modes for infrared emitters and laser concepts, 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 phonon polaritons can be employed as ultrafast and efficient carriers of thermal energy, providing alternative dissipation pathways for heat.