Phonon polaritons in low-symmetry crystals

Phonon polaritons are strongly coupled hybrid light-matter waves which emerge when infrared light interacts with infrared active phonons in polar dielectric crystals. These excitations have recently attracted much attention as a versatile tool for low-loss nanophotonic applications in a range spanning from mid- to far-infrared. Phonon polaritons exist inside the Reststrahlen band, i.e. the spectral region between the transverse and longitudinal optical phonon frequencies where the dielectric permittivity is negative. For anisotropic oxide crystals such as molybdenum trioxide, calcite, and beta-type gallium oxide, so-called hyperbolic polaritons emerge when the infrared permittivity has opposite signs along different principle directions, leading to highly directional and strongly confined polaritonic states. Here I will report on both momentum space and real space experimental observation of a new type of phonon polaritons entitled hyperbolic shear polaritons in beta-gallium oxide, which display frequency dependent nanoscale propagation direction and asymmetries emerging directly from the low crystal crystal symmetry.