  Polaritons are a hybrid class of quasiparticles originating from the strong
and resonant coupling between light and matter excitations. Recent years have
witnessed a surge of interest in novel polariton types, arising from
directional, long-lived material resonances, and leading to extreme optical
anisotropy that enables novel regimes of nanoscale, highly confined light
propagation. While such exotic propagation features may also be in principle
achieved using carefully designed metamaterials, it has been recently realized
that they can naturally emerge when coupling infrared light to directional
lattice vibrations, i.e., phonons, in polar crystals. Interestingly, a
reduction in crystal symmetry increases the directionality of optical phonons
and the resulting anisotropy of the response, which in turn enables new
polaritonic phenomena, such as hyperbolic polaritons with highly directional
propagation, ghost polaritons with complex-valued wave vectors, and shear
polaritons with strongly asymmetric propagation features. In this Review, we
develop a critical overview of recent advances in the discovery of phonon
polaritons in low-symmetry crystals, highlighting the role of broken symmetries
in dictating the polariton response and associated nanoscale-light propagation
features. We also discuss emerging opportunities for polaritons in
lower-symmetry materials and metamaterials, with connections to topological
physics and the possibility of leveraging anisotropic nonlinearities and
optical pumping to further control their nanoscale response.
