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Polaritons are a hybrid class of quasiparticles originating from the strongand resonant coupling between light and matter excitations. Recent years havewitnessed a surge of interest in novel polariton types, arising fromdirectional, long-lived material resonances, and leading to extreme opticalanisotropy that enables novel regimes of nanoscale, highly confined lightpropagation. While such exotic propagation features may also be in principleachieved using carefully designed metamaterials, it has been recently realizedthat they can naturally emerge when coupling infrared light to directionallattice vibrations, i.e., phonons, in polar crystals. Interestingly, areduction in crystal symmetry increases the directionality of optical phononsand the resulting anisotropy of the response, which in turn enables newpolaritonic phenomena, such as hyperbolic polaritons with highly directionalpropagation, ghost polaritons with complex-valued wave vectors, and shearpolaritons with strongly asymmetric propagation features. In this Review, wedevelop a critical overview of recent advances in the discovery of phononpolaritons in low-symmetry crystals, highlighting the role of broken symmetriesin dictating the polariton response and associated nanoscale-light propagationfeatures. We also discuss emerging opportunities for polaritons inlower-symmetry materials and metamaterials, with connections to topologicalphysics and the possibility of leveraging anisotropic nonlinearities andoptical pumping to further control their nanoscale response.

Extreme light confinement and control in low-symmetry phonon-polaritonic crystals