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 new polariton types, arising from directional, long-lived material resonances, and leading to extreme optical anisotropy that enables new regimes of nanoscale, highly confined light propagation. Although such exotic propagation features may also in principle be achieved by using carefully designed metamaterials, it has recently been realized that they can naturally emerge when coupling infrared light to directional lattice vibrations — 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 wavevectors, 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 using anisotropic nonlinearities and optical pumping to further control their nanoscale response.

极化子是一类混合类准粒子，源自光和物质激发之间的强共振耦合。近年来，人们对新型极化子的兴趣激增，这种极化子类型源于定向、长寿命的材料共振，并导致极端的光学各向异性，从而实现纳米级、高度受限的光传播的新机制。虽然这种奇特的传播特征原则上也可以通过使用精心设计的超材料来实现，但最近人们意识到，当将红外光与极性晶体中的定向晶格振动（声子）耦合时，它们可以自然出现。有趣的是，晶体对称性的减少增加了光学声子的方向性以及由此产生的响应的各向异性，这反过来又使得新的极化激元现象成为可能，例如具有高度方向性传播的双曲极化激元、具有复值波矢的鬼极化激元以及具有强烈不对称的传播特征。在这篇综述中，我们对低对称性晶体中声子极化子的发现的最新进展进行了批判性概述，强调了破缺对称性在决定极化子响应和相关纳米级光传播特征中的作用。我们还讨论了极化子在低对称性材料和超材料中的新兴机会，与拓扑物理学的联系以及使用各向异性非线性和光泵浦进一步控制其纳米级响应的可能性。