Photonic crystals are artificial materials characterized by a photonic band structure that governs the propagation of light waves. The photonic gap was originally introduced to inhibit spontaneous emission and facilitate photon localization. In this essay, I will highlight how, despite the established understanding of photonic crystals, they remain highly relevant today. Their design flexibility, the duality symmetry inherent in Maxwell’s equations, and their functionality across a wide frequency range all allow the exploration of new areas in physics, each revealing unique phenomena. Examples include band topology and topological effects in structured light fields, as well as geometric concepts such as quantum geometry and the properties of non-Euclidean spaces. Furthermore, photonic crystals provide a valuable platform for studying non-Hermitian physics, including exceptional points and the non-Hermitian skin effect. Beyond their fundamental significance, these properties hold promise for advancing photonic technologies. .
C. T. Chan (2025) studied this question.