Photonic Altermagnets: Magnetic Symmetries in Photonic Structures.

Altermagnetism, a newly discovered magnetic phase, blends the beneficial properties of ferromagnetism and antiferromagnetism, spurring extensive efforts to utilize it for spintronic applications. The unique transport properties, when transplanted to photonic systems, are anticipated to offer a new degree of freedom for controlling electromagnetic wave propagation, which alerts the study of altermagnetism in the photonic domain. Here, we show that altermagnetism can be mimicked using photonic crystals, demonstrating that the engineered photonic crystals can host spin space group symmetries through chiroptic media. Our approach allows for the creation of altermagnetic spin-split bands, and the corresponding transport properties provide an effective platform for circularly polarized light isolation without the need for geometrodynamic spin-orbit interaction. Beyond the concurrent solid-state materials, we anticipate our work to offer photonic crystals as a versatile platform to test altermagnetic spin-split band properties and inspire optical designs for photospintronic applications and unconventional even-parity wave pseudochirality.