Realization of a chiral photonic-crystal cavity with broken time-reversal symmetry

Chiral cavities offer an intriguing way to manipulate material properties by breaking fundamental symmetries. However, only a few chiral cavity implementations exhibiting broken time-reversal symmetry have been demonstrated, with most relying on either strong magnetic fields, circularly polarized Floquet driving, or ultrastrong coupling between cavity modes and matter excitations. Here, we present a one-dimensional terahertz photonic-crystal cavity that exhibits broken time-reversal symmetry. The cavity consists of a silicon wafer sandwiched between InSb wafers. By exploiting the nonreciprocal terahertz response of a magnetoplasma and the low electron effective mass in InSb, a circularly polarized cavity mode at 0.67 THz under a modest magnetic field of 0.3 T, with a quality factor exceeding 50 is realized. Temperature-, magnetic field-, and polarization-dependent measurements and simulations demonstrate the chiral cavity with broken time-reversal symmetry, providing a robust platform for exploring chiral light–matter interactions and vacuum dressed quantum condensed matter in the terahertz regime. Researchers realized the first truly chiral terahertz cavity with time-reversal-symmetry broken vacuum fields, with near-unity ellipticity at 0.66 THz and Q>50 under a 0.3 T field, offering a robust platform for chiral light–matter interactions.