Terahertz chiral subwavelength cavities breaking time-reversal symmetry via ultrastrong light-matter interaction

We demonstrate terahertz chiral subwavelength cavities that break time-reversal symmetry by coupling the degenerate linearly polarized modes of two orthogonal sets of nanoantenna arrays using the inter-Landau-level transition of a two-dimensional (2D) electron gas in a perpendicular magnetic field, realizing normalized light-matter coupling rates up to ${\mathrm{\ensuremath{\Omega}}}_R/{\ensuremath{\omega}}_{\mathrm{cav}}=0.78$ with a dispersion that is modified by the parasitic capacitive coupling between the orthogonal antennas. The deep subwavelength confinement of the nanoantennas means that the ultrastrong-coupling regime can be reached even with a small number of carriers compared to Fabry-P\'erot cavities, making it viable to be used with a variety of 2D materials. The nondegenerate circularly polarized ground state was only obtained after carefully optimizing the optical design to minimize the parasitic coupling to linearly polarized light.