Terahertz chiral metamaterial cavities breaking time-reversal symmetry

We demonstrate terahertz chiral metamaterial cavities that break time-reversal symmetry by coupling the degenerate linearly polarized modes of two orthogonal sets of nano-antenna arrays using the inter-Landau level transition of a two-dimensional electron gas in a perpendicular magnetic field, realizing normalized light-matter coupling rates up to $\Omega_R/\omega_{\mathrm{cav}} = 0.78$. The deep sub-wavelength confinement and gap of the nano-antennas means that the ultra-strong coupling regime can be reached even with a very small number of carriers, making it viable to be used with a variety of 2D materials, including graphene. In addition it possesses a non-degenerate chiral ground state that can be used to study the effect of circularly polarized electromagnetic quantum fluctuations by means of weakly-perturbing magneto-transport measurements.