High T c Superconducting THz Metamaterial for Ultrastrong Coupling in a Magnetic Field

We fabricate high temperature superconducting metasurfaces from yttrium–barium–copper oxide (YBCO) films via focused ion beam milling. The design of the complementary split ring resonators yields a high quality factor (up to Q = 31) resonance, which is fully switchable when exploiting the transition from the superconducting to normal conducting state, which is achieved by heating the sample above the critical temperature. Meanwhile, the resonance is continuously visible and frequency stable under the action of a magnetic field, up to B = 9 T. This makes it an ideal candidate for light–matter coupling experiments with magnetic field tunable Landau level transitions. We present two techniques to bring the metasurface in close vicinity of a 2D electron gas in an AlGaAs/GaAs QW without direct deposition, and we achieve a normalized vacuum Rabi frequency of Ω R /ω cyc = 24%.