Enhancement and tuning of the light-matter interaction represent key aspects for fundamental studies of cavity quantum electrodynamics (QED) and for applications both in classical and quantum devices. The coupling between the cavity photons and an elementary electronic excitation is quantified by the vacuum Rabi frequency Ω and the strong light-matter coupling regime is achieved when Ω is larger than the dephasing rates of the photons and electronic excitations. Recently a considerable research effort has been devoted to the study of the the ultrastrong ligh-matter coupling regime [1,2,3], which is obtained when the vacuum Rabi frequency becomes an appreciable fraction of the unperturbed frequency of the system ω. We recently demonstrated ultrastrong coupling regime in a system composed by an high-mobility two-dimensional electron gas (2DEG) coupled to terahertz (THz) metamaterial resonators [4]. The photonic modes of an array of split ring resonators are coupled to the inter- Landau level transition of the 2DEG, obtained by applying a magnetic field perpendicular to the plane of the quantum wells. With this scheme we demonstrated that the coupling ratio for this system scales as Ω/ωc ∼ √αnQWV where α is the fine structure constant and nQW is the number of 2DEGs and we reached a normalized coupling ratio of Ω/ω = 0.58 for a resonator frequency of 500 GHz and n=4 quantum wells. In order to enhance the coherence time of the magnetopolariton quasiparticle an improvement on the quality factor of the photonic resonance of the metamaterial is needed. We realized then superconducting metasurfaces [5] operating at 430 GHz based on Nb films of 100 nm of thickness. First we realized such metasurface a on semi-insulating GaAs where we clearly observed an abrupt change in the quality factor of the resonance at a temperature of 7.5 K (Fig.1(a)- . The quality factor of the resonance and its frequency increase slightly when going towards low temperatures. We then implemented the same Nb-based metasurface on a sample containing a single 2DEG based on modulation doped triangular quantum well. By applying the magnetic field necessary for the creation of Landau levels the quality factor of the resonator partially degrades but we could still clearly observe ultra strong coupling regime with a normalized coupling ratio of Ωω = 0.27 with Nb-based metasurfaces (Fig. 1(b)). This first result opens the way for further resonator optimization and frequency scaling aimed to the realization of ultrastrong coupling regime with long-coherence times and switchable superconducting cavities.
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