We probe ultrastrong light-matter coupling between metallic terahertz metasurfaces and Landau-level transitions in high-mobility two-dimensional electron and hole gases. We utilize heavy-hole cyclotron resonances in strained Ge and electron cyclotron resonances in InSb quantum wells, both within highly nonparabolic bands, and compare our results to well-known parabolic AlGaAs/GaAs quantum well systems. Tuning the coupling strength of the system by two methods, lithographically and by optical pumping, we observe a behavior clearly deviating from the standard Hopfield model previously verified in cavity quantum electrodynamics: An opening of a lower polaritonic gap.

We present our investigation on ultrastrong coupling of the Landau level transitions in a two dimensional electron gas to complementary split ring resonator arrays, as a terahertz metasurface. To achieve a higher coupling rate, the capacitive gap of the resonator is reduced systematically. This increment in the coupling rate is expected due to the enhanced vacuum field fluctuations in a smaller volume mode. However, our results indicate that the gap size reduction introduces a broadening in the upper polariton branch below a certain gap width.

Ultrastrong coupling (USC) regime has been an intriguing research topic in cavity quantum electrodynamics (QED) experiments due to its capability to modify the ground and the excited states of a coupled system[1-3]. We investigate the USC of Landau level (LL) transitions in a two dimensional electron gas to teraherz (THz) metasurfaces using THz time domain spectroscopy. Complementary THz split ring resonator (cSRR) arrays are used as metasurfaces[4]. A higher coupling rate can be achieved by reducing the capacitive gap of the resonator due to the increased vacuum field fluctuations. However, our results indicate that this reduction introduces an inhomogenous broadening of the upper polariton branch below a threshold gap width.

Complementary double split ring THz resonators fabricated on a 10 µm thin Si-membrane are studied. The linewidths of the fundamental LC mode and dipolar mode are drastically narrowing with increased resonator spacing. The extracted decay rate of the LC mode as a function of the resonator density shows a linear dependence, evidencing a collective superradiant effect of the resonator array. Furthermore, it is shown that a metamaterial can be designed for a low superradiant broadening of the resonance at high resonator densities, that is, in the metamaterial condition. The use of a thin membrane as a substrate is crucial, since it shifts the THz surface plasmon polaritons modes to much higher frequencies, preventing them to couple to the LC mode and unveiling the superradiant broadening mechanism for a large range of lattice spacings. At higher frequencies, not interfering with the high Q LC mode, other additional modes, which are ascribed to photonic crystal modes, form in the Si-membrane. The angle-dependent band structure is mapped and corresponding simulated electric field distributions are shown.

We study complementary THz metasurfaces with changing lattice constant. On thick substrates the LC-resonance anti-crosses with a surface plasmon polaritons mode. On 10 μm Simembranes, we reveal a superradiantly limited linewidth of the LC-resonance.

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/λ_cyc = 24%.