The possibility to achieve strong light-matter coupling in solid state systems , particularly semiconductor-based, has led to an impressive number of research efforts [2,3]. When the Rabi frequency ΩR is a considerable fraction of the unperturbed frequency ω the system operates in the so-called ”ultrastrong” coupling regime, and new cavity QED effects such as the modification of the vacuum ground state of the system  are predicted. Recently, it has been demonstrated that, using intersubband transitions in quantum wells, impressively high values of the coupling ratio ΩR/ ω ≃ 0.25 can be reached [5,6].
In this talk, I will present a strong light-matter coupling experiment performed at terahertz (THz) and sub-THz frequencies employing a new semiconductor system where the matter excitation is constituted by the cyclotron transition of a 2 dimensional electron gas (2DEG) and the cavity resonator is constituted by an array of electronic split-ring resonators . For this highly tunable and frequency-scalable system, theory predicts that very high values for the normalized coupling ratio ΩR/ ωc are possible, together with the scaling of such a ratio with the filling factor ν of the 2DEG . We observe strong coupling for different resonator frequencies in the range from 2.3 THz to 500 GHz. In agreement with the theory, we measure the highest coupling ratio at the frequency of 500 GHz, where ΩR/ ωc=0.58 for ν ≃ 15. In this condition the system is driven in the regime 2Ω = 1.2ωc where the full energy splitting between polaritonic branches is larger than the bare transition energy .
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