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Quantum Metamaterials in the Ultra Strong Coupling regime

Mid-Term Report Summary - MUSIC (Quantum Metamaterials in the Ultra Strong Coupling regime)

The project aims at studying the ultra-strong light-matter coupling in metamaterial-based structures. In a strong light-matter coupled system, there is a periodic exchange of energy between the light and matter excitations at the Rabi frequency, such that new quasi-particles, polaritons, are formed. The properties of such polaritons are especially interesting if the system operates in the so-called ultrastrong coupling regime, where the Rabi frequency becomes as large as the transition energy. Indeed, a salient feature of that regime is that the ground state of the system is now modified by the mixing with cavity photons.
One key aspect of this project is the attempt to detect directly these photons and their influence on other properties of the system.
To this end, we have realized a new time-domain electro-optic sampling system that can, for the first time, measure the photon statistics in the THz and its moments. In the first version of that experiment, published in Physical Review A, we have applied that technique to the measurement of the change in the intensity autocorrelation g(2) across the threshold of a quantum cascade laser. In a still unpublished work, we also demonstrated how that instrument is able to establish the intensity time profile of a terahertz quantum cascade optical frequency comb. These results have attracted the attention of the scientific community because they show how quantum optics can be performed using time-resolved technique in the THz.
The project is also exploring the ultra-strong coupling using different two dimensional materials, trying for example to prove the presence of a Dicke phase transition in the ultrastrong coupling between a graphene sheet with a THz resonator. In a first phase, to avoid the problem raised by the limited mobility of the graphene preventing the observation of polaritons, we fabricated metamaterials coupled to graphene ribbons. In this system, strong coupling was indeed observed between the graphene plasmon and the resonator cavity, and the results published in Nature Communication article. The work is now concentrating on having similar structures with better cooperativity and stronger coupling, such that this study could establish the presence or absence of the A2 terms in the Hamiltonian, and therefore the potential to observe the Dicke superradiance transition in this system. We also investigated another very non-parabolic system consisting of the ultrastrong coupling of metamaterial with hole gases in Si/SiGe heterostructures. The results on the cyclotron spectroscopy are published in New Journal of Physics and a paper on the ultra-strong coupling of this system with metamaterial resonators is in preparation.
Antother important signature would be the observation of the ultrastrong coupling using transport measurements. The observation of such features was a strong motivation for our development of a mK cryostat with state-of-the-art transport measurement capabilities. As an initial step, we have clarified the role of magnetoplasmons that naturally arise when the ultrastrongly coupled system comprises electron in a laterally confined geometry (i.e. Hall bar) coupled to resonators. The very attractive result is that the ultrastrong coupling can be achieved at zero magnetic fields, a result that may have some very important implications for transport experiments.