Sommaire:

Improvements in both the photonic confinement and the emitter design have led to a steady increase in the strength of the light-matter coupling in cavity quantum electrodynamics experiments, to the point that the light matter coupling, Ω, can become a non-negligible fraction of the bare dipole frequency ω0. In such an interaction-dominated regime, usually referred to as ultrastrong coupling regime, the state of the system is usually well described in terms of mixed light-matter excitations, called polaritons. I will show that, when the coupling between light and matter becomes strong enough, this picture breaks down. In particular, in the non-perturbative deep strong coupling regime, Ω > ω0, light and matter degrees of freedom totally decouple. A striking consequence of such a counterintuitive phenomenon is that the Purcell effect is reversed and the spontaneous emission rate, usually thought to increase with the light-matter coupling strength, plummets instead for large enough couplings. This phenomenon, which should become observable already for resonant normalized couplings, Ω /ω0, of the order of 1, that is less than a factor 2 from present day experiments, poses an intrinsic upper bound to the efficiency of strongly coupled light emitters. Nevertheless, it could be exploited to dynamically control the electromagnetic field distribution in optoelectronic devices, by modulating the intensity of the light-matter interaction by optical or electrical means.

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