Ultrastrong light matter coupling has raised high interest in recent years for the predicted unusual quantum properties of its ground state, which contains photons. We have investigated such physics in a system based on the cyclotron transition of a 2D confined electrons (or holes) gas in semiconductors coupled to the modes of highly subwavelength metallic resonators in the 200-1000 GHz range. The extreme reduction of the cavity volume and surface (Seff/λ0=3 x 10-7) led to the observation of ultrastrong coupling on a small (<100) number of electrons. Such extreme conditions reveal also a previously unobserved renormalization of the cyclotron effective mass, effectively breaking Kohn’s theorem. Kohn's theorem states the independence of the cyclotron resonance frequency from many-body effects in the case of a parabolic and translationally invariant system. For our resonator the translational invariance is clearly broken since the electric field is concentrated on a circular region of around r= 350 nm for a cyclotron radius of the order of 60 nm for a free space wavelength of 1 mm (300 GHz). In our case we can reveal many body effects on the cyclotron mass because we break the translational invariance of the system with the extreme photonic confinement provided by the cavity, observing an increase of the m*/m0 of 6% with respect to the uncoupled cyclotron mass. Experiments conduced on the same 2DEG with a standard split-ring resonator at the same frequency do not show any effective mass shift.
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