Mornhinweg, JoshuaDiebel, Laura KatharinaHalbhuber, MaikePrager, MichaelRiepl, JosefInzenhofer, TobiasBougeard, DominiqueHuber, RupertLange, Christoph2024-08-092024-08-092024-02-28http://hdl.handle.net/2003/42642http://dx.doi.org/10.17877/DE290R-24479Dressing electronic quantumstates with virtual photons creates exotic effects ranging from vacuum-field modified transport to polaritonic chemistry, and squeezing or entanglement of modes. The established paradigm of cavity quantum electrodynamics maximizes the light-matter coupling strength ΩR=ωc, defined as the ratio of the vacuumRabi frequency and the frequency of light, by resonant interactions. Yet, the finite oscillator strength of a single electronic excitation sets a natural limit to ΩR=ωc. Here, we enter a regime of record-strong light-matter interaction which exploits the cooperative dipole moments of multiple, highly non-resonant magnetoplasmon modes tailored by ourmetasurface. This creates an ultrabroadband spectrum of 20 polaritons spanning 6 optical octaves, calculated vacuum ground state populations exceeding 1 virtual excitation quantum, and coupling strengths equivalent to ΩR=ωc =3:19. The extreme interaction drives strongly subcycle energy exchange between multiple bosonic vacuum modes akin to high-order nonlinearities, and entangles previously orthogonal electronic excitations solely via vacuum fluctuations.en530Text