Atomically strong light pulses can drive sub-optical-cycle dynamics. When the Rabi frequency – the rate of energy exchange between light and matter – exceeds the optical carrier frequency, fascinating non-perturbative strong-field phenomena emerge, such as high-harmonic generation and lightwave transport. Here, we explore a related novel subcycle regime of ultimately strong light-matter interaction without a coherent driving field. We use the vacuum fluctuations of nanoantennas to drive cyclotron resonances of two-dimensional electron gases to vacuum Rabi frequencies exceeding the carrier frequency. Femtosecond photoactivation of a switch element inside the cavity disrupts this ‘deep-strong coupling’ more than an order of magnitude faster than the oscillation cycle of light. The abrupt modification of the vacuum ground state causes spectrally broadband polarisation oscillations confirmed by our quantum model. In the future, this subcycle shaping of hybrid quantum states may trigger cavity-induced quantum chemistry, vacuum-modified transport, or cavity-controlled superconductivity, opening new scenarios for non-adiabatic quantum optics.

We explore the nonlinear response of tailor-cut light-matter hybrid states in a novel regime, where both the Rabi frequency induced by a coherent driving field and the vacuum Rabi frequency set by a cavity field are comparable to the carrier frequency of light. In this previously unexplored strong-field limit of ultrastrong coupling, subcycle pump-probe and multiwave mixing nonlinearities between different polariton states violate the normal-mode approximation while ultrastrong coupling remains intact, as confirmed by our mean-field model. We expect such custom-cut nonlinearities of hybridized elementary excitations to facilitate nonclassical light sources, quantum phase transitions, or cavity chemistry with virtual photons.

Coupling multiple, highly non-resonant cavity and electronic modes, we reach unprecedented coupling strengths equivalent to ΩR/ωc=3.19. The new regime features over 10 polaritons spanning 6 octaves and vacuum ground state populations exceeding 1 photon.

We drive an ultrastrongly light-matter coupled system by strong, coherent terahertz (THz) waveforms which compete with the resonator vacuum electromagnetic field, leading to strong subcycle nonlinearities including eight-wave mixing and nonlinear polariton correlations.

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