Multi-mode Deep Strong Coupling in a Multi Quantum Well Fabry-Perot Cavity

We present multi-mode deep-strong coupling in a multi-quantum well (N=166) heterostructure. The heterostructure itself acts as a Fabry-Perot cavity, for which the even cavity modes strongly couple to the cyclotron resonance to form Landau polaritons. The experimentally observed vacuum Rabi splitting is larger than the mode spacing and well into the deep-strong coupling regime ($\eta>1$) resulting in a rich multi-mode polaritonic spectrum which is accurately reproduced by an all-to-all multi-photonic, multi-electronic Hopfield coupling model. Remarkably, light-matter decoupling is observed across the whole measurable spectrum, including in the low frequency limit ($\lambda>>L_{cav}$) where the normalized coupling strength reaches $\eta=8.1$. The system demonstrates a robust platform for exploring extreme coupling regimes and its chiral nature holds potential for chiral cavity and chiral mirror applications.