| Summary: | We embedded an all-hydrocarbon-based carbocation in a metallic microcavity that was tuned to resonance with an electronic transition of the carbocation. The measured Rabi splitting was 41% of the excitation energy, putting the system well into the ultrastrong coupling regime. Importantly, due to the intrinsic charge on the carbocation, the polaritons that form carry a significant charge fraction (0.55 e_{0}) and a large charge-to-mass ratio (∼2400 e_{0}/m_{0}). Moreover, the ground state of the ultrastrongly coupled system is calculated to carry about 1% of one elementary charge. These unique properties of our system, together with its convenient preparation, provide a practical platform to study charged polaritons in the ultrastrong coupling regime.
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