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Ultrastrong coupling is a distinct regime of electromagnetic interaction thatenables a rich variety of intriguing physical phenomena. Traditionally, thisregime has been reached by coupling intersubband transitions of multiplequantum wells, superconducting artificial atoms, or two-dimensional electrongases to microcavity resonators. However, employing these platforms requiresdemanding experimental conditions such as cryogenic temperatures, strongmagnetic fields, and high vacuum. Here, we use plasmonic nanorods arraypositioned at the antinode of the resonant optical Fabry-P\'erot microcavity toreach the ultrastrong coupling (USC) regime at ambient conditions and withoutthe use of magnetic fields. From optical measurements we extract the value ofthe interaction strength over the transition energy as high as g/{\omega}~0.55,deep in the USC regime, while the nanorods array occupies only ~4% of thecavity volume. Moreover, by comparing the resonant energies of the coupled anduncoupled systems, we indirectly observe up to ~10% modification of theground-state energy, which is a hallmark of USC. Our results suggest thatplasmon-microcavity polaritons are a promising new platform forroom-temperature USC realizations in the optical and infrared range.

Ultrastrong coupling between nanoparticle plasmons and cavity photons at ambient conditions