Ultrastrong coupling between nanoparticle plasmons and cavity photons at ambient conditions

TL;DR

This paper demonstrates ultrastrong coupling between nanoparticle plasmons and cavity photons at room temperature using plasmonic nanorods in a microcavity, achieving high interaction strength without cryogenic conditions.

Contribution

It introduces a novel plasmonic nanorod array platform for achieving ultrastrong coupling at ambient conditions, bypassing traditional experimental constraints.

Findings

Interaction strength ratio g/ω ~ 0.55 achieved

Ground-state energy modified by up to 10%

Ultrastrong coupling realized at room temperature

Abstract

Ultrastrong coupling is a distinct regime of electromagnetic interaction that enables a rich variety of intriguing physical phenomena. Traditionally, this regime has been reached by coupling intersubband transitions of multiple quantum wells, superconducting artificial atoms, or two-dimensional electron gases to microcavity resonators. However, employing these platforms requires demanding experimental conditions such as cryogenic temperatures, strong magnetic fields, and high vacuum. Here, we use plasmonic nanorods array positioned at the antinode of the resonant optical Fabry-P\'erot microcavity to reach the ultrastrong coupling (USC) regime at ambient conditions and without the use of magnetic fields. From optical measurements we extract the value of the 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 the cavity volume. Moreover, by comparing the resonant energies of the coupled and uncoupled systems, we indirectly observe up to ~10% modification of the ground-state energy, which is a hallmark of USC. Our results suggest that plasmon-microcavity polaritons are a promising new platform for room-temperature USC realizations in the optical and infrared range.