Ultrastrong coupling between nanoparticle plasmons and cavity photons at ambient conditions

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 a plasmonic nanorod array positioned at the antinode of a resonant optical Fabry-Pérot 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/ω ~ 0.55, deep in the USC regime, while the nanorod 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 platform for room-temperature USC realizations in the optical and infrared ranges, and may lead to the long-sought direct visualization of the vacuum energy modification.

Tunable self-assembled Casimir microcavities and polaritons

“…This results in symmetric and anti-symmetric hybrid eigenmodes with the splitting between the two determined by the thickness of the middle mirror. This view is similar to the symmetric and anti-symmetric polariton branches in the Rabi-like problem, following which, the three-mirror geometries were recently interpreted in terms of hybrid light-matter polaritonic states [31][32][33].…”

Water Oxidation under Modal Ultrastrong Coupling Conditions Using Gold/Silver Alloy Nanoparticles and Fabry–Pérot Nanocavities

“…Therefore, we employed Au‐Ag alloy NPs with a high oscillator strength and sufficient durability against oxidation reactions [28–31] to construct a photoanode under ultrastrong coupling conditions for efficient photochemical water splitting. From optical measurements, we found that this photoanode presents a splitting energy of up to 620 meV, which can be categorized into the ultrastrong coupling regime [32, 33, 37] . In addition, during photocurrent measurements using this photoanode, only Ag on the surface of the Au‐Ag alloy NPs eluted in the very early stage of the reaction, and a long‐term stable photocurrent was observed.…”

“…This result indicates that the AATA structure fulfils the ultrastrong coupling condition, which is defined as ħΩ>0.2 ħω 0, as discussed for both modal and light‐matter coupling. [ 32 , 37 ] Figures 2 b and 2c show the absorption and incident photon‐to‐current efficiency (IPCE) action spectra of the AATA and ATA photoanodes. Before starting the IPCE measurement, the photoanodes were irradiated with 580 nm light for 2 hours to remove Ag from the surface of the Au‐Ag alloy NPs.…”

Recent Progress on Optoplasmonic Whispering‐Gallery‐Mode Microcavities

“…In very recent studies, the combination of plasmonic nanostructures and photonic dielectrics has led to a good balance between confinement and absorption loss. [ 81–89 ] As such, enhanced light‐matter interactions sit at the core of hybrid optoplasmonic systems for the development of novel nanophotonic devices for sensing, light emission, and information processing, etc. [ 81,83,87,89 ] Photonic crystals, Fabry–Pérot cavities, photonic waveguides, and optical fibers have been integrated with plasmonic nanostructures, previously.…”