Placing a light emitter in an ultra-small optical cavity results in coupling between matter and light, generating new forms of emission that can be exploited in practical or fundamental applications; here, a system is described in which strong light–matter coupling occurs at room temperature and in ambient conditions by aligning single dye molecules in the optical cavities between gold nanoparticles and surfaces. When a light-emitter is placed in a high-quality optical cavity, where light remains confined for a long time, coupling between matter and light occurs. This coupling produces new forms of light that can be exploited in practical or fundamental applications such as low-power switches or quantum information networks. Jeremy Baumberg and colleagues demonstrate an unusual system, of interest for photochemistry studies, where the fully mixed, strong-coupling regime is reached for single dye molecules at room temperature and in ambient conditions. The system is assembled by aligning dye molecules using a host–guest chemistry approach in plasmonic gaps (the optical cavities) between gold nanoparticles and surfaces. The authors demonstrate strong statistical evidence for single-molecule strong coupling. This opens up the possibility of exploring the chemical structure of molecules with light-dressed states. Photon emitters placed in an optical cavity experience an environment that changes how they are coupled to the surrounding light field. In the weak-coupling regime, the extraction of light from the emitter is enhanced. But more profound effects emerge when single-emitter strong coupling occurs: mixed states are produced that are part light, part matter1,2, forming building blocks for quantum information systems and for ultralow-power switches and lasers3,4,5,6. Such cavity quantum electrodynamics has until now been the preserve of low temperatures and complicated fabrication methods, compromising its use5,7,8. Here, by scaling the cavity volume to less than 40 cubic nanometres and using host–guest chemistry to align one to ten protectively isolated methylene-blue molecules, we reach the strong-coupling regime at room temperature and in ambient conditions. Dispersion curves from more than 50 such plasmonic nanocavities display characteristic light–matter mixing, with Rabi frequencies of 300 millielectronvolts for ten methylene-blue molecules, decreasing to 90 millielectronvolts for single molecules—matching quantitative models. Statistical analysis of vibrational spectroscopy time series and dark-field scattering spectra provides evidence of single-molecule strong coupling. This dressing of molecules with light can modify photochemistry, opening up the exploration of complex natural processes such as photosynthesis9 and the possibility of manipulating chemical bonds10.

A cool route to nanospectroscopy: Confining light to a cavity is often used to enhance the interaction between the light and a particle stored within the cavity. Benz et al. worked with a self-assembled monolayer of biphenyl-4-thiol molecules sandwiched between a gold film and a gold nanoparticle. They used laser irradiation to move atoms in the nanoparticle and produced a “picocavity” that was stable at cryogenic temperatures. The authors were then able to obtain time-dependent Raman spectra from individual molecules. Such subwavelength cavities that can localize light to volumes well below 1 nm ³ will enable optical experiments on the atomic scale. Science , this issue p. 726

An emitter in the vicinity of a metal nanostructure is quenched by its decay through non-radiative channels, leading to the belief in a zone of inactivity for emitters placed within $<$10nm of a plasmonic nanostructure. Here we demonstrate and explain why in tightly-coupled plasmonic resonators forming nanocavities ``quenching is quenched'' due to plasmon mixing. Unlike isolated nanoparticles, such plasmonic nanocavities show mode hybridization which can massively enhance emitter excitation and decay via radiative channels, here experimentally confirmed by laterally dependent emitter placement through DNA-origami, and we explain why this enhancement of excitation and radiative decay can be strong enough to facilitate single-molecule strong-coupling as evident in dynamic Rabi-oscillations.

Fabricating nanocavities in which optically active single quantum emitters are precisely positioned is crucial for building nanophotonic devices. Here we show that self-assembly based on robust DNA-origami constructs can precisely position single molecules laterally within sub-5 nm gaps between plasmonic substrates that support intense optical confinement. By placing single-molecules at the center of a nanocavity, we show modification of the plasmon cavity resonance before and after bleaching the chromophore and obtain enhancements of ≥4 × 10³ with high quantum yield (≥50%). By varying the lateral position of the molecule in the gap, we directly map the spatial profile of the local density of optical states with a resolution of ±1.5 nm. Our approach introduces a straightforward noninvasive way to measure and quantify confined optical modes on the nanoscale.