Single-molecule strong coupling at room temperature in plasmonic nanocavities

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.