Reaching a Strong Coupling Regime between Quantum Emitters and Plasmonic Resonators using DNA

Light-matter interactions in condensed media at room-temperature are fundamentally limited by electron-phonon coupling. For instance, while the excitation cross-section of an isolated atom, or of a single quantum emitter at cryogenic temperatures, can reach one half of the wavelength of light squared (the so-called unitary limit); this value is reduced by 6 orders of magnitude for a fluorescent molecule or for a colloidal quantum dot at room temperature because of homogeneous phonon broadening. In order to render the exceptional optical properties of single quantum systems (such as single-photon emission and nonlinearities) efficiently accessible at room temperature and in condensed media, it is essential to enhance and optimize these interaction cross-sections.<br/>Over the last decade, gold nanostructures have shown amazing promise towards this goal thanks to their ability to enhance optical fields by several orders of magnitude in deeply sub-wavelength volumes. However, the nanoscale dimensions of these field enhancements mean that it is extremely difficult to address them in a controlled and reproducible way. To this end, we exploit DNA molecules to create plasmonic resonators with a control over both their nanoscale dimensions and their chemical environment. Using this strategy, we were able to enhance single-photon emission from fluorescent molecules by more than two orders of magnitude in a weak-coupling regime (ACS Nano 10, 4806 (2016)) and to reach a strong-coupling regime between a plasmonic resonator and 5 organic molecules, albeit with low reproducibility (ACS Nano 15, 14732 (2021)). We propose the DNA-based assembly of dimers of plasmonic nanocubes in order to provide reproducible single-molecule strong coupling and to make the coherent interaction of light with single quantum emitters feasible at room temperature (J. Phys. Chem. Lett. 13, 11996 (2022)).