Light-matter interactions in condensed media at room-temperature are fundamentally limited by electronphonon 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 (meaning that ~50% of incoming photons will interact for a diffraction-limited excitation); this value is reduced by 6-7 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. Over the last two decades, plasmonic resonators 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 or “hot-spots” mean that it is extremely
difficult to exploit them in a controlled and reproducible way. At Institut Langevin, we develop two approaches in order to achieve this: -We introduce, in a deterministic way, a controlled number of quantum emitters in the nanoscale hot-spot between two gold nanoparticles using a DNA-based self-assembly strategy. Using this approach, we were able to enhance single-photon emission from fluorescent molecules by more than two orders of magnitude in a
weak-coupling regime [1,2]. I will also discuss recent experiments where we reach a strong-coupling regime between a plasmonic resonator and five organic molecules.
-We actively control the seemingly random plasmonic hot-spots featured by disordered gold surfaces using far-field wavefront shaping. In practice, by tuning the phase of a pulsed excitation, we ensure the constructive interference of plasmonic modes that are delocalized over several microns on the surface; leading to a local
enhancement of the nonlinear luminescence of gold by more than two orders of magnitude [3].
[1] M. P. Busson, et al., Nat. Commun. 3, 962 (2012) – [2] S. Bidault, et al., ACS Nano 10, 4806 (2016)
[3] G. Roubaud, et al., Nano Lett. 20, 3291 (2020)
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Institut Langevin, ESPCI