Plasmonic radiance: probing structure at the Ångström scale with visible light.

Plasmonic modes with long radiative lifetimes combine strong nanoscale light confinement with a narrow spectral line width carrying the signature of Fano resonances, making them very promising for nanophotonic applications such as sensing, lasing, and switching. Their coupling to incident radiation, also known as radiance, determines their optical properties and optimal use in applications. In this work, we theoretically and experimentally demonstrate that the radiance of a plasmonic mode can be classified into three different regimes. In the weak coupling regime, the line shape exhibits remarkable sensitivity to the dielectric environment. We show that geometrical displacements and deformations at the Ångström scale can be detected optically by measuring the radiance. In the intermediate regime, the electromagnetic energy stored in the mode is maximal, with large electric field enhancements that can be exploited in surface enhanced spectroscopy applications. In the strong coupling regime, the interaction can result in hybridized modes with tunable energies.