From individual to strongly coupled metallic nanocavities

Localized plasmonic modes of metallic nanoparticles may hybridize like atomic orbitals forming a molecule. However, the rapid spatial decay of plasmonic fields outside the metal severely limits the range of these interactions to tens of nanometers. Herein, we demonstrate a strong coupling scheme between nanocavities carved in the same Silver metal films that is sustained by propagating surface plasmons within a hundreds of nanometers interval scale for a properly selected metal/wavelength combination. The nanotructures are patterned in Silver films by Focused Ion beam (FIB) with typical sizes in the 100 nm in all directions, also allowing to control the shape of the contours in different geometries [1]. Strong coupling drastically changes the symmetry of the charge distribution around the nanocavities, qualifying the highly symmetry-sensitive quadratic nonlinear optical response of the medium as a relevant probe [2,3]. We show by means of polarization resolved second-harmonic generation in a confocal microscope configuration that strongly coupled equilateral triangular nanocavities lose their individual three-fold symmetry to adopt the lower symmetry of the coupled system (see Figure).