From individual to strongly coupled metallic nanocavities

Summary form only given. 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). The coupled system then responds as a single dipolar entity, and the SHG emission can be either enhanced or depressed depending on the incoming beam polarization. We account for the evolution of the polarized secondharmonic response in terms of a phenomenological tensor symmetry model: the nonlinear anisotropy ratio ρ = χYXX (2) / χYYY (2) where χ.XX (2) and χ... (2) are the two in-plane quadratic nonlinear coefficients (with Y along the coupling axis and X perpendicular to it) is seen to guide a transition from an uncoupled to a coupled configuration, the magnitude of ρ measuring the strength of the coupling and being inferred from experiments [4]. An electromagnetic model based on the determination of plasmonic field distributions provides further insight, in particular with regard to the plasmon modes involved in the nonlinear process, and their dependence on the size of the triangles and their spacing. While this coupling phenomenon holds for nano-cavities, it is strongly contrasting with nanoparticles which cannot couple above a range in the ten's of nanometers scale, a situation which seems to rule-out Babinet's principle in the current nonlinear configuration. The investigation of such nano-plasmonic systems also lead us to revise the classically used quadratic nonlinear response to account for the non-local and non-homogeneous (e.g. non translationnaly invariant) nature of the medium.