Fano Transparency in Rounded Nanocube Dimers Induced by Gap Plasmon Coupling
Résumé
Homodimers of noble metal nanocubes form model plasmonic systems where the localized plasmon resonances sustained by each particle not only hybridize but also coexist with excitations of a different nature: surface plasmon polaritons confined within the Fabry−Perot cavity delimited by facing cube surfaces (i.e., gap plasmons). Destructive interference in the strong coupling between one of these highly localized modes and the highly radiating longitudinal dipolar plasmon of the dimer is responsible for the formation of a Fano resonance profile and the opening of a spectral window of anomalous transparency for the exciting light. We report on the clear experimental evidence of this effect in the case of 50 nm silver and 160 nm gold nanocube dimers studied by spatial modulation spectroscopy at the single particle level. A numerical study based on a plasmon mode analysis leads us to unambiguously identify the main cavity mode involved in this process and especially the major role played by its symmetry. The Fano depletion dip is red-shifted when the gap size is decreasing. It is also blue-shifted and all the more pronounced that the cube edge rounding is large. Combining nanopatch antenna and plasmon hybridization descriptions, we quantify the key role of the face-to-face distance and the cube edge morphology on the spectral profile of the transparency dip. T he optical excitation of localized surface plasmon resonances (LSPR) in single metallic nanoparticles or multicomponent nanostructures is responsible for efficient resonant far-field scattering and near-field concentration of light. 1,2 Such nanoantennas offer the opportunity to manipulate light at scales much shorter than the wavelength by making the best use of the LSPR sensitivity to variations in particle shape, size, chemical composition, and dielectric environment. 3,4 The electrostatic coupling between several plasmonic subunits is an additional tool for tailoring the optical response over a wide spectral range. 5,6 In this respect, much attention is paid to analogues of Fano resonances in classical electrostatics and especially those arising from the coupling between a " dark " plasmonic mode (weak dipole moment and narrow line width) and a degenerate mode that is highly radiative over a much broader spectral range (" bright " mode, large dipole moment). On either side of the resonance, the rapid phase shift of the " dark " mode polarizability relative to that of the bright mode and the exciting field may induce constructive or destructive interference in the net far field scattering process. They result in the formation of a dip in the broad spectral band of the far field scattered light with a typical dissymmetric profile and a narrow width related to the " dark " resonance lifetime. 7−11 Fano resonances open sharp and transparent windows in the plasmonic response of metallic nanostructures. 12−14 Their high sensitivity to relative changes in the nanostructure dielectric environment can be efficiently exploited for chemical and biological sensing. 15,16 Most of the studies in this field deal with noble metal nanoantennas built from the assembly and coupling of subunits of various size, shape, and orientation: oligomers of colloidal particles (DNA assembled, 17,18 heterodimers 19) and predominantly objects engraved by electron-beam lithography. 20−22 This method offers considerable flexibility for designing the spectral response of intricate structures with an advantageous symmetry breaking. 13,23 In the far field, they act as wide band