Coupling Bright and Dark Plasmonic Lattice Resonances

Abstract

We demonstrate the coupling of bright and dark surface lattice resonances (SLRs), which are collective Fano resonances in 2D plasmonic crystals. As a result of this coupling, a frequency stop gap in the dispersion relation of SLRs is observed. The different field symmetries of the low-and high-frequency SLR bands lead to pronounced differences in their coupling to free-space radiation. Standing waves of very narrow spectral width compared to localized surface-plasmon resonances are formed at the highfrequency band edge, while subradiant damping onsets at the low-frequency band edge, leading the resonance into darkness. We introduce a coupled-oscillator analog to the plasmonic crystal, which serves to elucidate the physics of the coupled plasmonic resonances and which is used to estimate very high quality factors for SLRs. DOI: 10.1103/PhysRevX.1.021019 Subject Areas: Nanophysics, PlasmonicsMetallic nanoparticles supporting surface-plasmon resonances allow light to be localized in nanoscale volumes, thereby opening exciting possibilities such as nanoscale control of emitters [1], large electromagnetic enhancements [2], and nonlinear nano-optics [3]. Much attention has been given to localized surface-plasmon resonances (LSPRs), which arise in individual particles when their conduction electrons are coherently driven by an electromagnetic field. Although localized surface plasmons in neighbor particles may mutually couple, their resonances are, in general, severely broadened due to strong radiative damping. Hence, LSPRs exhibit low quality factors Q. A recent development in nanoplasmonics deals with collective resonances in periodic arrays of metallic nanostructures, or plasmonic crystals. Such arrays support surface lattice resonances (SLRs), which are collective resonances mediated by diffractive coupling of localized plasmons. This coupling occurs near the critical frequency at which a diffraction order is radiating in the plane of the array, i.e., at the Rayleigh anomaly. SLRs were introduced by Carron [4], and the interest in this phenomenon was revived by Schatz and co-workers with a series of works on 1D and 2D arrays [5,6]. However, the experimental observation of SLRs remained elusive for many years [7]. Recent advances in nanofabrication and in the understanding of SLRs have allowed for their observation in periodic arrays of nanostructures with different geometries [8][9][10][11][12][13]. In contrast with LSPRs, SLRs possess much higher Q's, and the associated polaritons can propagate over tens of unit cells in the plasmonic crystal [12]. The relevance of SLRs for enhanced, directional, and polarized light emission [11,14] and sensing [15] has been recently demonstrated. Although the coupling of surface modes in periodic metallic structures has attracted much interest [16][17][18][19], especially for its connection with frequency stop gaps [20], coupled SLRs have not yet been discussed.In this paper, we demonstrate the mutual coupling of SLRs and the formation of a frequency stop gap in the dispersion ...

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