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Abstract
Considering the nanogap and lattice effects, there is an attractive structure in plasmonics: closely spaced metallic nanoarrays. In this work, we demonstrate experimentally and theoretically the lattice coupling of multipole plasmon modes for closely spaced gold nanorod arrays, offering a new insight into the higher order cavity modes coupled with each other in the lattice. The resonances can be greatly tuned by changes in inter-rod gaps and nanorod heights while the influence of the nanorod diameter is relatively insignificant. Experimentally, pronounced suppressions of the reflectance are observed. Meanwhile, the near-field enhancement can be further enhanced, as demonstrated through surface enhanced Raman scattering (SERS). We then confirm the correlation between the near-field and far-field plasmonic responses, which is significantly important for maximizing the near-field enhancement at a specific excitation wavelength. This lattice coupling of multipole plasmon modes is of broad interest not only for SERS but also for other plasmonic applications, such as subwavelength imaging or metamaterials.
Highlights
Periodic metallic nanostructures, i.e. plasmonic crystals, are of particular interest for these applications, as they can drastically improve the quality factor of LSPRs12 and provide further optimization of the optical response[13,14]
The gold lattice structure on the silicon template in this study is schematically depicted in Fig. 1a, which is defined by the inter-rod gap d, the gold nanorod diameter D and its height h
To give a general idea of these geometry parameters’ influence on the optical response of the lattice, the reflectance R is calculated with 5 nm wavelength spacing by 3D finite element method (FEM) simulations using the control variate method (See Methods)
Summary
Periodic metallic nanostructures, i.e. plasmonic crystals, are of particular interest for these applications, as they can drastically improve the quality factor of LSPRs12 and provide further optimization of the optical response[13,14]. Strong near-field enhancement like electromagnetic hot spots is commonly obtained by narrowing the gaps between metallic nanostructures[25,26] Considering these gap and lattice effects, there is an attractive structure: closely spaced metallic nanoarrays, which serves as an active plasmonic platform[27,28,29,30]. When the array is excited at the resonance wavelength, the near-field response in terms of surface enhanced Raman scattering (SERS)[28,31,32,33] can be further enhanced by nearly an order of magnitude. That the near-field and far-field responses correlate well with each other, which is of significant importance for maximizing the near-field enhancement at a specific excitation wavelength
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