Near-field Mapping Of Surface Plasmons By Finite Element Method

The plasmon hybridizations of metallic nanostructures can give rise to an enormous enhancement of their local electric fields.As one of the most attractive phenomena in plasmonics,the near-field enhancement serves as the fundamental mechanism for a wide variety of applications,from surface-enhanced spectroscopies,chemical and biological sensing,to single molecule detection,nonlinear optics,and so on.From the perspective of the electromagnetic mechanism of surface-enhanced Raman scattering(SERS),we have developed a visualized near-field mapping approach based on the finite element method(FEM)by applying Gauss' s law.It has been proved that this new approach is ideally suited to determining complicated plasmon modes directly.And then,a series of typical plasmonic systems have been extracted from practical applications,followed by systematic and intensive numerical studieson the fundamental properties of their plasmon hybridizations,the near-field enhancement and the spectral deviation between near-and far-field resonances.The results can provide a comprehensive physical image of surface plasmons,which benefits greatly both the optimized design and spectroscopy applications of potential plasmonic devices.For the the most important electromagnetic hot spot structure: the metallic nanoparticle(NP)dimer,aquantitative analysis of its nanogap effect has been performed.It is revealed that the near-field enhancement factor exhibits a weak power-law dependence on the gap size while both the near-and far-field resonance shifts decay exponentially as the gap size increases.When the gap is conductively bridged by a smooth thin metal junction,the bonding dipole plasmon(BDP)mode in the visible region degenerates rapidly,but the rising charge transfer plasmon mode enables a huge and tunable infrared near-field enhancement,offering a new approach to engineering potential plasmonic devices at infrared wavelengths.For the alternative structure consisting of metal NPs on a mirror,the coexistence of near-field NP-NP and NP-mirror couplings has been considered.It is confirmed that there are two different kinds of mirror-induced BDP modes which can support a much stronger near-field enhancement than that of the isolated BDP mode.For the attractive metallic nanoarrays,we have predicted universal near-field constructive and destructive interference patterns of the Fano resonance in 2D plasmonic crystals,visualizing its physical origin as a wave-interference phenomenon.Finally,based on above results and understanding,we have studied closely spaced gold nanorod arrays focusing on their near-and far-field plasmonic properties.Both the lattice coupling of multipole plasmon modes and its great tunability have been demonstratedtheoretically andexperimentally.