Optimized Design Of Plasmonic Nanoparticle Chains For Low-loss Subwavelength Energy Transport

Efficient transport of electromagnetic energy and miniaturization of photonic components are two paramount issues for the development of photonic integrated circuits(PICs).Conventional dielectric waveguides and photonic crystals can be used to guide light,but their sizes are limited by the diffraction effect.Subwavelength light confinement has been shown in high refractive index dielectric nanoparticle(NP)waveguides,mainly relevant to the hot research topic of silicon nanophotonics,although the compactness of such waveguide may be restricted due to the relatively large portion of energy propagating in the surrounding regions.An alternative way for manipulating light at the subwavelength scale lies in plasmonics.Owing to the unique properties of localized surface plasmon resonances(LSPRs),metallic nanostructures are able to confine and guide the electromagnetic energy below the diffraction limit,making them promising building blocks for PICs.However,the intrinsic loss in metals is inevitable,resulting into short propagation distances of optical signals,which severely hampers the practical applications of plasmonic waveguides.In this work,the closely spaced Au or Ag NPs are linearly arranged in analogy with Newton's cradle,forming various homogeneous and heterogeneous NP chains.COMSOL finite element method(FEM)simulations are performed to investigate the optical properties of the NP chains.The dielectric functions of Au and Ag are modeled by the Lorentz-Drude dispersion model fitting the experimental data in Palik's book.Using small NPs(R = 20 nm),the heterochain which intermediate component is Ag NPs and Au NPs arranged at the two ends has the lowest propagation loss,whether it is a trimer short-chain structure or a long-chain structure.The heterochain with Ag NPs at the two ends and Au NPs in the middle loses more than half of the total energy during propagation.To reduce the propagation loss in the plasmonic Newton's cradle,the optimal intermediate component should be three Ag NPs and it makes little difference to arrange Ag or Au NPs at the two ends.Fundamentally,such low propagation loss is mainly determined by the wavelength-dependent permittivity of the metals.As the excitation(or resonance)wavelength is tuned to the red and near infrared region,the imaginary part of the permittivity of Au and Ag remains to be small,and their real part is large enough to greatly suppress the penetration of local electric fields into the metal NPs,which also plays a critical role in reducing the propagation loss.Considering the high fabrication cost of the heterochain,the homochain composed of moderately large Ag NPs(< 100 nm)may be a better choice in practice.