Parallel Aligned Dipole–Multipole Plasmonic Hybridization

Abstract

Coupled plasmonic modes of noble metal nanoparticles are of interest in fields spanning chemistry, physics, materials science, and biology. We systematically investigated the dipole–multipole (D–M) coupling in asymmetric dimers of Au NRs through electromagnetic simulations. First, we calculated the hybridization between Au NR multipole and Au NR dipole in terms of far-field resonances, near-field profiles, and angular scattered radiation profiles at different arrangements. Notably, we found that the parallel aligned D–M plasmonic hybridization leads to asymmetric split resonances that include a multipolar resonance-dominated mode of higher energy and a dipolar resonance-dominated mode of lower energy. Then, we evaluated the “gap space effect” on the strength of D–M hybridization. Furthermore, we studied the dipole’s influence on the D–M coupling via systematically evaluating the optical properties of the dipolar rod’s effects on the hybridized system. Importantly, we discussed the carrier concentration and the damping velocity’s effect on each mode in terms of spectral shifts and near-field enhancement factors, and we found that dipole–octapole (DO) hybridization shows stronger interactions compared to dipole–quadrupole (DQ) hybridization. We hope our work may boost the fundamental understanding of the D–M plasmonic hybridization system and facilitate plasmonic–photonic innovations and nanoplasmonic applications, such as fluorescence imaging, energy storage, biosensors, metamaterials, etc.

Similarities of Quadrupolar Au NR Monomers and Antibonding Au NR Dimers as Building Blocks in Asymmetric Plasmonic Hybridization

“…This observation matches our previous conclusion that the parallel-aligned D-M plasmonic hybridization leads to the asymmetric split resonances that include the multipolar-resonance-dominated mode (|ω + >) of higher energy and the dipolar-resonance-dominated mode (|ω − >) of lower energy. 31 It is noteworthy that a multipolar monomer resonator (M long,l=2 ) shows similarities as a building block to that of an antibonding mode of a dimer of equivalent overall length (M half,l=1 −M half,l=1 ) in coupled plasmonic systems. Particularly, both of the M short,l=1 −M long,l=2 and M short,l=1 −M half,l=1 −M half,l=1 showed a similar split multipolarresonance-dominated mode (|ω + >) of higher energy and dipolar-resonance-dominated mode (|ω − >) of lower energy.…”

“…Similar to our previous study, we calculated the dielectric function of the M′ ω,γ short rod via DFT calculation with the CASTEP module of Material Studio (see Methods for details). 31 Especially, the complex dielectric function of M′ ω,γ can be calculated as a combination of intraband and interband sections. Particularly, the interband term of permittivity can be calculated from Kramers−Kronig transformation as follows:…”

“…Similar to our previous work, we set the E -field of incident light that polarized in the YZ plane, and its Poynting vector k in the YZ plane forms the angle of α relative to the longitudinal axis of Au NR. ,, Specifically, oblique (α = 45°) illumination was set in order to excite the even mode of the Au NR monomer ( M long,l=2 ) and the antibonding mode of the Au NR coupled dimer structure M half,l=1 – M half,l=1 .…”

“…In consistency with our previous report, the commercially available DFT computational software (Materials Studio) was applied to the calculation of Au complex dielectric functions . Basically, as shown in Figure S1a, a face-centered cubic (FCC) cell with the lattice lengths of a = b = c = 4.0783 Å was built with the angles of α = β = γ = 90°.…”

“…In consistency with our previous report, the commercially available DFT computational software (Materials Studio) was applied to the calculation of Au complex dielectric functions. 31 Basically, as shown in Figure S1a, a face-centered cubic (FCC) cell with the lattice lengths of a = b = c = 4.0783 Å was built with the angles of α = β = γ = 90°. Then, geometry optimization and calculations of optical properties were subsequently performed with the CASTEP (Cambridge Sequential Total Energy Package) module.…”