Substrate-Mediated Broadband Tunability in Plasmonic Resonances of Metal Nanoantennas on Finite High-Permittivity Dielectric Substrate

We investigate the optical response of a gold nanocube antenna supported by a high-permittivity dielectric nanocuboid substrate and propose schemes for broadband tailoring of its plasmonic resonances via alteration in image-charge screening. Based on finite-element-method (FEM) simulations-in agreement with filtered-coupled-dipole-approximations (FCDA)-we explore the tunability and spectral evolution of the substrate-supported nanocube's hybridized plasmon modes as functions of the relative permittivity and dimensions of the dielectric substrate. Besides numerical calculations, we also derive simple analytical expressions using image-charge theory to readily estimate the resonance spectral shift-gauging the intense particle-substrate interaction-for a substrate-supported nanocube. Strong localized electric field, around the nanocube's vertices and edges near the substrate, is observed due to the image charges induced in the substrate by the coupled bonding mode arising from hybridization of the primitive dipolar and quadrupolar modes of the nanocube. By introducing slots on the dielectric substrate in the areas around the nanocube's edges where electric field is highly concentrated, we achieve substrate's surface-mediated wideband tunability of plasmonic resonance as functions of the geometric parameters of the slots while maintaining the overall dimensions and material of the nanocuboid substrate. These slots enable dynamic tunability of plasmon resonance by placing graphene flakes on them, which facilitates electrical tailoring of nanocube's plasmon resonance over visible and near-infrared regions. Thus, these proposed schemes would allow one to widely tune the optical responses of any plasmonic nanoantennas using a slotted finite high-permittivity-dielectric substrate for numerous applications in nanophotonic integrated circuits and plasmonic devices.