Tunable plasmonic properties of spatially overlapping asymmetric nanoparticle dimers

Abstract: In this work, the plasmonic properties of nanoparticle dimers with optical responses over a wide spectral range have been investigated by varying the inter-particle gap, dimer geometry, gap morphology, nanoparticle composition, and refractive index of the surrounding medium. In particular, we have theoretically investigated the plasmonic properties of spatially overlapping symmetric gold nanodisks, shape-asymmetric gold nanodisk nanoplates, and compositionally asymmetric gold-silver nanodisk dimers by varying the gap separation from touching to overlapping regime. In such a configuration, we have observed the appearance of a dominant bonding dimer plasmon (BDP) mode that blue-shifts as gap separation turns from touching to overlapping. In addition, it is found that asymmetric dimer produces a broader resonance shift compared to symmetric dimer because of the hybridization of bright and dark plasmon modes, making it a viable option for sensing applications. It is also found that blue shifting of the plasmon mode occurred by changing the gap morphology of the contacting region of the dimer for fixed nanoparticle size and dimer overlapping. Moreover, we explored the influence of overlapping nanoparticle dimer thickness and observed a notable resonance shift by varying the thickness of the nanoparticle dimer. Finally, based on this tunable resonance shift, we explored the sensing applications of bonding dimer plasmon mode with optimized geometries. Thus, the computed figure of merit (FOM) of the overlapping symmetric, shape-asymmetric, and compositionally asymmetric nanoparticle dimers were found to be 1.55, 2.08, and 3.04, respectively, and comparative advantages among the three configurations with implications for surface-based sensing have been thoroughly discussed.