A novel plasmonic coupling mechanism in non-aligned metallic nanorod homodimers

The current study aims at comparing the impact of both the interparticle separation (g) and the longitudinal shift (h) on the plasmonic coupling features in a homodimer composed from two symmetric nanorods arranged in a side-to-side (SS) configuration. To achieve this, a Finite-Difference Time-Domain (FDTD) numerical tool is employed to calculate the optical properties of the dimer system as a function of both g and h under the illumination of both longitudinal and transverse lights. Despite the calculated spectral response of the transverse mode (TM) exhibiting very weak dependency on either g or h, the longitudinal mode (LM) demonstrates very interesting and distinguished optical characteristics in terms of the two structural parameters (g and h). When the value of g increases, the plasmonic coupling is significantly diminished and therefore both the resonance wavelength and the associated nearfield intensity of the observed LM are redshifted exponentially with g. As the two interacting rods are shifted away from each other by increasing h, the polarization state that dominates the plasmonic coupling is gradually switched from the facing sides pattern into the facing edges model and therefore both the charge density and the nearfield intensity of the LM are significantly enhanced. Surprisingly, depending on the relative value of h to the rod’s length, as h increases, a redshift followed by a blueshift in both the resonance wavelength and nearfield intensity is observed in the excited LM. The type of the plasmon shift is mainly controlled by the in-phase/out-of-phase arrangement between the induced multipoles and the dipoles. By maintaining a constant value of g, the introduced non-alignment condition sheds the light on an innovative method to precisely control and gradually alters the spectral response of dimer systems to reach the desired optical properties for specific nanoplasmonic-based applications.