Experimental study on the transition of plasmonic resonance modes in double-ring dimers by conductive junctions in the terahertz regime.

Plasmonic dimers that made from two subwavelength particles have drawn much attention in the recent years, which are quite promising in local field enhancement, sensing, high frequency conductance probing and electron tunneling. In this work, we experimentally investigate the mode transition effect of different plasmonic resonances in double-ring dimers when introducing conductive junction at the dimer gap in the terahertz regime. Without the junction, the dimers support a single dipolar bonding dimer plasmonic (BDP) mode. With the junction of a high conductance, two new resonance modes-a screened BDP (SBDP) mode and a charge transfer plasmonic (CTP) mode emerge. Such effect is proved to be unrelated to the shape of the rings, whether circular, square or triangular. However, the resonance statuses of the specific modes are different. Furthermore, we also experimentally study the controllable mode resonance behavior as the conductivity of the junction gradually changes by using superconducting material, and meanwhile numerically investigate the active mode transition behavior as well as the threshold effect. These results show great potential in applications of plasmonic sensing, spectral modulating and optical switching.