Three types of couplings between asymmetric plasmonic dimers.

We report extensive numerical studies on plasmonic dimers of different configurations and find that their coupling effects can be categorized into three types of phenomena. First, like ordinary mechanical systems, the plasmonic dimers can exhibit positive couplings that show anti-crossing behavior. Second, they can also be arranged to exhibit negative couplings that display opposite trends in resonant frequency shifts. Third, when there are surface currents in proximity to each other, the resonance frequencies of the dimers exhibit unusual redshifts that do not have any analogies in conventional systems. Our work suggests that in addition to the well-known electric and magnetic dipolar interactions, contributions from the inductance of displacement currents in the near field cannot be ignored. Overall, asymmetric plasmonic dimers exhibit better sensitivities than the symmetric counterparts and our extensive studies also enable us to identify the plasmonic dimer with the highest sensing capabilities.