Asymmetric L-shaped resonant optical antennas with plasmon length tuning and high-electric field enhancement

L-shaped resonant optical antennas (ROAs) are low-symmetry plasmonic nanostructures with the unique ability to show two tunable resonances in the optical and near-infrared wavelength regions. The plasmon length of the so-called longitudinal dipolar fundamental plasmon mode of these asymmetric L-shaped ROAs can be used as plasmon resonance building blocks to design polarization-sensitive devices. This paper introduces and numerically analyzes a novel design of asymmetric L-shape ROAs. The reported design offers two resonance modes, i.e., bimodal longitudinal antenna resonance behavior with a high enhancement factor. These two resonances can be selectively excited by changing the linear polarization angle. It is found that the coupled L-shaped ROA with a very small 2 nm gap width exhibits field enhancements 40 and 147.3 on scale |ETOT|/|EIN| for the high energy and low energy resonance, respectively. The obtained results and the analysis open a new route for multiple plasmon resonance devices with ultra-high field enhancement that can be easily integrated with future nano-optical circuits with multiple operational frequencies.