Simultaneously tuning the electric and magnetic plasmonic response using capped bi-metallic nanoantennas

We present a novel, capped bowtie nanoantenna capable of achieving simultaneous enhancement of electric and magnetic fields in a broad spectrum spanning visible to near-infrared frequencies. By controlling parameters including nanoantenna array spacing, cap thickness, and bowtie gap spacing, we show magnetic enhancements in excess of 3000 times the incident field, which are among the highest values reported to date. Further, electric field enhancements >10⁴ are obtained across the full parameter range. This is in contrast to diabolo antenna designs that achieve strong magnetic enhancement at the expense of mitigating the electrical resonance. We further show that this architecture achieves refractive index sensitivities of ∼700 nm RIU ^-1. The combination of large, tunable electric and magnetic-field enhancements makes the capped-nanoantenna platform highly attractive for magnetic plasmonics, metamaterial engineering, nonlinear optics, and sensing applications.