Nanobridged rhombic antennas supporting both dipolar and high-order plasmonic modes with spatially superimposed hotspots in the mid-infrared

Mid-infrared antennas (MIRAs) support highly-efficient optical resonance in the infrared, enabling multiple applications, such as surface-enhanced infrared absorption (SEIRA) spectroscopy and ultrasensitive mid-infrared detection. However, most MIRAs such as dipolar-antenna structures support only narrow-band dipolar-mode resonances while high-order modes are usually too weak to be observed, severely limiting other useful applications that broadband resonances make possible. In this study, we report a multiscale nanobridged rhombic antenna (NBRA) that supports two dominant resonances in the MIR, including a charge-transfer plasmon (CTP) band and a bridged dipolar plasmon (BDP) band which looks like a quadruple resonance. These assignments are evidenced by scattering-type scanning near-field optical microscopy (s-SNOM) imaging and electromagnetic simulations. The high-order mode only occurs with nanometer-sized bridge (nanobridge) linked to the one end of the rhombic arm which mainly acts as the inductance and the resistance by the circuit analysis. Moreover, the main hotspots associated with the two resonant bands are spatially superimposed, enabling boosting up the local field for both bands by multiscale coupling. With large field enhancements, multiband detection with high sensitivity to a monolayer of molecules is achieved when using SEIRA. Our work provides a new strategy possible to activate high-order modes for designing multiband MIRAs with both nanobridges and nanogaps for such MIR applications as multiband SEIRAs, IR detectors, and beam-shaping of quantum cascade lasers in the future.