Nanoscale Refractive Index Sensors with High Figures of Merit via Optical Slot Antennas.

Nanoscale refractive index (RI) sensors based on a single nanorod or nanoantenna typically suffer from a low figure of merit (FOM) due to the large full width at half-maximum of the plasmonic dipole resonance. Here, we demonstrate nanosensors with a high FOM and a sensing volume that is much smaller than λ3 using slot antennas. Two configurations, one based on a bowtie slot antenna (BSA) and one based on a slot antenna pair (SAP), are proposed. The RI information is obtained from the extinction dip that is due to the interference of surface plasmon polaritons (SPPs), which are launched at different nodes of a third-order resonant mode of the BSA or different antennas of the SAP. The high FOM is attributed to the dependence of the extinction spectrum on both the amplitude and the phase of the SPPs. There are important applications for these nanosensors, which can measure the local RI beyond the diffraction limit and can be flexibly integrated.