Refractive Index Sensing Based on Plasmonic Waveguide Side Coupled With Bilaterally Located Double Cavities

Based on a metal-dielectric-metal (MDM) plasmonic waveguide side coupled with a single cavity, we propose a resonator system by placing double cavities on each side of the MDM waveguide. The coupling between two cavities provides an additional degree of freedom to engineer the resonant transmission response. By decomposing the compound cavity modes into two decoupled resonances, we obtain the theoretical transmission response of the resonator system and optimize the geometric parameters to improve the performance of refractive index sensing. At last, we conduct numerical experiments on the theoretically optimized resonator system and find that the resonant wavelength is linearly related with the refractive index of the material filled in the cavities and waveguide. A high sensitivity of 1030 nm/RIU (refractive index unit) is realized in a nanoscale structure (several hundred nanometers). This refractive index sensing configuration is easy to be integrated into lab-on-a-chip and find potential application in plasmonic circuits.