Hybrid plasmonic–photonic multi-Fano resonance refractive index sensor using periodic gold nano-bar assisted microcavity coupling

In this work, a multi-Fano resonance optical refractive index sensor based on micro-cavity–microstructure coupling is numerically investigated. The proposed structure integrates periodic gold nano-bars with a dielectric micro-cavity to form a hybrid plasmonic–photonic platform capable of supporting multiple sharp resonances. The reflection spectra are systematically analyzed by varying key geometrical parameters, including the nano-bar radius, nano-bar length, and cavity thickness. The resonance wavelength shifts exhibit excellent linear dependence on the surrounding refractive index, and the sensitivity is extracted from the slope of the linear fitting curves. The results demonstrate strong performance tunability through geometric optimization. Among the supported modes, the highest sensitivity reaches 975 nm/RIU at (Mode 5), while the narrowest linewidth leads to a maximum figure of merit (FOM) of 139.28 at (Mode 5). Electric field distributions confirm the formation of hybrid plasmon–cavity modes responsible for the multi-Fano response. Owing to its high sensitivity, ultra-high FOM, and multi-mode capability, the proposed sensor shows strong potential for high-resolution biochemical and environmental refractive index sensing.