Highly sensitive metal-insulator-metal plasmonic refractive index sensor with a centrally coupled nanoring containing defects

A highly sensitive metal-insulator-metal plasmonic filter with a centrally coupled ring resonator containing silver nanorod defects is numerically investigated by using the finite element method and theoretically characterized by the temporal coupled-mode theory. The proposed structure can function as a plasmonic refractive index sensor compact in design with higher sensitivity and figure of merit (FOM). Simulation results reveal that the presence of silver nanorod defects in the ring resonator will significantly affect the sensor performance, which provides a tunable way to enhance the sensitivity and FOM. We found that the transmittance peak of plasmon can be controlled at constant particle size referring to its counterpart via its defect-change. The calculated refractive index sensitivity (i.e. the change in refractive index in the waveguide slot to the shift in the transmittance peak) is 3500 nm RIU-1 (RIU is refractive index unit), which remarkably enhances the sensitivity of 133.33% compared to its regular case. The central-coupling method used in this work shows the noticeably high sensitivity and FOM compared to its counterpart with the side-coupling method. For practical application, the modulation of transmittance (resonance) peaks of the proposed structure can be realized in a broad wavelength range, which may have promising applications in the on-chip plasmonic sensing and optical communication fields.