We propose a novel and compact plasmonic sensing structure based on a metal-insulator-metal (MIM) waveguide with a side-coupled hexagonal cavity. The sensing structure has been numerically and theoretically investigated using the finite-difference time-domain (FDTD) method and temporal coupled-mode theory. The numerical simulation results show that the resonance dips of the structure have a high resonant transmission contrast ratio and that the resonance wavelengths have a near-linear relationship with the refractive index of the dielectric material in the cavity. The numerical simulation results obtained from the transmission spectra are used to analyze the sensing characteristic of the structure.
The effects of the geometrical parameters on the transmission and sensing characteristics of the structure are analyzed in detail. The sensitivity can be tuned to a value as high as 1562.5 nm per refractive-index unit (RIU) with a high figure of merit of ~38.6 RIU-1 around the resonance wavelength of 1550 nm using the novel structure and by optimizing the structural parameters. In addition, the temperature-sensing characteristic of the structure based on the refractive-index sensor is also discussed in this paper. The proposed structure may potentially be applied in optical networks-on-chip and on-chip nanosensors.