High contrast and sensitive near-infrared refractive index sensors based on metal-dielectric-metal plasmonic metasurfaces

We report a numerical study of metal-dielectric-metal (MDM) plasmonic metasurface-based refractive index sensors that have high contrast and sensitivity in the near-infrared region. The MDM plasmonic metasurface is formed by stacking up silver (Ag) subwavelength disk array on thin silica (SiO2) spacer-layer and Ag film (acting as a reflector) on a silicon substrate. The MDM plasmonic metasurfaces with various heights, Ag disk diameters and SiO2 spacer-layer thicknesses are designated and numerically investigated using a finite-difference time-domain method. As results have shown, the optical properties of the device are weakly dependent on the height of Ag disks, but strongly dependent on the size of Ag disks and SiO2 spacer-layer thickness. By looking at the field distribution at the resonance, the strong confinement of incident light localized inside the SiO2 spacer-layer, thus extremely low reflection closed to 0%, corresponding to the absorption of up to 100% and high quality-factor (Q-factor) of ∼180 have been achieved. Because of operating in the near-infrared range, the proposed MDM plasmonic metasurface has low Ohmic loss, and then the refractive index biosensor based on that shows the figure-of-merit (FOM) to have the contrast and selectivity higher than that of other established MDM plasmonic biosensors. The sensitivities of 1109 nm/RIU and 1290 nm/RIU according to FOMs of 38.8 and 95.3, which correspond to the vapor and liquid sensors, respectively, have been achieved. Our systematic investigation provides useful guidelines for designing MDM plasmonic metasurface sensing devices.