Ultra-narrowband near-infrared tunable two-dimensional perfect absorber for refractive index sensing.

An ultra-narrow-bandwidth near-infrared perfect plasmonic absorber with a periodic structure composed of metal-insulator-metal configuration is numerically designed and analyzed for a refractive index sensor. A perfect absorptivity of 99.99% and a rather narrow bandwidth of 2 nm are observed in the near-infrared wave band when the light is vertically incident in the structure. The ultra-high absorption and ultra-narrow bandwidth are explained as local surface plasmon resonance stimulated in the structure. The absorber is put into use as a plasmonic refractive index sensor with wavelength sensitivity as high as 1500 nm/RIU and figure of merit as high as 750, which significantly surpass those of many plasmonic sensors proposed in the past few decades. The designed absorber is tunable by changing geometry parameters. Because of the excellent sensing properties mentioned above, the proposed structure can be widely applied in fields such as photodetectors, biosensors, and chemical molecule detection.