Infrared Perfect Ultra-narrow Band Absorber as Plasmonic Sensor

Abstract

We propose and numerically investigate a novel perfect ultra-narrow band absorber based on a metal-dielectric-metal-dielectric-metal periodic structure working at near-infrared region, which consists of a dielectric layer sandwiched by a metallic nanobar array and a thin gold film over a dielectric layer supported by a metallic film. The absorption efficiency and ultra-narrow band of the absorber are about 98 % and 0.5 nm, respectively. The high absorption is contributed to localized surface plasmon resonance, which can be influenced by the structure parameters and the refractive index of dielectric layer. Importantly, the ultra-narrow band absorber shows an excellent sensing performance with a high sensitivity of 2400 nm/RIU and an ultra-high figure of merit of 4800. The FOM of refractive index sensor is significantly improved, compared with any previously reported plasmonic sensor. The influences of structure parameters on the sensing performance are also investigated, which will have a great guiding role to design high-performance refractive index sensors. The designed structure has huge potential in sensing application.

Keywords: Absorption; Metamaterials; Optical sensing and sensors; Plasmonics.

Fig. 1

a Schematic of one unit cell in the proposed structure. b The cross section of the designed structure parameters

Fig. 2

a The reflection and absorption spectra of the proposed structure under different polarization configurations. b The magnetic field distribution and c the electric field distribution at the resonant wavelength of the structure. d The comparison chart of absorption spectrum between the designed structure and metallic grating structure. e The magnetic field distribution and f the electric field distribution at the resonant wavelength of the metallic grating structure. g Simulated absorbance spectra when the damping constant of the gold film is two and three times that of bulk gold

Fig. 3

a, b The dependence of reflective spectra of the designed structure on the index of dielectric. c The resonance wavelength as a function of dielectric spacer thickness t ₂. d Reflectivity of the resonance dip and FWHM as functions of dielectric spacer thickness t ₂

Fig. 4

The reflectance spectrum as a function of top layer nanobar a thickness t ₁, b width w, and c the distance between two top layer nanoribbons d, respectively. The refractive index of the surrounding environment is set as 1.02

Fig. 5

Reflectivity of the resonance dip and FWHM as functions of top layer nanobar a thickness t ₁, b width w, and c the distance between two top layer nanoribbons d, respectively. The refractive index of the surrounding environment is set as 1.02

Fig. 6

Schematic of the equivalent LC circuit for the structure shown in Fig. 1

Fig. 7

a-c Simulated reflective spectra of the sensor with different refractive index of environment. d Resonant wavelength of the sensor as a function of the surrounding refractive index

Fig. 8

FOM and FOM* as functions of top layer nanobar a thickness t ₁, b width w, and c the distance between two top layer nanoribbons d, respectively