A Refractive Index Sensor Based on H-Shaped Photonic Crystal Fibers Coated with Ag-Graphene Layers

Abstract

An Ag-graphene layers-coated H-shaped photonic crystal fiber (PCF) surface plasmon resonance (SPR) sensor with a U-shaped grooves open structure for refractive index (RI) sensing is proposed and numerically simulated by the finite element method (FEM). The designed sensor could solve the problems of air-holes material coating and analyte filling in PCF. Two big air-holes in the x-axis produce a birefringence phenomenon leading to the confinement loss and sensitivity of x-polarized light being much stronger than y-polarized. Graphene is deposited on the layer of silver in the grooves; its high surface to volume ratio and rich π conjugation make it a suitable dielectric layer for sensing. The effect of structure parameters such as air-holes size, U-shaped grooves depth, thickness of the silver layer and number of graphene layers on the sensing performance of the proposed sensor are numerical simulated. A large analyte RI range from 1.33 to 1.41 is calculated and the highest wavelength sensitivity is 12,600 nm/RIU. In the linear RI sensing region of 1.33 to 1.36; the average wavelength sensitivity we obtained can reach 2770 nm/RIU with a resolution of 3.61 × 10^-5 RIU. This work provides a reference for developing a high-sensitivity; multi-parameter measurement sensor potentially useful for water pollution monitoring and biosensing in the future.

Keywords: H-shaped optical fiber; graphene; liquid refractive index; photonic crystal fibers; surface plasmon resonance.

Figure 1

Schematic of the designed Ag-Graphene coated PCF-SPR sensor.

Figure 2

Dispersion relations and electric field distribution of core-guided modes and the plasmon mode with n_a = 1.33. (a) x-polarized core mode at λ = 500 nm; (b) y-polarized core mode at λ = 500 nm; (c) y-polarized core mode at λ = 590 nm (phase matching point); (d) x-polarized core mode at λ = 615 nm (phase matching point); (e) Plasmon mode at λ = 650 nm.

Figure 3

(a) Loss spectra of x-polarized and y-polarized with analyte RI = 1.33 and 1.34; (b) Amplitude sensitivity of x-polarized and y-polarized.

Figure 4

(a) Loss spectra of the designed sensor when n_a varies from 1.33 to 1.35; (b) Relationship between resonance wavelength and RI in the range of n_a from 1.33 to 1.41.

Figure 5

(a) Loss spectra of the designed sensor when d_c/Λ = 0.3, 0.4, 0.5 and 0.6 with n_a = 1.33; (b) Relationship between resonance wavelength and RI when d_c/Λ = 0.3, 0.4, 0.5 and 0.6 in the range of n_a from 1.33 to 1.41.

Figure 6

(a) Loss spectra of the designed sensor when d₃/Λ = 0.6, 0.7 and 0.8 with n_a = 1.33; (b) Amplitude sensitivities of the proposed sensor when d₃/Λ = 0.6, 0.7, 0.8 with n_a = 1.33; (c) Loss spectra of the designed sensor when H changed with n_a = 1.33; (d) Amplitude sensitivities of the proposed sensor when H changed with n_a = 1.33.

Figure 7

(a) Loss spectra of the designed sensor when t_Ag = 20, 30, 40 and 50 nm with n_a = 1.33; (b) Amplitude sensitivities of the proposed sensor when t_Ag = 20, 30, 40 and 50nm with n_a = 1.33; (c) Loss spectra of the designed sensor when L = 9, 12 and 15 with n_a = 1.33; (d) Amplitude sensitivities of the proposed sensor when L = 9, 12 and 15 with n_a = 1.33.