High Sensitivity Refractive Index Sensing Of D-shaped Photonic Crystal Fiber Based On Surface Plasmon Resonance

Photonic crystal fiber has attracted the attention of researchers in sensing field because of its flexibility in structure design.Photonic crystal fiber sensor has the advantages of anti-electromagnetic interference,remote on-line detection,and has a wide application prospect in biochemistry,environmental monitoring,food safety,medical diagnosis and other fields.The surface plasmon resonance sensing technology can realize the performance of free labeling,real-time detection and high sensitivity.The photonic crystal fiber sensing technology based on the mechanism of surface plasmon resonance is expected to further improve the sensitivity by combining the advantages of both.In recent years,the sensitivity of sensing detection is increasingly required.The use of sensitizing materials to improve the sensing performance of photonic crystal fiber based on surface plasmon resonance has important research value in the field of high sensitivity sensing.In this paper,a novel D-shaped photonic crystal fiber sensing structure based on the surface plasmon resonance effect has been proposed.The cladding air holes of the proposed sensing structure are arranged in a hexagonal shape.The two air holes nearest to the metal layer in the first layer of cladding are reduced to provide an efficient channel for the leakage of light wave energy.The two air holes second nearest to the metal layer in the same layer of cladding are enlarged to promote the coupling between the core mode and the surface plasma mode.Gold is used as the plasma material deposited on the partially removed cladding of the D-shaped PCF.By the finite element method,the optical properties of the proposed PCF-SPR sensing structure are investigated theoretically.The influence of structural parameters such as large air hole diameter D,small air hole diameter ds,pitch distanceΛ,polishing depth h and gold film thickness t_gon the sensing characteristics of the structure has been discussed.After optimization,the average refractive index sensitivity of the sensing structure is 24667 nm/RIU in the refractive index range of 1.39-1.42,with the linearity degree of 0.941.Based on the above sensing structure,Ti₃C₂T_x-MXene is used as a plasma sensitizing material to form a hybrid layer with the gold film.The sensitivity of the sensing structure is further improved by enhancing the interaction between the incident light,the plasma material and the target tracking molecule.The finite element method is used to analyze the optical properties of the proposed PCF-SPR sensing structure numerically.The influence of structural parameters such as large air hole diameter D,small air hole diameter ds,pitch distanceΛ,polishing depth h,gold film thickness t_gand the number of Ti₃C₂T_x-Mxene layers l_mon the sensing characteristics of the structure has been discussed.According to the optimized calculation,the average sensitivity of the sensing structure is51133 nm/RIU,the maximum sensitivity is up to 74600 nm/RIU and the linearity is 0.978 in the range of 1.39-1.42.The average sensitivity of the sensing structure is about 2.19 times higher than that of the structure with only gold film.Finally,the sensing structure has been applied to human immunoglobulin sensing,and the sensitivity of the concentration is calculated to be 9.98nm/（mg/m L）in the concentration range of 290-450 mg/m L,which has potential application in the field of biosensing.