Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61765003), the Natural Science Foundation of Jiangxi Province, China (Grant No. 20181BAB202029), and the Graduate Innovation Foundation of Gannan Normal University, China (Grant No. YCX19A043)
Received Date:06 September 2020
Accepted Date:24 November 2020
Available Online:24 March 2021
Published Online:05 April 2021
Abstract:Microstructured fiber (MF) sensors based on surface plasmon resonance (SPR) have been widely investigated because they have many merits including high sensitivity, label-free and real-time detection and so on, thus they possess extensive applications such as in food safety control, environmental monitoring, biomolecular analytes detection, antibody-antigen interaction, liquid detection and many others. However, most of reported SPR-based MF sensors can only work in the visible or near-infrared wavelength region. Hence, the investigation of high-performance mid-infrared SPR-based MF sensors is a challenge task. In this paper, with the aim of overcoming the above limitation, a new type of high-sensitivity SPR-based MF sensor coated with indium tin oxide (ITO) layer is proposed. The proposed sensor can work in both the near-infrared and mid-infrared wavelength region. Benefitting from its two-core and single analyte channel structure, our proposed sensor can effectively eliminate the interference among neighboring analyte channels, improving its signal-to-noise ratio, and achieving high-sensitivity detection in ultra-broadband wavelength range. By using the full-vector finite method with the PML boundary conditions, the sensing properties of our proposed sensor are numerically studied in detail. The numerical results show that the resonance wavelength of the proposed sensor shifts toward a long wavelength region as the refractive index of analyte increases from 1.423 to 1.513, and a similar phenomenon can be found if the thickness of the ITO layer increases from 40 nm to 60 nm. Nevertheless, the wavelength sensitivity of the proposed sensor decreases with the increase of the diameter of the hole located in the fiber core region. On the other hand, when the refractive index of analyte varies in a large range of 1.423–1.513, the proposed sensor can operate in an ultra-broad wavelength range of 1.548–2.796 μm, and the average wavelength sensitivity is as high as 13964 nm/refractive index unit (RIU). Moreover, the maximum wavelength sensitivity and refractive index resolution increase up to 17900 nm/RIU and 5.59 × 10–7 RIU, respectively. Hence, our proposed SPR-based MF sensor can be applied to environmental monitoring, biomolecular analyte detection and chemical detection. Keywords:fiber optics/ microstructured fiber/ surface plasmon resonance/ sensor
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2.传感器结构设计我们所设计的SPR-MF传感器的结构如图1所示. 该双芯MF包含3层按正三角形晶格排列的空气孔, 其晶格常数为Λ, 空气孔直径为d2. 光纤的两个纤芯位于第2层空气孔的相应位置, 并分别标记为1和2. 为了能够更好地调节纤芯基模的模有效折射率, 使传感器能够在更宽的波长范围内进行工作, 我们在纤芯的中心区域引入了2个填充待测样品的小孔, 其直径为d3. 此外, 为了提高传感器的信噪比和消除样品通道间的串扰, 该光纤的中心区域只有1个直径为d1, 内表面镀ITO(厚度为t)的大圆形空气孔用来作为待测样品的传感通道. na为待测样品的折射率, 其值在1.423—1.513之间; 空气的折射率为1. 光纤的衬底材料为SiO2, 其折射率由Sellmeier方程计算可得[13]: 图 1 SPR-MF传感器的横截面示意图 Figure1. Cross section of the proposed multi-core PCF sensor based on SPR.