A new type of ultra-broadband microstructured fiber sensor based on surface plasmon resonance

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<sup>–7</sup> RIU, respectively. Hence, our proposed SPR-based MF sensor can be applied to environmental monitoring, biomolecular analyte detection and chemical detection.