Study of a symmetrically structured SPR sensor and its dual-wavelength differential method

This study investigates a novel structure for surface plasmon resonance sensing and its dual-wavelength differential method. The surface plasmon wave is excited by a planar waveguide, which is prepared through the ion-exchange method. The distribution of refractive indices is fitted by the Fermi function. The sensing structure is based on a symmetric structure with a metal layer, a measured medium, and another metal layer. The condition for refractive index matching changes with the thicknesses of test samples, thus the test range can be adjusted using this structure. Given two appropriate wavelengths λ1 and λ2 for detection by the intensity method and an increase in the refractive index, the intensity variety at λ1 can be positive, whereas that at λ2 can be negative. When the refractive indices are determined based on differential values, sensitivity is improved. Solutions with refractive index values ranging from 1.33 to 1.428 are detected in the experiments using the single-wavelength method and the dual-wavelength differential method. Results show that the differential detection method enhances the adjustability and sensitivity of the SPR sensor in combination with a symmetric structure.