Tunable distributed SPR sensors with symmetrical structures excited by planar waveguides

Abstract

Measurement ranges of the traditional surface plasmon resonance (SPR) sensors, which use intensity detection method and are excited by waveguides, are limited by the equivalent refractive index of the excited waveguide, not only makes it non-adjustable but also difficult to realize multi-range detection. To solve the above problems, tunable distributed SPR sensors with symmetrical structures excited by planar waveguides are designed. By studying several metrics of a metal-medium-metal SPR sensing structure exited by a planar waveguide, the influence of a metallic material’s thickness, and the medium thickness between the gold films on the measurement characteristics, are accordingly optimized to design the detection system. Double attenuation peaks can be generated under certain conditions, which embodies distributed sensing characteristics. Here, changing the resonance condition by changing the thickness of the measured medium between the metal films makes the measured refractive index range tunable. The planar waveguide with the Fermi refractive index distribution is prepared and the symmetrical sensing structures are arranged longitudinally alongside the direction of light propagation to achieve multiple attenuation peaks, and thus distributed sensing in different measured ranges is realized. The structure of different medium thicknesses excited by planar waveguides is used to test the honey solution with refractive indices varying between 1.33299 and 1.53246 in exciting the surface plasma waves with double attenuation peaks. For any refractive index range, the linear correlation coefficients are all above 0.99232, which demonstrates that the refractive index detection can be carried out within multiple ranges. This result constitutes a theoretical and practical basis for tunable distributed online measurement of the intensity SPR sensors.