Maximizing spectral sensitivity of plasmonic photonic crystal fiber sensor

We propose a novel design for a plasmonic photonic crystal fiber (PCF) sensor. Our design includes a metal film incorporated within the PCF structure. Compared to previously discussed sensing configurations, the metal surface does not come in direct contact with the sample being tested in our design. The metal film serves to provide absorption due to the excitation of surface plasmon-polaritons (SPP). This absorption is enhanced when a PCF-guided mode resonantly couples to the SPP, or in other words when phase matching between the mode and the SPP is satisfied. We consider a configuration where a hollow PCF core is filled with a liquid sample, while the metal surfaces are kept tens of microns away within the PCF cladding. Our sensor detects small changes in the sample's refractive index, over a broad wavelength range, by taking advantage of the refractive index sensitivity of the resonance/phase-matching condition. We model the sensor and characterize its performance by using the COMSOL environment based on the finite element method. We evaluate and compare three elements: gold, silver, and copper, in order to increase the absorption at particular wavelengths. We also vary the thickness of the metal layer and optimize it to enhance the sensitivity. The results show a performance that exceeds a minimal sensor resolution over an ultra-broad spectral range while maintaining a reasonable amplitude sensitivity.