New characteristics of a resonant coupling between an analyte-filled core mode and a supermode of a liquid-core photonic crystal fiber based plasmonic sensor

A new fiber optic sensor recently proposed for sensing an analyte with a large refractive index, such as benzene, is investigated using a finite element method in order to optimize its sensitivity. The device exploits the resonant coupling between some modes in a microstructured optical fiber made by a SiO2 rod with several holes, some filled with air, some filled with a liquid analyte, and a central hollow core surrounded by a gold layer and filled with the analyte as well. The structure presents three resonant frequencies, at λ = 0.7105 μm, λ = 2.611 μm and λ = 1.094 μm, depending on the hollow core radius and the different couplings between guided and polariton modes. The first resonant coupling, at λ = 0.7105 μm, is due to a perfect phase matching condition between a mode in the analyte and a supermode; its advantages are a small value of the full width at half maximum (8.6 nm), and a high value of the signal-to-noise ratio (0.29). The second resonant coupling, at λ = 2.611 μm, is due to a loss matching condition between a supermode and a plasmon mode, and its main advantages are large spectral sensitivity and sensor resolution (1.3 × 10−6 RIU). This resonance has also some disadvantages due to a very large value of the full width at half maximum (102 nm for a symmetric line shape), and a small value of the signal-to-noise ratio (0.13). The third resonant coupling is at λ = 1.094 μm and is related again to a phase matching condition between a supermode and a plasmon mode, but for a smaller value of the radius of the central core filled with the analyte. Its advantages are a high value of the amplitude sensitivity (5741.2 RIU−1) and a better value of the sensor resolution (1.74 × 10−6 RIU). The major disadvantages of this structure are a small value of the shift (1.0 nm) towards longer wavelengths of the phase matching point for an increase of the analyte refractive index by 0.001 RIU, a small value of the signal-to-noise ratio (0.03) and a small value of the spectral sensitivity (1000 nm RIU−1).