Design and Analysis of D-shaped Fiber Optic Plasmonic Sensors using Planar and Grating Structure of Silver and Gold

In this paper, a D-shaped optical fiber plasmonic sensor using planar and grating structures of silver and gold metals is simulated using the finite element method under the wave optics module of COMSOL Multiphysics. Performance defining parameters: i) based on transmittance curve, viz. resonance wavelength (λr), shift in resonance wavelength (Δλr), minimum transmittance (Tmin), and bandwidth (BW), ii) based on electric field distribution of surface plasmon wave, viz. penetration depth (PD), and propagation length (PL) are obtained for the considered sensor structures. It is found that gold gives wider BW than silver (e.g., at 1.39 refractive index of sample: 480% for planar case and 241% for grating case), which deteriorates the sensor performance by degrading the detection accuracy. However, gold gives higher Δλr than silver (at 1.40-1.39 = 0.01 change in refractive index of sample: 18.33% for planar case and 16.39% for grating case) which improves the sensor performance and enhances the sensitivity. Grating slightly increases the BW and Δλr for both gold and silver. Further, with respect to silver, the sensor that contains gold demonstrates higher PD (e.g., 22.32% at 1.39 refractive index of sample for planar case) and lower PL (e.g., 22.74% at 1.39 refractive index of sample for planar case). Grating increases the PD (e.g., 10% for silver at 1.39 refractive index of sample), whereas it decreases the PL (e.g., 8.73% for silver at 1.39 refractive index of sample). Lower PL signifies the localization of the field, whereas higher PD enables the sensor to detect larger molecules. Therefore, the sensor with grating metals provides better sensitivity with reduced detection accuracy for the detection of comparatively larger molecules.