# Microchannel-Embedded D-Shaped Photonic Crystal Fiber-Based Highly Sensitive Plasmonic Biosensor
## 4 The Effect of Structural Parameters  
Since the thickness of the plasmonic layer controls the mode coupling between the core and SPP mode, the performance of the sensor is affected by this parameter, as shown in Figure 5. Loss spectra of analyte RIs 1.34 and 1.35 are shown in Figure 5a for the gold thickness of 30, 40, and 50 nm. As the gold thickness increases, the peak loss component decreases due to the decrease in the depth of penetration of the electric field in the metal layer. For example, peak losses are 7.10, 4.42, and 2.36 dB/cm for the gold thickness of 30, 40, and 50 nm, respectively, in the case of RI = 1.35. The change in loss spectra affects the amplitude sensitivity and FWHM as shown in Figure 5b in the case of analyte RI of 1.34. From Figure 5b, it is seen that amplitude sensitivity reaches a maximum for the gold thickness of 40 nm. However, the FWHM is found to be minimum for the gold thickness of 40 nm. The change in gold thickness hardly affects the wavelength sensitivity; therefore, FOM (i.e., FOM = 25.21 in the case of analyte RI = 1.34) must have the highest magnitude at the gold thickness of 40 nm. To determine the effect of thickness of the adhesive layer on the sensing performance, loss spectra are shown in Figure 6a for analyte RI of 1.33, 1.34, and 1.35. The penetration of electrical waves decreases with the increase in metal thickness. Therefore, in all cases of analyte RIs, the peak confinement loss increases with the decrease in the adhesive layer thickness. For example, the peak losses are 4.16, 4.22, and 4.62 dB/cm for analyte RI = 1.35 in the case of TiO\({}_{2}\) thicknesses of 8, 6, and 4 nm, respectively. The amplitude sensitivities also altered due to the change in loss spectra. However, the effect of the TiO\({}_{2}\) layer on the sensitivity is not significant. For example, for analyte RI of 1.34, the maximum amplitude sensitivities are 143.04, 145.89, and 146.21 RIU\({}^{-1}\) for the TiO\({}_{2}\) layer of 4, 6, and 8 nm. Figure 5: Effect of gold layer thickness: (**a**)loss spectra for analyte RI of 1.34, and 1.35, and (**b**)amplitude sensitivity and FWHM as a function of gold layer thickness for analyte RI = 1.34. Figure 6: Effect of adhesive layer thickness: change in (**a**) loss spectra for analyte RI of 1.33, 1.34, and (**b**) amplitude sensitivity with the increase in TiO\({}_{2}\) layer for analyte RI = 1.34. To investigate fabrication tolerance, the dependence of the sensing parameters is examined by varying all the structural parameters of the PCF sensor within \(\pm 10\%\) of the optimized value. Figure 9a,b shows loss spectra and amplitude sensitivity, respectively, for the change in pitch. All air hole diameters depend on the pitch, so a reduction in pitch results in the opening of the leakage path. Therefore, a reduced pitch leads to an increment of confinement loss and vice versa. For example, as the pitch is reduced by \(-10\%\), the peak loss increases by about \(40\%\). Since the resonant wavelength remains unchanged, the wavelength sensitivity stays unaffected by the change in pitch. However, the amplitude sensitivities are altered significantly by the alteration in pitch. For example, the amplitude sensitivity increases by \(1.48\%\) for the increase in pitch by \(+10\%\). Another structural parameter \(\mathrm{d_{s}}\) plays an important role in the sensing performance, as shown in Figure 9c,d. The air hole diameter \(\mathrm{d_{s}}\) regulates the evanescent wave that interacts with the plasmonic material. The smaller size of ds leads to a larger leakage path and more light to interact with the analyte. Thus, the energy transfer increases from the fundamental modes to the plasmonic modes for the reduction in ds. For example, the peak loss at the resonant wavelength rises by \(36\%\) from the optimum due to the \(-10\%\) decrease in \(\mathrm{d_{s}}\). Additionally, the amplitude sensitivity increases by only \(0.72\%\) for the \(-10\%\) decreases in \(\mathrm{d_{s}}\).