Figure 2 shows the dispersion relationship between the y-polarized fiber core mode and SPP mode when the refractive index of the object to be measured is n = 1.26, as well as the loss spectrum of y-polarization, and the electric field energy diagram is shown in the inset of the figure. The red curve is the y-polarization loss, and the blue and black curves are the real part of the effective refractive index of the SPP mode and the real part of the effective refractive index of the fundamental mode, respectively. From the figure, it can be found that when the wavelength increases from 1300 nm to 1600 nm, the real part of the effective refractive index of the fundamental mode and the real part of the effective refractive index of the SPP mode intersect at 1472 nm, at which time the phase matching occurs and the loss reaches the maximum of 113.1 dB/cm, and the energy is transferred from the fiber core to the surface of the gold nanofilm, forming a resonant coupling.
Figure 3(a) shows the confinement loss spectra of the sensor with different thickness gold nanofilm when the refractive indices are 1.26 and 1.32, and Fig. 3(b) shows the FOM corresponding to different gold nanofilm thicknesses. Because the gold nanofilm directly contact the object to be measured, the gold nanofilm have a great influence on the performance of the sensor. From Fig. 3(a), it can be found that when the gold nanofilm thickness t increases from 40 nm to 70 nm, the loss peak shifts toward the short wavelength direction and the resonance intensity changes less. This is because the gold nanofilm is too thick, which will cause the evanescent wave to be difficult to penetrate, resulting in a weak coupling strength, making the sensor sensitivity and loss decrease; conversely, the gold nanofilm is too thin, and the SPP mode will be suppressed due to radiation damping, and the manufacturing cost will be increased. From Fig. 3(b), it can be found that the FOM between the two is close when t = 50 nm and t = 70 nm. As the gold nanofilm is too thick will affect the sensitivity of the sensor, combined with Fig. 3(a) and Fig. 3(b) the gold nanofilm thickness t = 50nm selected in this paper.
Figure 4(a) shows the confinement loss spectra for different air hole diameters(d2) at refractive indices of 1.26 and 1.32, and (b) shows the FOM corresponding to different air hole diameters. From Fig. 4(a), it can be found that when the air hole diameter d2 increases from 2.0µm to 2.4µm, the loss peak gradually decreases and moves toward the long wavelength direction, and the resonance intensity becomes weaker. This is because with the increase of the air hole diameter, the energy is better confined within the fiber core and cannot be coupled with the surface plasmon excitations generated by the gold nanofilm, resulting in a weak coupling effect between the fundamental mode and the SPP mode, which is manifested by the decrease of the loss of the resonance peak. As can be seen in Fig. 4(b), when d2 is 2.0µm, 2.2µm, and 2.4µm, the corresponding FOMs are 10.359, 10.236, and 10.373, respectively. The highest FOM value is achieved when d2 = 2.4µm. The higher FOM makes the sensor detection more accurate. At the same time, the larger diameter air holes require less processing difficulty compared to small air holes, so the air hole diameter d2 = 2.4µm taken in this paper.
Figure 5 shows the confinement loss spectra for different open-ring diameters (d3) at refractive indices of 1.26 and 1.32. From Fig. 5, it can be found that the loss peak increases with a red shift when the diameter of the open-ring increases from 3.6µm to 4.0µm, and the loss peak increases with a blue shift when the diameter of the open-ring increases from 4.0µm to 4.4µm. This is due to the increase in the diameter of the opening ring leading to the increase in the area of the gold nanofilm, while the distance between the core and the gold nanofilm also gradually decreases, enhancing the coupling effect between the fundamental mode and the SPP mode, which is manifested by the increase in the resonance peak loss.
Figure 6 shows the confinement loss spectra of the proposed sensor at different analyte RI (1.20–1.34). The resonance wavelength is red-shifted as the refractive index of the object to be measured increases. This is because the refractive index of the object to be measured increases, changing the real part of the effective refractive index of the SPP mode. This leads to a gradual increase in the phase-matching wavelength between the fundamental mode and the SPP mode, enhancing the coupling effect and increasing loss. The loss reaches the maximum of 465.05 dB/cm when the refractive index of the object to be measured is 1.34. The corresponding resonance wavelengths when the refractive index of the object to be measured is increased from 1.20 to 1.34 are 1392 nm, 1411 nm, 1438 nm, 1472 nm, 1510 nm, 1556 nm, 1610 nm, and 1828 nm, respectively. Combining with Eq. (4), we can the maximum wavelength sensitivity of the sensor is 10900nm/RIU, and the sensor resolution is 9.17×10− 6 from Eq. (5).
Figure 7 shows the FOM corresponding to different refractive indices of the substances to be measured. FOM is a sensor parameter that estimates the sensing capacity of the sensor by analyzing the change in refractive index of the substance to be measured. Therefore, FOM is also an important sensing indicator. From Fig. 6, we can see that with the increase of the refractive index of the analyte, the wavelength sensitivity increases gradually, the loss spectrum shifts to the long wavelength direction, the loss curve narrows, the FWHM decreases, and the corresponding FOM increases, further reflecting the excellent sensing performance of the sensor. The highest value of FOM is 46.2RIU− 1.
Table 1 compares the performance of the proposed MOF-SPR sensor with that of the existing MOF-SPR sensors [24–26]in similar refractive index detection ranges. It can be seen that the proposed sensor has a higher wavelength sensitivity and a lower resolution in the similar detection range.
Table 1
Comparison of the proposed SPR-PCF sensor with existing SPR-PCF sensors
Reference
|
Detection
Range
|
Wavelength
range (nm)
|
Max RI
Sensitivity
(nm/RIU)
|
Resolution
(RIU)
|
14
|
1.18–1.30
|
1200–1800
|
9100
|
1.10×10− 5
|
17
|
1.25–1.30
|
500–1000
|
1932
|
5.18×10− 5
|
24
|
1.00-1.29
|
1600–2400
|
10000
|
1.00×10− 5
|
25
|
1.20–1.34
|
1600–2350
|
6100
|
2.63×10− 5
|
26
|
1.21–1.49
|
1000–1600
|
5200
|
1.92×10− 5
|
This senor
|
1.20–1.34
|
1300–1900
|
10900
|
9.17×10− 6
|