Surface plasmons are a coupling of electromagnetic oscillations generated by electromagnetic waves and free electrons at the interface between metal and dielectrics.^[1] Due to the sensitivity of surface plasma to the surrounding refractive index, the surface plasmon resonance (SPR)-based optical sensing of several quantities such as chemical, temperature, pressure, force, environmental monitoring, optical fluidic detection, food safety, and biological species have proved to be advantageous.^[2] Because the SPR-based technology can implement the promising and powerful optical detection of such as its high sensitivity, real-time sensing, and unmarked biomolecules, SPR has been used to study many different biomedical applications.^[3–5] With the development of miniaturized devices based on optical fibers, the shortcomings of traditional devices have been overcome. The SPR-based fiber optic plasmonic sensors (FOPS) have been very attractive for miniaturized sensors.^[6] Therefore, it is necessary to develop excellent new biochemical sensors in the FOPS field.

A lot of researches on SPR-based fiber sensors have been reported. In 2017, Rifat et al. reviewed photonic crystal fiber sensors based on surface plasmon and reported multifunctional optical fiber sensors.^[7] A novel sensing method for simultaneous measurement of seawater temperature and salinity through using C-type micro structured optical fibers was proposed by Zhao et al. in 2018.^[8] Zhang et al. measured hydrogen peroxide and glucose concentrations by using an SPR sensor with a corrosive plasma nanocoating on tilt fiber Bragg grafting.^[9] Yang et al. proposed a highly sensitive hollow fiber temperature liquid filling sensor based on surface plasmon resonance.^[10] Zhao et al. proposed a surface plasmon resonance sensor based on concave photonic crystal fibers for low refractive index detection in 2019.^[11] We have found the application of FOPS in medical diagnosis, but it is still difficult to identify specific molecules in molecular groups.^[6] With the development of SPR technology, the interface between metal and dielectric film could be deposited on the surface of optical fibers.^[12] However, in these studies only the SPR effect of a single precious metal was used to explore the change in refractive index. Considering different plasma bands of different metals, multiple signal peaks can be generated during detection to achieve multi-peaks sensing.

Therefore, in the present study, a novel dual-channel sensor is designed based on the flexible characteristics of PCF structure, different plasma bands for Au/Ag, and the necessity of multi-component detection. Owing to the unique structure of the micro structured optical fiber, the sensor can realize on-line detection. The sensor has a unique structure and can realize on-line detection. The numerical results show that the core mode in PCF sensor is affected by the SPR at the Au/Ag interface simultaneously, which makes it possible to monitor multiple substances near the Au/Ag films. Meanwhile, the detection of the positive and negative amplitude sensitivity makes the sensor detection stable and also make certain external disturbances eliminated. The unique open-loop structure can be designed according to the specific molecular size, allowing the specific size of molecules to enter into the structure. These excellent sensing characteristics are of great significance for the applications in environmental monitoring, biochemical sensing, etc. And Our theoretical research has integrated correct theoretical simulation methods for the design of optical fiber sensors.

According to the different plasma wave bands of different metals and polarization demodulation, the sensor was designed as shown in Fig. 1. We designed two open rings and coated Au/Ag films in the vertical and horizontal directions of the microstructured optical fiber, respectively. In the figure, t_u and t_g are the thickness of Au and Ag, respectively, d_u is the diameter of the micro-opening in the vertical analyte channel, d_g the diameter of the micro-opening in the vertical analyte channel, and p the distance between adjacent air holes. The diameters d_g and d_u are 1.2p and p, respectively. In order to make the surface plasma mode generated by the silver film fully coupled with the core mode, the air hole size in the x direction near the core is selected to be 0.3p, and the air aperture size in the y direction near the core is 0.45p. To maintain the characteristics of the single mode transmission of the designed sensor, and take the flexible adjustability of the PCF structure into account, we chose the size of the cladding holes to be 0.45p. The device is fabricated by a general stack and draw method.^[13] As for the polishing of the side-face MOF, the common method, i.e., the optical fiber grinding, such as a very common femtosecond laser or mechanical grinding method can be taken. The metal films can be coated by magnetron sputtering, which is a common method for metal coating.^[14]

Fig. 1.

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	Fig. 1. Schematics of proposed MOF-SPR sensor, showing (a) its cross-section and (b) three-dimensional schematics.

The dispersion relationship of silica can be described by the well-known Sellmeier equation.^[15] The dispersion relation between gold and silver can be described by Drude–Lorentz model.^[16,17] Because the guided loss of the guided mode also greatly influences the signal extraction of the sensor, the limiting loss in the x direction and y direction of the guided mode can be expressed by the following formula:

In the paper, a full-vector finite-element method is used to calculate the PCF sensor. Firstly, in order to detect the sample signal, we analyze the distribution of modal energy changes in the sample when the refractive index is 1.37 (as shown in Fig. 2), showing that the analysis is in accord with the mode coupling theory that the completely coupling occurs on the sample surface.^[18]

Fig. 2.

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	Fig. 2. Reversely crossed plots of effective refractive index versus wavelength.

With the refractive index of the external environment changing, the coupling conditions of core mode and SPP mode will vary due to the sensitivity of SPP mode. And the high loss peak will also change. This property can be applied to the sensor device. Hence, we analyze the loss of the y polarized mode when the refractive index of analyte varies from 1.37 to 1.4 (see Fig. 3). Obviously, as the refractive index increases, the loss peak of the y polarized core mode shifts to the longer wavelength in Fig. 3(a). Multiple peak losses occur in the y direction. At the same time, a new sub-peak appears after the first loss peak has vanished. Further analysis of its energy distribution reveals that the y polarized core mode is affected by SPP mode generated on the Au film and Ag film simultaneously, resulting in another increase of the loss curve that should have decreased, thereby increasing the number of signal peaks. Then the three peaks are fitted, and the results are shown in Figs. 3(b)–3(d).

Fig. 3.

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	Fig. 3. Sensing properties of y-polarized core mode, showing (a) multi-peak shift to long waveband with refractive index of the analyte increasing, variation of resonance wavelength of (b) the first peak, (c) the second peak, and (d) the third peak with effective refractive index.

The sensitivity of the first peak is 409.5 nm/RIU and expressed as ΔP₁. The sensitivity of the second peak is 1550 nm/RIU and expressed as ΔP₂, and the sensitivity of the third peak is 4240 nm/RIU and denoted as ΔP₃. The change of the analyte’s refractive index for the y-polarized core mode can be defined as

Table 1.

Table 1.

Table 1. Comparison of sensitivity between our results and other results from available literature. . Authors Range of RIU Sensitivity/(nm/RIU) Zhen-Kai Fan et al.[19] 1.40 to 1.43 3978 Rahul Kumar Gangwar et al.[20] 1.43 to 1.46 7700 Rifat et al.[21] 1.33 to 1.36 1000 Our work 1.37 to 1.40 4240

Table 1. Comparison of sensitivity between our results and other results from available literature. .

Since the film is coated in both directions, the x-polarized core mode will inevitably produce a similar phenomenon that the SPR coupling conditions change with the external analysis fluid. Considering that, we also analyzed the refractive index increasing from 1.37 to 1.41. Figure 4 shows that the loss of x-polarized core mode shifts to longer wavelengths as the refractive index increases. According to equation S(λ) = Δλ_peak/Δn_a, the slope of sensitivity’s fitting curve can be acquired.

Fig. 4.

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	Fig. 4. Plots of loss of x-polarized core mode versus wavelength for different indices of analyte.

Figure 5 shows that the sensitivity of the sensor can reach 4990 nm/RIU. From the fitting results, we can find that the refractive index becomes higher as the wavelength of the x-polarized mode shift grows faster and the linear correlation decreases. It can be explained as the fact that when the sensitive waveband is located around 2 μm, the gold film in the y-direction starts to react at the same time, and the loss change naturally becomes nonlinear.

Fig. 5.

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	Fig. 5. (a) linear and (b) nonlinear fittings of wavelength versus refractive index.

To more accurately describe the change in refractive index, we apply an exponential fit to a nonlinear fitting as shown in Fig. 5(a). Obviously, the fitting coefficient is improved, and the final fitting formula is Y = y₀ +A₁ · exp((x − x₀)/t₁). The corresponding coefficients are given in the inset of Fig. 5(b). Similarly, the refractive index from the change of the x-polarized core mode can be expressed as

Furthermore, we can use the polarization demodulation method to evaluate the change of the refractive index from the x and y directions. When the light enters into the polarization controller through the 3-dB coupler, it becomes the x-direction polarized light, and then it enters into the designed sensor. Afterward, the signal light goes out of the other end of the 3-dB coupler and ultimately enters into the optical spectrum analyzer (OSA) to gain the signal of x-polarized core mode. Second, the polarization controller is controlled to obtain the polarized light in the y direction entering into the sensor, and the signal is collected by a spectrometer to obtain the signal of y-polarized core mode. Finally, the signals in the x and y directions can be processed according to the following formula: Δn = (Δ n₁ + Δn₂)/2, where Δn₁ and Δn₂ represent the refractive index changes of the x- and y-polarized core modes, respectively, and the average refractive index changes are also calculated.

Overall, the third peak’s amplitude of y-polarized core mode gradually decreases while the signal of x-polarized core mode presents positive growth, and we analyze the amplitude sensitivities of these two signal peaks as shown in Fig. 6. According to the sensing setup mentioned above, we can collect the two orthogonal signals by OSA. Owing to the negative and positive amplitude sensitivity of y-polarized and x-polarized core modes, it can eliminate the external interference and further improve the stability of this system.

Fig. 6.

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	Fig. 6. (a) Linear and (b) nonlinear fitting of loss versus refractive index in a range from 1.37 to 1.4.

In view of the manufacturing process, we analyze the influence of structural parameters on sensing characteristics. Since the air holes in the x direction near the core are relatively small and easily collapse during drawing, the change in x and y sensing characteristics are shown in Fig. 7 when d_x is 0.25p, 0.3p, and 0.35p, separately.

Fig. 7.

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	Fig. 7. Variations of (a) x-polarization loss and (b) y-polarization loss with wavelength for three different values of dx.

In Fig. 7, it can be clearly observed that the loss peak of the x-polarized mode does not shift with d_x value increasing. It demonstrates that the change of d_x has little effect on sensing characteristics of the x-polarized mode as the loss becomes larger.

This phenomenon can be easily explained from the analysis of the cross-section of this structure. When the d_x value becomes smaller, the restriction capability of the core for the x-polarized core mode decreases, and the x-polarized core mode is more easily coupled with the SPP mode on silver film, so the loss becomes larger. In addition, the wavelength of the y-polarized core mode does not shift when the d_x value becomes small. However, the loss in the vicinity of 1.7 μm becomes larger. The reason is that the core mode is affected by SPP mode not only on Au but also on Ag due to the decrease of dx value, then the loss becomes larger. Like Fig. 7, figure 8(a) shows the changes of confined loss with d_y. In the shortwave range, the loss peak has almost no shift because the x-polarized core mode is affected by Ag and also by d_y near Au; the loss curve of x-polarized core mode is almost not affected. By contrast, the sub-peak in Fig. 8(b) is influenced by Au, so the loss of y-polarized core mode changes with the variation of d_y.

Fig. 8.

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	Fig. 8. Variations of (a) x-polarization loss and (b) y-polarization loss with wavelength for different values of dy.

Considering the influence of pressure in drawing process, we analyze the sensing characteristic changes when d_a value varies from 0.44p to 0.45p as shown in Fig. 9. It can be found that the loss peak has almost no shift in the shortwave range, but there is a small shift of the loss peak in the long wave range. The period of the air holes is longer than the light wavelength at the shorter wavelength, so the change of the light wave characteristics caused by the slight change of the light wave to the whole cycle is not very clear.

Fig. 9.

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	Fig. 9. Variations of (a) x-polarization loss and (b) y-polarization loss with wavelength for different values of da.

However, in the long wavelength range, the wavelength is comparable to the periodic size of the crystal structure. Therefore, the changes easily affect the coupling condition between the core mode and the SPP mode, which leads the high loss peak to change.

In addition, the thickness of the gold and silver coating is one of the most important factors that must be considered due to the limit of the actual coating process, thus we explore the changes of the silver film. As shown in Fig. 10, the sensor’s sensing characteristics change with the value of t. When the thickness of the film is 40 nm, a sub-peak appears in the y direction, and the coupling effects of gold and silver in both directions are more obvious. It can be concluded that the optimal thickness of the gold film is 40 nm, and the plasmon resonance is the most remarkable.

Fig. 10.

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	Fig. 10. Variations of (a) x-polarization loss and (b) y-polarization loss with wavelength at analytical solution’s refractive index of 1.37 for different values of t.

Now, comes a question: why can a multi-peak effect occur? We study the loss of the y-polarized core mode at longer wavelengths as shown in Figs. 11(a) and 11(b). Comparing this model, we find that Ag alone produces significant high-loss signal peaks respectively at 1690 nm and 2180 nm for the y-polarized core mode, while no high-loss signal peak appears at 1780 nm. In contrast, in the case of a single coating of the Au film, the y-polarized core mode has a high loss signal peak at 1780 nm. Therefore, we can confirm that the high-loss signal peaks of the y-polarized mode of the proposed sensor are caused by SPP mode generated by Ag film and Au film. How about the characteristic of the x-polarization mode? Here in this paper, the effects of separate Au and Ag films on the x-polarization core mode with Ag film and Au film are compared with each other (see Figs. 11(c) and 11(d)). In the case of only the Ag film, the x-polarized mode produces a high loss peak at 1800 nm and the signal peaks associated well with Au/Ag, while in the case of only the Au, the x-polarized mode appears only around 2280 nm. The apparent high loss range also explains that there is a high loss region near 2280 nm under the combined action of Au and Ag.

Fig. 11.

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	Fig. 11. Comparison between only one metal coated films and Au & Ag coated films, showing variations of y-polarization loss with wave length of (a) only Ag-coated film and (b) only Au-coated film, and variations of x-polarizaton loss with wavelength of (c) only Ag-coated film and (d) only Au-coated film.

In summary, the signal peaks of the sensor are from the coupling interaction of core mode and SPP mode generated by Ag film and Au film simultaneously.

In this paper, we present a sensor based on the multi-component detection of gold and silver SPR. The numerical studies reveal that high-loss multi-peaks vary with the refractive index appearing in different wavebands. The larger the phase difference, the stronger the resonance becomes. When the refractive index of the analyte increases from 1.37 to 1.4, the multimode sensitivity of the y-polarized core axis is 409.5 nm/RIU, 1550 nm/RIU, and 4240 nm/RIU, respectively. Furthermore, due to the special structure of the device, the two open-loop structures of the sensor can be designed and applied to the relevant biological experimental fields. The study of bimetallic SPR proves that the orthogonal bimetallic resonance can produce a multimodal effect. At the same time, due to the combined action of Au/Ag, the sensitivities of positive and negative amplitude in x and y directions greatly improve the ability of the sensor to resist external interference. The positive and negative amplitude sensitivity of y-polarized mode makes the sensor detection more stable and some interferences caused by other factors eliminated. Therefore, polarization demodulation technology can be used to measure the concentration of biological liquid more accurately. Through the investigation of the structural characteristics of the proposed sensor, we can find that the sensor has a good structural fault tolerance in practical fabrication. These excellent sensing properties indicate that the sensor has potential application prospects in the fields of drug, food safety, and other biosensors.