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Abstract
A surface plasmon resonance (SPR) chemical sensor comprising microstructured optical fiber (MOF) is designed for refractive index (RI) sensing in the visible to near-infrared (0.42-1.60 µm) region (NIR) as well as detection of gas-liquid pollutants. To realize mode coupling and facilitate manufacturing, gold with inert and plasmonic properties and an analyte sensing layer are introduced to the external surface of the MOF. The sensor is analyzed by the full-vector finite element method (FEM) and the wavelength and amplitude interrogation methods are adopted to evaluate the sensing characteristics. Our analysis reveals a maximum wavelength sensitivity (WS) of 15,000 nm/RIU, amplitude sensitivity (AS) of 1,603.37 RIU-1, and resolution (R) of 6.67×10−6 RIU in the determination of analyte RIs spanning an ultra-wide range between 1.00 and 1.45. Furthermore, the figure of merit (FOM) and signal-to-noise ratio (SNR) of the sensor are 295.01 RIU−1 and 2.95, respectively. On account of its simple structure, low cost, and industrial compatibility, this multi-functional sensor has tremendous potential in the chemical industry, environmental monitoring, and safety control such as harmful gas monitoring, industrial wastewater and domestic sewage detection.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Environmental protection and safety are vital to the modern society as pollutants produced by heavy industry, emission of toxic and greenhouse gas, as well as waste water continue to undermine people’s health and impede societal development [1,2]. Sensing technology plays a key role in environmental protection and detection of toxic gases such as nitrogen oxides (NO, NO2) and sulfur oxides (SO2, SO3) in the atmosphere as well as heavy metals (Hg2+, Mn4+, and Cr6+), hormonal metabolites (biological contaminants) and polycyclic aromatic hydrocarbons (PAHs) in polluted water. Therefore, it is desired to design novel sensors with strong ability to adapt to harsh environment, affordable for mass production, stability and reliability [3–6]. Nanostructured plasmonic sensors are promising because surface plasmon polaritions (SPPs) are easily stimulated by evanescent waves to produce excellent sensing effects [7]. Surface plasmon resonance (SPR) is an optical phenomenon in which free electrons in metals absorb the energy from incident light to generate collective oscillations [8,9]. It is extremely sensitive to tiny changes of the analyte refractive indexes (RIs) in the surroundings and subsequent wavelength shifts enable sensitive detection of analyte RIs [10,11]. In fact, owing to advantages such as the fast response, real-time dynamic monitoring, high sensitivity, label-free detection, and no sample damage, SPR sensing technology has been applied to chamical safety monitoring and environmental engineering [12–14].
The microstructured optical fiber (MOF) is a late-model optical fiber. Owing to the flexible design, adjustable chromaticity dispersion, and endlessly single-mode operation [15–17], there has been a great deal of effort to design MOF for different devices involving amplifier [18], filter [19], and sensor, which can applied in various fields such as strain [20], temperature [21], and refractive index [22]. On the heels of the continuous development of SPR sensing technology, SPR sensors based on MOF have also aroused much interest. Although different types of MOF-SPR sensors have been designed and investigated [23–26], the range of detectable analyte RIs is still quite limited. In general, the RIs of the detectable analytes are either more than 1.33 (high RIs detection) or less than 1.33 (low RIs detection), and there are only a few sensors that can detect RIs between 1.00 and 1.10. In other words, the gas-liquid state of the analyte can hardly be mixed and only one state can normally be detected by one sensor. Hence, so as to expand the application, MOF-SPR sensors with a wider RI range for simultaneous detection of gaseous and liquid analytes are highly desirable.
In this work, an SPR sensor based on MOF with an ultra-wide RI scope is designed and analyzed by the full-vector finite element method (FEM). The proposed sensor possesses fewer air holes arranged in the cladding to produce the upper and lower opening hexagonal symmetrical structure, which is convenient for manufacturing in practice. In addition, the gold film is coated on the external surface of the whole MOF structure, which avoids the difficulty of filling the air holes with analyte and is much easier to uniform coating compared to D-shaped structure to be polished [27]. In this structure, the evanescent wave penetrates the gold film to generate the SPR effect for efficient sensing. The MOF-SPR sensor can detect analytes in different states (gas and liquid) spanning a wide RIs range from 1.00 to 1.45. In low RIs detection, the working wavelength is in the visible regime from 420 nm to 640 nm and no special light source is needed. It is also applicable to the high RIs detection, particularly from 1.44 to 1.45. Our analysis reveals a wavelength sensitivity (WS) of 15,000 nm/RIU and amplitude sensitivity (AS) of 1,603.37 RIU-1. Because of the simple structure, flexible design, easy production, and multiple functionalities, the proposed sensor and strategy have large practical potential and also represent breakthrough and innovation in the design and functions of SPR sensors.
2. Modeling and manufacturing methods
The cross-section of the MOF-SPR sensor with fused quartz as the substrate is depicted in Fig. 1(a). The air holes in the MOF structure reduce the effective refractive index of the cladding, so that the majority of the incident light can be restrained to the fiber core during propagation. The Comsol multiphysics software is adopted to analyze the model by the full-vector finite element method (FEM). In addition, a circular perfectly matched layer (C-PML) as absorbing boundary condition is established by two-dimensional modeling, which is engaged to dwindle unnecessary electromagnetic reflection [28]. During the simulation process, the convergence tests are carried out by optimizing the mesh size and C-PML thickness to produce more accurate results. The computational area is divided by ultra-refined triangular grid, which contains 31,962 domain units and 2,190 boundary units. This precise and detailed division makes it easier to identify suitable modes. In the cladding, the diameters of the first air holes, second air hole, and third air holes are D1 = 0.7 µm, D2 = 0.8 µm, and D3 = 1.6 µm, respectively. The number of first air holes is six and three of them constitute a group, whereas two groups are arranged symmetrically. The second air hole is the central one and the number of the third air holes arranged hexagonally is eight. Gold is selected as the plasmonic medium due to its good oxidation resistance in aqueous solutions and broad shifts in the resonance wavelength. More importantly, to obtain uniformed gold layer thickness (TAu) on the surface region of the MOF, chemical vapor deposition (CVD) technique is employed which can offer minimal roughness [29,30]. At present, the manufacturing technology of MOF has been developed and improved rapidly. The MOF can be fabricated by the stack and draw method as shown in Fig. 1(b). In this method, many fine capillaries are stacked and inserted into the glass tube corresponding to the outer cladding to obtain a fiber perform. Finally, this fiber is drawn into the MOF with a cladding diameter of 125 µm. Especially, we can apply a pressure to maintain air holes in fiber drawing process and the air hole diameter can be also controlled [31].
In this MOF sensor, the relationship between the refractive index n of fused silica and transmission wavelength λ can be calculated by Sellmeier Equation as shown in Eq. (1) [32] :
The optimized thickness of the gold (TAu) film is found to be 40 nm. The dielectric constant value of gold can be demonstrated using the Drude–Lorentz model according Eq. (2) [33]:
In order to calculate the sensing performance of MOF-SPR sensors with different structures, there are two common analysis methods. One is the birefringence analysis method. When the designed structure is non-circularly symmetric or there is unbalanced pressure inside the structure, it will produce a birefringence phenomenon. This method is mainly used to calculate the refractive index difference between the two polarization fundamental modes [34]. The other is the loss spectrum analysis method, which is adopted in this work. In the process of utilizing Comsol multiphysics software to simulate and analyze the designed sensor model, the effective refractive index in the complex form of the mode field can be obtained. Generally, the imaginary part of the effective refractive index (Im(neff)) is related to the transmission loss of the mode. The sensitivity and other characteristics of MOF-SPR sensor can be obtained by calculating the resonant peak shift of the loss spectra. The confinement loss (CL) can be expressed with Eq. (3) [35]:
3. Modeling and manufacturing methods
Figures 2 (a)-(d) show the confinement loss (CL) and dispersion of the fundamental mode and plasmonic mode for both x-polarization and y-polarization.. The coupling intensity between the two modes is described by CL. The appropriate size design and the position arrangement of air holes in the cladding form an unequal polarization state. Compared to y-polarization, CL of the MOF is larger but smaller in the gold layer, indicating more energy transferred in x-polarization. Therefore, the coupling intensity of x-polarization is larger than that of y-polarization. Furthermore, the CL peaks of the MOF and gold layer appear at the intersection of the real part of the effective refractive index (Re(neff)) of the fundamental and plasmonic modes, implying that the phase matching condition is satisfied. Figures 2 (e)-(l) show the electric field distributions in the different modes. The mode is an extremely important characteristic of light wave propagation in the optical fiber. The number of modes allowed in the optical fiber is essentially determined by the normalized frequency (V) [36]. When V is very large, there will be multiple modes including the fundamental mode and higher-order modes. The order of the mode is mainly determined by the number of zero points of the transverse field of the waveguide. When the incident light propagates in the optical fiber, the modes of different orders will be excited. The smaller the incident angle of the light, the order of the excited mode is lower. Figures 2 (e) and (f-h) present the electric field distributions of the fundamental and higher-order modes for x-polarization, respectively. In order to study the optical characteristics of the MOF sensor, the fundamental mode is selected. (i) and (j) present the electric field distributions of the fundamental mode in resonance for x-polarization and y-polarization, respectively, indicating that the coupling effect is stronger for x-polarization and better sensing properties can be achieved. In addition, (k) and (l) refer to the plasmonic mode corresponding to the fundamental mode and higher order mode and hence, in the following analysis, CL of x-polarization is selected.
Fig. 2. (a-d) Dispersion and CL spectra of the fundamental mode and plasmonic mode; (e-l) Electric field distributions in different modes (nair = 1, na = 1.45, D1 = 0.7 µm, D2 = 0.8 µm, D3 = 1.9 µm, TAu = 40 nm).
4. Influence of structural parameters
The parameters for the MOF structure has a profound influence on the optical properties of the sensor. Gold is selected as the plasmonic materials due to the aforementioned advantages. Figure 3(a) presents the cross-sectional diagram showing the thickness of the gold film (TAu) and Fig. 3(b) shows the dependence of CL on the wavelength. When TAu increases from 40 nm to 60 nm, the resonant wavelength (RW) remains unchanged and CL decreases gradually as shown in Fig. 3(c). A larger TAu limits energy transmission in the MOF and reduces the coupling efficiency of the photons in the fiber core and free electrons in the surface of the gold film, consequently inhibiting surface plasmon resonance. Therefore, within the appropriate range, the thicker the metal film is coated on the cladding surface, the more likely it is to induce SPR effect. The optimal gold film thickness (TAu) is 40 nm.
Fig. 3. (a, d, g, j) Cross-sections of MOF structure for different TAu and D1-D3: (b, e, h, k) Dependence of CL on wavelengths for different structural parameters; (c, f, i, l) Optimal geometric parameters (nair = 1, na = 1.45, D1 = 0.7 µm, D2 = 0.8 µm, D3 = 1.6 µm, TAu = 40 nm).
The arrangement of air holes in the MOF also plays a crucial role in confinement and propagation of the incident light. Figures 3(d), (g), and (j) exhibit the cross-sectional diagrams and diameters of the three air holes in the MOF. In combination with Figs. 3(e) and (f), it is found that RW increases gradually with increasing D1, while CL decreases. It is attributed to that the Re(neff) of the fundamental mode is nearly unchanged, while the Re(neff) of plasmonic mode increases [14]. Figures 3(h) and (i) show that RW and CL increase when D2 changes from 0.6 µm to 0.8 µm, indicating that the larger central air hole in the fiber core facilitates the energy to be transferred to the upper and lower opening metal film surface. Figures 3(k) and (l) show that when D3 is between 1.6 µm and 2.0 µm, RW decreases gradually, but CL increases initially and then decreases exhibiting a blue-shifting trend. It is attributed to that the Re(neff) of plasmonic mode decreases, while the Re(neff) of fundamental mode is invariable [28]. Hence, the eight larger air holes in the periphery reduce the effective refractive index around the fiber core and increase the equivalent refractive index difference between the fiber core and cladding, which is conducive to transmission of incident light in the fiber core. Considering the effects of the structural parameters on CL and sensing properties, the optimal diameters of the three air holes are: D1 = 0.7 µm, D2 = 0.8 µm, and D3 = 1.6 µm. Therefore, the designed upper and lower opening hexagonal symmetrical structure allows more energy to be transferred between the fundamental mode and plasmonic mode, which provides the foundation and guarantee for phase matching. To be more exactly, it is helpful to the photons in the fiber core and the oscillating free electrons in the metallic surface inspiring each other to generate a stronger resonance effect on the interface, thus making the proposed sensor to achieve better sensing performance.
5. Construction of the sensor for photochemical detection
Figure 4 shows a simulation experimental system platform of photochemical sensing. Figure 4(a) describes the propagation mechanism of the evanescent wave in the fiber as well as the principle of excitation to produce resonance. The effective refractive index of the cladding can be reduced by filling a series of air holes in the MOF. The designed MOF is a step type optical fiber, which is beneficial to constrain the transmission of incident light in the fiber core to reduce transmission loss. Based on the light guiding mechanism of total internal reflection (TIR), the incident light transmits in the MOF and supports the evanescent field to excite SPR. At the core–metal interface, the guided/propagating ray excite surface plasmons at the metal–dielectrics medium (analyte) interface. Coupling of the evanescent wave with surface plasmons relys on various factors such as the wavelength, fiber parameters, probe geometry, metal layer properties, and sensing medium (analyte) [37,38]. In particular, the RIs of the external medium (analyte) have a significant impact on mode coupling. When phase matching conditions are satisfied, the evanescent wave transmitted to the surface of the metal can stimulate collective oscillations of internal free electrons to produce SPR effect. Figure 4(b) shows the experimental set-up for photochemical sensing. Light passes from a broadband light source (BBS) into the single-mode fiber (SMF) and impinges onto the interior of the MOF by means of the polarization controller (PC). The RI sensor with gold coated is placed in the chamber so that it contacts the analyte adequately. On the surface of the gold film, the flowing chemical molecules in the samples with different RIs cause shifts (blue or red) in the resonance wavelength detected by the optical spectrum analyzer (OSA) and analyzed by a computer [39,40]. In addition, the detection of gas is different from liquid, because gas detection needs to be carried out in a specific sealed environment. In the process of setting up the experiment, a gas generator is required to produce gas and transport it to the gas chamber. The flow meter is used to precisely control the flow rate of the gas. The humidity can be monitored and controlled by the hygrometer and the desiccant is put into the gas chamber to dry gas [41]. When the gas is transported into the gas chamber, it can be detected in real time by the above photochemical sensing experiment system. Figure 4(c) shows the series of samples filled with sealed tubes such as liquid pollutants and harmful gases in the atmosphere. In addition, other instruments are involved, for example, an ultrasonic bath and drying oven to clean and dry the containers and refractometer to measure analyte RIs. In this way, harmful chemicals in the environment can be effectively monitored and controlled to enhance the safety and quality of the measurement.
Fig. 4. (a) Light guiding mechanism and SPR resonance principle of MOF; (b) Experimental setup for photochemical sensing; (c) Detection of chemical substances and experimental apparatus.
6. Influence of analyte RIs on the wavelength and amplitude sensitivity
Figure 5(a, c, e, g) show the CL spectra of the fundamental mode for D1 = 0.7 µm, D2 = 0.8 µm, D3 = 1.6 µm, and TAu = 40 nm. With the analyte RIs are varied from 1.00 to 1.45, the resonance wavelength displays a red-shift. The intensity of the CL peak increases gradually and full-width at half-maximum (FWHM) becomes narrower indicative of stronger surface plasmon resonance. This is because when the analyte RIs go up, the energy confined to the fiber core is enhanced leading to coupling between the fundamental mode and plasmonic mode. The SPR generated on the gold film surface is easily affected by the analyte RIs in the surroundings. The wavelength location and CL peak intensity thus change with RIs and so sensitive RI sensing can be accomplished.
Fig. 5. Variations of CL spectra with wavelengths for different analyte RIs: (a) 1.00-1.20, (c) 1.21-1.32, (e) 1.33-1.39, and (g) 1.40-1.45; Dependences of the amplitude sensitivity on wavelengths for different analyte RIs: (b) 1.00-1.20, (d) 1.21-1.32, (f) 1.33-1.39, and (h) 1.40-1.45 (nair = 1, D1 = 0.7 µm, D2 = 0.8 µm, D3 = 1.6 µm, TAu = 40 nm).
Sensitivity is a crucial parameter and there are two detectable methods. The first method is wavelength modulation referred to the variation of the resonance absorption wavelength with analyte RIs for a fixed incident angle and broadband light source. The wavelength sensitivity (WS) can be expressed by the following equation [42].
In the MOF-SPR sensor, the RW shift from 1 nm to 150 nm is observed when the analyte RIs are changed from 1.00 to 1.01 and 1.44 to 1.45. The maximum WS of 15,000 nm/RIU is observed when the analyte RI is 1.00-1.45.
The second method is wavelength interrogation which it is a complex process due to spectral manipulation. Hence, the amplitude interrogation method is adopted because of the simplicity. The amplitude sensitivity (AS) at a specific wavelength is derived by the following equation [43]:
Another important parameter is the resolution (R) which reflects the ability to detect minute variations of the analyte RIs. The RI resolution (R) is calculated by the following equation [44]:
Table 1. Comparisons of the sensing properties with the reported sensors.
7. Sensing performance
Figure 6(A) shows the change of RW when the analyte RIs increase from 1.00 to 1.45 as the spectrum of the RW variations (Δλres) at high RIs. Δλres in the high RIs range of 1.40 to 1.45 is larger than that in the low RI range of 1.00 to 1.39, providing evidence that the sensor is much more sensitive to analytes with large RIs. Figure 6(B) shows the energy distributions of the corresponding fundamental mode for high analyte RIs. For na = 1.45, the light energy is distributed evenly in the fiber core and gold film surface but not concentrated in the fiber core as shown in (a) for na = 1.40. As RIs increase, the coupling strength between the fundamental mode and plasmonic mode increases and the energy transferred from the fiber core to the gold film surface increases resulting in stronger SPR. Figure 6(C) shows the important optical properties of the sensor including the full-width at half-maximum (FWHM), figure of merit (FOM) [49], signal-to-noise ratio (SNR), and detection limit (DL) [50] and more details are listed in Table 2.
Fig. 6. (A) Dependence of RW shown in (a) and Δλres in the inset; (b) Analyte RIs; (B) Energy field distribution with analyte RIs between 1.40 and 1.45; (C) Characteristics of the MOF-SPR sensor: (a) DL, (b) SNR, (c) FOM, and (d) FWHM (nair = 1, D1 = 0.7 µm, D2 = 0.8 µm, D3 = 1.6 µm, TAu = 40 nm).
Table 2. Properties of the MOF-SPR sensor.
8. Conclusion
A novel MOF-RIs sensor based on the SPR effect is designed and analyzed by FEM. Gold is selected as the plasmonic material due to the chemical stability in aqueous media. By optimizing the structural parameters, a maximum wavelength sensitivity (WS) of 15,000 nm/RIU and amplitude sensitivity (AS) of 1,603.37 RIU-1 can be realized for an ultra-wide analyte RI range spanning 1.00 to 1.45. Moreover, the sensor which can be used to detect noxious gases such as NO, NO2, SO2, and SO3 as well as liquid contaminants such as Hg2+, Mn4+, Cr6+, and PAHs can sense tiny variations in the analyte RIs up to 10−6. Owing to the flexible design, outstanding sensing properties, and industrial compatibility, the MOF-SPR sensor has great potential applications such as carbon dioxide concentration detection, ozone monitoring and water quality testing in the environmental fields.
Funding
Scientific Research Fund of Sichuan Province Science and Technology Department (2020YJ0137); City University of Hong Kong Strategic Research Grant (SRG)(7005505); Local Universities Reformation and Development Personnel Training Supporting Project from Central Authorities (140119001); Postdoctoral Scientific Research Development Fund of Heilongjiang Province (LBH-Q20081).
Acknowledgments
This work was jointly supported by Postdoctoral Scientific Research Development Fund of Heilongjiang Province [LBH-Q20081], Local Universities Reformation and Development Personnel Training Supporting Project from Central Authorities [140119001], Scientific Research Fund of Sichuan Province Science and Technology Department [2020YJ0137], and City University of Hong Kong Strategic Research Grant (SRG) [7005505].
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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}
}
}
}