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Barium titanate film based fiber optic surface plasmon sensor with high sensitivity
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Barium titanate film based fiber optic surface plasmon sensor with high sensitivity ⁎
Qi Wanga,b, , Li-Ye Niua, Jian-Ying Jinga, Wan-Ming Zhaoa a b
College of Information Science and Engineering, Northeastern University, Shenyang 110819, China State Key Laboratory of Synthetical Automation for Process Industries (Northeastern University), Shenyang 110819, China
H I GH L IG H T S
fiber/Au film/ BaTiO film SPR sensor is proposed for the first time. • The resonance spectra and sensitivity of the proposed sensor are simulated by using TMM. • The electric field distribution in the sensing region is analyzed by finite element method. • The improves the electric field strength of Au film surface. • BaTiO • BaTiO material improves the quality factor of traditional Au film SPR sensor. 3
3 3
A R T I C LE I N FO
A B S T R A C T
Keywords: Surface plasmon resonance Barium titanate Sensitivity Optical fiber sensor Electric field Biosensing
An optical fiber/Au film/barium titanate (BaTiO3) thin film surface plasmon resonance (SPR) sensor is proposed. Improving sensitivity by charge transfer between Au and BaTiO3. The calculation of transfer matrix method shows that BaTiO3 are coated on the surface of the sensing area, the simulation sensitivity of the sensor reaches 2820 nm/RIU. The finite element method simulation shows that BaTiO3 can improve the electric field intensity in the sensing region. The experimental results of refractive index show that the sensitivity of optical fiber/Au film/ BaTiO3 film SPR sensor is 2543 nm/RIU in the low refractive index range, 495 nm/RIU higher than that of traditional Au film SPR sensor. In the high refractive index range, the sensitivity of the sensor can reach 6040 nm/RIU. The sensor has broad application prospects in the field of biosensor.
1. Introduction In recent decades, surface enhancement technology based on SPR effect has been developed vigorously and attracted wide attention [1]. Many research directions have emerged, such as SPR biosensor technology, surface enhanced Raman scattering spectroscopy, spectroscopy, nanotechnology and so on [2–6]. With the continuous development of science and technology, the detection sensitivity requirements for various detection technologies are also increasing [7,8]. Because fiber optic SPR sensors have many advantages over prismatic SPR sensors in the biosensing field, fiber optic sensors can be widely used because they can achieve on-line inspection and have the advantages of no label, miniaturization, low cost and flexible design [9,10]. In this context, with the rapid development of material synthesis technology, SPR related research is not limited to the simple structure of a single precious metal material. Various composite noble metal nanostructures and high dielectric loss compounds ⁎
have been designed, developed and applied continuously [11–14]. The precious metal nanostructures include: gold nanospheres and silver nanospheres, gold nanorods and silver nanorods, gold nanometer and silver nanoplates, gold five-pointed stars and silver five-pointed stars, and gold nano-core shell structures [15–19]. The mechanism of action between noble metal nanostructures and various materials has also been gradually discovered. Compared with single noble metal film structures, these composite structures exhibit more colorful optical properties due to the interaction between nanoparticles and materials. The application prospects are also broader. New materials are used in fiber optic SPR sensors to improve sensor sensitivity [20]. At present, the detection performance of SPR sensors is improved mainly through precious metal nanostructures and new functional materials. Noble metal nanoparticles have local surface plasmon resonance (LSPR) effect, which can significantly enhance the local electric field strength. On this basis, the combination of sensitive detection based on SPR effect and precious metal nanoparticles can effectively
Corresponding author at: College of Information Science and Engineering, Northeastern University, Shenyang 110819, China. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.optlastec.2019.105899 Received 15 June 2019; Received in revised form 17 September 2019; Accepted 12 October 2019 0030-3992/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Qi Wang, et al., Optics and Laser Technology, https://doi.org/10.1016/j.optlastec.2019.105899
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solve many trace detection problems. In recent years, SPR sensors based on D-type optical fibers, plastic optical fibers, multimode optical fibers and photonic crystal fibers have been widely studied. Christopher, C. et al. studied compact plasma sensors based on gold sputtered U-bend plastic optical fibers SPR and LSPR. A compact U-shaped bending probe with 750 μm plastic optical fiber (PMMA) was used to excite the evanescent wave effectively on the plasma substrate to obtain high sensitivity, which proved its potential in the application of biological/chemical sensing [14]. Wang Qi and others have developed a long-distance surface plasmon resonance sensor based on D-type optical fiber with high quality factor and temperature self-compensation. The sensor shows good sensing performance in refractive index sensing experiment [15]. A large number of theoretical and experimental results have confirmed that the use of precious metal composite nanostructures can further improve the local electric field enhancement effect, which has great potential in the field of sensitive detection. Some new material functional materials are widely used in fiber optic sensing because of their high dielectric constant, and their functional materials include: molybdenum disulfide, tin dioxide and titanium dioxide [21–26]. Therefore, the use of SPR effect and the interaction mechanism between precious metals and functional materials, the development of sensitive detection and application research has important application value and practical significance. Based on this scientific issue, we need to find better functional materials for use in SPR sensing systems. Because BaTiO3 has a high dielectric constant and low dielectric loss, this paper proposes the use of BaTiO3 as a thin film material for enhancing SPR properties in fiber optic sensors. Previous work has confirmed the existence of charge transfer between Au and BaTiO3 [27]. The sensor structure used is a multimode fiber-single mode fiber-multimode fiber (MMF-SMF-MMF). In theory, the transfer matrix method is used to prove that the BaTiO3 in the fiber/Au film/barium titanate (Fiber/Au film/BaTiO3) sensor structure enhances the detection sensitivity. The finite element method is used to simulate the electric field intensity of the sensor model. It is proved that barium titanate can enhance the electric field intensity in the sensing area. It has been verified by subsequent tests that BaTiO3 has an enhanced effect on the detection performance of the sensor.
Sensing medium d BaTiO3 50nm Au D
Cladding(ncl)
Core (nco)
L=10mm MMF
SPF
MMF
Fig. 1. The sensor model.
(thickness 10 nm) can be coated on the surface of optical fibers at present. Therefore, only one layer of barium titanate is discussed in simulation and experiment). The resonance spectra and sensitivity curves of the proposed sensor are numerically simulated by the transfer matrix method. According to the Sellmeier relation [28], the dispersion characteristics of the core material are considered, as shown in Eq. (1).
ncore (λ ) =
1+
a1 λ2 a λ2 a λ2 + 2 2 2 + 2 3 2 λ2 − b12 λ − b2 λ − b3
(1)
At the entrance end of the fiber, the incident angle of p-polarized light is θ, and the transmission power Ptrans can be calculated at the exit end of the fiber, which is expressed by the following Eq. (2) [29]. π /2 N (θ ) ∫θcr Rp ref P (θ) dθ
Ptrans =
π /2 ∫θcr P (θ) dθ
(2)
In the sensing region, the fiber is incident on the fiber and metal interface, and the number of reflections of light at angle θ in the fiber sensing portion is represented by the following Eq. (3).
Nref (θ) =
L D tan θ
(3)
where L is the length of the sensing region portion and D is the fiber diameter. The reflected light intensity of the p-polarized light at the fiber-metal interface is Rp ; the optical power corresponding to the incident light of angle a is P (θ) ; the refractive index of the fiber cladding is ncl , the refractive index of the core is nco , and the critical angle is θcr = arc sin(ncl / nco) . The relationship between the refractive index of a metal and the wavelength can be obtained by the Drude-model Eq. (4) [30,31].
2. Theoretical investigation and simulated of the SPR sensors 2.1. Theoretical investigation Because BaTiO3 has good performance and high dielectric constant, we choose BaTiO3 as a thin film material to improve the performance of traditional SPR sensors. When a BaTiO3 film is added to the Au film on the surface of a single-mode fiber (core diameter 8.2 μm, cladding 125 μm), the resonance spectrum is red-shifted and the sensitivity of the sensor is improved. With the increase of the number of layers of BaTiO3 thin films, the resonance spectrum drifts continuously and the sensitivity improves continuously. In theory, the influence of BaTiO3 film on the optical fiber/Au film SPR sensor is proved by numerical calculation. In this paper, the following assumptions and simplifications are made on the theoretical model of optical fiber SPR sensor: first, only consider the case of light-excited SPR on the meridional plane; second, the propagation mode of incident light in the fiber is continuous, and one-to-one with the incident angle. Correspondingly, the range of incident angles is greater than the critical angle to an angle of 90 degrees. Through the above assumptions, the 4-layer dielectric plane SPR theory can be used to analyze the sensing characteristics of the transmissive fiber SPR. The proposed single-mode fiber/Au film/BaTiO3 sensor was numerically calculated by the N-layer reflection matrix method. The sensor model is shown in Fig. 1. The core diameter of single mode optical fiber core is 8.2 μm, the cladding diameter is 125 μm, the thickness of Au film is 50 nm, the length of sensing zone is L = 10 mm, and BaTiO3 film thickness 10 nm (d = 10 nm) (Due to the limitation of laboratory technical conditions, only one layer of BaTiO3 film
εm = εmr + iεmi = 1 −
λ2λ c λp2 (λ c + iλ )
(4)
where λp and λc are the wavelengths of the plasma wave and the light wave, respectively. λ is the wavelength of the incident light. The dielectric constant of the metal Au was calculated: λp = 168.26 nm, λc = 8934.2 nm. The Au film refractive index n2 is obtained by the following Eq. (5).
n2 =
εm (λ )
(5)
Barium titanate (BaTiO3) refractive index n3 obtained through experimental data, it can be calculated by Eq. (6) below [32].
n3 − 1 =
4.187 × λ2 λ2 − 0.2232
(6)
Sensitivity (S), full width of half peak (FWHM) and quality factor (Q) are three performance parameters of SPR biosensor. Sensitivity is defined as the ratio of the change of resonant wavelength (Δλ ) to the change of refractive index (Δn ) corresponding to the sensing medium with different refractive index. It can be expressed as Eq. (7) [33].
S=
Δλ Δn
(7)
FWHM is the spectral width of SPR curve corresponding to 50% 2
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Wavelength(nm)
(a) Optical fiber/Au film sensor
(b) Optical fiber/Au film/ BaTiO3 film sensor
1100
Fig. 2. Sensor resonance spectrogram.
transmittance. The bigger the quality factor is, the better the detection performance of the sensor is. The quality factor (Q) depends on the ratio of sensitivity to FWHM and can be expressed as Eq. (8) [33].
Q=
S FWHM
Table 1 Performance comparison of different SPR sensors.
(8)
Sensor type
Au film thickness (nm)
BaTiO3 film thickness (nm)
Sensitivity (nm/ RIU)
Fiber/Au film Fiber/Au Film/ BaTiO3
50 50
0 10
2280 2820
2.2. Numerical simulation of sensor model Table 2 Performance comparison of the two SPR sensors.
The fitting curves of resonance spectrum and sensitivity of optical fiber/Au film and optical fiber/Au film/BaTiO3 film sensors are obtained by transfer matrix method. The resonance spectrum of the optical fiber/Au film sensor is shown in Fig. 2(a). For optical fiber/Au film/BaTiO3 SPR sensor, the resonance spectrum is shown in Fig. 2(b). The sensor sensitivity fitting curve obtained by numerical calculation is shown in Fig. 3. Fig. 3(a): Sensitivity of fiber/Au film SPR sensor 2280 nm/RIU, linearity 97.59%; Fig. 3(b): Fiber/Au film/BaTiO3 SPR sensor, sensitivity of 2820 nm/RIU and linearity of 99.32%. From the resonance spectrum and the sensitivity fitting curve, the sensitivity of the fiber SPR sensor with the addition of BaTiO3 film is higher than that of the conventional Au film SPR sensor, and the resonance spectrum is red-shifted. The theoretical calculation results
Au film thickness (nm)
BaTiO3 film thickness (nm)
Sensitivity (nm/ RIU)
Q
Fiber/Au film Fiber/Au Film/ BaTiO3
50 50
0 10
2048.71 2543.33
13.95 17.97
show that the sensitivity of the sensor with BaTiO3 film is higher than that with optical fiber/Au film sensor, reaching 2820 nm/RIU. Table 1 shows the results of the sensor numerical calculation of BaTiO3 film. From the theoretical calculation in Table 2, it can be concluded that
Data Linear Fit of Data
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660
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630
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y=2280*x-2503.4 R-S:97.59% 570
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(a) Optical fiber/Au film sensor
(b) Optical fiber/Au film/ BaTiO3 film sensor Fig. 3. Sensitivity fitting curves. 3
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Wavelength(nm)
(a) Sensor resonance spectrogram
(b) Sensitivity fitting curves
Fig. 4. Theoretical simulation of high refractive index interval of optical fiber/Au film/BaTiO3 film sensor.
Fig. 5(b). The left longitudinal coordinate of the electric field diagram represents the distance from the center of the optical fiber, and the right color band represents the intensity of the electric field. The electric field distribution of the sensor is simulated by the finite element method, and the data of the electric field distribution are obtained by calculation. The electric field intensity comparison curves of the two sensors shown in Fig. 6 are drawn. The ordinate coordinate of coordinate axis represents the electric field intensity, and the abscissa Y represents the distance from the center of optical fiber. It can be seen from the electric field intensity diagram that the electric field intensity of the sensing area is enhanced by the surface modification of the optical fiber/Au film SPR sensor with BaTiO3. The surface electric field intensity of BaTiO3 is 1.5 times that of Au film. Previous studies have confirmed that the improvement of electric field intensity can improve the detection sensitivity of SPR sensor [34,35]. The transducer model is numerically calculated by transfer matrix method, and the sensitivity of the sensor is improved. It shows that BaTiO3 film can be used as sensitizing material for SPR sensor. The electric field of the sensor model is calculated by finite element method, and the electric field strength of the sensor is improved after the Au surface is modified with BaTiO3 film. The enhancement of sensor sensitivity is based on the enhancement of electric field intensity in the
the sensitivity of BaTiO3 film SPR sensor is higher than that of traditional Au film SPR sensor. The sensitivity characteristics of the optical fiber/Au film/ BaTiO3 film sensor in the low refractive index range are simulated above. The theoretical simulation of the sensor in the high refractive index range is carried out below. The simulation results are shown in Fig. 4. The sensitivity of the optical fiber/Au film/ BaTiO3 film sensor is 6730 nm/RIU. Compared with the simulation results above, the sensitivity of the sensor in the high refractive index range is obviously better than that in the low refractive index range. Next, the finite element method is used to calculate the electric field in the sensing area of the proposed sensor. In the finite element simulation, the wave optical module in the optical module is chosen. Then the refractive index of each part of the model and the boundary conditions of the model are set. The data needed in the simulation can be obtained through the previous theoretical analysis. Two kinds of sensor models are fabricated by coating Au film, Au film and barium titanate film on single-mode optical fibers. The electric field simulation of the two sensor models in the environment of 1.34 refractive index of external medium is shown in Fig. 5. The electric field intensity on the surface of Au film of optical fiber/Au film SPR sensor is obtained by simulation as shown in Fig. 5 (a). Electric field diagram of BaTiO3 surface of optical fiber/Au film/BaTiO3 film SPR sensor is shown in
Medium Au film
Medium BaTiO3 film Au film
(a) Optical fiber/Au film sensor
(b) Optical fiber/Au film/BaTiO3 film sensor
Fig. 5. Simulated electric field map of sensor area. 4
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BaTiO3 Film
Fig. 7. Sensor end face electron micrograph.
Co., Ltd.). In the experiment, the sensor is connected with light source and spectrometer by jump line, and the sensor is placed in a V-shaped glass tank. After the sensor is fixed, the light source and the corresponding spectrum analysis software are opened for experimental operation. First, distilled water droplets are added to the sensor of Vshaped glass tank by dropper, and then the resonance spectra are recorded. Then sodium chloride solution with different refractive index was added to observe and record the change of resonance spectrum position. Each time a different refractive index sodium chloride solution is measured, it is necessary to wash the liquid tank with distilled water to remove the last remaining sodium chloride liquid, and the entire measurement process is performed at room temperature. After the refractive index real sensing experiment is completed, the data is processed to calculate the refractive index sensitivity of the sensor. A schematic diagram of the refractive index test is shown in the Fig. 8.
Fig. 6. Electric field strength curves on Au Film Surface and BaTiO3 Surface.
sensing area by adding BaTiO3. The increase of electric field intensity is due to the charge transfer between BaTiO3 and Au film. Based on the above two theoretical calculations, the sensor proposed in this paper is reasonable. The rationality and effectiveness of the sensor are further verified by experiments. 3. Experimental 3.1. Sensor production Two segments of multi-mode fibers (MMF, diameter 62.5/125 μm, Changfei Fiber Optic Cable Co., Ltd.) are fused to two ends of 10 mm (SMF, diameter 8.2/125 um) single-mode fibers by means of optical fiber fusion machine (FITEL, S178). Then, a layer of Au thin film with thickness of about 50 nm was sputtered on the surface of the sensor by vacuum ion beam sputtering instrument (JS1600shunyi, Beijing Tongpioneering Technology Co., Ltd.). After the Au film was sputtered in the sensing region, a BaTiO3 (Shandong Xiya Chemical Industry Co., Ltd.) thin film having a thickness of 10 nm was fixed on the surface of the Au film. In order to immobilize BaTiO3 thin films, it is necessary to add nano-barium titanate powder into butyl titanate (Shandong Xiya Chemical Industry Co., Ltd.) solution diluted by ethanol. BaTiO3 nanoparticles were modified by butyl titanate as an intermediate, and p-aminophenylthiophenol (Shandong Xiya Chemical Industry Co., Ltd.) reagent was added to the dispersion solution of BaTiO3 nanoparticles. After stirring for 5 h, paminophenol was modified on the surface of BaTiO3 through hydrogen bond or other possible forces. The Au-plated sensor was immersed in the above solution for 12 h, and the thiol group in p-aminophenylthiophenol was connected with the Au film through the chemical bond Au-S [14]. Remove the sensor and rinse it with deionized water. Dry it naturally at room temperature. Complete the above steps, sensor production is completed. The sensor electron micrograph is shown in Fig. 7.
4. Analysis of results 4.1. Analysis of experimental results The refractive index test explored the sensing properties of fiber/Au film SPR sensors and fiber/Au film/barium titanate (BaTiO3) SPR sensors. The refractive index experimental resonance spectrum and sensitivity fitting curve of the fiber/Au film SPR sensor are shown in Fig. 9(a) and (b). Firstly, the sensor's sensing properties were tested in the low refractive index range. The refractive index range of the sodium chloride solution used in the experiment was 1.3332–1.3710. The fiber/ Au film SPR sensor’s sensitivity is 2048.71 nm/RIU and linearity 99.00%. The refractive index experimental resonance spectrum and sensitivity fitting curve of the fiber/Au film/ BaTiO3 SPR sensor are shown in Fig. 10 (a) and (b). The fiber/Au film/BaTiO3 SPR sensor, BaTiO3 film thickness of 10 nm sensitivity 2543.33 nm/RIU and linearity 99.44%. The experimental results are shown in Table 2. From the experimental data, it is known that the wavelength of the resonance light of
3.2. Refractive index experiment After the sensor is completed, the refractive index sensing experiment is started below. Sodium chloride solution with different refractive index was prepared before the experiment. The refractive index of the solution was measured by Abbe refractometer. The refractive index range of the sodium chloride solution was 1.3332–1.4140. The experimental light source is a halogen tungsten lamp (DH-2000, Weihai Optical Instruments (Shanghai) Co., Ltd.) with wavelength of 215–1100 nm. The resonance spectrum is measured by Ocean Spectrometer (MAYA 2000 Pro, Weihai Optical Instruments (Shanghai)
Sensing area MMF
SMF
MMF
Fig. 8. Refractive index experimental device diagram. 5
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Wavelength(nm)
(a) Resonance spectrum.
(b) Sensitivity fitting curve.
Fig. 9. The fiber/Au film SPR sensors.
the sensor to which the barium titanate thin film is added is red-shifted, and the sensitivity is improved by 495 nm/RIU with respect to the ordinary SPR sensor. The quality factor of barium titanate sensor is also improved. Explain the rationality of our proposed sensor. Calculate Q in Table 2, S represents the sensitivity of the sensor under the refractive index of 1.3332–1.3710. Both theoretical simulation and experimental results show that barium titanate thin film can improve the sensing performance of SPR sensor. The conditions of the sensor simulation model are ideal, but there are some problems in the actual fabrication of the sensor, such as the non-uniformity of Au film and barium titanate coating, the deviation of optical fiber fusion, the measurement error of refractive index solution and the deviation of resonance Valley peak seeking. The simulation results are different from the experimental results, and the simulation results of sensor sensitivity are better than the experimental results. Then, the sensing characteristics of the fiber/Au film/BaTiO3 (BaTiO3 film thickness 10 nm) SPR sensor in a large refractive index range are tested. The refractive index range of the glucose solution used in the experiment is 1.3332–1.4140. The resonance spectra and sensitivity fitting curves of the sensor can be obtained from experiments, as shown in Fig. 11 (a) and (b). The resonance spectrum shows that the resonance wavelength of the
BaTiO3 thin film sensor shifts greatly in the range of high refractive index. The sensor has high sensitivity in the range of high refractive index (the refractive index of sodium chloride solution: 1.3710–1.4140), and the sensitivity can reach 6040 nm/RIU. The fitting curve shows that the resonance wavelength and refractive index of the sensor are not completely linear. The sensitivity of the sensor in the high refractive index range is obviously higher than that in the low refractive index range. This is because the difference between the refractive index of the measured medium and the refractive index of the cladding decreases in the high refractive index range, resulting in an increase in the evanescent field of the cladding and triggering a stronger SPR phenomenon. At this time, when the refractive index of the external medium measured by the sensor changes in the high refractive index range, the resonance wavelength will have a greater red shift. This makes the fitting curve between the resonant wavelength and the refractive index of the sensor in the range of high refractive index and low refractive index not completely linear. The sensor presented in this paper is compared with the work done by predecessors, as shown in Table 3. The data in the table show that the sensor proposed in this paper has good performance.
750 100
Data Linear Fit of Data
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Transmittance(%)
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y=2543.33*x-2743.42 R-S:99.44%
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1.34
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1.36
Refractive Index
(a) Resonance spectrum
(b) Sensitivity fitting curve
Fig. 10. The fiber/Au film/BaTiO3 SPR sensors. 6
1.37
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Fig. 11. The fiber/Au film/BaTiO3 SPR sensors.
4.2. Repeatability of experiments In order to verify the sensitivity of the sensor and the repeatability of the experiment, two identical optical fibers are used to perform the same operation. Two identical optical fiber/Au film/barium titanate SPR sensors were fabricated. The refractive index sensing experiments of these two sensors are carried out, and the sensitivity fitting curves are obtained, as shown in Fig. 12. The following Table 4 can be obtained from the sensitivity fitting curve of the sensor. The original sensor in the table represents the same type of sensor proposed earlier. Sensors 1 and 2 represent newly fabricated sensors for verifying sensor sensitivity and experimental repeatability. From the sensitivity data in the table, it can be concluded that the sensor presented in this paper has good repeatability.
5. Conclusion Fig. 12. Sensitivity Fitting Curve of fiber/Au film/BaTiO3 SPR sensors.
BaTiO3 with high dielectric constant is proposed as a thin film material to improve the sensitivity of SPR sensor and BaTiO3 improve the quality factor (Q) of the sensor. The influence of BaTiO3 thin film on the detection performance of traditional SPR sensor was studied by using transfer matrix method. The simulation results show that when the thickness of the Au film is 50 nm and the thickness of the BaTiO3 film is 10 nm, the simulation sensitivity of the sensor reaches 2820 nm/ RIU, which is 1.24 times of that of the traditional Au film SPR sensor. Then the refractive index sensing experiments of optical fiber/Au film SPR sensor and optical fiber/Au film/BaTiO3 film SPR sensor are carried out, and the experimental results are systematically analyzed. The experimental results of refractive index show that the sensitivity of
optical fiber SPR sensor can be improved by introducing BaTiO3 thin film. The sensitivity of optical fiber/Au film/BaTiO3 thin film SPR sensor is 495 nm/RIU higher than that of optical fiber/Au film SPR sensor. The sensor has high sensitivity in the range of high refractive index (the refractive index of glucose solution: 1.3710–1.4140), and the sensitivity can reach 6040 nm/RIU. The experimental results are consistent with the numerical simulation results, which further illustrates the rationality of the sensor we designed. Fiber optic SPR sensor has great advantages in biomass detection. The addition of BaTiO3 film improves the detection
Table 3 Comparison of SPR sensors. Sensor
Refractive index range
Sensitivity (nm/RIU)
Method use
Ref.
POF/Au SPF/Au/ZnS-SiO2/Au FBG/Ag SPF/Ag
1.33–1.41 1.332–1.38 1.333–1.38 1.34–1.36 1.36–1.38 1.333–1.360 1.3332–1.3710 1.3332–1.3710 1.3332–1.3710 1.3332–1.3710 1.3710–1.4140
2533 2017 2557 2062 3169 1603 2217 3074 2049 2543 6040
SPR WCSPR SPR SPR
[32] [36] [34] [35]
LRSPR SPR SPR + LSPR SPR SPR
[37] [38] [38] This work This work
SPF/Ag/ MgF2 SPF/Au SPF/Au/ gold NPS Fiber/Au film Fiber/Au film/BaTiO3
7
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Table 4 Performance comparison. Sensor
Refractive index range
Sensitivity (nm/ RIU)
Linearity (%)
The original sensor
1.3332–1.3710 1.3710–1.4140 1.3332–1.3710 1.3710–1.4140 1.3332–1.3710 1.3710–1. 4140
2543 6040 2573 6011 2606 6035
99.44 95.92 98.40 95.99 98.79 96.10
Sensor 1 Sensor 2
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performance of the sensor. The sensor proposed in this paper has great application prospects in the field of biosensor and low concentration biomass detection. Although the sensor proposed in this paper has good performance, there is still much room for improvement in sensitivity. To improve the performance of sensors, it is necessary to design new sensor structures, select special materials with superior performance and other thin film materials. Declaration of Competing Interest In view of this paper, the following statement is made: “to the best of our knowledge, the named authors have no conflict of interest, financial or otherwise.” Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities under Grant N180402023 and N172002001, the National Natural Science Foundation of China under Grant 51607028. References [1] Q. Wang, W.M. Zhao, A comprehensive review of lossy mode resonance-based fiber optic sensors, Opt. Laser. Eng. 100 (2018) 47–60. [2] S.W. Zhang, B.P. Zhang, S. Li, et al., SPR enhanced photocatalytic properties of Audispersed amorphous BaTiO3 nanocomposite thin films, J. Alloy. Compd. 654 (2016) 112–119. [3] Yong Zhao, Rui-jie Tong, Feng Xia, Yun Peng, Current status of optical fiber biosensor based on surface plasmon resonance, Biosens. Bioelectron. 142 (2019) 111505. [4] M. Liu, X. Yang, B. Zhao, J. Hou, S. Ping, Square array photonic crystal fiber-based surface plasmon resonance refractive index sensor, Mod. Phys. Lett. B 31 (2017) 1750352. [5] L. Duan, X. Yang, Y. Lu, J. Yao, Hollow-fiber-based surface plasmon resonance sensor with large refractive index detection range and high linearity, Appl. Opt. 56 (2017) 9907–9912. [6] S.J. Wang, B.P. Zhang, SPR propelled visible-active photocatalysis on Au-dispersed Co3O4 films, Appl. Catal. A-Gen 467 (2013) 585–592. [7] F. Long, A.N. Zhu, H.C. Shi, Recent advances in optical biosensors for environmental monitoring and early warning, Sensors 13 (10) (2013) 13928–13948. [8] J.N. Anker, W.P. Hall, O. Lyandres, et al., Biosensing with plasmonic nanosensors, Nat. Mater. 7 (2008) 442–453. [9] S. Kim, J. Lee, S.J. Lee, et al., Ultra-sensitive detection of IgE using biofunctionalized nanoparticle-enhanced SPR, Talanta 81 (2010) 1755–1759. [10] Qi Wang, Jian-Ying Jing, Bo-Tao Wang, Highly sensitive SPR biosensor based on graphene oxide and staphylococcal protein A co-modified TFBG for human IgG detection, IEEE Trans. Instrum. Measure. 68 (9) (2019) 3350–3357. [11] I.E. Sendroiu, L.K. Gifford, A. Lupták, et al., Ultrasensitive DNA microarray biosensing via in situ RNA transcription-based amplification and nanoparticle-enhanced SPR imaging, J. Am. Chem. Soc. 133 (2011) 4271–4275. [12] S. Ko, T.J. Park, H.S. Kim, et al., Directed self-assembly of gold binding polypeptideprotein A fusion proteins for development of gold nanoparticle-based SPR
8
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Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Barium titanate film based fiber optic surface plasmon sensor with high sensitivity ⁎
Qi Wanga,b, , Li-Ye Niua, Jian-Ying Jinga, Wan-Ming Zhaoa a b
College of Information Science and Engineering, Northeastern University, Shenyang 110819, China State Key Laboratory of Synthetical Automation for Process Industries (Northeastern University), Shenyang 110819, China
H I GH L IG H T S
fiber/Au film/ BaTiO film SPR sensor is proposed for the first time. • The resonance spectra and sensitivity of the proposed sensor are simulated by using TMM. • The electric field distribution in the sensing region is analyzed by finite element method. • The improves the electric field strength of Au film surface. • BaTiO • BaTiO material improves the quality factor of traditional Au film SPR sensor. 3
3 3
A R T I C LE I N FO
A B S T R A C T
Keywords: Surface plasmon resonance Barium titanate Sensitivity Optical fiber sensor Electric field Biosensing
An optical fiber/Au film/barium titanate (BaTiO3) thin film surface plasmon resonance (SPR) sensor is proposed. Improving sensitivity by charge transfer between Au and BaTiO3. The calculation of transfer matrix method shows that BaTiO3 are coated on the surface of the sensing area, the simulation sensitivity of the sensor reaches 2820 nm/RIU. The finite element method simulation shows that BaTiO3 can improve the electric field intensity in the sensing region. The experimental results of refractive index show that the sensitivity of optical fiber/Au film/ BaTiO3 film SPR sensor is 2543 nm/RIU in the low refractive index range, 495 nm/RIU higher than that of traditional Au film SPR sensor. In the high refractive index range, the sensitivity of the sensor can reach 6040 nm/RIU. The sensor has broad application prospects in the field of biosensor.
1. Introduction In recent decades, surface enhancement technology based on SPR effect has been developed vigorously and attracted wide attention [1]. Many research directions have emerged, such as SPR biosensor technology, surface enhanced Raman scattering spectroscopy, spectroscopy, nanotechnology and so on [2–6]. With the continuous development of science and technology, the detection sensitivity requirements for various detection technologies are also increasing [7,8]. Because fiber optic SPR sensors have many advantages over prismatic SPR sensors in the biosensing field, fiber optic sensors can be widely used because they can achieve on-line inspection and have the advantages of no label, miniaturization, low cost and flexible design [9,10]. In this context, with the rapid development of material synthesis technology, SPR related research is not limited to the simple structure of a single precious metal material. Various composite noble metal nanostructures and high dielectric loss compounds ⁎
have been designed, developed and applied continuously [11–14]. The precious metal nanostructures include: gold nanospheres and silver nanospheres, gold nanorods and silver nanorods, gold nanometer and silver nanoplates, gold five-pointed stars and silver five-pointed stars, and gold nano-core shell structures [15–19]. The mechanism of action between noble metal nanostructures and various materials has also been gradually discovered. Compared with single noble metal film structures, these composite structures exhibit more colorful optical properties due to the interaction between nanoparticles and materials. The application prospects are also broader. New materials are used in fiber optic SPR sensors to improve sensor sensitivity [20]. At present, the detection performance of SPR sensors is improved mainly through precious metal nanostructures and new functional materials. Noble metal nanoparticles have local surface plasmon resonance (LSPR) effect, which can significantly enhance the local electric field strength. On this basis, the combination of sensitive detection based on SPR effect and precious metal nanoparticles can effectively
Corresponding author at: College of Information Science and Engineering, Northeastern University, Shenyang 110819, China. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.optlastec.2019.105899 Received 15 June 2019; Received in revised form 17 September 2019; Accepted 12 October 2019 0030-3992/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Qi Wang, et al., Optics and Laser Technology, https://doi.org/10.1016/j.optlastec.2019.105899
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solve many trace detection problems. In recent years, SPR sensors based on D-type optical fibers, plastic optical fibers, multimode optical fibers and photonic crystal fibers have been widely studied. Christopher, C. et al. studied compact plasma sensors based on gold sputtered U-bend plastic optical fibers SPR and LSPR. A compact U-shaped bending probe with 750 μm plastic optical fiber (PMMA) was used to excite the evanescent wave effectively on the plasma substrate to obtain high sensitivity, which proved its potential in the application of biological/chemical sensing [14]. Wang Qi and others have developed a long-distance surface plasmon resonance sensor based on D-type optical fiber with high quality factor and temperature self-compensation. The sensor shows good sensing performance in refractive index sensing experiment [15]. A large number of theoretical and experimental results have confirmed that the use of precious metal composite nanostructures can further improve the local electric field enhancement effect, which has great potential in the field of sensitive detection. Some new material functional materials are widely used in fiber optic sensing because of their high dielectric constant, and their functional materials include: molybdenum disulfide, tin dioxide and titanium dioxide [21–26]. Therefore, the use of SPR effect and the interaction mechanism between precious metals and functional materials, the development of sensitive detection and application research has important application value and practical significance. Based on this scientific issue, we need to find better functional materials for use in SPR sensing systems. Because BaTiO3 has a high dielectric constant and low dielectric loss, this paper proposes the use of BaTiO3 as a thin film material for enhancing SPR properties in fiber optic sensors. Previous work has confirmed the existence of charge transfer between Au and BaTiO3 [27]. The sensor structure used is a multimode fiber-single mode fiber-multimode fiber (MMF-SMF-MMF). In theory, the transfer matrix method is used to prove that the BaTiO3 in the fiber/Au film/barium titanate (Fiber/Au film/BaTiO3) sensor structure enhances the detection sensitivity. The finite element method is used to simulate the electric field intensity of the sensor model. It is proved that barium titanate can enhance the electric field intensity in the sensing area. It has been verified by subsequent tests that BaTiO3 has an enhanced effect on the detection performance of the sensor.
Sensing medium d BaTiO3 50nm Au D
Cladding(ncl)
Core (nco)
L=10mm MMF
SPF
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Fig. 1. The sensor model.
(thickness 10 nm) can be coated on the surface of optical fibers at present. Therefore, only one layer of barium titanate is discussed in simulation and experiment). The resonance spectra and sensitivity curves of the proposed sensor are numerically simulated by the transfer matrix method. According to the Sellmeier relation [28], the dispersion characteristics of the core material are considered, as shown in Eq. (1).
ncore (λ ) =
1+
a1 λ2 a λ2 a λ2 + 2 2 2 + 2 3 2 λ2 − b12 λ − b2 λ − b3
(1)
At the entrance end of the fiber, the incident angle of p-polarized light is θ, and the transmission power Ptrans can be calculated at the exit end of the fiber, which is expressed by the following Eq. (2) [29]. π /2 N (θ ) ∫θcr Rp ref P (θ) dθ
Ptrans =
π /2 ∫θcr P (θ) dθ
(2)
In the sensing region, the fiber is incident on the fiber and metal interface, and the number of reflections of light at angle θ in the fiber sensing portion is represented by the following Eq. (3).
Nref (θ) =
L D tan θ
(3)
where L is the length of the sensing region portion and D is the fiber diameter. The reflected light intensity of the p-polarized light at the fiber-metal interface is Rp ; the optical power corresponding to the incident light of angle a is P (θ) ; the refractive index of the fiber cladding is ncl , the refractive index of the core is nco , and the critical angle is θcr = arc sin(ncl / nco) . The relationship between the refractive index of a metal and the wavelength can be obtained by the Drude-model Eq. (4) [30,31].
2. Theoretical investigation and simulated of the SPR sensors 2.1. Theoretical investigation Because BaTiO3 has good performance and high dielectric constant, we choose BaTiO3 as a thin film material to improve the performance of traditional SPR sensors. When a BaTiO3 film is added to the Au film on the surface of a single-mode fiber (core diameter 8.2 μm, cladding 125 μm), the resonance spectrum is red-shifted and the sensitivity of the sensor is improved. With the increase of the number of layers of BaTiO3 thin films, the resonance spectrum drifts continuously and the sensitivity improves continuously. In theory, the influence of BaTiO3 film on the optical fiber/Au film SPR sensor is proved by numerical calculation. In this paper, the following assumptions and simplifications are made on the theoretical model of optical fiber SPR sensor: first, only consider the case of light-excited SPR on the meridional plane; second, the propagation mode of incident light in the fiber is continuous, and one-to-one with the incident angle. Correspondingly, the range of incident angles is greater than the critical angle to an angle of 90 degrees. Through the above assumptions, the 4-layer dielectric plane SPR theory can be used to analyze the sensing characteristics of the transmissive fiber SPR. The proposed single-mode fiber/Au film/BaTiO3 sensor was numerically calculated by the N-layer reflection matrix method. The sensor model is shown in Fig. 1. The core diameter of single mode optical fiber core is 8.2 μm, the cladding diameter is 125 μm, the thickness of Au film is 50 nm, the length of sensing zone is L = 10 mm, and BaTiO3 film thickness 10 nm (d = 10 nm) (Due to the limitation of laboratory technical conditions, only one layer of BaTiO3 film
εm = εmr + iεmi = 1 −
λ2λ c λp2 (λ c + iλ )
(4)
where λp and λc are the wavelengths of the plasma wave and the light wave, respectively. λ is the wavelength of the incident light. The dielectric constant of the metal Au was calculated: λp = 168.26 nm, λc = 8934.2 nm. The Au film refractive index n2 is obtained by the following Eq. (5).
n2 =
εm (λ )
(5)
Barium titanate (BaTiO3) refractive index n3 obtained through experimental data, it can be calculated by Eq. (6) below [32].
n3 − 1 =
4.187 × λ2 λ2 − 0.2232
(6)
Sensitivity (S), full width of half peak (FWHM) and quality factor (Q) are three performance parameters of SPR biosensor. Sensitivity is defined as the ratio of the change of resonant wavelength (Δλ ) to the change of refractive index (Δn ) corresponding to the sensing medium with different refractive index. It can be expressed as Eq. (7) [33].
S=
Δλ Δn
(7)
FWHM is the spectral width of SPR curve corresponding to 50% 2
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transmittance. The bigger the quality factor is, the better the detection performance of the sensor is. The quality factor (Q) depends on the ratio of sensitivity to FWHM and can be expressed as Eq. (8) [33].
Q=
S FWHM
Table 1 Performance comparison of different SPR sensors.
(8)
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BaTiO3 film thickness (nm)
Sensitivity (nm/ RIU)
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2.2. Numerical simulation of sensor model Table 2 Performance comparison of the two SPR sensors.
The fitting curves of resonance spectrum and sensitivity of optical fiber/Au film and optical fiber/Au film/BaTiO3 film sensors are obtained by transfer matrix method. The resonance spectrum of the optical fiber/Au film sensor is shown in Fig. 2(a). For optical fiber/Au film/BaTiO3 SPR sensor, the resonance spectrum is shown in Fig. 2(b). The sensor sensitivity fitting curve obtained by numerical calculation is shown in Fig. 3. Fig. 3(a): Sensitivity of fiber/Au film SPR sensor 2280 nm/RIU, linearity 97.59%; Fig. 3(b): Fiber/Au film/BaTiO3 SPR sensor, sensitivity of 2820 nm/RIU and linearity of 99.32%. From the resonance spectrum and the sensitivity fitting curve, the sensitivity of the fiber SPR sensor with the addition of BaTiO3 film is higher than that of the conventional Au film SPR sensor, and the resonance spectrum is red-shifted. The theoretical calculation results
Au film thickness (nm)
BaTiO3 film thickness (nm)
Sensitivity (nm/ RIU)
Q
Fiber/Au film Fiber/Au Film/ BaTiO3
50 50
0 10
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show that the sensitivity of the sensor with BaTiO3 film is higher than that with optical fiber/Au film sensor, reaching 2820 nm/RIU. Table 1 shows the results of the sensor numerical calculation of BaTiO3 film. From the theoretical calculation in Table 2, it can be concluded that
Data Linear Fit of Data
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(b) Optical fiber/Au film/ BaTiO3 film sensor Fig. 3. Sensitivity fitting curves. 3
1.37
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Fig. 4. Theoretical simulation of high refractive index interval of optical fiber/Au film/BaTiO3 film sensor.
Fig. 5(b). The left longitudinal coordinate of the electric field diagram represents the distance from the center of the optical fiber, and the right color band represents the intensity of the electric field. The electric field distribution of the sensor is simulated by the finite element method, and the data of the electric field distribution are obtained by calculation. The electric field intensity comparison curves of the two sensors shown in Fig. 6 are drawn. The ordinate coordinate of coordinate axis represents the electric field intensity, and the abscissa Y represents the distance from the center of optical fiber. It can be seen from the electric field intensity diagram that the electric field intensity of the sensing area is enhanced by the surface modification of the optical fiber/Au film SPR sensor with BaTiO3. The surface electric field intensity of BaTiO3 is 1.5 times that of Au film. Previous studies have confirmed that the improvement of electric field intensity can improve the detection sensitivity of SPR sensor [34,35]. The transducer model is numerically calculated by transfer matrix method, and the sensitivity of the sensor is improved. It shows that BaTiO3 film can be used as sensitizing material for SPR sensor. The electric field of the sensor model is calculated by finite element method, and the electric field strength of the sensor is improved after the Au surface is modified with BaTiO3 film. The enhancement of sensor sensitivity is based on the enhancement of electric field intensity in the
the sensitivity of BaTiO3 film SPR sensor is higher than that of traditional Au film SPR sensor. The sensitivity characteristics of the optical fiber/Au film/ BaTiO3 film sensor in the low refractive index range are simulated above. The theoretical simulation of the sensor in the high refractive index range is carried out below. The simulation results are shown in Fig. 4. The sensitivity of the optical fiber/Au film/ BaTiO3 film sensor is 6730 nm/RIU. Compared with the simulation results above, the sensitivity of the sensor in the high refractive index range is obviously better than that in the low refractive index range. Next, the finite element method is used to calculate the electric field in the sensing area of the proposed sensor. In the finite element simulation, the wave optical module in the optical module is chosen. Then the refractive index of each part of the model and the boundary conditions of the model are set. The data needed in the simulation can be obtained through the previous theoretical analysis. Two kinds of sensor models are fabricated by coating Au film, Au film and barium titanate film on single-mode optical fibers. The electric field simulation of the two sensor models in the environment of 1.34 refractive index of external medium is shown in Fig. 5. The electric field intensity on the surface of Au film of optical fiber/Au film SPR sensor is obtained by simulation as shown in Fig. 5 (a). Electric field diagram of BaTiO3 surface of optical fiber/Au film/BaTiO3 film SPR sensor is shown in
Medium Au film
Medium BaTiO3 film Au film
(a) Optical fiber/Au film sensor
(b) Optical fiber/Au film/BaTiO3 film sensor
Fig. 5. Simulated electric field map of sensor area. 4
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BaTiO3 Film
Fig. 7. Sensor end face electron micrograph.
Co., Ltd.). In the experiment, the sensor is connected with light source and spectrometer by jump line, and the sensor is placed in a V-shaped glass tank. After the sensor is fixed, the light source and the corresponding spectrum analysis software are opened for experimental operation. First, distilled water droplets are added to the sensor of Vshaped glass tank by dropper, and then the resonance spectra are recorded. Then sodium chloride solution with different refractive index was added to observe and record the change of resonance spectrum position. Each time a different refractive index sodium chloride solution is measured, it is necessary to wash the liquid tank with distilled water to remove the last remaining sodium chloride liquid, and the entire measurement process is performed at room temperature. After the refractive index real sensing experiment is completed, the data is processed to calculate the refractive index sensitivity of the sensor. A schematic diagram of the refractive index test is shown in the Fig. 8.
Fig. 6. Electric field strength curves on Au Film Surface and BaTiO3 Surface.
sensing area by adding BaTiO3. The increase of electric field intensity is due to the charge transfer between BaTiO3 and Au film. Based on the above two theoretical calculations, the sensor proposed in this paper is reasonable. The rationality and effectiveness of the sensor are further verified by experiments. 3. Experimental 3.1. Sensor production Two segments of multi-mode fibers (MMF, diameter 62.5/125 μm, Changfei Fiber Optic Cable Co., Ltd.) are fused to two ends of 10 mm (SMF, diameter 8.2/125 um) single-mode fibers by means of optical fiber fusion machine (FITEL, S178). Then, a layer of Au thin film with thickness of about 50 nm was sputtered on the surface of the sensor by vacuum ion beam sputtering instrument (JS1600shunyi, Beijing Tongpioneering Technology Co., Ltd.). After the Au film was sputtered in the sensing region, a BaTiO3 (Shandong Xiya Chemical Industry Co., Ltd.) thin film having a thickness of 10 nm was fixed on the surface of the Au film. In order to immobilize BaTiO3 thin films, it is necessary to add nano-barium titanate powder into butyl titanate (Shandong Xiya Chemical Industry Co., Ltd.) solution diluted by ethanol. BaTiO3 nanoparticles were modified by butyl titanate as an intermediate, and p-aminophenylthiophenol (Shandong Xiya Chemical Industry Co., Ltd.) reagent was added to the dispersion solution of BaTiO3 nanoparticles. After stirring for 5 h, paminophenol was modified on the surface of BaTiO3 through hydrogen bond or other possible forces. The Au-plated sensor was immersed in the above solution for 12 h, and the thiol group in p-aminophenylthiophenol was connected with the Au film through the chemical bond Au-S [14]. Remove the sensor and rinse it with deionized water. Dry it naturally at room temperature. Complete the above steps, sensor production is completed. The sensor electron micrograph is shown in Fig. 7.
4. Analysis of results 4.1. Analysis of experimental results The refractive index test explored the sensing properties of fiber/Au film SPR sensors and fiber/Au film/barium titanate (BaTiO3) SPR sensors. The refractive index experimental resonance spectrum and sensitivity fitting curve of the fiber/Au film SPR sensor are shown in Fig. 9(a) and (b). Firstly, the sensor's sensing properties were tested in the low refractive index range. The refractive index range of the sodium chloride solution used in the experiment was 1.3332–1.3710. The fiber/ Au film SPR sensor’s sensitivity is 2048.71 nm/RIU and linearity 99.00%. The refractive index experimental resonance spectrum and sensitivity fitting curve of the fiber/Au film/ BaTiO3 SPR sensor are shown in Fig. 10 (a) and (b). The fiber/Au film/BaTiO3 SPR sensor, BaTiO3 film thickness of 10 nm sensitivity 2543.33 nm/RIU and linearity 99.44%. The experimental results are shown in Table 2. From the experimental data, it is known that the wavelength of the resonance light of
3.2. Refractive index experiment After the sensor is completed, the refractive index sensing experiment is started below. Sodium chloride solution with different refractive index was prepared before the experiment. The refractive index of the solution was measured by Abbe refractometer. The refractive index range of the sodium chloride solution was 1.3332–1.4140. The experimental light source is a halogen tungsten lamp (DH-2000, Weihai Optical Instruments (Shanghai) Co., Ltd.) with wavelength of 215–1100 nm. The resonance spectrum is measured by Ocean Spectrometer (MAYA 2000 Pro, Weihai Optical Instruments (Shanghai)
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Fig. 8. Refractive index experimental device diagram. 5
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1.37
Refractive Index
Wavelength(nm)
(a) Resonance spectrum.
(b) Sensitivity fitting curve.
Fig. 9. The fiber/Au film SPR sensors.
the sensor to which the barium titanate thin film is added is red-shifted, and the sensitivity is improved by 495 nm/RIU with respect to the ordinary SPR sensor. The quality factor of barium titanate sensor is also improved. Explain the rationality of our proposed sensor. Calculate Q in Table 2, S represents the sensitivity of the sensor under the refractive index of 1.3332–1.3710. Both theoretical simulation and experimental results show that barium titanate thin film can improve the sensing performance of SPR sensor. The conditions of the sensor simulation model are ideal, but there are some problems in the actual fabrication of the sensor, such as the non-uniformity of Au film and barium titanate coating, the deviation of optical fiber fusion, the measurement error of refractive index solution and the deviation of resonance Valley peak seeking. The simulation results are different from the experimental results, and the simulation results of sensor sensitivity are better than the experimental results. Then, the sensing characteristics of the fiber/Au film/BaTiO3 (BaTiO3 film thickness 10 nm) SPR sensor in a large refractive index range are tested. The refractive index range of the glucose solution used in the experiment is 1.3332–1.4140. The resonance spectra and sensitivity fitting curves of the sensor can be obtained from experiments, as shown in Fig. 11 (a) and (b). The resonance spectrum shows that the resonance wavelength of the
BaTiO3 thin film sensor shifts greatly in the range of high refractive index. The sensor has high sensitivity in the range of high refractive index (the refractive index of sodium chloride solution: 1.3710–1.4140), and the sensitivity can reach 6040 nm/RIU. The fitting curve shows that the resonance wavelength and refractive index of the sensor are not completely linear. The sensitivity of the sensor in the high refractive index range is obviously higher than that in the low refractive index range. This is because the difference between the refractive index of the measured medium and the refractive index of the cladding decreases in the high refractive index range, resulting in an increase in the evanescent field of the cladding and triggering a stronger SPR phenomenon. At this time, when the refractive index of the external medium measured by the sensor changes in the high refractive index range, the resonance wavelength will have a greater red shift. This makes the fitting curve between the resonant wavelength and the refractive index of the sensor in the range of high refractive index and low refractive index not completely linear. The sensor presented in this paper is compared with the work done by predecessors, as shown in Table 3. The data in the table show that the sensor proposed in this paper has good performance.
750 100
Data Linear Fit of Data
Resonance Wavelength(nm)
Transmittance(%)
95 90 85 80 75 70 400
1.3332 1.3400 1.3508 1.3600 1.3710 500
600
700
800
900
1000
720
690
y=2543.33*x-2743.42 R-S:99.44%
660
1.33
Wavelength(nm)
1.34
1.35
1.36
Refractive Index
(a) Resonance spectrum
(b) Sensitivity fitting curve
Fig. 10. The fiber/Au film/BaTiO3 SPR sensors. 6
1.37
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105
Data Linear Fit of Data Linear Fit of Data
1000
Resonance Wavelength(nm)
100
Transmittance(%)
95 90 85 80 75 70 400
1.3332 1.3400 1.3508 1.3600 1.3710 1.3861 1.4020 1.4140 500
950 900
y=6040.42*x-7553.55 R-S:95.92%
850
6040.42nm/RIU 1.3710-1.4140
800 750 700
y=2543.33*x-2743.42 R-S:99.44% 2543.33nm/RIU 1.3332-1.3710
650
600
700
800
900
1000
1.33
1100
1.34
1.35
1.36
1.37
1.38
1.39
1.40
1.41
1.42
Refractive Index (b) Sensitivity fitting curve
Wavelength(nm) (a) Resonance spectrum
Fig. 11. The fiber/Au film/BaTiO3 SPR sensors.
4.2. Repeatability of experiments In order to verify the sensitivity of the sensor and the repeatability of the experiment, two identical optical fibers are used to perform the same operation. Two identical optical fiber/Au film/barium titanate SPR sensors were fabricated. The refractive index sensing experiments of these two sensors are carried out, and the sensitivity fitting curves are obtained, as shown in Fig. 12. The following Table 4 can be obtained from the sensitivity fitting curve of the sensor. The original sensor in the table represents the same type of sensor proposed earlier. Sensors 1 and 2 represent newly fabricated sensors for verifying sensor sensitivity and experimental repeatability. From the sensitivity data in the table, it can be concluded that the sensor presented in this paper has good repeatability.
5. Conclusion Fig. 12. Sensitivity Fitting Curve of fiber/Au film/BaTiO3 SPR sensors.
BaTiO3 with high dielectric constant is proposed as a thin film material to improve the sensitivity of SPR sensor and BaTiO3 improve the quality factor (Q) of the sensor. The influence of BaTiO3 thin film on the detection performance of traditional SPR sensor was studied by using transfer matrix method. The simulation results show that when the thickness of the Au film is 50 nm and the thickness of the BaTiO3 film is 10 nm, the simulation sensitivity of the sensor reaches 2820 nm/ RIU, which is 1.24 times of that of the traditional Au film SPR sensor. Then the refractive index sensing experiments of optical fiber/Au film SPR sensor and optical fiber/Au film/BaTiO3 film SPR sensor are carried out, and the experimental results are systematically analyzed. The experimental results of refractive index show that the sensitivity of
optical fiber SPR sensor can be improved by introducing BaTiO3 thin film. The sensitivity of optical fiber/Au film/BaTiO3 thin film SPR sensor is 495 nm/RIU higher than that of optical fiber/Au film SPR sensor. The sensor has high sensitivity in the range of high refractive index (the refractive index of glucose solution: 1.3710–1.4140), and the sensitivity can reach 6040 nm/RIU. The experimental results are consistent with the numerical simulation results, which further illustrates the rationality of the sensor we designed. Fiber optic SPR sensor has great advantages in biomass detection. The addition of BaTiO3 film improves the detection
Table 3 Comparison of SPR sensors. Sensor
Refractive index range
Sensitivity (nm/RIU)
Method use
Ref.
POF/Au SPF/Au/ZnS-SiO2/Au FBG/Ag SPF/Ag
1.33–1.41 1.332–1.38 1.333–1.38 1.34–1.36 1.36–1.38 1.333–1.360 1.3332–1.3710 1.3332–1.3710 1.3332–1.3710 1.3332–1.3710 1.3710–1.4140
2533 2017 2557 2062 3169 1603 2217 3074 2049 2543 6040
SPR WCSPR SPR SPR
[32] [36] [34] [35]
LRSPR SPR SPR + LSPR SPR SPR
[37] [38] [38] This work This work
SPF/Ag/ MgF2 SPF/Au SPF/Au/ gold NPS Fiber/Au film Fiber/Au film/BaTiO3
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Table 4 Performance comparison. Sensor
Refractive index range
Sensitivity (nm/ RIU)
Linearity (%)
The original sensor
1.3332–1.3710 1.3710–1.4140 1.3332–1.3710 1.3710–1.4140 1.3332–1.3710 1.3710–1. 4140
2543 6040 2573 6011 2606 6035
99.44 95.92 98.40 95.99 98.79 96.10
Sensor 1 Sensor 2
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