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Refractive index sensor based on silica microfiber doped with Ag microparticles
Optics and Laser Technology 94 (2017) 40–44
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Optics and Laser Technology journal homepage: www.elsevier.com/locate/jolt
Full length article
Refractive index sensor based on silica microfiber doped with Ag microparticles Jin Li a,b,⇑, Hanyang Li c, Haifeng Hu a, Chengbao Yao d a
College of Information Science and Engineering, Northeastern University, Shenyang 110819, China Laser Physics Centre, Australian National University, Canberra, Australian Capital Territory 2601, Australia c Key Lab of In-fiber Integrated Optics, Ministry Education of China, Harbin Engineering University, Harbin 150080, China d Key Laboratory of Photonic and Electric Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China b
a r t i c l e
i n f o
Article history: Received 10 November 2016 Received in revised form 6 March 2017 Accepted 14 March 2017
Keywords: Microfiber Fiber sensor Ag microparticles
a b s t r a c t We proposed a composite microfiber using silica capillary and Ag microparticles, and demonstrated its optical sensing characteristics by changing the refractive index of surrounding environment using different concentrations of NaCl solution. Either the diameter or doping density of Ag microparticles contributes to the sensitivity change. The experimental results reveal that the diameter uniformity and distribution of Ag microparticles exerted impact on the sensing performance. The highest sensitivity of 7246 dB/RIU has been experimentally demonstrated in this work. The selectivity of this composite microfiber sensor will be explored by decorating its surface with some enzyme or other sensitive materials. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The micro/nano-fibers (MNF) have triggered a fire-new research area in novel sensors [1,2]. These miniature sensors have drawn an extensive interest in drawbacks detection, environment monitoring, chemical reaction, and biometric identification [3–6]. Especially in recent years, a variety of micro/nano-materials have been introduced to significantly improve their sensing performances, such as selectivity, sensitivity, resolution and response time. These materials include graphene, metal particles, metal oxide particles, and carbon nanotubes, which were decorated by hydroxyl or carboxyl on the surface of MNF [7–10]. The corresponding sensitivities have been demonstrated much higher than that of other fiber sensors, such as the photonic crystal fiber, tapered fiber, fiber grating and fiber interferometer [11]. Both surface plasmon resonance (SPR) (at the interface of metal micro/nanoparticles and dielectric) and evanescent field effect (around the MNF) have been used to enhance the reaction efficiency between light and analyte [12,13]. SPR is the charge density oscillation excited on the surface of metal micro/nanostructure, which has been playing an important role in optical sensing technology in recent years [14,15]. The MNF provides the large evanescent fields ⇑ Corresponding author at: College of Information Science and Engineering, Northeastern University, No. 11, Lane 3, Wenhua Road, Heping District, Shenyang 110819, China. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.optlastec.2017.03.024 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
and confines the light tightly on its surface, which enables its sensitive response to the tiny change of surrounding environment [16–18]. Generally, the MNF was prepared from polymer materials with the disadvantages of light loss and poor resistance to chemical corrosion or high temperature. In this work, we proposed a novel Ag microparticles doped silica microfiber (ASF). The sensing properties were studied by using the NaCl solution with the concentration ranges from 0% to 10%, the corresponding sensitivities are demonstrated as high as 7292 dB/RIU (in simulation) and 7246 dB/RIU (in experiment), respectively.
2. Microfiber preparation and experiment setup The schematic of ASF for sensing the concentration of NaCl solution is shown in Fig. 1(a), where the ASF with a length of 10 mm was used as the sensing head and connected with two single mode fiber (SMF) tapers at its both ends. The two SMF tapers were fixed by two fiber clamps and respectively equipped on two fiber adjustments with the accuracy of 0.1 lm in three dimensions (X, Y, Z axis). In experiment, the ASF and SMF tapers were placed on a MgF2 substrate to reduce the light loss, because the refractive index (RI) of MgF2 is as low as 1.37, which is far less than that of silica microfiber (with the RI of 1.45). Both the layout and geometrical morphologies of ASF were monitored and recorded by a microscope. Inset of Fig. 1 shows the micrograph of ASF used in
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Fig. 2. Comparison of (a) Simulated transmission and (b) Reflection spectra of ASF with different doping density of Ag microparticles (N = 0, N = 2, N = 4 and N = 6, respectively).
Fig. 1. (a) Schematic diagram of ASF sensor system. Inset: the micrograph of ASF; (b) Preparing process of ASF.
the experiment. The diameter of ASF is 2.3 lm. The Ag microparticles (black dots) were centered in ASF. To obtain ASF, the Ag microparticles (spherical shape, diameter: 1.5–2.0 lm, Alfa Aesar, A Johnson Matthey Company) were filled into the silica capillary (outer diameter: 160 lm, air-core diameter: 100 lm, Polymicro Technologies Company). The preparing process is shown in Fig. 1(b). Firstly, the silica capillary with the polyimide coating layer (12 lm) was moved away by heating in the flame of a Bunsen burner; then, several sections of the silica capillaries were buried in the Ag microparticles, which was then ultrasonically vibrated and filled; finally, the silica capillary was stretched to prepare the ASF, whose diameter can be manipulated by changing the heating temperature and stretching velocity. In the heating region, the air in the capillary was extruded and the silica microfiber with solid core was obtained. 3. Sensor properties simulation The simulation model was built and analyzed using FDTD solution 8.6 (FDTD: Finite Difference Time Domain). In the modeling structure, the diameter and length of ASF are 2.3 lm and 300 lm, respectively. The Ag microparticles were linearly inserted along the central axis. To study the scattering properties of ASF, the simulated transmission and reflection spectra of ASF with different doping density of Ag microparticles were theoretically simulated, as presented in Fig. 2. Where R0(T0), R2(T2), R4(T4), R6(T6) refer to the reflection (transmission) spectra of ASF with the doping density of Ag microparticles of N = 0, N = 2, N = 4 and N = 6, respectively. The intensity of both transmission and reflection spectra decreased with the increasing of the density attributed to the strong scattering of the metal microparticles. The extinction ratio of reflection spectra was relatively lower and did not suitable for sensing. The intensity of the transmission spectra decreases significantly with the doping intensity due to the serious backward scattering of the metal microparticles. When the diameter of the spherical particle closes to the incident wavelength, the absorption will dominate over the scattering efficiency [19]. Furthermore, for the adequate doping density (N = 4), both the extinction ratio and relative intensity are big enough to sense the variation of the sur-
Fig. 3. (a) Simulated transmission and (b) Reflection spectra of ASF with different doping density of Ag microparticles (N = 0, N = 1, N = 2, N = 3, N = 4, N = 5 and N = 6, respectively).
rounding environment. The corresponding offset relative intensity for the transmission and reflection spectra have been compared, as illustrated in Fig. 3(a) and (b), respectively. The silica microfiber promises the light guiding with a high efficiency. Smaller doping density (N < 3) revealed the sine function waveform of whispering gallery mode with the low extinction ratio because of the serious
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absorption of metals. When the doping density increases (N = 4–6), the SPR effect played the most important role and resulted the enhancement of the extinction ratio. To explore the impact of Ag microparticles’ density on the sensing performance. Here, the diameter was fixed as 1.5 lm, the transmission spectra, and sensing characteristics of ASF with different doping density for different concentrations of NaCl solution were simulated, as shown in Fig. 4(a–f). Fig. 4(a–c) refer to the corresponding transmission spectra of ASF with different doping density of Ag microparticles (N = 4, N = 5 and N = 6, respectively). Generally, on one hand, too high doping density will cause the high transmission loss because of the strong light scattering on the surface of particles; on the other hand, the SPR intensity excited on the surface of metal particles will become more significant for the higher doping density. Therefore, the doping density should be carefully balanced. Here, the transmission spectra as a function of the NaCl solution with the concentrations from 0% (refers to distilled water) to 10% were calculated by a step of 2%. When the concentration increased continuously, the power of corresponding transmission peaks reduced significantly. The sensing curves of peaks’ intensity (the power values along the gray making rods) are presented in Fig. 4(d–f). The ASF with four Ag microparticles (N = 4) indicates a best linearity (0.9912) and highest sensitivity ( 7292 dB/RIU), as shown in Fig. 4(d). A higher doping density of Ag microparticles results in a worse linearity, which reduces to 0.79 (N = 5) or 0.8628 (N = 6), as shown in Fig. 4(e) and (f). In addition to the doping density, the diameters of Ag microparticles also play an important role and should be studied. The length of ASF and the numbers of Ag microparticles were fixed as 300 lm and N = 4, respectively. The transmission spectra of ASF with different diameters of Ag microparticles were analyzed when the concentration of NaCl solution changed from 0% to 10%. The corresponding results are illustrated in Fig. 5. Obviously, the power decreases with the diameter increasing (from 1.5 lm, 1.6 lm, 1.8 lm to 2.0 lm), which resulted by the enhancement of light reflectivity due to the bigger reflection area. The calculated results show that the sensing characteristic curves keep a good linearity (0.982–0.9984). Apart from the power reduction of transmission
spectra, the sensitivity decreases from 7292 dB/RIU (diameter = 1.5 lm) to 7074 dB/RIU (diameter = 1.6 lm). When the diameter of Ag microparticles further increases, the sensitivity decreases seriously. The sensitivities are 7886 dB/RIU and 6771 dB/RIU for the diameters of 1.5 lm and 1.6 lm, respectively, attributed to the serious transmission loss, as well as the decreasing of evanescent field along the ASF depended on the bigger diameter of Ag microparticles. 4. Experimental results and analysis To experimentally demonstrate the sensing properties, the ASF with the diameter of 2.3 lm was prepared to determine the concentration of NaCl solution. During the preparation process, 0.02 mg Ag microparticles with the diameters range from 1.5 lm to 2 lm were filled and distributed ultrasonically in one section of silica capillary with the length of 10 mm. The doping density (numbers) of Ag microparticles in ASF was 14/mm. One section of ASF with the length of 350 lm was chosen, in which four Ag microparticles were doped uniformly along its central axis, as shown in the inset micrograph of Fig. 1(a). In the experiment, a light source with the wavelength of 1520–1560 nm was used to launch into the ASF and the transmission spectra for different concentration of NaCl solution (0 –10%) were recorded, as shown in Fig. 6. NaCl solutions used in the experiment are formulated by mixing the different mass ratio of NaCl crystal with distilled water. The reference values of the corresponding RI were measured by an Abbe refractometer, which were different from 1.333 to 1.3505 according to the NaCl concentrations of 0–10%. In the experiment, NaCl solution was instilled in the sensing area of MNF, and the transmission spectra were recorded. Then, one concentration of NaCl solution was sucked by an absorbent paper, and the sensing area was cleaned twice by the distilled water before the next measurement. The working wavelength depends on the morphology parameters of ASF, such as the diameter, diameter uniformity, and surface smoothness of both the Ag microparticles and silica microfiber; as well as the distribution of Ag microparticles in silica microfiber. However, the working wavelength was fixed for a fabricated ASF
Fig. 4. Modeling structure of ASF RI sensor. (a–c) Transmission spectra and (d–f) sensing characteristic curves of ASF for NaCl solution with different concentration from 0% to 10%. Doped numbers of Ag microparticles are (a and d) N = 4, (b and e) N = 5, (c and f) N = 6, respectively.
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Fig. 5. Sensing characteristic curves of ASF for NaCl solution with different concentration from 0% to 10%. Diameters of Ag microparticles are (a) 1.5 lm, (b) 1.6 lm, (c) 1.8 lm and (d) 2.0 lm.
Fig. 7. Sensing characteristic curves change with numbers reduction of Ag microparticles (N = 3, N = 2, N = 1) in ASF.
centration increasing of NaCl solution, which agrees with the calculated results. The experimental sensitivity of characteristic peak is 7246 dB/RIU with a linearity of 0.98922, as shown in Fig. 6(b). The changing trend of experimental sensing curve was coincident with the calculated results. Fig. 6. (a) Experimental transmission spectra of ASF for NaCl solution with different concentration from 0% (distilled water) to 10%; (b) Intensity of characteristic peak as a function of NaCl solution concentration.
with the fixed morphology parameters. This wavelength could be experimentally determined by immerging it into distilled water before measuring NaCl solution. The experimental results in Fig. 6 indicate that the sensing performance can be demonstrated by the power decreasing of characteristic peak. Fig. 6(a) illustrates that the power of the characteristic peak decreases with the con-
5. Discussions and analysis In order to explore the function of the doped Ag microparticles, we demonstrated the RI sensing properties of the ASF with less Ag microparticles (N = 3, N = 2 and N = 1), as shown in Fig. 7. It was demonstrated that the sensitivity increased with the reduction of the Ag microparticles. When the density changed from N = 3, N = 2 to N = 1, the corresponding sensitivities increased from
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8903 dB/RIU, 8251 dB/RIU to 7515 dB/RIU, respectively. Furthermore, the corresponding curves indicated the excellent linearity performance. It can be attributed to the reduction of the scattering loss by the Ag microparticles. Take into consideration of linearity and sensitivity, the smaller density shows the more excellent performance. However, some details seem confusion. The experimental sensitivity was far lower than that of calculated results attributed to the following reasons and should be improved in future work. Firstly, the ASF used in the experiment was prepared by hand without other mechanical devices, which resulted in inevitable randomness; During the preparation of the ASF, the size uniform of Ag microparticles, as well as their uniform dispersion in the silica capillary cannot been controlled meticulously; Secondly, the fabrication process of the SMF-ASF-SMF sensing structure needs further improvement; The corresponding coupling area was easily disturbed, or even destroyed sometimes, resulting in the low coupling efficiency. In order to couple the light into ASF and enhance the stability of the composite fiber structure, the coupling region between SMF and ASF was encapsulated by UV glues. The spectra characteristics of simulation and experimental results are different in some way. In general, the simulation results accuracy of micro/nano-structure depends on the mesh fineness. Furthermore, although the Ag microparticles were fixed in MNF. It is easily deformed due to its low melting point during the heating and drawing process. The Ag microparticles cannot be fixed accurately in the center axis of MNF, which is not completely consistent with the simulation. 6. Conclusions In conclusion, a composite ASF prepared by silica capillary and Ag microparticles was proposed. The transmission spectra and sensing characteristics were demonstrated theoretically and experimentally. The theoretical results proved that the doped density and diameter of Ag microparticles exerted a significant impact on the sensing characteristic of ASF. For the proper doping density (N = 4) and diameter (1.5 lm), the highest sensitivity has been demonstrated as 7292 dB/RIU with a good linearity. The experimental results indicate a sensitivity of 7246 dB/RIU for the NaCl solution with the concentration changed in a range of 0–10%. The changing trend is coincident with each other between the simulated and experimental results. The changing trend of the theoretical and experimental results is coincident with each other. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) under Grants (number 61405032,
61403074, 61605031, 11504072); and Doctoral Scientific Research Startup Foundation of Liaoning Province under Grant (number 201501144); and Fundamental Research Funds for the Central Universities under Grants (number N150404022, N150401001). Jin Li acknowledges the financial support from the China Scholarship Council for his Visiting Scholarship No. 201606085023. References [1] J. Li, Y.N. Duan, H.F. Hu, Y. Zhao, Q. Wang, Flexible NWs sensors in polymer, metal oxide and semiconductor materials for chemical and biological detection, Sens. Actuat. B-Chem. 219 (2015) 65. [2] S. Nodehia, W.S. Mohammedb, H. Ahmada, S.W. Harun, Fabrication and characterization of high order filter based on resonance in hybrid multi-knots microfiber structure, Opt. Laser Technol. 78 (2016) 120. [3] D. Kinet, P. Megret, K.W. Goossen, L. Qiu, D. Heider, C. Caucheteur, Fiber bragg grating sensors toward structural health monitoring in composite materials: challenges and solutions, Sensors 14 (2014) 7394. [4] S.C. Yan, B.C. Zheng, J.H. Chen, F. Xu, Y.Q. Lu, Optical electrical current sensor utilizing a graphene-microfiber-integrated coil resonator, Appl. Phys. Lett. 107 (2015) 053502. [5] C.R. Liao, D.N. Wang, Y. Wang, Microfiber in-line Mach-Zehnder interferometer for strain sensing, Opt. Lett. 38 (2013) 757. [6] M.F. Jaddoa, A.A. Jasim, M.Z.A. Razak, S.W. Harun, H. Ahmad, Highly responsive NaCl detector based on inline microfiber Mach-Zehnder interferometer, Sens. Actuat. A: Phys. 237 (2016) 56. [7] H.Y. Lin, C.H. Huang, G.L. Cheng, N.K. Chen, H.C. Chui, Tapered optical fiber sensor based on localized surface plasmon resonance, Opt. Exp. 20 (2012) 21693. [8] C. Li, T.G. Ning, X.D. Wen, J. Li, C. Zhang, C.B. Zhang, Magnetic field and temperature sensor based on a no-core fiber combined with a fiber Bragg grating, Opt. Laser Technol. 72 (2015) 104. [9] P. Zu, C.C. Chan, T.X. Gong, Y.X. Jin, W.C. Wong, X.Y. Dong, Magneto-optical fiber sensor based on bandgap effect of photonic crystal fiber infiltrated with magnetic fluid, Appl. Phys. Lett. 101 (2012) 241118. [10] K. Gasior, T. Martynkien, W. Urbanczyk, Effect of constructional parameters on the performance of a surface plasmon resonance sensor based on a multimode polymer optical fiber, Appl. Opt. 53 (2014) 8167. [11] Y. Zhao, Z.Q. Deng, Q. Wang, Fiber optic SPR sensor for liquid concentration measurement, Sens. Actuat. B-Chem. 192 (2014) 229. [12] V. Bhardwaj, V.K. Singh, Fabrication and characterization of cascaded tapered Mach-Zehnder interferometer for refractive index sensing, Sens. Actuat. A: Phys. 244 (2016) 30. [13] H. Ahmad, M.J. Faruki, M.Z.A. Razak, Z.C. Tiu, M.F. Ismail, Evanescent field interaction of tapered fiber with graphene oxide in generation of widebandwidth mode-locked pulses, Opt. Laser Technol. 88 (2017) 166. [14] J. Li, H.Y. Li, K.Y. Wang, X.N. Zhang, C.B. Yao, Y.D. Zhang, P. Yuan, Plasmon resonance of Ag micro–sphere in fiber taper, Opt. Exp. 21 (2013) 21414. [15] M. Cao, M. Wang, N. Gu, Plasmon singularities from metal nanoparticles in active media: influence of particle shape on the gain threshold, Plasmonics 7 (2012) 347. [16] G.Y. Chen, M. Ding, T.P. Newson, G. Brambilla, A review of microfiber and nanofiber based optical sensors, Open Opt. J. 7 (2013) 32. [17] J. Li, Y.D. Zhang, H.Y. Li, C.B. Yao, P. Yuan, The optical response of the Ag nanosphere with two spindle-shaped cavities in a sub-wavelength quartz fiber, Europhys. Lett. 102 (2013) 67012. [18] J.Y. Lou, Y. Wang, L.M. Tong, Microfiber optical sensors: a review, Sensors (Basel) 14 (2014) 5823. [19] X.F. Fan, W.T. Zheng, D.J. Singh, Light scattering and surface plasmons on small spherical particles, Light: Sci. Appl. 3 (2014) e179.
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/jolt
Full length article
Refractive index sensor based on silica microfiber doped with Ag microparticles Jin Li a,b,⇑, Hanyang Li c, Haifeng Hu a, Chengbao Yao d a
College of Information Science and Engineering, Northeastern University, Shenyang 110819, China Laser Physics Centre, Australian National University, Canberra, Australian Capital Territory 2601, Australia c Key Lab of In-fiber Integrated Optics, Ministry Education of China, Harbin Engineering University, Harbin 150080, China d Key Laboratory of Photonic and Electric Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China b
a r t i c l e
i n f o
Article history: Received 10 November 2016 Received in revised form 6 March 2017 Accepted 14 March 2017
Keywords: Microfiber Fiber sensor Ag microparticles
a b s t r a c t We proposed a composite microfiber using silica capillary and Ag microparticles, and demonstrated its optical sensing characteristics by changing the refractive index of surrounding environment using different concentrations of NaCl solution. Either the diameter or doping density of Ag microparticles contributes to the sensitivity change. The experimental results reveal that the diameter uniformity and distribution of Ag microparticles exerted impact on the sensing performance. The highest sensitivity of 7246 dB/RIU has been experimentally demonstrated in this work. The selectivity of this composite microfiber sensor will be explored by decorating its surface with some enzyme or other sensitive materials. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The micro/nano-fibers (MNF) have triggered a fire-new research area in novel sensors [1,2]. These miniature sensors have drawn an extensive interest in drawbacks detection, environment monitoring, chemical reaction, and biometric identification [3–6]. Especially in recent years, a variety of micro/nano-materials have been introduced to significantly improve their sensing performances, such as selectivity, sensitivity, resolution and response time. These materials include graphene, metal particles, metal oxide particles, and carbon nanotubes, which were decorated by hydroxyl or carboxyl on the surface of MNF [7–10]. The corresponding sensitivities have been demonstrated much higher than that of other fiber sensors, such as the photonic crystal fiber, tapered fiber, fiber grating and fiber interferometer [11]. Both surface plasmon resonance (SPR) (at the interface of metal micro/nanoparticles and dielectric) and evanescent field effect (around the MNF) have been used to enhance the reaction efficiency between light and analyte [12,13]. SPR is the charge density oscillation excited on the surface of metal micro/nanostructure, which has been playing an important role in optical sensing technology in recent years [14,15]. The MNF provides the large evanescent fields ⇑ Corresponding author at: College of Information Science and Engineering, Northeastern University, No. 11, Lane 3, Wenhua Road, Heping District, Shenyang 110819, China. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.optlastec.2017.03.024 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
and confines the light tightly on its surface, which enables its sensitive response to the tiny change of surrounding environment [16–18]. Generally, the MNF was prepared from polymer materials with the disadvantages of light loss and poor resistance to chemical corrosion or high temperature. In this work, we proposed a novel Ag microparticles doped silica microfiber (ASF). The sensing properties were studied by using the NaCl solution with the concentration ranges from 0% to 10%, the corresponding sensitivities are demonstrated as high as 7292 dB/RIU (in simulation) and 7246 dB/RIU (in experiment), respectively.
2. Microfiber preparation and experiment setup The schematic of ASF for sensing the concentration of NaCl solution is shown in Fig. 1(a), where the ASF with a length of 10 mm was used as the sensing head and connected with two single mode fiber (SMF) tapers at its both ends. The two SMF tapers were fixed by two fiber clamps and respectively equipped on two fiber adjustments with the accuracy of 0.1 lm in three dimensions (X, Y, Z axis). In experiment, the ASF and SMF tapers were placed on a MgF2 substrate to reduce the light loss, because the refractive index (RI) of MgF2 is as low as 1.37, which is far less than that of silica microfiber (with the RI of 1.45). Both the layout and geometrical morphologies of ASF were monitored and recorded by a microscope. Inset of Fig. 1 shows the micrograph of ASF used in
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Fig. 2. Comparison of (a) Simulated transmission and (b) Reflection spectra of ASF with different doping density of Ag microparticles (N = 0, N = 2, N = 4 and N = 6, respectively).
Fig. 1. (a) Schematic diagram of ASF sensor system. Inset: the micrograph of ASF; (b) Preparing process of ASF.
the experiment. The diameter of ASF is 2.3 lm. The Ag microparticles (black dots) were centered in ASF. To obtain ASF, the Ag microparticles (spherical shape, diameter: 1.5–2.0 lm, Alfa Aesar, A Johnson Matthey Company) were filled into the silica capillary (outer diameter: 160 lm, air-core diameter: 100 lm, Polymicro Technologies Company). The preparing process is shown in Fig. 1(b). Firstly, the silica capillary with the polyimide coating layer (12 lm) was moved away by heating in the flame of a Bunsen burner; then, several sections of the silica capillaries were buried in the Ag microparticles, which was then ultrasonically vibrated and filled; finally, the silica capillary was stretched to prepare the ASF, whose diameter can be manipulated by changing the heating temperature and stretching velocity. In the heating region, the air in the capillary was extruded and the silica microfiber with solid core was obtained. 3. Sensor properties simulation The simulation model was built and analyzed using FDTD solution 8.6 (FDTD: Finite Difference Time Domain). In the modeling structure, the diameter and length of ASF are 2.3 lm and 300 lm, respectively. The Ag microparticles were linearly inserted along the central axis. To study the scattering properties of ASF, the simulated transmission and reflection spectra of ASF with different doping density of Ag microparticles were theoretically simulated, as presented in Fig. 2. Where R0(T0), R2(T2), R4(T4), R6(T6) refer to the reflection (transmission) spectra of ASF with the doping density of Ag microparticles of N = 0, N = 2, N = 4 and N = 6, respectively. The intensity of both transmission and reflection spectra decreased with the increasing of the density attributed to the strong scattering of the metal microparticles. The extinction ratio of reflection spectra was relatively lower and did not suitable for sensing. The intensity of the transmission spectra decreases significantly with the doping intensity due to the serious backward scattering of the metal microparticles. When the diameter of the spherical particle closes to the incident wavelength, the absorption will dominate over the scattering efficiency [19]. Furthermore, for the adequate doping density (N = 4), both the extinction ratio and relative intensity are big enough to sense the variation of the sur-
Fig. 3. (a) Simulated transmission and (b) Reflection spectra of ASF with different doping density of Ag microparticles (N = 0, N = 1, N = 2, N = 3, N = 4, N = 5 and N = 6, respectively).
rounding environment. The corresponding offset relative intensity for the transmission and reflection spectra have been compared, as illustrated in Fig. 3(a) and (b), respectively. The silica microfiber promises the light guiding with a high efficiency. Smaller doping density (N < 3) revealed the sine function waveform of whispering gallery mode with the low extinction ratio because of the serious
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absorption of metals. When the doping density increases (N = 4–6), the SPR effect played the most important role and resulted the enhancement of the extinction ratio. To explore the impact of Ag microparticles’ density on the sensing performance. Here, the diameter was fixed as 1.5 lm, the transmission spectra, and sensing characteristics of ASF with different doping density for different concentrations of NaCl solution were simulated, as shown in Fig. 4(a–f). Fig. 4(a–c) refer to the corresponding transmission spectra of ASF with different doping density of Ag microparticles (N = 4, N = 5 and N = 6, respectively). Generally, on one hand, too high doping density will cause the high transmission loss because of the strong light scattering on the surface of particles; on the other hand, the SPR intensity excited on the surface of metal particles will become more significant for the higher doping density. Therefore, the doping density should be carefully balanced. Here, the transmission spectra as a function of the NaCl solution with the concentrations from 0% (refers to distilled water) to 10% were calculated by a step of 2%. When the concentration increased continuously, the power of corresponding transmission peaks reduced significantly. The sensing curves of peaks’ intensity (the power values along the gray making rods) are presented in Fig. 4(d–f). The ASF with four Ag microparticles (N = 4) indicates a best linearity (0.9912) and highest sensitivity ( 7292 dB/RIU), as shown in Fig. 4(d). A higher doping density of Ag microparticles results in a worse linearity, which reduces to 0.79 (N = 5) or 0.8628 (N = 6), as shown in Fig. 4(e) and (f). In addition to the doping density, the diameters of Ag microparticles also play an important role and should be studied. The length of ASF and the numbers of Ag microparticles were fixed as 300 lm and N = 4, respectively. The transmission spectra of ASF with different diameters of Ag microparticles were analyzed when the concentration of NaCl solution changed from 0% to 10%. The corresponding results are illustrated in Fig. 5. Obviously, the power decreases with the diameter increasing (from 1.5 lm, 1.6 lm, 1.8 lm to 2.0 lm), which resulted by the enhancement of light reflectivity due to the bigger reflection area. The calculated results show that the sensing characteristic curves keep a good linearity (0.982–0.9984). Apart from the power reduction of transmission
spectra, the sensitivity decreases from 7292 dB/RIU (diameter = 1.5 lm) to 7074 dB/RIU (diameter = 1.6 lm). When the diameter of Ag microparticles further increases, the sensitivity decreases seriously. The sensitivities are 7886 dB/RIU and 6771 dB/RIU for the diameters of 1.5 lm and 1.6 lm, respectively, attributed to the serious transmission loss, as well as the decreasing of evanescent field along the ASF depended on the bigger diameter of Ag microparticles. 4. Experimental results and analysis To experimentally demonstrate the sensing properties, the ASF with the diameter of 2.3 lm was prepared to determine the concentration of NaCl solution. During the preparation process, 0.02 mg Ag microparticles with the diameters range from 1.5 lm to 2 lm were filled and distributed ultrasonically in one section of silica capillary with the length of 10 mm. The doping density (numbers) of Ag microparticles in ASF was 14/mm. One section of ASF with the length of 350 lm was chosen, in which four Ag microparticles were doped uniformly along its central axis, as shown in the inset micrograph of Fig. 1(a). In the experiment, a light source with the wavelength of 1520–1560 nm was used to launch into the ASF and the transmission spectra for different concentration of NaCl solution (0 –10%) were recorded, as shown in Fig. 6. NaCl solutions used in the experiment are formulated by mixing the different mass ratio of NaCl crystal with distilled water. The reference values of the corresponding RI were measured by an Abbe refractometer, which were different from 1.333 to 1.3505 according to the NaCl concentrations of 0–10%. In the experiment, NaCl solution was instilled in the sensing area of MNF, and the transmission spectra were recorded. Then, one concentration of NaCl solution was sucked by an absorbent paper, and the sensing area was cleaned twice by the distilled water before the next measurement. The working wavelength depends on the morphology parameters of ASF, such as the diameter, diameter uniformity, and surface smoothness of both the Ag microparticles and silica microfiber; as well as the distribution of Ag microparticles in silica microfiber. However, the working wavelength was fixed for a fabricated ASF
Fig. 4. Modeling structure of ASF RI sensor. (a–c) Transmission spectra and (d–f) sensing characteristic curves of ASF for NaCl solution with different concentration from 0% to 10%. Doped numbers of Ag microparticles are (a and d) N = 4, (b and e) N = 5, (c and f) N = 6, respectively.
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Fig. 5. Sensing characteristic curves of ASF for NaCl solution with different concentration from 0% to 10%. Diameters of Ag microparticles are (a) 1.5 lm, (b) 1.6 lm, (c) 1.8 lm and (d) 2.0 lm.
Fig. 7. Sensing characteristic curves change with numbers reduction of Ag microparticles (N = 3, N = 2, N = 1) in ASF.
centration increasing of NaCl solution, which agrees with the calculated results. The experimental sensitivity of characteristic peak is 7246 dB/RIU with a linearity of 0.98922, as shown in Fig. 6(b). The changing trend of experimental sensing curve was coincident with the calculated results. Fig. 6. (a) Experimental transmission spectra of ASF for NaCl solution with different concentration from 0% (distilled water) to 10%; (b) Intensity of characteristic peak as a function of NaCl solution concentration.
with the fixed morphology parameters. This wavelength could be experimentally determined by immerging it into distilled water before measuring NaCl solution. The experimental results in Fig. 6 indicate that the sensing performance can be demonstrated by the power decreasing of characteristic peak. Fig. 6(a) illustrates that the power of the characteristic peak decreases with the con-
5. Discussions and analysis In order to explore the function of the doped Ag microparticles, we demonstrated the RI sensing properties of the ASF with less Ag microparticles (N = 3, N = 2 and N = 1), as shown in Fig. 7. It was demonstrated that the sensitivity increased with the reduction of the Ag microparticles. When the density changed from N = 3, N = 2 to N = 1, the corresponding sensitivities increased from
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8903 dB/RIU, 8251 dB/RIU to 7515 dB/RIU, respectively. Furthermore, the corresponding curves indicated the excellent linearity performance. It can be attributed to the reduction of the scattering loss by the Ag microparticles. Take into consideration of linearity and sensitivity, the smaller density shows the more excellent performance. However, some details seem confusion. The experimental sensitivity was far lower than that of calculated results attributed to the following reasons and should be improved in future work. Firstly, the ASF used in the experiment was prepared by hand without other mechanical devices, which resulted in inevitable randomness; During the preparation of the ASF, the size uniform of Ag microparticles, as well as their uniform dispersion in the silica capillary cannot been controlled meticulously; Secondly, the fabrication process of the SMF-ASF-SMF sensing structure needs further improvement; The corresponding coupling area was easily disturbed, or even destroyed sometimes, resulting in the low coupling efficiency. In order to couple the light into ASF and enhance the stability of the composite fiber structure, the coupling region between SMF and ASF was encapsulated by UV glues. The spectra characteristics of simulation and experimental results are different in some way. In general, the simulation results accuracy of micro/nano-structure depends on the mesh fineness. Furthermore, although the Ag microparticles were fixed in MNF. It is easily deformed due to its low melting point during the heating and drawing process. The Ag microparticles cannot be fixed accurately in the center axis of MNF, which is not completely consistent with the simulation. 6. Conclusions In conclusion, a composite ASF prepared by silica capillary and Ag microparticles was proposed. The transmission spectra and sensing characteristics were demonstrated theoretically and experimentally. The theoretical results proved that the doped density and diameter of Ag microparticles exerted a significant impact on the sensing characteristic of ASF. For the proper doping density (N = 4) and diameter (1.5 lm), the highest sensitivity has been demonstrated as 7292 dB/RIU with a good linearity. The experimental results indicate a sensitivity of 7246 dB/RIU for the NaCl solution with the concentration changed in a range of 0–10%. The changing trend is coincident with each other between the simulated and experimental results. The changing trend of the theoretical and experimental results is coincident with each other. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) under Grants (number 61405032,
61403074, 61605031, 11504072); and Doctoral Scientific Research Startup Foundation of Liaoning Province under Grant (number 201501144); and Fundamental Research Funds for the Central Universities under Grants (number N150404022, N150401001). Jin Li acknowledges the financial support from the China Scholarship Council for his Visiting Scholarship No. 201606085023. References [1] J. Li, Y.N. Duan, H.F. Hu, Y. Zhao, Q. Wang, Flexible NWs sensors in polymer, metal oxide and semiconductor materials for chemical and biological detection, Sens. Actuat. B-Chem. 219 (2015) 65. [2] S. Nodehia, W.S. Mohammedb, H. Ahmada, S.W. Harun, Fabrication and characterization of high order filter based on resonance in hybrid multi-knots microfiber structure, Opt. Laser Technol. 78 (2016) 120. [3] D. Kinet, P. Megret, K.W. Goossen, L. Qiu, D. Heider, C. Caucheteur, Fiber bragg grating sensors toward structural health monitoring in composite materials: challenges and solutions, Sensors 14 (2014) 7394. [4] S.C. Yan, B.C. Zheng, J.H. Chen, F. Xu, Y.Q. Lu, Optical electrical current sensor utilizing a graphene-microfiber-integrated coil resonator, Appl. Phys. Lett. 107 (2015) 053502. [5] C.R. Liao, D.N. Wang, Y. Wang, Microfiber in-line Mach-Zehnder interferometer for strain sensing, Opt. Lett. 38 (2013) 757. [6] M.F. Jaddoa, A.A. Jasim, M.Z.A. Razak, S.W. Harun, H. Ahmad, Highly responsive NaCl detector based on inline microfiber Mach-Zehnder interferometer, Sens. Actuat. A: Phys. 237 (2016) 56. [7] H.Y. Lin, C.H. Huang, G.L. Cheng, N.K. Chen, H.C. Chui, Tapered optical fiber sensor based on localized surface plasmon resonance, Opt. Exp. 20 (2012) 21693. [8] C. Li, T.G. Ning, X.D. Wen, J. Li, C. Zhang, C.B. Zhang, Magnetic field and temperature sensor based on a no-core fiber combined with a fiber Bragg grating, Opt. Laser Technol. 72 (2015) 104. [9] P. Zu, C.C. Chan, T.X. Gong, Y.X. Jin, W.C. Wong, X.Y. Dong, Magneto-optical fiber sensor based on bandgap effect of photonic crystal fiber infiltrated with magnetic fluid, Appl. Phys. Lett. 101 (2012) 241118. [10] K. Gasior, T. Martynkien, W. Urbanczyk, Effect of constructional parameters on the performance of a surface plasmon resonance sensor based on a multimode polymer optical fiber, Appl. Opt. 53 (2014) 8167. [11] Y. Zhao, Z.Q. Deng, Q. Wang, Fiber optic SPR sensor for liquid concentration measurement, Sens. Actuat. B-Chem. 192 (2014) 229. [12] V. Bhardwaj, V.K. Singh, Fabrication and characterization of cascaded tapered Mach-Zehnder interferometer for refractive index sensing, Sens. Actuat. A: Phys. 244 (2016) 30. [13] H. Ahmad, M.J. Faruki, M.Z.A. Razak, Z.C. Tiu, M.F. Ismail, Evanescent field interaction of tapered fiber with graphene oxide in generation of widebandwidth mode-locked pulses, Opt. Laser Technol. 88 (2017) 166. [14] J. Li, H.Y. Li, K.Y. Wang, X.N. Zhang, C.B. Yao, Y.D. Zhang, P. Yuan, Plasmon resonance of Ag micro–sphere in fiber taper, Opt. Exp. 21 (2013) 21414. [15] M. Cao, M. Wang, N. Gu, Plasmon singularities from metal nanoparticles in active media: influence of particle shape on the gain threshold, Plasmonics 7 (2012) 347. [16] G.Y. Chen, M. Ding, T.P. Newson, G. Brambilla, A review of microfiber and nanofiber based optical sensors, Open Opt. J. 7 (2013) 32. [17] J. Li, Y.D. Zhang, H.Y. Li, C.B. Yao, P. Yuan, The optical response of the Ag nanosphere with two spindle-shaped cavities in a sub-wavelength quartz fiber, Europhys. Lett. 102 (2013) 67012. [18] J.Y. Lou, Y. Wang, L.M. Tong, Microfiber optical sensors: a review, Sensors (Basel) 14 (2014) 5823. [19] X.F. Fan, W.T. Zheng, D.J. Singh, Light scattering and surface plasmons on small spherical particles, Light: Sci. Appl. 3 (2014) e179.