Open Access
Add to BibSonomy
Abstract
An opening up dual-core microstructured optical fiber based surface plasmon resonance sensor is numerically investigated for the measurement of a broad refractive index (RI) range. An open sensing channel is designed to facilitate the gold coating and accelerate the analyte infiltration. Results indicate that the sensitivity curve shows a nearly linear feature in two parts, and the maximal sensitivity is 4900 nm/RIU when the RI of the analyte is close to that of the background material of the fiber. Moreover, the sensitivities in low RI range and the signal to noise ratio can be improved by introducing air holes into the core center.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to the extreme sensitivity and the absence of labeling procedure, the surface plasmon resonance (SPR) technology has been widely adopted to detect the physical, chemical and biological quantities [1–6]. The prism-SPR sensor is most frequently used in commercially available devices, which utilize the prism to couple light to the surface plasmon polariton (SPP) at the metal surface. However, the large size, non-flexible and complicated design of these devices make them difficult to optimize the sensing systems and to perform distributed sensing [1]. In order to overcome these drawbacks, the optical fiber-based SPR sensor have been proposed [1,2]. The use of an optical fiber instead of a bulky prism could provide a high degree of integration and allows it to be used in a very small volume. However, in these fiber-based designs, to enhance the coupling between the core modes and the SPP modes, these fibers have to be processed to enable their cores exposing to the sensing regions by using various approaches such as side-polishing, stripping cladding and tapering [1,2]. In general, these processing destroy fiber integrity, and making the sensors more fragile. In addition, the sensitivity of these sensors is hard to be further improved for aqueous analytes because of the phase matching problem [5–8]. To solve these problems, the microstructured optical fiber (MOF)-based SPR sensors have been widely studied, which are constructed by coating the MOF holes with metal films [5–9]. Because of the flexible design of microstructures, the MOFs cannot only facilitate the phase matching but also provide new properties for SPR sensing [3–13]. Generally, according to the detecting range of the refractive index (RI), the MOF-SPR sensors can be divided into two categories, low RI and high RI sensors. In the low RI MOF-SPR sensors, the analytes are filled into the metal-coated holes in the cladding of the MOFs [7–9]. To maintain the condition of total reflection, the RI of the analytes should be lower than that of the background material of the MOFs. In practice, for the silica MOF-SPR sensors, the maximum detectable na are typically below 1.42 [9]. In the high RI MOF-SPR sensors, the analytes are filled into the holes in the MOF core area to form a new waveguide [10,11]. To satisfy the condition of total reflection, the na must be higher than the RI of the fiber materials. However, in these proposed MOF-SPR sensors [7–11], analytes filling of the tiny holes of the MOFs is typically driven by the capillary action from the distal ends. This is an extremely slow process and making it difficult to perform real-time sensing or distributed sensing. Besides, limited by the condition of total reflection, it is a challenge to design the MOF-SPR sensors that can detect a wide RI range (the RI can be either lower or higher than the fiber materials) which is always desirable in the practical applications. Recently, a few special designs have been shown to have the capability for detection of a wide RI range [12,13]. However, in these designs, the analytes filling of the MOF holes are also required.
To avoid the analytes filling operation, the common approach is to use the opening up MOF structures, such as D-shaped or exposed-core MOF-SPR sensors [14–20]. So far, these opening up MOFs have previously been mainly applied in SPR sensing for measurement of the low RI [14–16,18–20] and high RI ranges [17]. However, for measurement of a wider RI range, it has not been reported yet. In this paper, we propose an opening up dual-core MOF-based SPR sensor for measurement of a large RI range. The opening up part act as the sensing channels that can be directly in contact with the analytes, and providing a possibility for real-time sensing or distributed sensing.
2. Sensor structure
The proposed sensor is fabricated from an opening up dual-core MOF with air holes arranged in a hexagonal way. As shown in Fig. 1, the open slot of the MOF is coated a gold layer and acts as a sensing channel, and consequently avoids the analytes filling operation. To facilitate phase matching with the SPP modes, a small air hole is introduced into the center of the each core to lower the average RI of the core modes. Such opening up structure can be fabricated from the MOFs by using femtosecond laser micromachining or focused ion beam milling [21–23], or can be directly drawn by creating an opening at the preform stage of the fiber fabrication [24,25]. With the capacity for quick response and fast filling, the opening up MOF has been shown to be practical for real time and distributed sensing applications [26,27].
Fig. 1 (a) Schematic diagram of the proposed opening up dual-core MOF-SPR sensor; (b) Cross-section of the SPR sensor part (sizes are not to scale).
The mode characters and coupling properties of the proposed sensor are analyzed by using the finite element method (FEM). For FEM simulation, the distance between the holes is Λ = 2 µm [7–9,12], and the diameters of the cladding holes are d = 0.5Λ. The depth (h) and the width (w) of the sensing channel are 5.75Λ and 0.5Λ, respectively. The thickness of the gold film is 40 nm [7–9,12], and the permittivity of the gold ε(ω) can be calculated by using the Drude-Lorentz model [28]
where the parameters are obtained from Ref [28]. The RIs of fiber material and air holes are assumed to be 1.45 and 1, respectively.3. Results and discussion
3.1 Coupling properties
The dual-core MOF can support two interacting fundamental modes, and forming four core supermodes which include (a) x-polarized even (symmetric) mode, (b) x-polarized odd (anti-symmetric) mode, (c) y-polarized even mode and (d) y-polarized odd mode, as shown in Fig. 2. While the metal-coated open slot with two parallel planes only support the x-polarized odd SPP mode, as shown in the inset B of Fig. 3(b), and it can only coupe to the x-polarized odd core mode because they have the same polarized direction [14,15,17,18].
Fig. 2 Electric field distributions of the (a) x-polarized even, (b) x-polarized odd, (c) y-polarized even and (d) y-polarized odd core supermodes with na = 1.44 at 1200 nm (The red arrows represent the direction of the electric fields, similarly hereinafter).
Fig. 3 (a) Re(neff) curves of the x-polarized odd core mode and x-polarized SPP mode, loss spectra of the x-polarized odd core mode with na = 1.44, 1.45 and 1.46; (b) Electric field distributions of the x-polarized odd core mode A at 900 nm, x-polarized SPP mode B at 950 nm and x-polarized core mode C at 974 nm with na = 1.44.
In order to demonstrate the potential of sensor for sensing a wide RI range, we plot the neff curves of the x-polarized odd core modes and x-polarized SPP modes in Fig. 3(a) when the RI of the analyte (na) is 1.44, 1.45 and 1.46 respectively. Taking the case of na = 1.44, the resonance coupling happens at 974 nm where the real part of neff [Re(neff)] of the core mode [Inset A of Fig. 3(b)] and that of the SPP mode [Inset B of Fig. 3(b)] are equal [Phase-matching point C in Fig. 3(a)]. This coupling process can be identified by observing an obvious resonance peak in the loss spectra of the core mode, as shown in Fig. 3(a), because the energy transfer to the SPP modes which can be clearly shown by the electric field distributions C in Fig. 3(b). For comparison, in Fig. 3(a) we also present the loss spectrum of the x-polarized odd core mode at na = 1.44 for the case when no gold layer is coated.
3.2 Sensor sensitivity
Generally, the neff of a SPP mode is close to that of a bordering analyte (na), therefore the increase of na could increase the neff of the SPP modes [see dashed and dotted red lines in Fig. 3(a)], and leading to the shift of the resonance peak (phase-matching point) toward longer wavelengths as shown in Fig. 3(a). Commonly, the change of na (Δna) can be detected by measuring the shift of the peak (Δλpeak), and the sensitivity in term of RI units (RIU) can be defined as [8]
For instance, when the na increases from 1.44 to 1.45 (Δna = 0.01), as shown in Fig. 3(a), the Δλpeak is 47 nm and the corresponding sensitivity is 4700 nm/RIU according to the Eq. (2). By using the same method, we present the peak wavelengths and the sensitivities of the sensor in the RI range 1.33–1.61 in Fig. 4. It can be found that the sensitivities show a typical peak distribution, and the maximal sensitivity is 4900 nm/RIU in the RI range of 1.45–1.46. This phenomenon is caused by the fact that the sensing channel is inserted into the fiber cladding in this design. When the na is close to the RI of the fiber core, the analyte-core RI contrast become lower, and more evanescent field will penetrate into the sensing channel, and thus resulting in higher sensitivities. It is worthy to note that the sensitivity curve exhibits a nearly linear feature in two parts, the lower RI range of 1.33–1.46 and higher RI range of 1.46–1.61. The linear sensitivity is very desirable for calibration process in the practical applications, which is difficult to obtain with traditional MOF-based SPR sensors [9,11,13,15,17,18].Fig. 4 (a) Peak wavelengths and (b) Sensitivities of the SPR sensor at various na when dc is 0Λ, 0.2Λ and 0.4Λ respectively.
3.3 Discussion
In low RI MOF-SPR sensors, to facilitate the phase matching between the core mode and SPP mode, a common approach is to introduce air holes into the core center, which can lower the neff of the core modes. In this design, we also introduce a small air hole into the center of each fiber core and investigate its influence on the sensing performance. Figure 5(a) shows the Re(neff) curves of the core modes and SPP mode when the na is 1.47 and the diameter of the air hole (dc) is 0Λ, 0.2Λ, and 0.4Λ respectively. The larger dc reduces the neff of the core modes, and thus leads to the fact that the phase-matching points (peak wavelengths) shift to longer wavelengths, as shown in Figs. 5(a), 5(c) and 4(a). The larger dc also extrudes more mode fields from the fiber core [see inset A–C in Fig. 5(b)], and resulting in higher mode losses which can be seen from Fig. 5(c). Another effect of the extruded mode fields is that it can increase the mode presence near the metal interface, hence increasing the coupling intensity between the core modes and SPP modes [see inset D–F in Fig. 5(b)], and resulting in higher mode losses of the resonance peaks which can be also seen from Fig. 5(c). The losses caused by the center holes at the resonance wavelengths is higher than that at the non-resonance wavelengths, and therefore lead to narrower spectral widths of the resonance peaks and higher signal-to-noise ratio (SNR) as shown in Fig. 5(c). In order to further investigate the effects of dc variation on the sensing performance, in Fig. 4 we also present sensitivities of the sensors with the dc = 0.2, and 0.4 respectively. The sensitivity increases when the dc increases in the lower RI range of 1.33–1.46, and weakly depend on the size of dc in the higher RI range of 1.46–1.61. In general, the introduced air hole at the core center not only increases the sensitivities in the low RI sensing range, but also provides narrower resonance spectral width that can give better sensing resolution and SNR. Although it causes additional losses in the loss spectra, it also means that only shorter length of the sensing channel is needed to achieve detectable.
Fig. 5 (a) Re(neff) curves, (b) Electric field distributions of the relevant core modes and (c) Loss spectra of the SPR sensor at na = 1.47 when the dc is 0Λ, 0.2Λ and 0.4Λ respectively.
The fabrication of opening up MOFs as well as the operation of metal coating on the hole surfaces of the MOFs have been achieved by various methods [21–25,29,30]. With these technologies, our designed structure can simplify the sensor fabrication in comparision to the internally metal coated MOF SPR sensors. The proposed SPR sensor could be successfully implemented by using a coaxial or off axial input beam with a Gaussian profile [31,32], and provide a more competitive solution in the biological and chemical sensing applications.
4. Conclusion
In this paper, we propose an opening up dual-core MOF based SPR sensor to measure a broad RI range. The opening up structure cannot only simplify the sensor fabrication but also provide the capacity for real-time sensing or distributed sensing. With the large sensing range and the linear sensitivity, the proposed sensor can be better not only to measure the RI or RI-dependent physical parameters such as pressure, humidity, concentration, temperature, and so forth, but also to characterize and quantify the chemical molecule or biological molecule due to adsorption, chemical reaction or binding reaction.
Funding
Natural Science Foundation of Tianjin, China (15JCYBJC17000); Science and Technology Research Project of Hebei Higher Education, China (ZD2017021).
References
1. J. Homola, S. S. Yee, and G. Gauglitz, “Surface plasmon resonance sensors: Review,” Sens. Actuators B Chem. 54(1–2), 3–15 (1999). [CrossRef]
2. E. Klantsataya, P. Jia, H. Ebendorff-Heidepriem, T. M. Monro, and A. François, “Plasmonic fiber optic refractometric sensors: From conventional architectures to recent design trends,” Sensors (Basel) 17(12), 1–23 (2016). [CrossRef] [PubMed]
3. P. Singh, “SPR biosensors: Historical perspectives and current challenges,” Sens. Actuators B Chem. 229, 110–130 (2016). [CrossRef]
4. S. Zeng, D. Baillargeat, H. P. Ho, and K. T. Yong, “Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications,” Chem. Soc. Rev. 43(10), 3426–3452 (2014). [CrossRef] [PubMed]
5. Y. Zhao, Z. Deng, and J. Li, “Photonic crystal fiber based surface plasmon resonance chemical sensors,” Sens. Actuators B Chem. 202, 557–567 (2014). [CrossRef]
6. A. A. Rifat, R. Ahmed, A. K. Yetisen, H. Butt, A. Sabouri, G. Amouzad Mahdiraji, S. H. Yun, and F. R. Mahamd Adikan, “Photonic crystal fiber based plasmonic sensors,” Sens. Actuators B Chem. 243, 311–325 (2017). [CrossRef]
7. A. Hassani and M. Skorobogatiy, “Design of the microstructured optical fiber-based surface plasmon resonance sensors with enhanced microfluidics,” Opt. Express 14(24), 11616–11621 (2006). [CrossRef] [PubMed]
8. A. Hassani and M. Skorobogatiy, “Design criteria for the microstructured optical fiber-based surface plasmon resonance sensors,” J. Opt. Soc. Am. B 24(6), 1423–1429 (2007). [CrossRef]
9. X. Yu, Y. Zhang, S. S. Pan, P. Shum, M. Yan, Y. Leviatan, and C. M. Li, “A selectively coated photonic crystal fiber based surface plasmon resonance sensors,” J. Opt. 12(1), 015005 (2010). [CrossRef]
10. B. H. Liu, Y. X. Jiang, X. S. Zhu, X. L. Tang, and Y. W. Shi, “Hollow fiber surface plasmon resonance sensor for the detection of liquid with high refractive index,” Opt. Express 21(26), 32349–32357 (2013). [CrossRef] [PubMed]
11. N. Luan and J. Yao, “High refractive index surface plasmon resonance sensor based on a silver wire filled hollow fiber,” IEEE Photonics J. 8(1), 1 (2016). [CrossRef]
12. B. Shuai, L. Xia, Y. Zhang, and D. Liu, “A multi-core holey fiber based plasmonic sensor with large detection range and high linearity,” Opt. Express 20(6), 5974–5986 (2012). [CrossRef] [PubMed]
13. N. Luan and J. Yao, “A hollow-core photonic crystal fiber-based SPR sensor with large detection range,” IEEE Photonics J. 9(3), 1 (2017). [CrossRef]
14. N. Luan, R. Wang, W. Lv, and J. Yao, “Surface plasmon resonance sensor based on D-shaped microstructured optical fiber with hollow core,” Opt. Express 23(7), 8576–8582 (2015). [CrossRef] [PubMed]
15. A. A. Rifat, R. Ahmed, G. A. Mahdiraji, and F. M. Adikan, “Highly sensitive D-shaped photonic crystal fiber-based plasmonic biosensor in visible to near-IR,” IEEE Sens. J. 17(9), 2776–2783 (2017). [CrossRef]
16. C. Liu, W. Su, Q. Liu, X. Lu, F. Wang, T. Sun, and P. K. Chu, “Symmetrical dual D-shape photonic crystal fibers for surface plasmon resonance sensing,” Opt. Express 26(7), 9039–9049 (2018). [CrossRef] [PubMed]
17. N. Luan, L. Zhao, Y. Lian, and S. Lou, “A high refractive index plasmonic sensor based on D-shaped photonic crystal fiber with laterally accessible hollow-core,” IEEE Photonics J. 10(5), 6803707 (2018). [CrossRef]
18. N. Luan, R. Wang, W. Lv, and J. Yao, “Surface plasmon resonance sensor based on exposed-core microstructured optical fibres,” Electron. Lett. 51(9), 714–715 (2015). [CrossRef]
19. E. Klantsataya, A. François, H. Ebendorff-Heidepriem, P. Hoffmann, and T. M. Monro, “Surface plasmon scattering in exposed core optical fiber for enhanced resolution refractive index sensing,” Sensors (Basel) 15(10), 25090–25102 (2015). [CrossRef] [PubMed]
20. N. Luan and J. Yao, “Surface plasmon resonance sensor based on exposed-core microstructured optical fiber placed with a silver wire,” IEEE Photonics J. 8(1), 4800508 (2016). [CrossRef]
21. A. van Brakel, C. Grivas, M. N. Petrovich, and D. J. Richardson, “Micro-channels machined in microstructured optical fibers by femtosecond laser,” Opt. Express 15(14), 8731–8736 (2007). [CrossRef] [PubMed]
22. F. Wang, W. Yuan, O. Hansen, and O. Bang, “Selective filling of photonic crystal fibers using focused ion beam milled microchannels,” Opt. Express 19(18), 17585–17590 (2011). [CrossRef] [PubMed]
23. C. Martelli, P. Olivero, J. Canning, N. Groothoff, B. Gibson, and S. Huntington, “Micromachining structured optical fibers using focused ion beam milling,” Opt. Lett. 32(11), 1575–1577 (2007). [CrossRef] [PubMed]
24. F. M. Cox, R. Lwin, M. C. J. Large, and C. M. B. Cordeiro, “Opening up optical fibres,” Opt. Express 15(19), 11843–11848 (2007). [CrossRef] [PubMed]
25. R. Kostecki, H. Ebendorff-Heidepriem, C. Davis, G. McAdam, S. C. Warren-Smith, and T. M. Monro, “Silica exposed-core microstructured optical fibers,” Opt. Mater. Express 2(11), 1538–1547 (2012). [CrossRef]
26. S. C. Warren-Smith, E. Sinchenko, P. Stoddart, and T. Monro, “Distributed fluorescence sensing using exposed core microstructured optical fiber,” IEEE Photonics Technol. Lett. 22(18), 1385–1387 (2010). [CrossRef]
27. S. C. Warren-Smith, H. Ebendorff-Heidepriem, T. C. Foo, R. Moore, C. Davis, and T. M. Monro, “Exposed-core microstructured optical fibers for real-time fluorescence sensing,” Opt. Express 17(21), 18533–18542 (2009). [CrossRef] [PubMed]
28. A. Vial, A.-S. Grimault, D. Macías, D. Barchiesi, and M. L. de la Chapelle, “Improved analytical fit of gold dispersion: Application to the modeling of extinction spectra with a finite-difference time-domain method,” Phys. Rev. B Condens. Matter Mater. Phys. 71(8), 085416 (2005). [CrossRef]
29. P. J. A. Sazio, A. Amezcua-Correa, C. E. Finlayson, J. R. Hayes, T. J. Scheidemantel, N. F. Baril, B. R. Jackson, D. J. Won, F. Zhang, E. R. Margine, V. Gopalan, V. H. Crespi, and J. V. Badding, “Microstructured optical fibers as high-pressure microfluidic reactors,” Science 311(5767), 1583–1586 (2006). [CrossRef] [PubMed]
30. J. A. Harrington, “A review of IR transmitting, hollow waveguides,” Fiber Integr. Opt. 19(3), 211–217 (2000). [CrossRef]
31. B. M. Shalaby, V. Kermene, D. Pagnoux, A. Desfarges-Berthelemot, and A. Barthélémy, “Phase-locked supermode emissions from a dual multicore fiber laser,” Appl. Phys. B 105(2), 213–217 (2011). [CrossRef]
32. B. M. Shalaby, V. Kermene, D. Pagnoux, A. Desfarges-Berthelemot, A. Barthélémy, A. Popp, M. A. Ahmed, A. Voss, and T. Graf, “19-cores Yb-fiber laser with mode selection for improved beam brightness,” Appl. Phys. B 100(4), 859–864 (2010). [CrossRef]
