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Refractive index sensors based on Ag-metalized nanolayer in microstructured optical fibers
Optik 123 (2012) 1167–1170
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Refractive index sensors based on Ag-metalized nanolayer in microstructured optical fibers Wei Wei, Xia Zhang ∗ , Xin Guo, Long Zheng, Jing Gao, Weipeng Shi, Qi Wang, Yongqing Huang, Xiaomin Ren State Key Lab of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, P.O. Box 66 (Room 741), Beijing 100876, China
a r t i c l e
i n f o
Article history: Received 28 February 2011 Accepted 14 July 2011
Keywords: Surface plasmon resonance Microstructured optical fiber Sensors Ag nanolayer
a b s t r a c t We propose refractive index sensors based on Ag-metalized nanolayer in microstructured optical fibers. The surface plasmon resonance modes and the sensing properties are theoretically analyzed using finite element method (FEM). In the calculation, Drude–Lorentz model is used to describe the Metal Dielectric constant. The calculation results show that the sensitivity of Ag-metalized SPR sensor can reach 1500 nm/RIU corresponding to a resolution of 6.67 × 10−5 RIU. Comparing with conventional detecting material-Au under the same structure, the sensitivity and 3 dB bandwidth of our device are better. © 2011 Elsevier GmbH. All rights reserved.
1. Introduction Surface plasmon resonance (SPR) is one of the most promising optical techniques which has broad applications in biology, environment, chemistry, medicine, etc. [1–3]. In SPR technique, a light causes the excitation of electron density oscillations which is known as surface plasmon wave (SPW) at the metal (usually Ag and Au)-dielectric interface [3]. When the phase of the incident light matches with that of SPW, a resonance occurs which results in a sharp decrease in the reflected light intensity [4]. Comparing to traditional fibers, microstructured optical fibers (MOF [5]) have lots of fantastic properties and it has been a hot research issue in recent years [6–8]. Due to the management of the arrangement and size of air holes, we can control the optical property by designing the cross-section structure of MOF according to the requirement to optimize the distribution of light field in MOF. Recently, MOF-based SPR sensors attract much attention and have been used in refractive index sensing of aqueous analyte [9,10]. In the above-mentioned MOF-based SPR sensors, Au is used as the metal material to excite SPR. In this paper we propose a novel design scheme of SPR sensors based on Agmetalized nanolayer in MOF in which Ag is the metal material to excite SPR and less holes arrayed in MOF. Furthermore the
∗ Corresponding author. Tel.: +86 10 62284004 86; fax: +86 10 62284004 88. E-mail address: [email protected] (X. Zhang). 0030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2011.07.044
simple structure of our scheme will contribute to lower accurate drawing process that MOF request. Then we use finite element method to investigate the property of SPR sensors based on Ag-metalized nanolayer in MOF. During the calculation, Drude–Lorentz model is introduced to describe the metal dielectric constant, which is much closer to the real metal status. The calculation results show that the sensitivity and 3 dB width of Ag-metalized sensor are better than Au-metalized sensor.
2. Geometry of a MOF-based SPR sensor The structure of the new MOF-based SPR sensor is shown in Fig. 1. It is composed of a center hole and two layer holes surrounding it. Pitches of the first and second layer hexagonal lattices are = 2 m and Ag = 3.46 m. Holes in the core and the first layer are filled with air nair = 1.0 and the diameters are dc = 0.45 and d1 = 0.5 respectively. The small center air hole is used to lower the refractive index of a core-guided mode to facilitate phase matching with Plasmon. Six big holes which plated with silver film in the second layer are channels of analyte. Their diameters are d2 = 1.2 and the thickness of the silver film is 40 nm. We assume that the MOF is a glass made with refractive index given by the Sellmeier formula. This sensor has not only a simple structure, but also a good performance. It also satisfies the two key requirements that the phase of core-guided mode matching with that of a plasmon wave and optimized microfluidics can be achieved perfectly [4].
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Fig. 1. Schematic of MOF-based SPR sensor. The holes in the center and the first layer are filled with air. Six big holes in the second layer are channels of analyte. They are metalized with a 40 nm layer of silver. The large diameter of Six big holes can not only make them contact more light field, but also simplify the flow of the analyte through them. The small air filled hole in the fiber core is used to lower the refractive index of a core guided mode to facilitate phase matching with a plasmon.
3. SPs excitation by the core-guided mode of MOF We use finite element method (Rsoft) with Perfect Matched Layer (PML) boundaries to find complex propagation constants of the guided modes and analyze the plasmonic nanostructures. The authors [4] describe the dielectric constant of Au in the visible and near-IR region by Drude model, which is applicable in the range of 600–900 nm [11]. So in our calculations, the dielectric constant of silver layer is given by Drude–Lorentz model.
ε(ω) = 1 +
k
εk −ω2 − ak (iω) + bk
where εk , ak and bk are constants that provide the best fit for various metals (gold and silver) when compared with optical constant data of these metals given by Palik et al. [12]. We plot in dB/cm losses curves of the core-guided mode in a 400–1500 nm wavelength range for the two values of the analyte index nanalyte = 1.33 and nanalyte = 1.34, as shown in Fig. 2(a). There are four SPR peaks of Ag which respectively located at 510 nm, 790 nm, 900 nm and 1120 nm in the losses curves. For comparison, the dash dot line shows the confinement loss of the core-guided mode in the absence of a metal coating. The first SPR peak which located at 510 nm is more sensitive than the other three SPR peaks. So in the next sections we focus on this region. The mode field distribution of the core-guided mode at the wavelength corresponding to the first SPR peak is shown in Fig. 2(b). It is obvious that SPs excitation is on a boundary of a metalized hole closet to the fiber core. It is interesting to note that the shape of a metalized surface modifies SPR excitation spectrum. Thus, planar metalized surface supports only one SPR peak, while cylindrical metal layer can support several different SPR peaks [13–15]. Fig. 2(c) and (d) shows the mode field distributions of the core-guided mode at the second, third and forth SPR peak. SPs excitation at the boundary of a metalized hole is also clearly visible. However, most of the SPs intensity distributing away from the fiber core leads to a decreased sensitivity of the core-guided mode losses at the second SPR peak to the changes in the analyte. In principle, by monitoring the changes in excitation of several SPR resonances in cylindrical metallic layers, one can improve sensor sensitivity.
Fig. 2. (a) Loss curves of the core-guided mode exhibiting four loss peaks corresponding to the excitation of various SPs modes in the Ag-metalized holes. The solid line-nanalyte = 1.33, the dash line-nanalyte = 1.34. For comparison, the dash dot line shows the confinement loss of a core-guided mode in the absence of a metal coating. (b)–(e) The energy flux of the core-guided mode at the first, second, third and forth SPR peak with Ag layer at = 510 nm (neff = 1.450422 + 1.046 × 10−4 i), = 790 nm (neff = 1.428719 + 1.869 × 10−4 i), = 900 nm (neff = 1.42081 + 3.818 × 10−4 i) and = 1120 nm (neff = 1.404009 + 1.105 × 10−3 i) respectively.
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Fig. 4. (a) Calculated loss spectra of the MOF (Ag-metalized) core-guided mode exhibiting first loss peak corresponding to the various analyte refractive indices from 1.325 to 1.345. (b) The relationship between nanalyte and resonance peak wavelength in Ag-metalized holes.
Fig. 3. Effect of the fiber structure parameters on the efficiency of SPs excitation, in corporating a 40 nm thickness silver layer. (a) Loss curves of the core-guided mode for various values of the center hole diameter dc = 0.35, 0.4 and 0.45, assuming the fixed diameters of the first and second layer holes d1 = 0.5, d2 = 1.2. (b) Loss curves of the core-guided mode for various values of the holes in the first layer diameter d1 = 0.5, 0.55 and 0.6, assuming the fixed diameters of the center and second layer holes dc = 0.45, d2 = 1.2. (c) Loss curves of the core-guided mode for various values of the holes in the second layer diameter d2 = 1, 1.1 and 1.2, assuming the fixed diameters of the center and first layer holes dc = 0.45, d1 = 0.5.
4. Influencing factors of SPs excitation As the first SPR peak is most sensitive to changes of analyte index in the four SPR peaks, we focus on the first SPR peak which is located at the vicinity of 510 nm. When the diameter of the center hole varies between 0.35, 0.4 and 0.45, change in the position of the SPR peak is shown in Fig. 3(a). The diameters of the holes in the first layer and the second layer are relatively fixed at d1 = 0.5 and d2 = 1.2. In Fig. 3(a), one can observe an overall decrease in the confinement losses of the core-guided mode for the smaller diameter of the center hole. This can be explained by noting that the smaller size of the center hole weakens expulsion of the modal field from the fiber core. This, in turn, leads to the smaller modal presence near the metallic interface, hence lower propagation losses. Another consequence of the modal expansion from the fiber core and into the air-filled microstructure is reduction of the modal refractive index, leading to the shift of the SPR peak towards to longer wavelengths.
Fig. 3(b) shows loss curves of the core-guided mode of the fiber in the vicinity of the first SPR peak for various diameters of the holes in the first layer d1 = 0.5, d1 = 0.55 and d1 = 0.6. And the diameters of the center hole and the holes in the second layer are relatively fixed at dc = 0.45 and d2 = 1.2. In Fig. 3(b), one can observe an overall increase in the confinement losses of the coreguided mode for the smaller size holes in the first layer. We can explain it by noting that the smaller size holes in the first layer lead to the higher refractive index of the microstructured cladding. This in turn, decrease the core-cladding refractive index contrast, hence decreasing modal confinement in the core region, and resulting in higher confinement losses due to coupling to a metal surface in the second layer [16]. Loss curves of the holes in the second layer are shown in Fig. 3(c) in the case of the diameter of the holes varies between 1, 1.1 and 1.2. The center hole and the holes in the first layer are relatively fixed at dc = 0.45 and d1 = 0.5. In Fig. 3(c), one can observe an overall increase in the confinement losses of the core-guided mode for the larger size holes in the second layer. This can be explained by noting that the larger size of holes makes metallic surface closer to the mode field. This can also increase contact area between metallic surface and mode field and lead to more light coupling to metallic surface. As a result, the confinement increases. 5. Sensitivity of MOF-based SPR sensor To investigate the refractive index variation of SPR sensor, we take aqueous analyte as example. The sensitivity of SPR sensor is defined as:
dpeak S (nm/RIU) = dna
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bandwidth of half maximum value) of Ag-coated sensor is smaller than that of Au-coated sensor obviously. The sensitivity of the refractive index sensor is mainly affected by the structure of the MOF, as well as the refractive index of the metal nanolayer in the MOF. Considering the MOF with same structure, the dielectric constant of the metal material is the determining factor of the sensing sensitivity. By comparing the refractive indices of Ag and Au at the resonant wavelength [17], the Ag-metalized sensor exhibits higher sensitivity than the Au-metalized sensor. 6. Conclusion The SPR sensors based on Ag-metalized nanolayer in MOF which has a novel structure is analyzed using finite element method. Through the above calculation, we know how the structure parameters influence the SPR excitation. The calculations also show the corresponding sensitivity of the Agcoated sensors is 1500 nm/RIU, which is higher than that of the Au-coated sensors (1260 nm/RIU). In addition, 3 dB bandwidth of Ag-coated sensors is smaller than that of Au-coated sensors. Acknowledgments
Fig. 5. (a) Calculated loss spectra of the MOF (Au-metalized) core-guided mode exhibiting first loss peak corresponding to the various analyte refractive indices from 1.325 to 1.345. (b) The relationship between nanalyte and resonance peak wavelength in Au-metalized holes.
The shifts of the first SPR peak are shown in Fig. 4(a) corresponding to the analyte refractive index varies from 1.325 to 1.345. And Fig. 4(b) shows the relationship between nanalyte and SPR peak wavelength in Ag-metalized holes. From the above, we can calculate the sensitivity of silver coated sensor is 1500 nm/RIU. We assume that a 0.1 nm change in the position of a SPR peak can be detected reliably. The corresponding sensor resolution is 6.67 × 10−5 RIU. Under the same structure of MOF and the same analyzing model, we investigate the characteristic of Au-metalized sensor for comparison. From Fig. 5(a) and (b), we can calculate the sensitivity of gold coated sensor is 1260 nm/RIU and the corresponding resolution is 7.94 × 10−5 RIU. It is obvious that the sensitivity of Ag coated sensor is higher than Au coated sensor. Furthermore, as shown in Fig. 6, 3 dB bandwidth (3 dB bandwidth, here means the
Fig. 6. Schematic of 3 dB width. The average 3 dB bandwidth values of Ag and Au are 51 nm and Au103 nm.
This work was supported by National Basic Research Program of China (2010CB327605), National Natural Science Foundation of China (61077049), Program for New Century Excellent Talents in University of China (NCET-08-0736), and the 111 Project of China (B07005). References [1] J. Homola, S. Yee, G. Gauglitz, Surface plasmon resonance sensors: review, Sens. Actuat. B: Chem. 54 (1/2) (1999) 3–15. [2] W.L. Barnes, A. Dereux, T.W. Ebbesen, Surface plasmon subwavelength optics, Nature 424 (2003) 824–830. [3] B.D. Gupta, R.K. Verma, Surface plasmon resonance-based fiber optic sensors: principle, probe designs, and some applications, J. Sens. (2009) 1. [4] A. Hassani, M. Skorobogatiy, Design criteria for microstructured-opticalfiber-based surface-plasmon-resonance sensors, J. Opt. Soc. Am. B 24 (2007) 1423–1429. [5] P. Russell, Photonic crystal fibers, Science 299 (5605) (2003) 358–362. [6] H. Ademgil, S. Haxha, Highly nonlinear birefringent photonic crystal fiber, Opt. Commun. 282 (2009) 2831–2835. [7] C.-C. Wang, F. Zhang, R. Geng, C. Liu, T.-G. Ning, Z. Tong, S.-S. Jian, Photonic crystal fiber for fundamental mode operation of multicore fiber lasers and amplifiers, Opt. Commun. 281 (2008) 5364–5371. [8] X. Yu, P. Shum, G.B. Ren, N.Q. Ngo, Photonic crystal fibers with high index infiltrations for refractive index sensing, Opt. Commun. 281 (2008) 4555–4559. [9] M. Hautakorpi, M. Mattinen, H. Ludvigsen, Surface-plasmon-resonance sensor based on three-hole microstructured optical fiber, Opt. Express 16 (12) (2008) 8427–8432. [10] H. Alireza, S. Maksim, Photonic crystal fiber-based plasmonic sensors for the detection of biolayer thickness, JOSA B Vol. 26 (8) (2009) 1550–1557. [11] K. Kurihara, K. Nakamura, E. Hirayama, K. Suzuki, An absorption-based surface plasmon resonance sensor applied to sodium ion sensing based on an ion-selective optode membrane, Anal. Chem. 74 (2002) 6323–6329. [12] E.D. Palik, Handbook of optical constants of solids, Academic Press, San Diego, 1998. [13] B.T. Kuhlmey, K. Pathmanandavel, R.C. McPhedran, Multipole analysis of photonic crystal fibers with coated inclusions, Opt. Express 14 (22) (2006) 10851–10864. [14] S.J. Al-Bader, M. Imtaar, Optical fiber hybrid-surface Plasmon polaritons, J. Opt. Soc. Am. B 10 (1993) 83–88. [15] A. Diez, M.V. Andres, J.L. Cruz, In-line fiber-optic sensors based on the excitation of surface plasma modes in metal-coated tapered fibers, Sens. Actuat. B 73 (2001) 95–99. [16] J. Hou, D. Bird, A. George, S. Maier, B. Kuhlmey, J.C. Knight, Metallic mode confinement in microstructured fibres, Opt. Express 16 (9) (2008) 5983–5990. [17] D.E. Gray, American Institute of Physics Handbook, McGraw-Hill, New York, 1972.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Refractive index sensors based on Ag-metalized nanolayer in microstructured optical fibers Wei Wei, Xia Zhang ∗ , Xin Guo, Long Zheng, Jing Gao, Weipeng Shi, Qi Wang, Yongqing Huang, Xiaomin Ren State Key Lab of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, P.O. Box 66 (Room 741), Beijing 100876, China
a r t i c l e
i n f o
Article history: Received 28 February 2011 Accepted 14 July 2011
Keywords: Surface plasmon resonance Microstructured optical fiber Sensors Ag nanolayer
a b s t r a c t We propose refractive index sensors based on Ag-metalized nanolayer in microstructured optical fibers. The surface plasmon resonance modes and the sensing properties are theoretically analyzed using finite element method (FEM). In the calculation, Drude–Lorentz model is used to describe the Metal Dielectric constant. The calculation results show that the sensitivity of Ag-metalized SPR sensor can reach 1500 nm/RIU corresponding to a resolution of 6.67 × 10−5 RIU. Comparing with conventional detecting material-Au under the same structure, the sensitivity and 3 dB bandwidth of our device are better. © 2011 Elsevier GmbH. All rights reserved.
1. Introduction Surface plasmon resonance (SPR) is one of the most promising optical techniques which has broad applications in biology, environment, chemistry, medicine, etc. [1–3]. In SPR technique, a light causes the excitation of electron density oscillations which is known as surface plasmon wave (SPW) at the metal (usually Ag and Au)-dielectric interface [3]. When the phase of the incident light matches with that of SPW, a resonance occurs which results in a sharp decrease in the reflected light intensity [4]. Comparing to traditional fibers, microstructured optical fibers (MOF [5]) have lots of fantastic properties and it has been a hot research issue in recent years [6–8]. Due to the management of the arrangement and size of air holes, we can control the optical property by designing the cross-section structure of MOF according to the requirement to optimize the distribution of light field in MOF. Recently, MOF-based SPR sensors attract much attention and have been used in refractive index sensing of aqueous analyte [9,10]. In the above-mentioned MOF-based SPR sensors, Au is used as the metal material to excite SPR. In this paper we propose a novel design scheme of SPR sensors based on Agmetalized nanolayer in MOF in which Ag is the metal material to excite SPR and less holes arrayed in MOF. Furthermore the
∗ Corresponding author. Tel.: +86 10 62284004 86; fax: +86 10 62284004 88. E-mail address: [email protected] (X. Zhang). 0030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2011.07.044
simple structure of our scheme will contribute to lower accurate drawing process that MOF request. Then we use finite element method to investigate the property of SPR sensors based on Ag-metalized nanolayer in MOF. During the calculation, Drude–Lorentz model is introduced to describe the metal dielectric constant, which is much closer to the real metal status. The calculation results show that the sensitivity and 3 dB width of Ag-metalized sensor are better than Au-metalized sensor.
2. Geometry of a MOF-based SPR sensor The structure of the new MOF-based SPR sensor is shown in Fig. 1. It is composed of a center hole and two layer holes surrounding it. Pitches of the first and second layer hexagonal lattices are = 2 m and Ag = 3.46 m. Holes in the core and the first layer are filled with air nair = 1.0 and the diameters are dc = 0.45 and d1 = 0.5 respectively. The small center air hole is used to lower the refractive index of a core-guided mode to facilitate phase matching with Plasmon. Six big holes which plated with silver film in the second layer are channels of analyte. Their diameters are d2 = 1.2 and the thickness of the silver film is 40 nm. We assume that the MOF is a glass made with refractive index given by the Sellmeier formula. This sensor has not only a simple structure, but also a good performance. It also satisfies the two key requirements that the phase of core-guided mode matching with that of a plasmon wave and optimized microfluidics can be achieved perfectly [4].
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Fig. 1. Schematic of MOF-based SPR sensor. The holes in the center and the first layer are filled with air. Six big holes in the second layer are channels of analyte. They are metalized with a 40 nm layer of silver. The large diameter of Six big holes can not only make them contact more light field, but also simplify the flow of the analyte through them. The small air filled hole in the fiber core is used to lower the refractive index of a core guided mode to facilitate phase matching with a plasmon.
3. SPs excitation by the core-guided mode of MOF We use finite element method (Rsoft) with Perfect Matched Layer (PML) boundaries to find complex propagation constants of the guided modes and analyze the plasmonic nanostructures. The authors [4] describe the dielectric constant of Au in the visible and near-IR region by Drude model, which is applicable in the range of 600–900 nm [11]. So in our calculations, the dielectric constant of silver layer is given by Drude–Lorentz model.
ε(ω) = 1 +
k
εk −ω2 − ak (iω) + bk
where εk , ak and bk are constants that provide the best fit for various metals (gold and silver) when compared with optical constant data of these metals given by Palik et al. [12]. We plot in dB/cm losses curves of the core-guided mode in a 400–1500 nm wavelength range for the two values of the analyte index nanalyte = 1.33 and nanalyte = 1.34, as shown in Fig. 2(a). There are four SPR peaks of Ag which respectively located at 510 nm, 790 nm, 900 nm and 1120 nm in the losses curves. For comparison, the dash dot line shows the confinement loss of the core-guided mode in the absence of a metal coating. The first SPR peak which located at 510 nm is more sensitive than the other three SPR peaks. So in the next sections we focus on this region. The mode field distribution of the core-guided mode at the wavelength corresponding to the first SPR peak is shown in Fig. 2(b). It is obvious that SPs excitation is on a boundary of a metalized hole closet to the fiber core. It is interesting to note that the shape of a metalized surface modifies SPR excitation spectrum. Thus, planar metalized surface supports only one SPR peak, while cylindrical metal layer can support several different SPR peaks [13–15]. Fig. 2(c) and (d) shows the mode field distributions of the core-guided mode at the second, third and forth SPR peak. SPs excitation at the boundary of a metalized hole is also clearly visible. However, most of the SPs intensity distributing away from the fiber core leads to a decreased sensitivity of the core-guided mode losses at the second SPR peak to the changes in the analyte. In principle, by monitoring the changes in excitation of several SPR resonances in cylindrical metallic layers, one can improve sensor sensitivity.
Fig. 2. (a) Loss curves of the core-guided mode exhibiting four loss peaks corresponding to the excitation of various SPs modes in the Ag-metalized holes. The solid line-nanalyte = 1.33, the dash line-nanalyte = 1.34. For comparison, the dash dot line shows the confinement loss of a core-guided mode in the absence of a metal coating. (b)–(e) The energy flux of the core-guided mode at the first, second, third and forth SPR peak with Ag layer at = 510 nm (neff = 1.450422 + 1.046 × 10−4 i), = 790 nm (neff = 1.428719 + 1.869 × 10−4 i), = 900 nm (neff = 1.42081 + 3.818 × 10−4 i) and = 1120 nm (neff = 1.404009 + 1.105 × 10−3 i) respectively.
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Fig. 4. (a) Calculated loss spectra of the MOF (Ag-metalized) core-guided mode exhibiting first loss peak corresponding to the various analyte refractive indices from 1.325 to 1.345. (b) The relationship between nanalyte and resonance peak wavelength in Ag-metalized holes.
Fig. 3. Effect of the fiber structure parameters on the efficiency of SPs excitation, in corporating a 40 nm thickness silver layer. (a) Loss curves of the core-guided mode for various values of the center hole diameter dc = 0.35, 0.4 and 0.45, assuming the fixed diameters of the first and second layer holes d1 = 0.5, d2 = 1.2. (b) Loss curves of the core-guided mode for various values of the holes in the first layer diameter d1 = 0.5, 0.55 and 0.6, assuming the fixed diameters of the center and second layer holes dc = 0.45, d2 = 1.2. (c) Loss curves of the core-guided mode for various values of the holes in the second layer diameter d2 = 1, 1.1 and 1.2, assuming the fixed diameters of the center and first layer holes dc = 0.45, d1 = 0.5.
4. Influencing factors of SPs excitation As the first SPR peak is most sensitive to changes of analyte index in the four SPR peaks, we focus on the first SPR peak which is located at the vicinity of 510 nm. When the diameter of the center hole varies between 0.35, 0.4 and 0.45, change in the position of the SPR peak is shown in Fig. 3(a). The diameters of the holes in the first layer and the second layer are relatively fixed at d1 = 0.5 and d2 = 1.2. In Fig. 3(a), one can observe an overall decrease in the confinement losses of the core-guided mode for the smaller diameter of the center hole. This can be explained by noting that the smaller size of the center hole weakens expulsion of the modal field from the fiber core. This, in turn, leads to the smaller modal presence near the metallic interface, hence lower propagation losses. Another consequence of the modal expansion from the fiber core and into the air-filled microstructure is reduction of the modal refractive index, leading to the shift of the SPR peak towards to longer wavelengths.
Fig. 3(b) shows loss curves of the core-guided mode of the fiber in the vicinity of the first SPR peak for various diameters of the holes in the first layer d1 = 0.5, d1 = 0.55 and d1 = 0.6. And the diameters of the center hole and the holes in the second layer are relatively fixed at dc = 0.45 and d2 = 1.2. In Fig. 3(b), one can observe an overall increase in the confinement losses of the coreguided mode for the smaller size holes in the first layer. We can explain it by noting that the smaller size holes in the first layer lead to the higher refractive index of the microstructured cladding. This in turn, decrease the core-cladding refractive index contrast, hence decreasing modal confinement in the core region, and resulting in higher confinement losses due to coupling to a metal surface in the second layer [16]. Loss curves of the holes in the second layer are shown in Fig. 3(c) in the case of the diameter of the holes varies between 1, 1.1 and 1.2. The center hole and the holes in the first layer are relatively fixed at dc = 0.45 and d1 = 0.5. In Fig. 3(c), one can observe an overall increase in the confinement losses of the core-guided mode for the larger size holes in the second layer. This can be explained by noting that the larger size of holes makes metallic surface closer to the mode field. This can also increase contact area between metallic surface and mode field and lead to more light coupling to metallic surface. As a result, the confinement increases. 5. Sensitivity of MOF-based SPR sensor To investigate the refractive index variation of SPR sensor, we take aqueous analyte as example. The sensitivity of SPR sensor is defined as:
dpeak S (nm/RIU) = dna
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bandwidth of half maximum value) of Ag-coated sensor is smaller than that of Au-coated sensor obviously. The sensitivity of the refractive index sensor is mainly affected by the structure of the MOF, as well as the refractive index of the metal nanolayer in the MOF. Considering the MOF with same structure, the dielectric constant of the metal material is the determining factor of the sensing sensitivity. By comparing the refractive indices of Ag and Au at the resonant wavelength [17], the Ag-metalized sensor exhibits higher sensitivity than the Au-metalized sensor. 6. Conclusion The SPR sensors based on Ag-metalized nanolayer in MOF which has a novel structure is analyzed using finite element method. Through the above calculation, we know how the structure parameters influence the SPR excitation. The calculations also show the corresponding sensitivity of the Agcoated sensors is 1500 nm/RIU, which is higher than that of the Au-coated sensors (1260 nm/RIU). In addition, 3 dB bandwidth of Ag-coated sensors is smaller than that of Au-coated sensors. Acknowledgments
Fig. 5. (a) Calculated loss spectra of the MOF (Au-metalized) core-guided mode exhibiting first loss peak corresponding to the various analyte refractive indices from 1.325 to 1.345. (b) The relationship between nanalyte and resonance peak wavelength in Au-metalized holes.
The shifts of the first SPR peak are shown in Fig. 4(a) corresponding to the analyte refractive index varies from 1.325 to 1.345. And Fig. 4(b) shows the relationship between nanalyte and SPR peak wavelength in Ag-metalized holes. From the above, we can calculate the sensitivity of silver coated sensor is 1500 nm/RIU. We assume that a 0.1 nm change in the position of a SPR peak can be detected reliably. The corresponding sensor resolution is 6.67 × 10−5 RIU. Under the same structure of MOF and the same analyzing model, we investigate the characteristic of Au-metalized sensor for comparison. From Fig. 5(a) and (b), we can calculate the sensitivity of gold coated sensor is 1260 nm/RIU and the corresponding resolution is 7.94 × 10−5 RIU. It is obvious that the sensitivity of Ag coated sensor is higher than Au coated sensor. Furthermore, as shown in Fig. 6, 3 dB bandwidth (3 dB bandwidth, here means the
Fig. 6. Schematic of 3 dB width. The average 3 dB bandwidth values of Ag and Au are 51 nm and Au103 nm.
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