- Email: [email protected]
Three-core photonic crystal fiber surface plasmon resonance sensor
Optical Fiber Technology 46 (2018) 306–310
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Three-core photonic crystal fiber surface plasmon resonance sensor a
a
b,⁎
Kai Tong , Fucheng Wang , Meiting Wang a b
a
, Peng Dang , Yunxuan Wang
a
T
School of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China College of Liren, Yanshan University, Qinhuangdao 066004, China
ARTICLE INFO
ABSTRACT
Keywords: Surface plasmon resonance Photonic crystal fiber Biosensor Finite element method Refractive index sensitivity
A three-core photonic crystal fiber (PCF) surface plasmon resonance (SPR) biosensor with high performance is presented. The structure of the biosensor is easy to implement and has good repetition. The outer layer of the fiber structure is gold film. The analyte is placed on the surface of the gold film. The Full vector finite element method is used to numerical simulation and analysis the transmission mode and characteristics of the three-core PCF-SPR biosensor. The influence of sensor structure parameters on sensing performance is investigated. The structure parameters are optimized. The optimum structure parameters are given. The refractive index (RI) sensitivity of the sensor is discussed there. The results show that the average refractive index sensitivity of the sensor is 3435 nm/RIU in the sensing range of 1.33–1.40, and the resolution is 2.91 × 10−6 RIU. The sensor has the advantages of high sensitivity and high resolution. And it can be applied to real-time detection of the biomolecules and small drug-molecules.
1. Introduction Surface plasmon resonance (SPR) sensing technology has been widely used in biochemistry, medical pharmacy and environmental monitoring due to its high sensitivity and high resolution [1]. It has become a focus hotspot in biosensor technology [2]. Optical fiber has the advantages of small volume, anti-electromagnetic interference, long distance transmission and strong environmental adaptability [3,4]. Photonic crystal fiber (PCF) has the characteristics of flexible and adjustable structure as a special optical fiber. In recent years, the biosensing technology based on PCF has received continuous attention in the field of biochemical sensing [5]. The usual methods are making metal film on the inner surface of optical fiber air holes or fill metal wires, through these designs, optical fiber SPR sensor can be realized [6]. However, because it is difficult to infiltrate the analyte in the inner air holes for practical, the manufacture of these designs is very challenging. Therefore, several externally coated metallic film SPR sensors have been introduced. A muliti-hole optical fiber SPR sensor based on gold and TiO2 was proposed by Gao et al. [7]. The detection range of the sensor is small and the sensitivity is relatively low. A two-layer circular lattice photonic crystal fiber is proposed by Hasan et al. [8] in 2017, the proposed sensor shows maximum sensitivity of 2200 nm/RIU in the sensing range between 1.33 and 1.36. In 2018, a surface plasmon resonance based D-shaped photonic crystal fiber sensor has been reported with a sensitivity of 3300 nm/RIU in the analyte refractive index
⁎
1.36–1.38 [9].On the other hand, externally coated metallic film SPR sensors with multi-core have seldom been rarely reported. A three-core photonic crystal fiber biosensor is proposed in this paper to improve the sensitivity and the performance. The optimum structure parameters are given. The average refractive index sensitivity of the sensor is 3435 nm/RIU in the sensing range of 1.33–1.40, and the resolution is 2.91 × 10−6 RIU. The metal layer can be integrated outside the fiber structured by using sputtering technique, high pressure chemical vapor deposition (CVD) technique [10,11]. The sensing layer is located on the surface of the metal layer, so that the measured analyte can be easily flowed or dripped onto the surface of the metal thin film layer. 2. Structural design and theoretical model The structure of the three-core photonic crystal fiber biosensor is shown in Fig. 1. The basic structure is a photonic crystal fiber. This kind of fiber composed of a single material can be fabricated using the stack &draw method. As can be seen from Fig. 1(a), the capillary of the second layer is replaced by solid rods to form the three cores of the PCF. The three cores are recorded as 1, 2, 3 in counter-clockwise direction respectively. The small center air hole plays an important role in tuning the coupling between fundamental mode and SPP mode [12,13] with a diameter of dc. The special air holes with a diameter of d1 on the outer layer are conducive to the excitation of SPP mode. All the three layers
Corresponding author. E-mail address: [email protected] (M. Wang).
https://doi.org/10.1016/j.yofte.2018.11.014 Received 8 August 2018; Received in revised form 12 October 2018; Accepted 9 November 2018 Available online 15 November 2018 1068-5200/ © 2018 Elsevier Inc. All rights reserved.
Optical Fiber Technology 46 (2018) 306–310
K. Tong et al.
Fig. 1. Structure of three-core PCF-SPR sensor.
air holes are arranged in a hexagon arrangement with a lattice pitch Ʌ, and the air hole diameter of the cladding is d. The capillary pre-fabricated structure is shown in Fig. 1(b). The required three-core PCF structure can be obtained under the conditions of precise drawing temperature, drawing speed, and delivery rod speed [14]. Schematic diagram of the proposed sensor set-up is presented in Fig. 1(c). The analyte adsorbed on the metal horizontal surface through simple flow or drip. The incident light is provided by a wide band light source (BBS) and then injected into D-shaped PCF-SPR biosensor after being polarized by a polarization controller. Finally, the output spectrum is detected by optical spectrum analyzer (OSA). The substrate material of photonic crystal fiber is pure silica whose material dispersion can be calculated by Sellmeier equation [15]:
n=
1+
A i2 2 N i=1 2 li2
the Sellmeier equation is closer to its true refractive index value. The surface plasmon resonance metal material is Au with thickness of dg. The dielectric constant of Au is given by the Drude dispersion model [16]: Au
=
2 D
( + j D)
(
2
.
L
2 L)
+j
L
(2)
where, high-frequency dielectric constant ε∞ = 5.9673. The angular frequency can be expressed as ω = 2πc/λ, where c is the velocity of light. ωD and γD are plasma frequency and damping frequency respectively, ωD/2π = 2113.6THZ,γD/2π = 15.92THZ. The weighting factor is Δε = 1.09. ГL and ΩL are expressed as ГL/2 = 104.86 Hz, ΩL/ 2π = 650.07 THz [17],respectively. 3. Mode analysis of sensor
(1)
where, λ is the wavelength of incident light, N = 3, Ai and li are dispersion coefficients, A1 = 0.6961663, A2 = 0.4079426, A3 = 0.8974794, l1 = 0.0684043, l2 = 0.1162414, l3 = 9.896161. The relationship between the refractive index of SiO2 and the wavelength of incident light is nonlinear. The refractive index of SiO2 calculated by
The Full vector finite element method (FEM) is used to analyze the transmission mode and the sensing characteristic of the photonic crystal fiber surface plasmon resonance sensor. Because the structure of sensor is three-core, the transmission mode is more complex than the single-core structure. Fig. 2 shows the effective refractive index and electric field
Fig. 2. Field profile of sensor at λ = 650 nm.(a) Super mode S1; (b) Super mode S2; (c) Fundamental mode.
307
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K. Tong et al.
distribution of the relevant mode distribution of PCF with the refractive index (RI) of analyte is na = 1.37 and the incident wavelength is λ = 650 nm. Super model is formed by the superposition of the light fields of each core. But the direction of the electric field of each core is not the same, and the number of super mode is consistent with the number of core [18]. As shown in Fig. 2, the red arrow in the diagram represents the direction of the electric field. Fig. 2(a)–(c) are recorded as super-mode S1, super-mode S2 and fundamental mode, respectively. The corresponding effective refractive index is 1.447 + i9.016 × 10–6, 1.447 + i9.006 × 10–6, 1.448 + i5.594 × 10–6. It can be seen that the real part of the effective refractive index of the fundamental mode is the largest. Although the difference in the effective refractive index of the three modes is very small, the SPR resonance mode is quite different. The imaginary part of the effective refractive index of the fundamental mode is much smaller than S1 mode and S2 mode, and the direction of the electric field in the core of the fundamental mode is the same. Moreover, the S1 mode and the S2 mode do not excite the SPP mode to resonate with it. Therefore, only the characteristics of the fundamental mode and the corresponding SPP mode need to be analyzed there. The sensor is a single mode fiber due to triple symmetry structure. For the fundamental mode, they have the same propagation constant from the perspective of polarization. In other words, the polarization characteristics are the same. The x-polarization mode and y-polarization mode transmit at the same effective refractive index and there is no birefringence phenomenon [19].Only the characteristics of the y-polarization mode are analyzed here. In the field of waveguide optics, the confinement loss is calculated according to the following equation:
(dB/cm) = 8.686 × (2 / )Im(neff ) × 10 4
Fig. 4. Fundamental loss spectrums for d1 = 0.2 Λ, 0.25 Λ and 0.3 Λ with na = 1.37(Λ = 2 μm, dc = 0.15 Λ, d = 0.25 Λ).
most intense. 4. Sensor structural parameters optimization The parameters of the designed sensor are optimized using the Full vector finite element method based COMSOL Multiphysics software with the Perfect Matching Layer (PML) as boundary conditions. The fundamental loss spectrums are shown in Fig. 4 with the outer layer air hole diameter d1 is 0.2 Λ, 0.25 Λ and 0.3 Λ, respectively. It can be seen that the resonance wavelengths remain unchanged. The peak intensity increases with d1 decreases. In other words, the SPR intensity is enhanced. This is because the energy confined to the core weakens when the value of d1 decreases, therefore the resonance intensity between core mode and SPP mode is weakened. The fundamental mode loss spectrum has the smallest FWHM when d1 = 0.25 Λ, that is the best parameters of the outer layer air hole. The influence of the hole size pitch Λ on the sensing performance is shown in Fig. 5(a) with the value is 1.9 µm, 2.0 µm and 2.1 µm, respectively. It can be seen that both the peak intensity and the resonance wavelength decrease as Λ increases. This is because the energy confined to the core weakens when the value of Λ increases and the resonance intensity between core mode and SPP mode is weakened. The peak intensity and resonant wavelength of the fundamental mode loss spectrums with different the value of Λ are shown in Fig. 5(b) when the RI of the analyte is 1.36 and 1.37, respectively. The difference of resonant wavelengths Δλ between adjacent refractive index media is the largest when Λ = 2µm. Therefore, the best parameter of the air hole pitch is Λ = 2 µm. At the same time, it can be seen that the corresponding resonant wavelength and peak intensity is 1.9 μm, 2 μm and 2.1 μm are 690 nm (28.55 dB/cm), 686 nm (27.52 dB/cm), and 684 nm (26.22 dB/cm), respectively. The small change in the value indicates that the variation of the hole pitch (Λ) has a small influence on the sensing performance of the sensor. The gold is chosen as the surface plasmon resonance excitation material due to the advantages of chemical stability and wavelength drift. The thickness of gold film is also an important parameter that affects the performances of sensor. The fundamental loss spectrums are shown in Fig. 6 with the thickness is 35 nm, 40 nm and 45 nm, respectively. As the dg increases, the resonant wavelength red-shift presented and the intensity of the SPR weakens. The Fundamental peak loss and resonant wavelength is shown in Fig. 6(b) with dg under all numerical values with na = 1.37 and 1.38. The thicker the gold film layer is, the larger the damping loss is. However, the difference of resonant wavelengths Δλ between adjacent refractive index media increases with the increases of gold thickness. Therefore, the best thickness of gold is set as 40 nm.
(3)
where, Im(neff) is the imaginary part of the effective refractive index. From Eq. (3), it can be seen that the transmission loss is proportional to the imaginary part of the effective refractive index. The proposed sensor’s electric field profile and phase matching property are shown in Fig. 3. The red line and blue line in the figure represent the real part of the effective index of the fundamental mode and the SPP mode, respectively. The corresponding electric field diagrams are shown in Fig. 3(a) and (b). The electric field distribution of the SPP mode at non-resonant wavelengths is shown in Fig. 3(a). All the energy is concentrated on the thin gold layer. The electric field distribution of the SPP mode at non-resonant wavelengths is shown in Fig. 3(b) and all the energy is concentrated in the gold film. The electric field distribution of the fundamental mode at the resonance wavelength is shown in Fig. 3(c). The energy is confined to the core and some energy is distributed on the surface of gold—the SPR is successfully excited. The resonant wavelength is 686 nm where the energy coupling is
Fig. 3. Dispersion relations with analyte na = 1.37. (na = 1.37, Λ = 2 μm, dc = 0.15 Λ, d1 = 0.5 Λ, d = 0.25 Λ, h = 2.6 μm, dg = 40 nm). 308
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Fig. 5. (a) Fundamental loss spectrums for Λ = 1.9 µm, 2.1 µm and 2.1 µm with na = 1.37; (b) Fundamental peak loss and resonant wavelength for various values of Λ with na = 1.36 and 1.37 (dc = 0.15 Λ, d = 0.5 Λ, d1 = 0.25 Λ).
Fig. 6. (a) Fundamental loss spectrums for dg = 35 nm, 40 nm and 45 nm with na = 1.37; (b) Fundamental peak loss and resonant wavelength for various values of dg with na = 1.36 and 1.37. (Λ = 2 µm, dc = 0.15 Λ, d1 = 0.25 Λ, d = 0.5 Λ).
Fig. 7. (a) Fundamental loss spectrums for dc/Λ = 0.1, 0.15 and 0.2 with na = 1.37; (b) Fundamental peak loss and resonant wavelength for various values of dc/Λ with na = 1.36 and 1.37. (Λ = 2 µm, d1 = 0.25 Λ, d = 0.5 Λ, dg = 40 nm).
The fundamental loss spectrums are shown in Fig. 7(a) with the center air hole diameter dc is 0.1 Λ, 0.15 Λ and 0.2 Λ, respectively. It can be seen that the peak intensity increases with dc increases. In other words, the SPR intensity is enhanced. This is because the effective refractive index of the core mode is closer to the refractive index of the analyte when the size of the center hole increases [20]. Fig. 7(b) presents peak intensity and resonant wavelength of the fundamental mode loss spectrums for two different analyte RI. Since the difference Δλ in
resonance wavelength is the largest at dc = 0.15 Λ. Therefore, the center hole is the best parameters when dc = 0.15 Λ. The loss spectrum of the fundamental mode is shown in Fig. 8 by varying the external analyte refractive index from 1.33 to 1.40. When the refractive index of the analyte increases, both the resonance wavelength and the peak intensity increase, which indicates that the SPR intensity is enhanced. The energy confined in the core increases with the refractive index of the analyte increases, then the coupling between 309
Optical Fiber Technology 46 (2018) 306–310
K. Tong et al.
6. Conclusions The photonic crystal fiber surface plasmon resonance has a great potential in the field of nano-optics research due to its unique properties. The proposed three-core PCF-SPR is simple and easy to use. COMSOL software is used to numerical simulation of the PCF-SPR sensor’s transmission mode and sensing characteristics. The structural parameters of the sensor are optimized, and the influence of each parameter on the sensing performance is shown. The loss curves corresponding to the optimal parameters are plotted, and the resonant wavelengths of the refractive index are clearly displayed. The sensing characteristics of PCF-SPR sensor are described by wavelength interrogation method. The study shows that the sensor not only has a higher sensitivity, but also has a larger detection range. For externally coated metallic film SPR sensors, the improvement of the performance by introduction of multi-core provides some new ideas and theoretical basis for the research of optical devices. The proposed sensor is easier to implement at the present level of technology. Fig. 8. Fundamental loss spectrums by varying.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 61172044), National Natural Science Foundation of Hebei (No. F2017203316) and Science Technology Research and Development Program of Qinhuangdao (No.201602A272). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.yofte.2018.11.014. References [1] B.D. Gupta, R.K. Verma, Surface plasmon resonance-based fiber optic sensors: principle, probe designs, and some applications, J. Sens. 2009 (2) (2009) 12. [2] J. Homola, Surface plasmon resonance sensors for detection of chemical and biological species, Cheminform 39 (18) (2008) 462–493. [3] C. Jacobsen, et al., Low-loss photonic crystal fibers for transmission systems and their dispersion properties, Opt. Express 12 (7) (2004) 1372–1376. [4] S. Silva, P. Rozia, O. Frazoa, Refractive index measurement of liquids based on microstructured optical fibers, Photonics 1 (4) (2014) 516–529. [5] H. Chen, H. Yan, G. Shan, Design of two-dimensional bending vector sensor based on selective infiltration of photonic crystal, Fiber Chin. J. Lasers 43 (1) (2016) 0105003. [6] W. Qin, S. Li, et al., Analyte-filled core selfcalibration microstructured optical fiber based plasmonic sensor for detecting high refractive index aqueous analyte, Opt. Lasers 58 (4) (2014) 1–8. [7] D. Gao, C. Guan, et al., Multi-hole-fiber based surface plasmon resonance sensor operated at near-infrared wavelengths, Opt. Commun 313 (4) (2014) 94–98. [8] M.R. Hasan, S. Akter, A highly sensitive gold-coated photonic crystal fiber biosensor based on surface plasmon resonance, Photonics 4 (1) (2017) 0168. [9] J. Lu, Y. Li, Y. Han, D-shaped photonic crystal fiber plasmonic refractive index sensor based on gold grating, Appl. Opt. 57 (2018) 5268. [10] P. Malinský, P. Slepicka, et al., Early stages of growth of gold layers sputter deposited on glass and silicon substrates, Nanoscale Res. Lett 7 (1) (2012) 1–7. [11] P.J. Sazio, A. Amezcua Correa, et al., Microstructured optical fibers as high-pressure microfluidic reactors, Science 311 (25) (2006) 1583–1586. [12] A. Hassani, M. Skorobogatiy, Design of the microstructured optical fiber-based surface plasmon resonance sensors with enhanced microfluidics, Opt. Express 14 (24) (2006) 11616–11621. [13] X. Yu, Y. Zhang, S.S. Pan, et al., A selectively coated photonic crystal fiber based surface plasmon resonance sensors, J. Opt. 12 (1) (2010) 015–020. [14] A. Ahmmed, G. Rifat, Amouzad Mahdiraji, Photonic crystal fiber-based surface plasmon resonance sensor with selective analyte channels and graphene-silver deposited core, Sensor 15 (5) (2015) 11499–11510. [15] E.K. Akowuah, T. Gorman, H. Ademgil, et al., Numerical analysis of a photonic crystal fiber for biosensing applications, IEEE J. Quantum Electron 48 (11) (2012) 1403–1410. [16] A. Hassani, M. Skorobogatiy, Design criteria for micro-structured optical fiber based surface plasmon resonance sensors, J. Opt. Soc. Am. Opt. Phys. 24 (6) (2007) 1423–1429. [17] Qin Wei, Shuguang Li, et al., Numerical analysis of a photonic crystal fiber based on two polarized modes for bio-sensing applications, Chin. Phys. B 22 (7) (2013) 3050–3055. [18] A. Nagasaki, K. Saitoh, et al., Polarization characteristics of photonic crystal fibers selectively filled with metal wires into cladding air holes, Opt. Express 19 (4) (2011) 3801–3810. [19] R. Otupiri, E.K. Akowuah, et al., A novel birefrigent photonic crystal fibre surface plasmon resonance biosensor, IEEE Photon. J. 6 (4) (2014) 1–11. [20] Xianchao Yang, Lu. Ying, et al., Analysis of Graphene-Based photonic crystal fiber sensor using birefringence and surface Plasmon Resonance, Plasmonics 12 (2) (2017) 489–496. [21] Chao Liu, Famei Wang, et al., Design and theoretical analysis of a photonic crystal fiber based on surface plasmon resonance sensing, J. Nanophoton. 9 (1) (2015) 930501–9305010.
Fig. 9. Linear fitting of the fundamental the na from 1.33 to 1.40 mode resonant wavelength and analyte.
the core mode and the SPP mode is enhanced. This is because the effective fraction of the SPP mode is mainly affected by the analyte RI [21].The effective refractive index of the SPP mode increases with the analyte RI increases, but the effective refractive index of the fundamental mode is not affected by the analyte RI. Therefore, the resonance wavelength exhibits a red-shift. The SPR induced by three-core structure is susceptible to the analyte RI. 5. Sensitivity of biosensor The sensing characteristics of photonic crystal fiber surface plasmon resonance sensor are described by wavelength interrogation method. The incident light is a beam of broadband light. The output spectrum of the sensor is obtained by the spectrometer when the incident light is passed through the PCF-SPR sensor, and then the refractive index information of the analyte is obtained. The sensitivity of wavelength interrogation is determined by S ( )= peak / na where, Δna is the analyte RI variation and the λpeak is the peak shift. The linear fitting between the resonant wavelength of core mode and the analyte RI is shown in Fig. 9. The result shows that the average refractive index sensitivity of the sensor is 3435 nm/RIU, and the resolution is 2.91 × 10−6 RIU (the wavelength resolution is 10 pm). This biosensor has a higher sensitivity and larger detection range compared with [7–9]. 310
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Three-core photonic crystal fiber surface plasmon resonance sensor a
a
b,⁎
Kai Tong , Fucheng Wang , Meiting Wang a b
a
, Peng Dang , Yunxuan Wang
a
T
School of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China College of Liren, Yanshan University, Qinhuangdao 066004, China
ARTICLE INFO
ABSTRACT
Keywords: Surface plasmon resonance Photonic crystal fiber Biosensor Finite element method Refractive index sensitivity
A three-core photonic crystal fiber (PCF) surface plasmon resonance (SPR) biosensor with high performance is presented. The structure of the biosensor is easy to implement and has good repetition. The outer layer of the fiber structure is gold film. The analyte is placed on the surface of the gold film. The Full vector finite element method is used to numerical simulation and analysis the transmission mode and characteristics of the three-core PCF-SPR biosensor. The influence of sensor structure parameters on sensing performance is investigated. The structure parameters are optimized. The optimum structure parameters are given. The refractive index (RI) sensitivity of the sensor is discussed there. The results show that the average refractive index sensitivity of the sensor is 3435 nm/RIU in the sensing range of 1.33–1.40, and the resolution is 2.91 × 10−6 RIU. The sensor has the advantages of high sensitivity and high resolution. And it can be applied to real-time detection of the biomolecules and small drug-molecules.
1. Introduction Surface plasmon resonance (SPR) sensing technology has been widely used in biochemistry, medical pharmacy and environmental monitoring due to its high sensitivity and high resolution [1]. It has become a focus hotspot in biosensor technology [2]. Optical fiber has the advantages of small volume, anti-electromagnetic interference, long distance transmission and strong environmental adaptability [3,4]. Photonic crystal fiber (PCF) has the characteristics of flexible and adjustable structure as a special optical fiber. In recent years, the biosensing technology based on PCF has received continuous attention in the field of biochemical sensing [5]. The usual methods are making metal film on the inner surface of optical fiber air holes or fill metal wires, through these designs, optical fiber SPR sensor can be realized [6]. However, because it is difficult to infiltrate the analyte in the inner air holes for practical, the manufacture of these designs is very challenging. Therefore, several externally coated metallic film SPR sensors have been introduced. A muliti-hole optical fiber SPR sensor based on gold and TiO2 was proposed by Gao et al. [7]. The detection range of the sensor is small and the sensitivity is relatively low. A two-layer circular lattice photonic crystal fiber is proposed by Hasan et al. [8] in 2017, the proposed sensor shows maximum sensitivity of 2200 nm/RIU in the sensing range between 1.33 and 1.36. In 2018, a surface plasmon resonance based D-shaped photonic crystal fiber sensor has been reported with a sensitivity of 3300 nm/RIU in the analyte refractive index
⁎
1.36–1.38 [9].On the other hand, externally coated metallic film SPR sensors with multi-core have seldom been rarely reported. A three-core photonic crystal fiber biosensor is proposed in this paper to improve the sensitivity and the performance. The optimum structure parameters are given. The average refractive index sensitivity of the sensor is 3435 nm/RIU in the sensing range of 1.33–1.40, and the resolution is 2.91 × 10−6 RIU. The metal layer can be integrated outside the fiber structured by using sputtering technique, high pressure chemical vapor deposition (CVD) technique [10,11]. The sensing layer is located on the surface of the metal layer, so that the measured analyte can be easily flowed or dripped onto the surface of the metal thin film layer. 2. Structural design and theoretical model The structure of the three-core photonic crystal fiber biosensor is shown in Fig. 1. The basic structure is a photonic crystal fiber. This kind of fiber composed of a single material can be fabricated using the stack &draw method. As can be seen from Fig. 1(a), the capillary of the second layer is replaced by solid rods to form the three cores of the PCF. The three cores are recorded as 1, 2, 3 in counter-clockwise direction respectively. The small center air hole plays an important role in tuning the coupling between fundamental mode and SPP mode [12,13] with a diameter of dc. The special air holes with a diameter of d1 on the outer layer are conducive to the excitation of SPP mode. All the three layers
Corresponding author. E-mail address: [email protected] (M. Wang).
https://doi.org/10.1016/j.yofte.2018.11.014 Received 8 August 2018; Received in revised form 12 October 2018; Accepted 9 November 2018 Available online 15 November 2018 1068-5200/ © 2018 Elsevier Inc. All rights reserved.
Optical Fiber Technology 46 (2018) 306–310
K. Tong et al.
Fig. 1. Structure of three-core PCF-SPR sensor.
air holes are arranged in a hexagon arrangement with a lattice pitch Ʌ, and the air hole diameter of the cladding is d. The capillary pre-fabricated structure is shown in Fig. 1(b). The required three-core PCF structure can be obtained under the conditions of precise drawing temperature, drawing speed, and delivery rod speed [14]. Schematic diagram of the proposed sensor set-up is presented in Fig. 1(c). The analyte adsorbed on the metal horizontal surface through simple flow or drip. The incident light is provided by a wide band light source (BBS) and then injected into D-shaped PCF-SPR biosensor after being polarized by a polarization controller. Finally, the output spectrum is detected by optical spectrum analyzer (OSA). The substrate material of photonic crystal fiber is pure silica whose material dispersion can be calculated by Sellmeier equation [15]:
n=
1+
A i2 2 N i=1 2 li2
the Sellmeier equation is closer to its true refractive index value. The surface plasmon resonance metal material is Au with thickness of dg. The dielectric constant of Au is given by the Drude dispersion model [16]: Au
=
2 D
( + j D)
(
2
.
L
2 L)
+j
L
(2)
where, high-frequency dielectric constant ε∞ = 5.9673. The angular frequency can be expressed as ω = 2πc/λ, where c is the velocity of light. ωD and γD are plasma frequency and damping frequency respectively, ωD/2π = 2113.6THZ,γD/2π = 15.92THZ. The weighting factor is Δε = 1.09. ГL and ΩL are expressed as ГL/2 = 104.86 Hz, ΩL/ 2π = 650.07 THz [17],respectively. 3. Mode analysis of sensor
(1)
where, λ is the wavelength of incident light, N = 3, Ai and li are dispersion coefficients, A1 = 0.6961663, A2 = 0.4079426, A3 = 0.8974794, l1 = 0.0684043, l2 = 0.1162414, l3 = 9.896161. The relationship between the refractive index of SiO2 and the wavelength of incident light is nonlinear. The refractive index of SiO2 calculated by
The Full vector finite element method (FEM) is used to analyze the transmission mode and the sensing characteristic of the photonic crystal fiber surface plasmon resonance sensor. Because the structure of sensor is three-core, the transmission mode is more complex than the single-core structure. Fig. 2 shows the effective refractive index and electric field
Fig. 2. Field profile of sensor at λ = 650 nm.(a) Super mode S1; (b) Super mode S2; (c) Fundamental mode.
307
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K. Tong et al.
distribution of the relevant mode distribution of PCF with the refractive index (RI) of analyte is na = 1.37 and the incident wavelength is λ = 650 nm. Super model is formed by the superposition of the light fields of each core. But the direction of the electric field of each core is not the same, and the number of super mode is consistent with the number of core [18]. As shown in Fig. 2, the red arrow in the diagram represents the direction of the electric field. Fig. 2(a)–(c) are recorded as super-mode S1, super-mode S2 and fundamental mode, respectively. The corresponding effective refractive index is 1.447 + i9.016 × 10–6, 1.447 + i9.006 × 10–6, 1.448 + i5.594 × 10–6. It can be seen that the real part of the effective refractive index of the fundamental mode is the largest. Although the difference in the effective refractive index of the three modes is very small, the SPR resonance mode is quite different. The imaginary part of the effective refractive index of the fundamental mode is much smaller than S1 mode and S2 mode, and the direction of the electric field in the core of the fundamental mode is the same. Moreover, the S1 mode and the S2 mode do not excite the SPP mode to resonate with it. Therefore, only the characteristics of the fundamental mode and the corresponding SPP mode need to be analyzed there. The sensor is a single mode fiber due to triple symmetry structure. For the fundamental mode, they have the same propagation constant from the perspective of polarization. In other words, the polarization characteristics are the same. The x-polarization mode and y-polarization mode transmit at the same effective refractive index and there is no birefringence phenomenon [19].Only the characteristics of the y-polarization mode are analyzed here. In the field of waveguide optics, the confinement loss is calculated according to the following equation:
(dB/cm) = 8.686 × (2 / )Im(neff ) × 10 4
Fig. 4. Fundamental loss spectrums for d1 = 0.2 Λ, 0.25 Λ and 0.3 Λ with na = 1.37(Λ = 2 μm, dc = 0.15 Λ, d = 0.25 Λ).
most intense. 4. Sensor structural parameters optimization The parameters of the designed sensor are optimized using the Full vector finite element method based COMSOL Multiphysics software with the Perfect Matching Layer (PML) as boundary conditions. The fundamental loss spectrums are shown in Fig. 4 with the outer layer air hole diameter d1 is 0.2 Λ, 0.25 Λ and 0.3 Λ, respectively. It can be seen that the resonance wavelengths remain unchanged. The peak intensity increases with d1 decreases. In other words, the SPR intensity is enhanced. This is because the energy confined to the core weakens when the value of d1 decreases, therefore the resonance intensity between core mode and SPP mode is weakened. The fundamental mode loss spectrum has the smallest FWHM when d1 = 0.25 Λ, that is the best parameters of the outer layer air hole. The influence of the hole size pitch Λ on the sensing performance is shown in Fig. 5(a) with the value is 1.9 µm, 2.0 µm and 2.1 µm, respectively. It can be seen that both the peak intensity and the resonance wavelength decrease as Λ increases. This is because the energy confined to the core weakens when the value of Λ increases and the resonance intensity between core mode and SPP mode is weakened. The peak intensity and resonant wavelength of the fundamental mode loss spectrums with different the value of Λ are shown in Fig. 5(b) when the RI of the analyte is 1.36 and 1.37, respectively. The difference of resonant wavelengths Δλ between adjacent refractive index media is the largest when Λ = 2µm. Therefore, the best parameter of the air hole pitch is Λ = 2 µm. At the same time, it can be seen that the corresponding resonant wavelength and peak intensity is 1.9 μm, 2 μm and 2.1 μm are 690 nm (28.55 dB/cm), 686 nm (27.52 dB/cm), and 684 nm (26.22 dB/cm), respectively. The small change in the value indicates that the variation of the hole pitch (Λ) has a small influence on the sensing performance of the sensor. The gold is chosen as the surface plasmon resonance excitation material due to the advantages of chemical stability and wavelength drift. The thickness of gold film is also an important parameter that affects the performances of sensor. The fundamental loss spectrums are shown in Fig. 6 with the thickness is 35 nm, 40 nm and 45 nm, respectively. As the dg increases, the resonant wavelength red-shift presented and the intensity of the SPR weakens. The Fundamental peak loss and resonant wavelength is shown in Fig. 6(b) with dg under all numerical values with na = 1.37 and 1.38. The thicker the gold film layer is, the larger the damping loss is. However, the difference of resonant wavelengths Δλ between adjacent refractive index media increases with the increases of gold thickness. Therefore, the best thickness of gold is set as 40 nm.
(3)
where, Im(neff) is the imaginary part of the effective refractive index. From Eq. (3), it can be seen that the transmission loss is proportional to the imaginary part of the effective refractive index. The proposed sensor’s electric field profile and phase matching property are shown in Fig. 3. The red line and blue line in the figure represent the real part of the effective index of the fundamental mode and the SPP mode, respectively. The corresponding electric field diagrams are shown in Fig. 3(a) and (b). The electric field distribution of the SPP mode at non-resonant wavelengths is shown in Fig. 3(a). All the energy is concentrated on the thin gold layer. The electric field distribution of the SPP mode at non-resonant wavelengths is shown in Fig. 3(b) and all the energy is concentrated in the gold film. The electric field distribution of the fundamental mode at the resonance wavelength is shown in Fig. 3(c). The energy is confined to the core and some energy is distributed on the surface of gold—the SPR is successfully excited. The resonant wavelength is 686 nm where the energy coupling is
Fig. 3. Dispersion relations with analyte na = 1.37. (na = 1.37, Λ = 2 μm, dc = 0.15 Λ, d1 = 0.5 Λ, d = 0.25 Λ, h = 2.6 μm, dg = 40 nm). 308
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Fig. 5. (a) Fundamental loss spectrums for Λ = 1.9 µm, 2.1 µm and 2.1 µm with na = 1.37; (b) Fundamental peak loss and resonant wavelength for various values of Λ with na = 1.36 and 1.37 (dc = 0.15 Λ, d = 0.5 Λ, d1 = 0.25 Λ).
Fig. 6. (a) Fundamental loss spectrums for dg = 35 nm, 40 nm and 45 nm with na = 1.37; (b) Fundamental peak loss and resonant wavelength for various values of dg with na = 1.36 and 1.37. (Λ = 2 µm, dc = 0.15 Λ, d1 = 0.25 Λ, d = 0.5 Λ).
Fig. 7. (a) Fundamental loss spectrums for dc/Λ = 0.1, 0.15 and 0.2 with na = 1.37; (b) Fundamental peak loss and resonant wavelength for various values of dc/Λ with na = 1.36 and 1.37. (Λ = 2 µm, d1 = 0.25 Λ, d = 0.5 Λ, dg = 40 nm).
The fundamental loss spectrums are shown in Fig. 7(a) with the center air hole diameter dc is 0.1 Λ, 0.15 Λ and 0.2 Λ, respectively. It can be seen that the peak intensity increases with dc increases. In other words, the SPR intensity is enhanced. This is because the effective refractive index of the core mode is closer to the refractive index of the analyte when the size of the center hole increases [20]. Fig. 7(b) presents peak intensity and resonant wavelength of the fundamental mode loss spectrums for two different analyte RI. Since the difference Δλ in
resonance wavelength is the largest at dc = 0.15 Λ. Therefore, the center hole is the best parameters when dc = 0.15 Λ. The loss spectrum of the fundamental mode is shown in Fig. 8 by varying the external analyte refractive index from 1.33 to 1.40. When the refractive index of the analyte increases, both the resonance wavelength and the peak intensity increase, which indicates that the SPR intensity is enhanced. The energy confined in the core increases with the refractive index of the analyte increases, then the coupling between 309
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6. Conclusions The photonic crystal fiber surface plasmon resonance has a great potential in the field of nano-optics research due to its unique properties. The proposed three-core PCF-SPR is simple and easy to use. COMSOL software is used to numerical simulation of the PCF-SPR sensor’s transmission mode and sensing characteristics. The structural parameters of the sensor are optimized, and the influence of each parameter on the sensing performance is shown. The loss curves corresponding to the optimal parameters are plotted, and the resonant wavelengths of the refractive index are clearly displayed. The sensing characteristics of PCF-SPR sensor are described by wavelength interrogation method. The study shows that the sensor not only has a higher sensitivity, but also has a larger detection range. For externally coated metallic film SPR sensors, the improvement of the performance by introduction of multi-core provides some new ideas and theoretical basis for the research of optical devices. The proposed sensor is easier to implement at the present level of technology. Fig. 8. Fundamental loss spectrums by varying.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 61172044), National Natural Science Foundation of Hebei (No. F2017203316) and Science Technology Research and Development Program of Qinhuangdao (No.201602A272). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.yofte.2018.11.014. References [1] B.D. Gupta, R.K. Verma, Surface plasmon resonance-based fiber optic sensors: principle, probe designs, and some applications, J. Sens. 2009 (2) (2009) 12. [2] J. Homola, Surface plasmon resonance sensors for detection of chemical and biological species, Cheminform 39 (18) (2008) 462–493. [3] C. Jacobsen, et al., Low-loss photonic crystal fibers for transmission systems and their dispersion properties, Opt. Express 12 (7) (2004) 1372–1376. [4] S. Silva, P. Rozia, O. Frazoa, Refractive index measurement of liquids based on microstructured optical fibers, Photonics 1 (4) (2014) 516–529. [5] H. Chen, H. Yan, G. Shan, Design of two-dimensional bending vector sensor based on selective infiltration of photonic crystal, Fiber Chin. J. Lasers 43 (1) (2016) 0105003. [6] W. Qin, S. Li, et al., Analyte-filled core selfcalibration microstructured optical fiber based plasmonic sensor for detecting high refractive index aqueous analyte, Opt. Lasers 58 (4) (2014) 1–8. [7] D. Gao, C. Guan, et al., Multi-hole-fiber based surface plasmon resonance sensor operated at near-infrared wavelengths, Opt. Commun 313 (4) (2014) 94–98. [8] M.R. Hasan, S. Akter, A highly sensitive gold-coated photonic crystal fiber biosensor based on surface plasmon resonance, Photonics 4 (1) (2017) 0168. [9] J. Lu, Y. Li, Y. Han, D-shaped photonic crystal fiber plasmonic refractive index sensor based on gold grating, Appl. Opt. 57 (2018) 5268. [10] P. Malinský, P. Slepicka, et al., Early stages of growth of gold layers sputter deposited on glass and silicon substrates, Nanoscale Res. Lett 7 (1) (2012) 1–7. [11] P.J. Sazio, A. Amezcua Correa, et al., Microstructured optical fibers as high-pressure microfluidic reactors, Science 311 (25) (2006) 1583–1586. [12] A. Hassani, M. Skorobogatiy, Design of the microstructured optical fiber-based surface plasmon resonance sensors with enhanced microfluidics, Opt. Express 14 (24) (2006) 11616–11621. [13] X. Yu, Y. Zhang, S.S. Pan, et al., A selectively coated photonic crystal fiber based surface plasmon resonance sensors, J. Opt. 12 (1) (2010) 015–020. [14] A. Ahmmed, G. Rifat, Amouzad Mahdiraji, Photonic crystal fiber-based surface plasmon resonance sensor with selective analyte channels and graphene-silver deposited core, Sensor 15 (5) (2015) 11499–11510. [15] E.K. Akowuah, T. Gorman, H. Ademgil, et al., Numerical analysis of a photonic crystal fiber for biosensing applications, IEEE J. Quantum Electron 48 (11) (2012) 1403–1410. [16] A. Hassani, M. Skorobogatiy, Design criteria for micro-structured optical fiber based surface plasmon resonance sensors, J. Opt. Soc. Am. Opt. Phys. 24 (6) (2007) 1423–1429. [17] Qin Wei, Shuguang Li, et al., Numerical analysis of a photonic crystal fiber based on two polarized modes for bio-sensing applications, Chin. Phys. B 22 (7) (2013) 3050–3055. [18] A. Nagasaki, K. Saitoh, et al., Polarization characteristics of photonic crystal fibers selectively filled with metal wires into cladding air holes, Opt. Express 19 (4) (2011) 3801–3810. [19] R. Otupiri, E.K. Akowuah, et al., A novel birefrigent photonic crystal fibre surface plasmon resonance biosensor, IEEE Photon. J. 6 (4) (2014) 1–11. [20] Xianchao Yang, Lu. Ying, et al., Analysis of Graphene-Based photonic crystal fiber sensor using birefringence and surface Plasmon Resonance, Plasmonics 12 (2) (2017) 489–496. [21] Chao Liu, Famei Wang, et al., Design and theoretical analysis of a photonic crystal fiber based on surface plasmon resonance sensing, J. Nanophoton. 9 (1) (2015) 930501–9305010.
Fig. 9. Linear fitting of the fundamental the na from 1.33 to 1.40 mode resonant wavelength and analyte.
the core mode and the SPP mode is enhanced. This is because the effective fraction of the SPP mode is mainly affected by the analyte RI [21].The effective refractive index of the SPP mode increases with the analyte RI increases, but the effective refractive index of the fundamental mode is not affected by the analyte RI. Therefore, the resonance wavelength exhibits a red-shift. The SPR induced by three-core structure is susceptible to the analyte RI. 5. Sensitivity of biosensor The sensing characteristics of photonic crystal fiber surface plasmon resonance sensor are described by wavelength interrogation method. The incident light is a beam of broadband light. The output spectrum of the sensor is obtained by the spectrometer when the incident light is passed through the PCF-SPR sensor, and then the refractive index information of the analyte is obtained. The sensitivity of wavelength interrogation is determined by S ( )= peak / na where, Δna is the analyte RI variation and the λpeak is the peak shift. The linear fitting between the resonant wavelength of core mode and the analyte RI is shown in Fig. 9. The result shows that the average refractive index sensitivity of the sensor is 3435 nm/RIU, and the resolution is 2.91 × 10−6 RIU (the wavelength resolution is 10 pm). This biosensor has a higher sensitivity and larger detection range compared with [7–9]. 310