Asymmetrical photonic crystal fiber based on the surface plasmonresonance sensor and analysis by the lower-birefringence peakmethod
Jian Hana, Weiquan Sua, Chao Liua,⁎, Famei Wangb, Chunhong Xua, Qiang Liua,Lin Yanga, Xianli Lia, Tao Sunc,⁎, Paul K. Chud
a School of Electronics Science, NortheastPetroleumUniversity, Daqing 163318, PR Chinab School of Materials Science and Chemical Engineering, Harbin Engineering, Harbin 150001, PR ChinacMedia Lab, Massachusetts Institute of Technology, Cambridge, MA 02139, USAdDepartment of Physics and Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong,China
A lower-birefringence peak method is used to investigate a new birefringence photonic crystalfiber (PCF) based on the surface plasmon resonance (SPR) sensor with an asymmetrical config-uration. The novel method can determine the resonance wavelength and resonance intensity bythe appearance of the lower peak and overcome deficiencies of the reported zero-birefringencepoint method. Upper and lower gaps are designed in the air holes to enhance the birefringence ofthe sensor as well as coupling efficiency between the plasmonic mode and core-guided mode. Thedependence of the sensing performance on the structure parameters is discussed considering thedepth of penetration of the evanescent wave and location of light confinement analysis. Amaximum spectral sensitivity of 16,700 nm/RIU can be obtained in a broad refractive index (RI)detection range from 1.33 to 1.42 and the corresponding RI resolution is 5.99×10−6RIU.
1. Introduction
The surface plasmon wave is generated from charge density oscillation on a metal layer and the evanescent wave is a kind ofelectromagnetic wave arising from incident p-polarized light or TM (transverse magnetic) wave [1–3]. Surface plasmon resonance(SPR) can be achieved at the metal-dielectric interface by coupling the evanescent wave with the surface plasmon wave [4,5]. TheSPR sensing technology has drawn a great deal of attention due to advantages of being label-free, highly sensitive, and real-time[6–9] and thus has great application potentials in the fields of biomedicine, biomaterials, and chemical sensing. SPR has become aversatile tool in monitoring refractive indexes (RI) of analytes, filtering light at certain frequencies and detecting the formation ofnanoscale biofilms [[6–9],10–15].
Many types of operating platforms such as the prism and optical fiber have been developed for SPR excitation. Although theprism-based SPR sensor was proposed as early as 1982 to detect chemicals with good sensitivity and high resolution [16], it suffersfrom the bulky configuration. In 1993, Jorgenson et al. [17] presented the first optical fiber SPR sensing configuration without therequired light-coupling prism. The proposed fiber sensor allows for a small sample volume and miniaturizes the sensing element.
https://doi.org/10.1016/j.ijleo.2019.05.070Received 3 April 2019; Received in revised form 19 May 2019; Accepted 23 May 2019
However, the SPR sensors employing conventional fibers have limited sensitivity due to the single configuration. Recently, thephotonic crystal fiber (PCF) and photonic quasi-crystal fiber (PQF) have attracted much research interest due to their distinctivemicrostructures and excellent optical properties, including near-zero flattened dispersion [18], high birefringence [19,20], ultra-lowconfinement loss and large effective mode area [21] as well as large cutoff ratio for endlessly single-mode propagation [22]. The PCFcomprising metallized holes provides a novel platform for SPR excitation and is a great breakthrough in the SPR sensing field as aresult of unique properties and flexibility of the microstructure design [8,23–25]. The PCF-based SPR (PCF-SPR) sensor not onlyovercomes the bulky size and low sensitivity drawbacks of the prism-based and conventional optical fiber-based SPR sensors, but alsopossesses very high sensitivity and resolution in refractive index detection. Typically, refractive indexes between 1.33 and 1.42constitute the important detection range for various analytes. An et al. [26] studied the quasi-D-shaped PCF-SPR sensor for RIdetection range of 1.33–1.42 and achieved a maximum spectral sensitivity of 3877 nm/RIU. Rifat et al. [27] proposed a highlysensitive PCF-SPR sensor with a selectively coated metal layer and the maximum spectral sensitivity reached 11,000 nm/RIU for theRI detection range of 1.33–1.42. Yang et al. [23] investigated a grapefruit PCF-SPR sensor with an exposed core to detect analyte RI of1.33–1.42 and the maximum spectral sensitivity was 13,500 nm/RIU. For the RI detection range of 1.33–1.42, the maximum spectralsensitivity of PCF-SPR sensors available is still lower than 15,000 nm/RIU. Besides, the resonance properties of most of the reportedPCF-SPR sensors have been calculated by the conventional confinement loss method. For instance, Yu et al. [28] used a zero-birefringence point method to determine the resonance wavelength of the asymmetrical PCF-SPR sensor. Nevertheless, this method isincapable of determining the resonance intensity and cannot be applied to some asymmetrical sensors because of the non-existentzero-birefringence point.
Herein, we describe an asymmetrical birefringent PCF-SPR sensor for the RI detection range of 1.33–1.42 and a novel lower-birefringence peak method to study the resonance properties of the sensor. The lower peak is caused by the irregular change in thereal part curve of the effective index of the y-polarized mode. A maximum spectral sensitivity of 16,700 nm/RIU is achieved when theanalyte RI is varied from 1.41 to 1.42 and the corresponding RI resolution is 5.99×10−6 RIU. Our results reveal that the lower-birefringence peak method can overcome the two limitations of indescribable resonance intensity and non-existent zero-birefringencepoint compared to the reported zero-birefringence point method.
2. Theoretical model
This study focuses on 2D simulation of the novel birefringent PCF sensor based on SPR and the sensing characteristics areinvestigated by the COMSOL Multiphysics software based on the finite element method (FEM). The software has prominent ad-vantages in both the computational efficiency and memory utilization. Fig. 1 depicts the schematic diagram of the birefringent PCF-SPR sensor. The proposed PCF can be achieved by using a stack and draw method [27] and a thin metallic coating can be depositedonto the surface of the PCF by using a physical vapor deposition or wet chemical method [29]. Pure silica glass is used as thebackground materials and the RI of silica glass is calculated by the Sellmeier dispersion relation [30,31]:
− =−
+−
+−
n λλ
λλ
λλ
1 0.6961663(0.0684043)
0.4079426(0.1162414)
0.897479(9.896161)
,22
2 2
2
2 2
2
2 2 (1)
where n and λ are the RI of silica glass and wavelength of the incident light, respectively. At the edge of the fiber, an array of circularair holes with the same radius of r1= 0.6 μm are introduced to lower the average RI of the edge. The RI of air (nair) is fixed to be 1.d1= 2Λsin(π/16) is the distance between adjacent air holes. These air holes are positive for confining the light into the fiber core. Itis noteworthy that there are no air holes at the upper and lower edges, thus enhancing the birefringence of the sensor as well ascoupling efficiency between the plasmonic mode and core-guided mode. d2= 2Λsin(π/8) is the size of opening. Additionally, there isa big central air hole with a radius of r2= 4 μm. The distance between the edge air holes and central air hole is Λ=11 μm. An activeplasmonic silver layer is deposited outside the fiber structure, which is beneficial to PCF fabrication compared to the silver layerdeposited on the inner surface of the fiber hole. The thickness of the silver layer is tAg = 45 nm and the relative permittivity of silver
Fig. 1. Cross-section of the birefringent PCF-SPR sensor.
J. Han, et al. Optik - International Journal for Light and Electron Optics 189 (2019) 121–129
122
(εr) between 188 nm and 2066 nm is defined by the L4 model (extended Drude model) [32]:
∑= + ++ +
∞
=
ε ω ε σ εiω
Cω A iω B
( ) / .p
p
P pr
0
1
4
2(2)
The parameters for Eq. (2) are listed in Table 1. The entire sensor is surrounded by the analyte with RI (nana) varying from 1.33 to1.42 and an artificial and perfectly matched layer (PML) is used as the boundary condition to absorbing the radiation energy [33].The other two boundary conditions, the perfect electric conductor and perfect magnetic conductor, are added to the outer boundariesof the theoretical model.
Table 1Parameters of the L4 model of Ag permittivity.
Fig. 2. Mode field distributions of the core-guided modes: (a) y-polarized mode; (b) x-polarized mode.
Fig. 3. Electric field intensity distribution along the y-axis (λ=1096 nm, nana= 1.42, r1= 0.6 μm, r2= 4 μm, Λ=11 μm, and tAg =45 nm).
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3. Results and discussion
Fig. 2 shows the mode field distributions of the core-guided modes and the black arrows represent the direction of the electricfields. Figs. 2(a) and (b) display the mode distribution in the y-polarized direction (y-polarized mode) and mode distribution in the x-polarized direction (x-polarized mode), respectively. The coupling efficiency between the plasmonic mode and core-guided mode isclearly stronger in the y-polarized mode than the x-polarized mode. With regard to the designed asymmetrical PCF structure, the realpart of the effective index of the y-polarized mode is different from that of the x-polarized mode. The birefringence (B) is determinedby the difference between the real parts of the effective indexes of two orthogonal polarized modes [20]:
= −B n n|Re( ) Re( )|,effx
effy
(3)
Fig. 4. (a) Birefringence and real part curves of the effective indexes of two orthogonal polarized modes; (b) Birefringence curve without the silverlayer (nana= 1.42, r1= 0.6 μm, r2 = 4 μm, Λ=11 μm, and tAg =45 nm).
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where Re (neffx ) and Re(neff
y ) represent the real part of the effective index of the x-polarized mode and y-polarized mode, respectively.There is a difference between the metallized birefringence PCF and ordinary birefringence PCF and so = −B n nRe( ) Re( )eff
xeffy is used
to replace Eq. (3).Fig. 3 presents the electric field intensity distribution along the y-axis in the y-polarized mode when the SPR is excited. Peaks 1
and 2 correspond to the positions of the silver-analyte interface (y=13.796 μm) and fiber core, respectively. It is seen that a part ofthe energy leaks into the silver-analyte interface (peak 1) from the fiber core, indicating coupling between the plasmonic mode andcore-guided mode.
Fig. 4(a) shows the relationship between the birefringence and real parts of the effective indexes of the two orthogonal polarizedmodes. Unlike the single peak in the confinement loss spectrum, the birefringence curve exhibits two peaks, an upper peak and lowerpeak as shown in Fig. 4(a). The real parts of the effective indexes decrease gradually as the wavelength increases. However, there isan irregular change in the real part curve of the y-polarized mode resulting in the appearance of the lower peak. Fig. 4(b) plots thebirefringence curve when the silver layer of the sensor is removed and there is no peak in the curve. Therefore, this lower peak isinevitably related to the silver layer, which may be attributed to the intrinsic structure of metallized birefringence PCF. In themetallized birefringence model, the lower peak wavelength and birefringence difference between the two peaks can be defined as theresonance wavelength and resonance intensity, respectively. It is noteworthy that the birefringence of pure PCF is visibly lower thanthat of metallized PCF by comparing Fig. 4(a) and (b), which can be explained by that the birefringence increases as the structuralasymmetry increases and the metallized PCF has a stronger rotational asymmetry [20].
Silver and gold are extensively used as plasmonic materials to excite SPR because of the abundance of free conduction electrons[1,34]. Fig. 5 displays the birefringence curves of the core-guided modes when the SPR exciting layers are made of silver and gold,respectively. For the same analyte RI, the sensor with a silver coating has a smaller resonance wavelength than the gold coating.Additionally, the sensor with a silver coating exhibits a sharper birefringence peak and larger resonance intensity thus enhancing thesensing accuracy. Therefore, silver is selected as the plasmonic material in this structure.
Fig. 6 shows the birefringence curves of the core-guided mode when the thickness of silver layer is varied from 55 nm to 45 nm.The resonance wavelength shifts to the short wavelength and the resonance intensity rises as the thickness of the silver layer de-creases. This can be explained as follows. SPR occurs at the silver-analyte interface as confirmed by the electronic field distribution inFig. 3. The thicker silver layer causes more damping in the evanescent wave penetration towards the silver-analyte interface giving
Fig. 5. Birefringence curves of the core-guided modes of silver and gold with the analyte RI being 1.41 and 1.42 (r1= 0.6 μm, r2 = 4 μm,Λ=12 μm, and tAg/Au =45 nm).
Fig. 6. Dependence of the birefringence curves of the core-guided mode on the silver layer thickness (nana = 1.42, r1 = 0.6 μm, r2 = 4 μm, andΛ=12 μm).
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rise to a smaller resonance intensity. The depth of penetration (dp) of the evanescent wave can be defined as [35]:
= =dkβ
λπβ
12
,p(4)
where k and β are the wave number and decay constant, respectively. The penetration depth is directly proportional to the wave-length of the incident light and therefore, in order to penetrate the thicker silver layer, a longer wavelength is required.
Fig. 7 shows the dependence of the birefringence curves of the core-guided mode on the radii of the edge air holes and central airhole. The resonance wavelength is hardly affected by the edge air holes radius, whereas the resonance wavelength exhibits a slightred-shift with increasing central air hole radius. Furthermore, the resonance intensity increases as the radii of the edge air holes andcentral air hole increase. Fig. 8(a) shows the birefringence curves of the core-guided mode for different distances between the edge airholes and central air hole. The resonance wavelength exhibits a blue-shift and the resonance intensity decreases gradually when thedistance increases from 11 μm to 13 μm. The variation in the resonance intensity can be explained by the light distribution. Figs. 8(b),(c), and (d) display the light distributions at the resonance wavelength when the distances between the edge air holes and central airhole are 11 μm, 12 μm, and 13 μm, respectively. In this sensor, coupling between the plasmonic mode and core-guided mode occursmostly in the upper and lower gaps as shown in Figs. 8(b), (c), and (d). When Λ is 11 μm, light confinement in the location near thegaps improves compared to that when Λ is 12 μm or 13 μm thereby enhancing the coupling efficiency. This is why the resonanceintensity decreases as Λ increases. The same argument is applicable to explain the variation in the resonance intensity as shown inFigs. 7(a) and (b).
Fig. 9 shows the birefringence curves of the core-guided mode for different analyte RIs. When the analyte RI increases from 1.33to 1.42, the resonance intensity increases gradually and the birefringence curve shifts towards longer wavelengths, corresponding tothe resonance wavelengths of 611 nm for 1.33, 646 nm for 1.35, 696 nm for 1.37, 776 nm for 1.39, 929 nm for 1.41, and 1096 nm for1.42. For the PCF based on the SPR sensor, the spectral sensitivity (S(λ)) is one of the major performance parameters and can becalculated by [36,37]:
=S λ ΔλΔn
( ) (nm/RIU),ana (5)
where Δλ is the shift of the resonance wavelength and Δnana is the difference between two analyte RI. According to Eq. (5), amaximum spectral sensitivity of 16,700 nm/RIU is obtained for the analyte RI of 1.41–1.42. The analyte RI resolution (R) which isanother performance indicator of the PCF-SPR sensor can be expressed as follows [38]:
= =R Δn Δλ Δλ Δλ S λ/ / ( ),ana min min (6)
Fig. 7. Birefringence curves of the core-guided mode for different radii: (a) Edge air holes and (b) Central air hole (r1= 0.6 μm for (b), r2= 4 μm for(a), nana= 1.42, tAg= 45 nm, and Λ=12 μm).
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where Δλmin is the wavelength resolution. Supposing Δλmin =0.1 nm, the maximum RI resolution is 5.99×10−6 RIU for the analyteRI of 1.41–1.42.
Fig. 10 shows the numerical relationship between the resonance wavelength and analyte RI. The discrete data points show thatthe resonance wavelength is nonlinear with increasing analyte RI. A nonlinear curve fit is used to obtain the regression equation asshown in the following:
= × × +λ n2.38906 10 exp(0.02894
) 598.96814,peakana-19
(7)
where λpeak is the resonance wavelength and nana is the analyte RI varying between 1.33 and 1.42. The calculated R-square value ofthe fitted curve is as large as 0.99478 corroborating the excellent fit. The equation plays a prominent role in the design of the PCF-SPR sensor.
Fig. 8. (a) Birefringence curves of the core-guided mode for different distances between the edge air holes and central air hole; (b), (c), and (d) showthe distributions of light for Λ=11 μm, Λ=12 μm, and Λ=13 μm, respectively (nana= 1.42, tAg= 45 nm, r1= 0.6 μm, and r2 = 4 μm).
Fig. 9. Birefringence curves of the core-guided mode for the analyte RI of 1.33–1.42 (r1= 0.6 μm, r2= 4 μm, tAg =45 nm, and Λ=11 μm).
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4. Conclusion
A new birefringent PCF-SPR sensing configuration is designed and investigated by the lower-birefringence peak method. Ourresults show that the lower-birefringence peak method can overcome the deficiencies of the reported zero-birefringence point methodand replace the conventional confinement loss method. The structure parameters such as the silver layer thickness, air holes radii,and distance between air holes affect the sensing performance and the influence of the penetration depth penetration and location oflight confinement is analyzed. The sensing configuration shows a maximum spectral sensitivity of 16,700 nm/RIU and broad RIdetection range from 1.33 to 1.42. A nonlinear curve fit is used to obtain the regression equation of the resonance wavelength andanalyte RI showing a high R-square value of 0.99478. The designed SPR sensor has potential applications in biomedical and chemicalfields especially real-time and fast sensing.
Impact statement
1 We use a novel lower-birefringence peak method to study an asymmetrical PCF-SPR sensor.2 The lower-birefringence peak method can overcome the two limitations of the reported zero-birefringence point method.3 Upper and lower gaps are designed in the air holes to enhance the birefringence of the sensor and coupling efficiency.4 High sensitivity are obtained both in wavelength interrogation.
Acknowledgements
This work was jointly supported by the National Natural Science Foundation of China [Grant No. 51474069, 41472126], ChinaPostdoctoral Science Foundation funded project [Grant No. 2016M591510], Natural Science Foundation of Heilongjiang Province[Grant No. E2016007, E2017010], Northeast Petroleum University Innovation Foundation For Postgraduate [Grant No. JYCX-CX09-2018], as well as Hong Kong Innovation and Technology Fund (ITF) [Grant No. ITS/452/17FP]. The authors acknowledge theCOMSOL Multiphysics software.
References
[1] A.K. Mishra, S.K. Mishra, B.D. Gupta, SPR based fiber optic sensor for refractive index sensing with enhanced detection accuracy and figure of merit in visibleregion, Opt. Commun. 344 (2015) 86–91.
[2] X. Zhang, Y. Qi, P. Zhou, H. Gong, B. Hu, C. Yan, Refractive index sensor based on Fano resonances in plasmonic waveguide with dual side-coupled ringresonators, Photonic Sens. 8 (2018) 367–374.
[3] X. Wang, X. Bai, Z. Pang, H. Yang, Y. Qi, X. Wen, Surface-enhanced Raman scattering effect of a composite structure with gold nano-cubes and gold filmseparated by polymethylmethacrylate film, Acta Phys. Sin. 68 (2019) 037301.
[4] T. Hu, Y. Zhao, A. Song, Fiber optic SPR sensor for refractive index and temperature measurement based on MMF-FBG-MMF structure, Sensor. Actuat. B-Chem.237 (2016) 521–525.
[5] Z. Yi, L. Liu, L. Wang, C. Cen, X. Chen, Z. Zhou, X. Ye, Y. Yi, Y. Tang, Y. Yi, P. Wu, Tunable dual-band perfect absorber consisting of periodic cross-crossmonolayer graphene arrays, Results Phys. 13 (2019) 102217.
[6] C. Liu, L. Yang, X. Lu, Q. Liu, F. Wang, J. Lv, T. Sun, H. Mu, P.K. Chu, Mid-infrared surface plasmon resonance sensor based on photonic crystal fibers, Opt.Express 25 (2017) 14227–14237.
[7] X.X. Wang, X.L. Bai, Z.Y. Pang, H. Yang, Y.P. Qi, Investigation of surface plasmons in Kretschmann structure loaded with a silver nano-cube, Results Phys. 12(2019) 1866–1870.
[8] W. Qin, S. Li, Y. Yao, X. Xin, J. Xue, Analyte-filled core self-calibration microstructured optical fiber based plasmonic sensor for detecting high refractive indexaqueous analyte, Opt. Laser. Eng. 58 (2014) 1–8.
[9] C. Liu, L. Yang, Q. Liu, F. Wang, Z. Sun, T. Sun, H. Mu, P.K. Chu, Analysis of a Surface Plasmon Resonance Probe Based on Photonic Crystal Fibers for LowRefractive Index Detection, Plasmonics 13 (2018) 779–784.
[10] N. Luan, J. Yao, Refractive index and temperature sensing based on surface plasmon resonance and directional resonance coupling in a PCF, IEEE Photonics J. 9(2017) 1–5.
[11] C. Liang, G. Niu, X. Chen, Z. Zhou, Z. Yi, X. Ye, T. Duan, Y. Yi, S. Xiao, Tunable triple-band graphene refractive index sensor with good angle-polarization
Fig. 10. Birefringence curves of the core-guided mode for the analyte RI of 1.33–1.42 (r1= 0.6 μm, r2= 4 μm, tAg = 45 nm, and Λ=11 μm).
J. Han, et al. Optik - International Journal for Light and Electron Optics 189 (2019) 121–129
tolerance, Opt. Commun. 436 (2019) 57–62.[12] B. Han, Y. Zhang, X. Wang, D. Yang, Y. Liu, J. Sun, Y. Wang, High-sensitive Fiber anemometer based on surface plasmon resonance effect in photonic crystal
Fiber, IEEE Sens. J. 19 (2019) 3391–3398.[13] Q. Liu, B. Yan, J. Liu, U-shaped photonic quasi-crystal fiber sensor with high sensitivity based on surface plasmon resonance, Appl. Phys. Express 12 (2019)
052014.[14] Y. Guo, J. Li, S. Li, S. Zhang, Y. Liu, Broadband single-polarization filter of D-shaped photonic crystal fiber with a micro-opening based on surface plasmon
resonance, Appl. Opt. 57 (2018) 8016–8022.[15] B. Yan, A. Wang, E. Liu, W. Tan, J. Xie, R. Ge, J. Liu, Polarization filtering in the visible wavelength range using surface plasmon resonance and a sunflower-type
photonic quasi-crystal fiber, J. Phys. D Appl. Phys. 51 (2018) 155105.[16] C. Nylander, B. Liedberg, T. Lind, Gas detection by means of surface plasmon resonance, Sensor. Actuat. 3 (1992) 79–88 (1982–1983).[17] R.C. Jorgenson, S.S. Yee, A fiber-optic chemical sensor based on surface plasmon resonance, Sensor. Actuators B-Chem. 12 (1993) 213–220.[18] W. Cai, E. Liu, B. Feng, H. Liu, Z. Wang, W. Xiao, T. Liang, Wang S, J. Liu, J. Liu, Dispersion properties of a photonic quasi-crystal fiber with double cladding air
holes, Optik 127 (2016) 4438–4442.[19] W. Cai, E. Liu, B. Feng, W. Xiao, H. Liu, Z. Wang, S. Wang, T. Liang, J. Liu, J. Liu, Dodecagonal photonic quasi-crystal fiber with high birefringence, J. Opt. Soc.
Am. A 33 (2016) 2108–2114.[20] H. Liu, W. Xiao, W. Cai, E. Liu, B. Feng, Z. Wang, T. Liang, S. Wang, J. Liu, Photonic quasi-crystal fiber with high birefringence, Opt. Eng. 55 (2016) 036101.[21] E. Liu, W. Tan, B. Yan, J. Xie, R. Ge, J. Liu, Broadband ultra-flattened dispersion, ultra-low confinement loss and large effective mode area in an octagonal
photonic quasi-crystal fiber, J. Opt. Soc. Am. A 35 (2018) 431–436.[22] E. Liu, B. Yan, W. Tan, J. Xie, R. Ge, J. Liu, Guiding characteristics of sunflower-type fiber, Superlattice. Microst. 115 (2018) 123–129.[23] X. Yang, Y. Lu, M. Wang, J. Yao, An exposed-core grapefruit fibers based surface plasmon resonance sensor, Sensors 15 (2015) 17106–17114.[24] N. Luan, R. Wang, W. Lv, J. Yao, Surface plasmon resonance sensor based on D-shaped microstructured optical fiber with hollow core, Opt. Express 23 (2015)
8576–8582.[25] N.M.Y. Zhang, D.J.J. Hu, P.P. Shum, Z. Wu, K. Li, T. Huang, L. Wei, Design and analysis of surface plasmon resonance sensor based on high-birefringent
microstructured optical fiber, J. Opt. 18 (2016) 065005.[26] G. An, S. Li, H. Wang, X. Zhang, X. Yan, Quasi-D-shaped optical fiber plasmonic refractive index sensor, J. Opt. 20 (2018) 035403.[27] A.A. Rifat, F. Haider, R. Ahmed, G.A. Mahdiraji, F.R.M. Adikan, A.E. Miroshnichenko, Highly sensitive selectively coated photonic crystal fiber-based plasmonic
sensor, Opt. Lett. 43 (2018) 891–894.[28] X. Yu, S. Zhang, Y. Zhang, H.P. Ho, P. Shum, H. Liu, D. Liu, An efficient approach for investigating surface plasmon resonance in asymmetric optical fibers based
on birefringence analysis, Opt. Express 18 (2010) 17950–17957.[29] J. Boehm, A. Francois, H.E. Heidepriem, T.M. Monro, Chemical deposition of silver for the fabrication of surface plasmon microstructured optical fibre sensors,
Plasmonics 6 (2011) 133–136.[30] J.N. Dash, R. Das, R. Jha, AZO coated microchannel incorporated PCF-Based SPR sensor: a numerical analysis, IEEE photon, Technol. Lett. 30 (2018) 1032–1035.[31] R. Otupiri, E.K. Akowuah, S. Haxha, Multi-channel SPR biosensor based on PCF for multi-analyte sensing applications, Opt. Express 23 (2015) 15716–15727.[32] F. Hao, P. Nordlander, Efficient dielectric function for FDTD simulation of the optical properties of silver and gold nanoparticles, Chem. Phys. Lett. 446 (2007)
115–118.[33] P. Bing, Z. Li, S. Yuan, J. Yao, Y. Lu, Surface plasmon resonance biosensor based on large size square-lattice photonic crystal fiber, J. Mod. Optic. 63 (2016)
793–797.[34] C. Rhodes, S. Franzen, J.P. Maria, M. Losego, D.N. Leonard, B. Laughlin, G. Duscher, Surface plasmon resonance in conducting metal oxides, J. Appl. Phys. 100
(2006) 054905.[35] D.C. Prieve, J.Y. Walz, Scattering of an evanescent surface wave by a microscopic dielectric sphere, Appl. Opt. 32 (1993) 1629–1641.[36] Z. Yi, J. Chen, C. Cen, X. Chen, Z. Zhou, Y. Tang, X. Ye, S. Xiao, W. Luo, P. Wu, Tunable graphene-based plasmonic perfect metamaterial absorber in the THz
region, Micromachines 10 (2019) 194.[37] A.K. Mishra, S.K. Mishra, R.K. Verma, Doped single-wall carbon nanotubes in propagating surface plasmon resonance-based fiber optic refractive index sensing,
