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Abstract
An asymmetrical configuration of hexagonal lattice photonic crystal fiber (PCF) based surface plasmon resonance (SPR) refractive index sensor is proposed. Instead of the typical confinement loss method, the lower birefringence peak method is considered to explore the sensing performance. The asymmetry in the core region induces birefringence that enhances the coupling efficiency between the core and surface plasmon polariton (SPP) mode. To form the strong SPR effect, both the gold layer and analyte layer are deposited on the external surface of the PCF. The proposed birefringent sensor exhibits the maximum wavelength sensitivity of 22,000 nm/RIU within broad analyte refractive index (RI) from 1.33 to 1.42. The sensing characteristics are carried out with the variation of several PCF structural parameters. Owing to enhanced sensitivity, the proposed sensor can be a potential candidate for biological and biomolecular analyte detection.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, surface plasmon resonance (SPR) based sensing technology has become an attractive research area because of its excellent properties of label-free monitoring and real-time analyte detections [1]. Various bio-sensing applications such as bioimaging, bioengineering, biochemistry, biological and biomolecular analytes detection are adhered to the SPR phenomenon [2–3]. Due to the collective vibration of free electrons along the metal-dielectric interface, the optical phenomenon of SPR can be generated effectively. In prior SPR techniques, prism coupling sensors are mainly employed based on the principle of attenuated total reflection (ATR) [4]. Unfortunately, these sensors have some significant limitations such as bulky configuration, miniaturization roughness, and inappropriate for remote sensing applications [5]. To get over such restrictions, optical fiber-based SPR sensors have been introduced [6]. Optical fiber sensors are simple in design and cost-effective. Because of the transmission accuracy and improved resolution, these sensors can be used for far distance sensing applications [7–8]. However, these sensors are not flexible for potentially changing the design parameters, and light can be guided through the fiber with a narrow incident angle only. With compared to the conventional optical fibers, photonic crystal fibers (PCFs) are preferred for SPR sensing because of their excellent birefringence characteristics. By properly changing the structural parameters of the PCF, it is possible to control the induced birefringence [9–10]. Moreover, model properties of the PCF (propagation loss and single mode guidance) can be tailored effectively, which are not possible in conventional optical fibers. Furthermore, both the sensing performance and detection range can be sufficiently improved by varying the PCF design parameters such as the number of rings, air hole dimension, pitch parameter, etc.
Selecting of a plasmonic material is another crucial issue for PCF based SPR sensor. Due to the chemical stability in the aqueous environment, gold is widely used as a plasmonic material [11–14]. Moreover, silver, copper, and aluminum are also employed as active plasmonic materials to generate the SPR effect [15]. However, these materials are susceptible to oxidation in the aqueous environment [16]. So far, titanium nitride (TiN) [17], indium tin oxide (ITO) [18], and titanium oxide (TiO2) [19] are also employed as alternative plasmonic materials. To date, various designs of PCF-SPR sensors have been demonstrated in the literature such as internal metal coating [20,21], external metal coating [22–24], slotted configurations [25,26], and D-shaped [27–29] configurations. In internal metal-coated PCF sensors, analytes are selectively filled at the inside of the air-holes, and a metal layer is also deposited in the air-holes. In practice, it is challenging to selectively deposit the analyte and metal layer into the inner air-holes. Rifat et al. proposed a plasmonic sensor for analyte refractive index (RI) detection with the selective coating of sensing medium. However, it is difficult during fabrication to infiltrate analyte into selective air-hole [30]. Externally coated PCF sensors can potentially overcome these drawbacks. Unlike internally metal coated PCFs, both metal and analyte layers are placed on the outer surface of the PCF in external coated PCF sensor to implement a straightforward sensing mechanism. On that perspective, Saiful et al. proposed a circular-shaped dual polarized PCF-SPR sensor which showed improved sensing performance regarding wavelength sensitivity and resolution [11]. Haque et al. proposed a modified D-shape PCF-SPR sensor for a broad range of RI detection. However, to make the flat surface of the D-shape PCF, excessive surface polishing is required which is also a time-consuming process for fabrication [31].
In optical sensing applications, birefringence in photonic crystal fiber is an effective property [32]. Birefringence means the difference of the real part of the effective RI of the two polarization modes which can be created in a sensor with the asymmetrical structure of cladding. The previous PCF-SPR sensor performances have been investigated using the confinement loss method [33]. The significant deficiency of this method is incoherent to some asymmetrical sensors due to the zero birefringence properties. Moreover, this method is incapable of explaining the resonance intensity of the sensor. To overcome these constraints, a lower birefringence peak method has been applied to PCF-SPR sensing. Liu et al. proposed an internal metal coated birefringent PCF-SPR sensor for a broad range of lower analyte RI detection which showed the maximum wavelength sensitivity of 6300 nm/RIU [34]. Although this sensor showed improved performance, the penetration of liquid inside the air-holes and the deposition of metal into the inner air-holes make this sensor infeasible for fabrication.
In this manuscript, we propose and discuss the performance of an asymmetrical PCF based SPR sensor for analyte refractive index detection using the lower birefringence peak method. To overcome the deficiencies of the internal coating PCF sensor, both metal and analyte layer are deposited on the external portion of the PCF. The effects of the structural parameters on sensing performance are investigated with the birefringence property and resonance intensity. Due to the variation of analyte RIs from 1.33 to 1.42, the maximum wavelength sensitivity is achieved about 22,000 nm/RIU, and the corresponding maximum sensor resolution is found about 4.55×10−6 RIU. Because of the improved sensitivity and excellent polynomial fitting characteristics, the proposed sensor can be applied for the various bio-targets detection.
2. Experimental consideration and theoretical model
The typical sensing system in the experiment is illustrated in Fig. 1(a). The incident light launched from a light source transmits through the polarizer followed by a polarizer controller, and then a linearly polarized light is introduced into the proposed sensor. After interacting with the external targets, the transmission spectrum is detected by the optical spectrum analyzer (OSA), and the data are analyzed by the computer to assess the sensing performance of the proposed birefringent sensor. Here, the lower birefringence peak method by measuring the variation in resonance intensity is used to evaluate the sensing performance. In this case, when the lower peak of the birefringence moves to the shorter wavelength, it is called blue shift. On the contrary, when it advances towards the longer wavelength, it is regarded as red shift. When the refractive index of the surrounding medium changes, the resonance wavelength experiences a red or a blue shift. The change of resonance wavelength or resonance intensity is directly proportional to the variation of analytes refractive index. Therefore, the refractive index of the unknown liquid sample can be detected by measuring the amount of wavelength shift or resonance intensity. Figure 1(b) depicts the two-dimensional cross-section view of the proposed birefringent PCF-SPR sensor. The PCF lattice is formed with all circular air holes. The edge air-holes are arranged in a hexagonal structure with a comparatively larger air hole in the center. Four edge air holes are scaled down, and two air holes are omitted from the right and left edges of the center air hole in order to enhance the birefringence effect. The distance between the central and edge air holes is Λ = 2 µm. The diameter of the central air hole is dc = 0.9 Λ. The larger edge air-hole diameter is d = 0.5 Λ and the smaller edge air-hole diameter is d1 = 0.25 Λ. The Sellmeier equation is used to determine the wavelength dependent refractive index of fused silica, which is given by [9],
Fig. 1. (a) Schematic of the typical experimental set up of a sensing system using the proposed sensor and (b) cross-sectional view of the proposed birefringent PCF-SPR sensor with tg = 45 nm, Λ = 2 µm, dc = 1.8 µm, d = 1 µm, and d1 = 0.5 µm.
3. Results and discussion
Figure 2 represents the optical field distributions of the y- and x- polarized core-guided mode and SPP mode. Comparing the coupling strength between the fundamental mode and plasmonic mode, a relatively stronger coupling is visible in Fig. 2(a) than Fig. 2(b). It is evident from the figure that for y-polarized direction, the evanescent field efficiently penetrates towards the metallic surface. Therefore, a surface plasmon wave (SPW) is generated, which propagates along the metal-dielectric surface. The effective refractive indices difference of two orthogonal polarization modes is called birefringence (B) which can be defined by the following equation [35],
where ${\mathop{\rm Re}\nolimits} ({n_{eff}^x} )$ and ${\mathop{\rm Re}\nolimits} ({n_{eff}^y} )$ defines the x- polarized and y-polarized effective mode index. Figure 3(a) shows the relationship between the birefringence and the real part of the effective RI (neff) of x- and y- polarization modes. It can be seen that with the increment of wavelength, neff decreases gradually. Unlike the single peak in the confinement loss curve, birefringence spectrum shows two peaks: a lower peak and an upper peak. Due to the asymmetrical structure of the proposed PCF, the y-polarized effective mode index is dissimilar from that of the x-polarized effective mode index. In metalized birefringence PCF, the wavelength of the lower peak is considered as the resonance wavelength and the difference between the upper and lower peak is called the resonance intensity. In this particular case, the resonance wavelength is 0.73 µm and the resonance intensity is 2.24×10−4 for na = 1.36, tg = 45 nm, Λ = 2 µm, dc = 1.8 µm, d = 1 µm, and d1 = 0.5 µm. Figure 3(b) depicts the birefringence curve with omitting the gold layer from the outer surface of the PCF. It is revealed from the figure that birefringence increases with wavelength and no birefringence peak appears. The physical reason behind this phenomenon can be illustrated as follows. When there is no gold layer on the outer surface of the PCF, the incoming light cannot interact with the metal surface hence there is no SPP mode (illustrated in the inset of Fig. 3(b)). As a result, birefringence increases with wavelength, and it never provides a sudden peak.Fig. 2. Optical field distributions of the (a) y-polarized core mode, (b) x-polarized core mode, and (c) SPP mode for λ = 0.73 µm, na = 1.36, tg = 45 nm, Λ = 2 µm, dc = 1.8 µm, d = 1 µm and d1 = 0.5 µm. The color bars show the normalized electric field intensity.
Fig. 3. (a) Wavelength dependent birefringence and effective RI for both polarized modes and (b) birefringence with omitting gold layer with na=1.36, tg=45 nm, Λ=2 µm, dc=1.8 µm, d = 1 µm, and d1=0.5 µm. Inset figure shows fundamental field distribution without gold coating.
Figure 4(a) and (b) show the birefringence curves of the core-guided mode when analyte RI is changed from 1.33 to 1.42. Due to the increment of analyte RI, the resulting upper birefringence peak increases monotonically. Moreover, significant red shift is observed when analyte RI is changed from lower to higher values. Additionally, the differences between the upper and lower peak i.e., the resonance intensity is increased with analyte RI. Figure 4(c) and (d) show the normalized resonance intensity for different analyte RI ranging from 1.33 to 1.42. The associated color bar indicates the highest and lowest value of birefringence. For example, in Fig. 4(c) black color bar shows the lowest birefringence peak (i.e., the resonance wavelength) and yellow-white bar shows the highest birefringence (i.e., the upper peak wavelength). As clearly evident from this figure, resonance intensity gets stronger with higher analyte RI. This is due to the fact that the light interaction between the fundamental core mode and the plasmonic mode is enhanced for higher analyte RI, which is illustrated in the contour plots of Fig. 5. When analyte RI is 1.33, it shows a weak interaction of the evanescent field with metal-dielectric interface. However, the strength of interaction with plasmonic material increases for analyte RI of 1.34 and 1.35. Due to this reason, resonance intensity is enlarged.
Fig. 4. Birefringence curves of the core-guided mode: (a) analyte RI variation from 1.33 to 1.38, (b) analyte RI variation from 1.39 to 1.42 and (c) normalized resonance intensity for varying analyte RI from 1.33 to 1.37, and (d) normalized resonance intensity for varying analyte RI from 1.38 to 1.42 with tg = 45 nm, Λ = 2 µm, dc = 1.8 µm, d = 1 µm, and d1 = 0.5 µm.
Wavelength sensitivity is a crucial performance parameter for PCF-SPR sensor which is expressed by the following equation [11],
where Δλpeak is the resonance wavelength peak shift with the changing of any na and Δna is the two nearby na variation in the sensing medium. In this work, we changed the value of analyte RI from 1.33 to 1.34, 1.34 to 1.35 and so on, which indicate Δna of 0.01. Using Eq. (4), we have tabulated wavelength sensitivities from 1.33 to 1.41. Another performance parameter is the sensor resolution which can be defined by the following formula [12],
Table 1. Performance analysis with varying the analyte RI from 1.33 to 1.42.
The performance of the proposed PCF-SPR sensor is highly dependent on the gold layer thickness. Therefore, it is required to investigate the effect of varying gold layer thickness on the birefringence characteristics. The birefringence curve of the fundamental core-mode with changing gold layer thickness from 45 to 49 nm is depicted in Fig. 6(a) for analyte RI of 1.36. The resonance lower peak point is observed at 0.73, 0.74, and 0.74 µm for a gold layer thickness of 45, 47, and 49 nm, respectively. It can be evident that the resonance peak moves to the higher wavelength, and the corresponding resonance intensity also decreases gradually. This phenomenon can be illustrated as follows. SPR phenomenon creates at the gold-analyte interface which is shown in optical field distribution in Fig. 2. However, strong coupling occurs between the core mode and plasmonic mode for a thin layer of gold while a thicker layer decreases the coupling efficiency [36]. More damping of the evanescent wave occurs with a thicker gold layer which is the reason for smaller resonance intensity. The penetration depth of the evanescent wave can be expressed as [37],
where β and k are the decay constant and wave number. The wavelength of the incident light is directly proportional to the penetration depth. Therefore, a higher wavelength is needed to penetrate a thicker gold layer. Figure 6(b) shows the normalized resonance intensity for varying gold layer thickness from 45 to 49 nm. As expected, the resonance wavelength is same for all gold layer thickness, which can be identified by the black color. The smallest gold layer thickness (45 nm) provides the strongest resonance intensity. When the gold layer thickness is increased, the black color fades away which is the indication of smaller resonance intensity.Fig. 6. (a) Birefringence curve with varying gold layer thickness tg from 45 to 49 nm for analyte RI of 1.36, and (b) normalized resonance intensity for varying gold layer thickness with na = 1.36, Λ = 2 µm, dc = 1.8 µm, d = 1 µm, and d1 = 0.5 µm
The birefringence curve with varying the distances between the central air hole and edge air-holes (Λ) is shown in Fig. 7(a). Resonance wavelengths at lower peak are observed at 0.73, 0.72, and 0.72 µm for Λ = 2, 2.5, and 3 µm, respectively. Due to the increasing of distances from 2 to 3 µm, the intensity decreases gradually. At Λ = 2 µm, the induced birefringence is higher due to stronger asymmetry in the core. As a consequent, it exhibits the strongest resonance coupling than any other values of Λ. Variation of resonance wavelength and resonance intensity for different Λ values is shown in Fig. 7(b). It can be observed that resonance wavelength is 0.73 µm for Λ = 2 µm (deep black color in the figure), which is agreed with Fig. 7(a). The deep black color starts fading when Λ is increased to higher values, which means a reduction of resonance intensity. In this analysis, the lower value of Λ is expected to have greater resonance intensity that increases the detection accuracy.
Fig. 7. (a) Birefringence curve with varying the distances between central air hole and edge air-holes (Λ) for analyte RI of 1.36 and (b) resonance intensity for varying Λ value with na = 1.36, tg = 45 nm, dc = 1.8 µm, d = 1 µm, and d1 = 0.5 µm.
The diameter of the central air-hole (dc) has a significant impact on birefringence and resonance intensity, which is depicted in Fig. 8(a). Resonance wavelengths at lower birefringence peak are observed at 0.73, 0.73, and 0.72 µm for a central air-hole diameter of 1.8, 1.2, and 0.6 µm, respectively. It can be seen from Fig. 8(a) that the resonance intensity reduces when dc value is scaled down. Actually, the value of dc determines the amount of birefringence. Obviously, when the center air-hole is omitted, it exhibits the minimum birefringence. As a result, the corresponding resonance intensity is the lowest. Therefore, for strong birefringence effect in the core region, the diameter of central air-hole should be kept as high as possible. Resonance intensity for different values of dc is illustrated in Fig. 8(b). According to this figure, higher resonance intensity can be obtained by using a larger value of dc.
Fig. 8. (a) Birefringence curve with varying the center air-hole diameter (dc) for analyte RI of 1.36, and (b) resonance intensity for varying dc with na = 1.36, tg = 45 nm, Λ = 2 µm, d = 1 µm, and d1 = 0.5 µm.
Figure 9(a) shows the birefringence curves due to the variation of the larger edge air-holes diameter d. According to this figure, the resonance wavelength is remained the same at 0.72 µm when the diameter d is varied from 0.4 to 1 µm. Likewise, the upper peak is also found at the identical wavelength of 0.68 µm, while the resonance intensity increases gradually with the scaling up of d. The variation of resonance intensity for different values of d is presented in Fig. 9(b). As evident from the figure, resonance wavelength (black color) is remained fixed at 0.72 µm which is similar to Fig. 9(a). We found higher resonance intensity for a larger value of d. Birefringence as a function of wavelength for increasing the smaller edge air-holes diameter d1 is shown in Fig. 9(c). As opposed with the Fig. 9(a), increasing the value of d1 results in a reduction of the resonance intensity. However, both the upper and lower birefringence peak is remained unchanged for varying the value of d1. Figure 9(d) shows the variation of resonance intensity for increasing of d1. Unlike Fig. 9(b), resonance intensity decreases with scaling up of d1.
Fig. 9. (a) Birefringence curve with varying the larger edge air-holes diameter d, (b) effect on the resonance intensity for the variation of d, (c) birefringence curve with varying the smaller edge air-holes diameter d1, and (d) effect on the resonance intensity for the variation of d1.
A symmetric PCF exhibits very low birefringence, which is difficult to utilize in sensing applications. Therefore, an asymmetry of a PCF is intentionally created in order to induce a high birefringence. In this PCF sensor, there are four design parameters (Λ, dc, d and d1) that control the level of asymmetry. More asymmetry in the fiber structure results in a higher value of birefringence. Simulation analysis in the previous section demonstrates that reducing the value of Λ and d1 provides higher level of asymmetry since difference of effective refractive index increases correspondingly. On the contrary, the same effect can be obtained for the lower values of dc and d. It should be noted that when varying a particular design parameter of the PCF during simulation, other parameters were kept unchanged. In this way, it is not possible to understand the impact of structural asymmetry accurately. Therefore, we have changed all design parameters simultaneously in such as a way that reveals either increase or decrease of the asymmetry. The effect of changing asymmetry on sensing performance is shown in Fig. 10. As clearly evident from Fig. 10 when structural asymmetry is enhanced (in this case Λ = 1.8 µm, dc = 2 µm, d = 1.2 µm, and d1 = 0.4 µm), resonance intensity increases significantly. On the other hand, resonance intensity reduces notably when structural asymmetry is decreased (in this case Λ = 2.2 µm, dc = 1.6 µm, d = 0.8 µm, and d1 = 0.6 µm). In the optimum condition (in this case Λ = 2 µm, dc = 1.8 µm, d = 1 µm, and d1 = 0.5 µm), the birefringence curve lies between the asymmetry increase and decrease case. The resonance intensity is higher than the asymmetry decrease case but lower than the asymmetry increase case.
Fig. 10. Effect of changing asymmetry on the performance of the sensor for tg = 45 nm and na = 1.36.
Figure 11 shows the polynomial fitting curve as a function of analyte RI. The calculated R-square value is 0.9740, which implies an excellent fitting characteristic. Here, the polynomial fitting regression equation is λr = 103.79 na2 - 279.16 na ± 188.39, where λr indicates the resonance wavelength and the na indicates the analyte refractive index.
Fig. 11. Polynomial fitting curve considering resonance wavelength of the fundamental mode for analyte RI from 1.33 to 1.42.
The proposed hexagonal PCF-SPR sensor can be practically fabricated with the stack-and-draw technique where the air-holes can be realized with employing thicker wall capillaries, and the missing air-holes can be realized with using solid-rods as shown in Fig. 12. There are several techniques to deposit the outer plasmonic material such as thermal evaporation, and wet-chemistry deposition [38]. However, the major shortcoming of these techniques is excessive surface roughness during deposition. On that perspective, to minimize the surface roughness and to have a uniformly nano-layer coating, chemical vapor deposition (CVD) and atomic layer deposition (ALD) method are well accepted. Fused silica is used as the background material, which has an ultra-low thermal sensitivity. The RIs variation with temperature for fused silica is 1.28×10−5/°C only. Therefore, the temperature effect can be ignored in normal environments without severe temperature variations. It is well known that for far distance SPR sensing system, large sensor length is preferable. In practice, the sensor length is fixed to millimeter scale, which is required to balance the fabrication feasibility with sensitivity. However, the sensor length can be potentially increased to centimeter scale using fiber holder and three-dimensional translation stages [11]. Table 2 shows a comparison among the existing sensors with the proposed sensor.
Fig. 12. The stacked preform view of the proposed PCF. Missing air-holes are filled by solid rods, smaller air-holes contain thicker wall capillaries, and larger air-holes contain thin wall capillaries.
Table 2. Performance comparison among the existing PCF-SPR sensor with the proposed sensor
4. Conclusion
A new birefringent PCF-SPR sensor has been proposed for external sensing of liquid sample RIs from 1.33 to 1.42. Lower birefringence peak method has been used to investigate the sensor characteristics in order to remove the limitations of typical confinement loss method. Tuning the design parameters such as gold layer thickness, air-hole diameter, and the distances between the center and the edge air hole the sensing performances were analyzed. The proposed sensor showed the maximum wavelength sensitivity of 22,000 nm/RIU and the sensor resolution of 4.55×10−6 RIU within analyte RIs from 1.33 to 1.41. Due to easy design and high sensitivity, the designed sensor can be applicable to the unknown bio-targets detection in a lab-on-fiber technology.
References
1. M. Piliarik, J. Homola, Z. Manıkova, and J. Ctyroky, “Surface plasmon resonance sensor based on a single-mode polarization-maintaining optical fiber,” Sens. Actuators, B 90(1-3), 236–242 (2003). [CrossRef]
2. M. R. Hasan, S. Akter, A. A. Rifat, S. Rana, K. Ahmed, R. Ahmed, H. Subbaraman, and D. Abbott, “Spiral photonic crystal fiber based dual-polarized surface plasmon resonance biosensor,” IEEE Sens. J. 18(1), 133–140 (2018). [CrossRef]
3. K. Tong, F. Wang, M. Wang, P. Dang, Y. Wang, and J. Sun, “D-shaped photonic crystal fiber biosensor based on silver-graphene,” Optik 168, 467–474 (2018). [CrossRef]
4. E. Kretschmann and Z. H. Raether, “Radiative decay of non radiative surface plasmons excited by Light,” Z. Naturforsch. A. Phys. Sci. 23(12), 2135–2136 (1968). [CrossRef]
5. X. Yang, Y. Lu, B. Liu, and J. Yao, “Analysis of Graphene-Based Photonic Crystal Fiber Sensor Using Birefringence and Surface Plasmon Resonance,” Plasmonics 12(2), 489–496 (2017). [CrossRef]
6. H. Yuan, W. Ji, S. Chu, S. Qian, F. Wang, J. F. Masson, X. Han, and W. Peng, “Fiber-optic surface plasmon resonance glucose sensor enhanced with phenylboronic acid modified Au nanoparticles,” Biosens. Bioelectron. 117, 637–643 (2018). [CrossRef]
7. Á. González-Vila, A. Ioannou, M. Loyez, M. Debliquy, D. Lahem, and C. Caucheteur, “Surface plasmon resonance sensing in gaseous media with optical fiber gratings,” Opt. Lett. 43(10), 2308–2311 (2018). [CrossRef]
8. A. D. S. Arcas, F. D. S. Dutra, R. C. S. B. Allil, and M. M. Werneck, “Surface plasmon resonance and bending loss-based U-shaped plastic optical fiber biosensors,” Sensors 18(2), 648 (2018). [CrossRef]
9. M. R. Hasan, M. A. Islam, A. A. Rifat, and M. I. Hasan, “A single-mode highly birefringent dispersion-compensating photonic crystal fiber using hybrid cladding,” J. Mod. Opt. 64(3), 218–225 (2017). [CrossRef]
10. A. A. Rifat, G. A. Mahdiraji, Y. M. Sua, R. Ahmed, Y. G. Shee, and F. R. Mahamd Adikan, “Highly sensitive multi-core flat fiber surface plasmon resonance refractive index sensor,” Opt. Express 24(3), 2485–2495 (2016). [CrossRef]
11. M. S. Islam, J. Sultana, A. A. Rifat, R. Ahmed, A. Dinovitser, B. W. H. Ng, H. Ebendorff-Heidepriem, and D. Abbott, “Dual-polarized highly sensitive plasmonic sensor in the visible to near-IR spectrum,” Opt. Express 26(23), 30347–30361 (2018). [CrossRef]
12. S. Chakma, M. A. Khalek, B. K. Paul, K. Ahmed, M. R. Hasan, and A. N. Bahar, “Gold-coated photonic crystal fiber biosensor based on surface plasmon resonance: design and analysis,” Sensing and Bio-Sensing Research 18, 7–12 (2018). [CrossRef]
13. A. A. Rifat, R. Ahmed, G. A. Mahdiraji, and F. R. M. Adikan, “Highly sensitive d-shaped photonic crystal fiber-based plasmonic biosensor in visible to near-IR,” IEEE Sens. J. 17(9), 2776–2783 (2017). [CrossRef]
14. X. Liu, M. Chang, N. Chen, X. Zhang, S. Zhuang, and J. Xu, “Design of a Metal-Filled Photonic-Crystal Fiber Polarization Filter Based on Surface Plasmon Resonance at 1.31 and 1.55 µm,” IEEE Photon. J. 10(5), 7203913 (2018). [CrossRef]
15. K. M. McPeak, S. V. Jayanti, S. J. P. Kress, S. Meyer, S. Iotti, A. Rossinelli, and D. J. Norris, “Plasmonic films can easily be better: rules and recipes,” ACS Photonics 2(3), 326–333 (2015). [CrossRef]
16. M. A. Ordal, L. L. Long, R. J. Bell, S. E. Bell, R. R. Bell, R. W. Alexander, and C. A. Ward, “Optical properties of fourteen metals in the infrared and far infrared: Al, Co, Cu, Au, Fe, Pb, Mo, Ni, Pd, Pt, Ag, Ti, V, and W,” Appl. Opt. 24(24), 4493–4499 (1985). [CrossRef]
17. A. E. Khalil, A. H. El-Saeed, M. A. Ibrahim, M. E. Hashish, M. R. Abdelmonem, M. F. O. Hameed, M. Y. Azab, and S. S. A. Obayya, “Highly sensitive photonic crystal fiber biosensor based on titanium nitride,” Opt. Quantum Electron. 50(3), 158 (2018). [CrossRef]
18. T. Huang, “Highly sensitive spr sensor based on d-shaped photonic crystal fiber coated with indium tin oxide at near infrared wavelength,” Plasmonics 12(3), 583–588 (2017). [CrossRef]
19. M. R. Hasan, S. Akter, M. S. Rahman, and K. Ahmed, “Design of a surface plasmon resonance refractive index sensor with high sensitivity,” Opt. Eng. 56(08), 1 (2017). [CrossRef]
20. W. Qin, S. Li, Y. Yao, X. Xin, and J. Xue, “Analyte-filled core self-calibration microstructured optical fiber based plasmonic sensor for detecting high refractive index aqueous analyte,” Opt. Lasers Eng. 58(14), 1–8 (2014). [CrossRef]
21. Z. Fan, S. Li, Q. Liu, G. An, H. Chen, J. Li, D. Chao, H. Li, J. Zi, and W. Tian, “High sensitivity of refractive index sensor based on analyte-filled photonic crystal fiber with surface plasmon resonance,” IEEE Photonics J. 7(3), 1–9 (2015). [CrossRef]
22. C. Liu, L. Yang, X. Lu, Q. Liu, F. Wang, J. Lv, T. Sun, H. Mu, and P. K. Chu, “Mid-infrared surface plasmon resonance sensor based on photonic crystal fibers,” Opt. Express 25(13), 14227–14237 (2017). [CrossRef]
23. G. An, G. A. Shuguang, L. X. Yan, X. Zhang, Z. Yuan, H. Wang, Y. Zhang, X. Hao, Y. Shao, and Z. Han, “Extra-broad photonic crystal fiber refractive index sensor based on surface plasmon resonance,” Plasmonics 12(2), 465–471 (2017). [CrossRef]
24. M. R. Hasan, S. Akter, K. Ahmed, and D. Abbott, “Plasmonic refractive index sensor employing niobium nanofilm on photonic crystal fiber,” IEEE Photon. Tech. Letters 30(4), 315–318 (2018). [CrossRef]
25. E. K. Akowuah, T. Gorman, H. Ademgil, S. Haxha, G. K. Robinson, and J. V. Oliver, “Numerical analysis of a photonic crystal fiber for bio sensing applications,” IEEE J. Quantum Electron. 48(11), 1403–1410 (2012). [CrossRef]
26. A. Hassani and M. Skorobogatiy, “Photonic crystal fiber-based plasmonic sensors for the detection of bio layer thickness,” J. Opt. Soc. Am. B 26(8), 1550 (2009). [CrossRef]
27. N. Luan, L. Zhao, Y. Lian, and S. Lou, “A High Refractive Index Plasmonic Sensor Based on D-Shaped Photonic Crystal Fiber with Laterally Accessible Hollow-Core,” IEEE Photonics J. 10(5), 1–7 (2018). [CrossRef]
28. W. Zhang, Z. Lian, T. Benson, X. Wang, and S. Lou, “A refractive index sensor based on a D-shaped photonic crystal fiber with a nanoscale gold belt,” Opt. Quantum Electron. 50(1), 29 (2018). [CrossRef]
29. L. Peng, F. Shi, G. Zhou, S. Ge, Z. Hou, and C. Xia, “A Surface Plasmon Biosensor Based on a D-Shaped Microstructured Optical Fiber with Rectangular Lattice,” IEEE Photonics J. 7(5), 1–9 (2015). [CrossRef]
30. A. A. Rifat, F. Haider, R. Ahmed, G. A. Mahdiraji, F. R. M. Adikan, and A. E. Miroshnichenko, “Highly sensitive selectively coated photonic crystal fiber-based plasmonic sensor,” Opt. Lett. 43(4), 891–894 (2018). [CrossRef]
31. E. Haque, M. A. Hossain, F. Ahmed, and Y. Namihira, “Surface plasmon resonance sensor based on modified d-shaped photonic crystal fiber for wider range of refractive index detection,” IEEE Sensors J. 18(20), 8287–8293 (2018). [CrossRef]
32. H. Liu, W. Xiao, W. Cai, and E. Liu, “Photonic quasi-crystal fiber with high birefringence,” Opt. Eng. 55(3), 036101 (2016). [CrossRef]
33. F. Haider, R. A. Aoni, R. Ahmed, and A. E. Miroshnichenko, “Highly amplitude-sensitive photonic crystal-fiber-based plasmonic sensor,” J. Opt. Soc. Am. B 35(11), 2816–2821 (2018). [CrossRef]
34. C. Liu, W. Su, F. Wang, X. Li, Q. Liu, H. Mu, T. Sun, P. K. Chu, and B. Liu, “Birefringent PCF based SPR sensor for a broad range of low refractive index detection,” IEEE Photonics Technol. Lett. 30(16), 1471–1474 (2018). [CrossRef]
35. P. Kumar, A. Soni, and J. S. Roy, “Ethanol doped photonic crystal fibers with nearly flattened zero dispersion and high birefringence,” Int. J. Microw. Opt. Technol. 13(6), 544–551 (2018).
36. X. Chen, L. Xia, and C. Li, “Surface plasmon resonance sensor based on a novel d-shaped photonic crystal fiber for low refractive index detection,” IEEE Photonics J. 10(1), 1–9 (2018). [CrossRef]
37. D. C. Prieve and J. Y. Walz, “Scattering of an evanescent surface wave by a microscopic dielectric sphere,” Appl. Opt. 32(9), 1629–1641 (1993). [CrossRef]
38. A. A. Rifat, M. R. Hasan, R. Ahmed, H. Butt, and A. E. Miroshnichenkoa, “Photonic crystal fiber based plasmonic biosensor with external sensing approach,” J. Nanophotonics 12(1), 012503 (2017). [CrossRef]
39. Y. Chen, Q. Xie, X. Li, H. Zhou, X. Hong, and Y. Geng, “Experimental realization of D-shaped photonic crystal fiber SPR sensor,” J. Phys. D: Appl. Phys. 50(2), 025101 (2017). [CrossRef]
40. P.-B. Bing, Z.-Y. Li, J.-Q. Yao, Y. Lu, Z.-G. Di, and X. Yan, “Theoretical and experimental researches on a PCF based SPR sensor,” Optoelectron. Lett. 8(4), 245–248 (2012). [CrossRef]
41. C.-L. Tien, H.-Y. Lin, and S.-H. Su, “High sensitivity refractive index sensor by D-shaped fibers and titanium dioxide nanofilm,” Adv. Condens. Matter Phys. 2018, 1–6 (2018). [CrossRef]
