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An air-core photonic crystal fiber based plasmonic sensor for high refractive index sensing
Journal Pre-proof An air-core photonic crystal fiber based plasmonic sensor for high refractive index sensing Alok Kumar Paul, Md. Samiul Habib, Nguyen Hoang Hai, S.M. Abdur Razzak
PII: DOI: Reference:
S0030-4018(20)30171-1 https://doi.org/10.1016/j.optcom.2020.125556 OPTICS 125556
To appear in:
Optics Communications
Received date : 9 December 2019 Revised date : 15 February 2020 Accepted date : 17 February 2020 Please cite this article as: A.K. Paul, Md.S. Habib, N.H. Hai et al., An air-core photonic crystal fiber based plasmonic sensor for high refractive index sensing, Optics Communications (2020), doi: https://doi.org/10.1016/j.optcom.2020.125556. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
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An Air-Core Photonic Crystal Fiber Based Plasmonic Sensor for High Refractive Index Sensing Alok Kumar Paul,1 Md. Samiul Habib,1 Nguyen Hoang Hai,2 and S. M. Abdur Razzak,1 1
Department of Electrical & Electronic Engineering, Rajshahi University of Engineering & Technology, Kazla, Rajshahi-6204, Bangladesh (e-mail: [email protected]) 2
School of Electronics and Telecommunication, Hanoi University of Science and Technology, Hanoi 100000, Vietnam
Abstract: Most surface plasmon resonance (SPR) based photonic crystal fiber (PCF) sensors have been designed to detect the analyte refractive index (RI) ranging from 1.33‒1.41. In this paper, we propose a new approach to extend the detection range of the sensor, and it can be fabricated with the existing fabrication techniques. Contrary to the existing PCF based sensors, our designed sensor is based on an air-core PCF where analyte is placed to the core of the PCF through a vertical side opening channel and is able to detect RI of the analyte higher than that of the PCF background. We use a chemically stable plasmonic material (i.e., gold), where the plasmonic material and analyte are placed in such a way that there is no direct contact between them, thus reducing the interference effect. According to our simulation, we show that the spectral sensitivity and resolution are achieved as 11,700 nm/RIU and 8.55×10–6 RIU, respectively, for the analyte RI of 1.42. We believe that our proposed sensor can be used to detect highly active chemical and biological liquid samples.
Index Terms: Photonic crystal fiber, optical fiber sensors, surface plasmon resonance.
1. Introduction
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Sensors should be highly sensitive for the refractive index (RI) where the sensing region consists of bio-fluids and biomolecules that change their bulk RI. Due to the precise response for the small variation of external RI, surface plasmon resonance (SPR) sensors are now playing an important role for direct and real-time observations of bio-molecular interactions [1]. Among various SPR sensors, photonic crystal fiber (PCF) based sensors have gained increasing attention in the scientific community because of their excellent guiding properties, design freedom and also for the miniaturized size. Owing to the miniaturized size and design flexibility, they are easily to incorporate with the SPR and are broadly used as sensing scheme for its extremely sensitive behavior [2]. Over the past few years, a number of SPR sensors have been proposed by several groups for detecting the analytes ranging from 1.33-1.41 [3]–[10]. Existing PCF based SPR sensors can be divided into five categories e.g., internal and external metal coated based PCF sensors, nanowire based sensors, micro-fluidic slotted, and D-shaped configuration based sensors [11]. For example, in external metal coated PCF sensors, the free electrons of the metal are excited by the incident photon, which thus increase the confinement loss, and at particular condition, the loss is maximum known as resonance condition. If the analyte’s RI is changed, then resonance condition also changes, and this mechanism is used for detecting the various analytes by observing the red and blue shift at the resonance wavelength [12]. However, two major limitations have been observed for further enlargement of these sensors. Firstly, one of the major problems of PCF based sensors is the fabrication issue: Internal metal coated and nanowirebased PCF sensors are difficult to fabricate because it requires coating of the plasmonic material and filling of analyte to the micro-meter sized air-hole’s surface [13]. To metallize the PCFs, chemical vapor deposition (CVD) technique is the well-established method and used to anoint the inner walls of the air-holes with metal [14]. Besides, in case of metallic nanowire infiltration into the fiber holes, the molten metal is pumped through the fiber holes [15]. But, these processes are complex as the air-holes are in micro-meter scaled. In contrast, micro-fluidic slotted based PCF sensors contain metal in the outer surface, however, this process is not easier because these sensors require micro meter sized slots to create the channel. Among various configurations, external metal coated PCF sensor is easier to fabricate [14]. The major drawback of such sensors is that they are limited to a few number of air-hole rings that lead to high propagation loss. Secondly, most of the SPR sensors are limited to detect the high RI measurement of the bio-molecules: They cannot detect the analytes higher than the analytes RI of 1.41 [16]. As an example, Liu et al., have recently developed a biosensor that can detect the analytes RI up to 1.41 [17]. In addition, dual core hexagonal lattice high index PCF sensor with the detection range of 1.33-1.40 has been proposed recently [18]. For the satisfactory requirement of total internal reflection (TIR), the RI of core should be greater than that of cladding. So far, the existing sensors are designed to infiltrate the analyte of the selected channels, RI contrast is not maintained properly to meet the requirement of TIR [19]. However, the material coating and filling of analytes to the micro-meter sized fiber holes are also mandatory in these
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sensors that do not comply for real time sensing. Moreover, several groups have been proposed plasmonic sensors with various configurations, where analyte and plasmonic metal are placed outside the fiber structure to reduce the fabrication complexity [6], [12], [14], [20]. Though, the plasmonic material coating and analyte filling of the sensors provide an idea of real time sensing, these types of sensors are limited to detect the analytes whose RI is higher than that of 1.41. For example, if analyte is filled with RI of 1.43, effective RI of core will be lower than that of cladding, and hence no TIR mechanism is satisfied. Recently, hollowcore PCF based SPR sensors have been proposed by N. Luan et al,. [21], [22], where the analyte is placed to the fiber core to create the waveguide. As the analyte RI is greater than that of cladding, the TIR is satisfied and the light guides through core region of the PCF. However, in such hollow-core PCF sensors, plasmonic material and analyte are placed into the fiber core, resulting in an interference effect. To overcome the aforementioned problems, we propose an air-core PCF based plasmonic sensor, in which plasmonic material and the analyte are placed ~ 2.5 µm apart (this distance is a few wavelengths larger than the operating wavelength), which would reduce the interference effect. We show that the proposed sensor can be able to detect the analyte RI ranging from 1.42 to 1.54 (to the best of our knowledge this is the highest detection range using SPR), which cannot be realized with the conventional PCF based sensors. The proposed sensor is analyzed through wavelength sensitivity and amplitude sensitivity with the finite element method (FEM) based simulation tool.
2. Design Methodology
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Fig. 1 shows the sensors cross-sectional view with hexagonal lattice PCF in the transverse plane (i.e., x-y plane) with a vertical side opening channel. The diameter of the air-core is dc = 4.50 µm, thickness of side opening channel is ts = 0.50 µm and gold layer thickness is set to tg = 40 nm. The first ring and second ring are placed at 3.0 µm and 4.50 µm away from the center of the air-core, respectively, with diameter of air-holes of each rings is d = 1.0 µm. The proposed sensor can be fabricated in two steps: air-core PCF fabrication and creation of vertical side opening channel. To fabricate the aircore PCF, stack and draw method can be applied to form the PCF, as mentioned in [23]. In [23], the diameter of the fabricated PCF’s air-core was 14.80 µm. Moreover, an extremely sub-wavelength air-core with diameter ~ 110 nm was fabricated using stack and draw method [24]. Since the diameter of the air-core of our sensor is 4.50 µm, we believe that the proposed PCF can be fabricated using the standard stack and draw method. In the second step, the vertical opening channel can be fabricated using the focused ion beam milling and femtosecond laser micromachining [25]. Finally, the proposed structure can be manufactured by polishing two opposite side of the PCF with depositing gold film by CVD technique [17]. The permittivity of the gold is retrieved from the well-known Drude-Lorenz model [19]. To design the proposed sensor and get the best performance the following steps have been used: Step-1: Analyzing the existing SPR based PCF sensor. Step-2: Defining parameters, i.e. pitch, air-core diameter, air-hole diameter, thickness of side opening channel, gold layer thickness etc. Step-3: Determining the loss profile for different analyte RI. Step-4: Analyzing the sensor performance in terms of wavelength sensitivity, amplitude sensitivity, sensor resolution and so on. Step-5: If satisfactory, quit simulation. If not, then go to next step. Step-6: Change the design parameters. Step-7: Again, determining the loss profile for different analyte RI. Step-8: Analyzing the sensor performance in terms of wavelength sensitivity, amplitude sensitivity, sensor resolution and so on. Step-9: Go to Step-5.
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Fig. 1. The cross sectional view of the proposed sensor in the transverse plane.
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3. Results and Discussions 3.1 Coupling properties (c)
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Fig. 2. Optical field distribution of proposed sensor, core guided mode: (a) x- and (b) y-polarization; SPP mode: (c) x- and (d) y-polarization at 1.424 µm for analyte RI of 1.43.
The optical field distribution of the fundamental core mode and surface plasmon polariton (SPP) mode of the proposed sensor is shown in Fig. 2. Note, the fundamental core mode and SPP mode are plotted for both x- and y-polarization. As an example, the field distribution of the modes is shown at the resonance condition, and is calculated at 1.424 µm wavelength with analyte RI of 1.43. It can be seen from the Figs. 2 (a) and (b) that y-polarized core mode is stronger than the x-polarized mode, which is due to the fact that the plasmonic material is placed in the vertical direction, in our case in y-axis, see Fig. 1. Throughout this paper, we consider only the y-polarized mode to investigate the performance of the proposed sensor. Note, we set the diameter of the air-core such that the sensor always satisfies the operation of TIR either at high or low analyte RI. To understand the phase matching behavior, we calculate the dispersion relation between the fundamental core mode and SPP mode, which is depicted in Fig. 3. We have plotted the effective RI (neff) of both modes (core and SPP) for na = 1.43. It can be seen from the figure that real part of effective RI of both modes are matched at the wavelength of 1.424 µm, called resonance wavelength. At this wavelength the maximum energy is transferred to the SPP mode. As the imaginary part of the effective RI (neff) is responsible for the confinement loss, confinement loss will be maximum at the resonance wavelength, and can be calculated from the equation mentioned in [12].
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To realize the sensing mechanisms, the analyte RI is slightly changed from 1.42 to 1.43, we observe the changes in their real part of neff for the fundamental modes, and SPP modes and resonance peaks which are shown in Figs. 4 (a) and (b). From the figure it is seen that, the small change in the analyte RI changes the real part of neff for the core modes and SPP modes, hence, altering the position of the phase matching point. As a result, different core mode loss spectrums are obtained. Consequently, by detecting the changing of resonance wavelength and core mode loss spectra, the analyte changes can be detected. Moreover, amplitude interrogation method and wavelength interrogation method are the two well-known techniques for performance measuring approaches for SPR sensor [17].
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3.2 Wavelength sensitivity The difference of resonance wavelength with change in analyte RI is called wavelength sensitivity for SPR sensor. Basically, more amount of shift in resonance wavelength with a small change in analyte RI stands for high wavelength sensitivity which is highly desired. The wavelength sensitivity can be measured using wavelength interrogation method and can be calculated as [13], peak [nm/RIU] (1) S ( ) n a where, δλpeak and δna indicates resonance peak difference and analyte RI difference, respectively. The resonance peak can be calculated from the loss spectra for the corresponding analyte RI. Then, difference of resonance wavelength is gained from the loss curves of two successive analytes. The loss spectra as a function of wavelength for different analyte RI is shown in Figs. 5(a-b). From the figure it can be clearly stated that resonance wavelength changes its position with the change in analyte RI. For better understanding, we plot the loss intensity as a function of wavelength and analyte RI, as shown in Figs. 5(c-d). We plot the intensity in logarithmic scale (normalized), showing the important intensity characteristics. Note that each loss curve is normalized with its maximum value, and hence, the maximum intensity stands in each resonance wavelength for each analyte. In addition, the blue shift of the resonance wavelength occurs for higher analyte RI due to rise in index contrast between core and cladding as core RI increases for higher analyte RI. The resonance is occurred at the wavelength of 1.541 µm, 1.424 µm, 1.323 µm, 1.243 µm, 1.168 µm, 1.095 µm, and 1.041 µm for the analyte RI of 1.42-1.48 with increased rate of 0.01, respectively as shown in the Fig. 5(a). When analyte RI is changed from 1.42 to 1.43, the maximum resonance wavelength shift is observed of 117 nm. According to the Eq. 1., the calculated wavelength sensitivity is 11,700 nm/RIU for the analyte RI of 1.42. Similarly, the wavelength sensitivities are 10,100 nm/RIU, 8000 nm/RIU, 7500 nm/RIU, 7300 nm/RIU, and 5400 nm/RIU for the analyte RI of 1.43, 1.44, 1.45, 1.46, and 1.48, respectively. In Fig. 5(b), we show the loss spectrum for analyte RI ranging from 1.49 to 1.54. Note, as similar to Fig. 5(a), one can clearly see that the loss intensity decreases with increasing the analyte RI, which is due to the increment of index contrast between core and cladding. In Fig. 5(b), we see that resonance wavelength is occurred at 0.9918 µm, 0.9488 µm, 0.9136 µm, 0.8824 µm, 0.8569 µm, and 0.8353 µm for the analyte RI of 1.49 to 1.54 with wavelength sensitivity of 4920 nm/RIU, 4300 nm/RIU, 3520 nm/RIU,3120 nm/RIU, 2550 nm/RIU, 2160 nm/RIU, respectively. The resonance wavelengths and wavelength sensitivities as a function of analyte RI are summarized in Fig. 6. As expected, resonance wavelength is shifted to the lower wavelength with increasing analyte RI.
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Fig. 6. The resonance wavelengths and the wavelength sensitivities of the proposed sensor with different na.
3.3 Amplitude sensitivity Another analyte detection method is the amplitude interrogation method, the analyte RI (na) can be detected by observing the change in transmitted optical power at a fixed wavelength. The amplitude sensitivity is defined as [19],
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here, α(λ, na) presents the confinement loss as a function of the wavelength λ and the na. Fig. 7(a) shows the amplitude sensitivity curve calculated from above equation for the analyte RI from 1.42 to 1.47. As seen from Fig. 7(a), the maximum amplitude sensitivity is obtained for the analyte RI of 1.42 and which is 159.7 RIU-1. Furthermore, the amplitude sensitivities are 96.91 RIU-1, 93.64 RIU-1, 65.16 RIU-1, 35.05 RIU-1, and 18.75 RIU-1 for the analyte RI of 1.43, 1.44, 1.45, 1.46, and 1.47, respectively, as shown in Fig. 7(a). In Fig. 7(b), we plot the amplitude sensitivity for analyte RI of 1.49 to 1.53, showing amplitude sensitivity of 8.94 RIU-1,19.79 RIU-1, 28.44 RIU-1, 89.62 RIU-1, 200.7 RIU-1. The main feature of this approach is that it is economical due to no need of spectral manipulation, while the shortcoming is that in this approach, operational range is smaller to get maximum sensitivity.
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3.4 Sensor resolution Sensor resolution is very important as it provides how much the sensor can able to sense the change in analyte RI. The resolution for a PCF SPR RI sensor can be calculated as follows [20], R na min / peak [RIU]
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where, Δλpeak, Δλmin and Δna indicate that the resonance peak difference, minimum spectral resolution, and analyte RI difference, respectively. Therefore, the maximum wavelength resolution based on Eq. 3. of the proposed RI sensor is 8.55×10–6 RIU for analyte RI of 1.42, has ability to detect the analyte RI with a change in the order of 10–6. From the recent literature, it is shown that most PCF SPR sensors [6], [12], [14], [20], are designed in such a way that plasmonic material and analyte are placed in direct contact, which creates interference from the materials in some chemical and biological sensing [25]. In this work, we design an air-core PCF and analyte is infiltrated in the core region with a side opening channel. The plasmonic metal is put outside (flat surface) the structure, hence, no directly interaction with analyte and reduces interference effect. Another key feature of this sensor is that it can also be able to detect the analytes when the analyte RI is slightly lower than the background material RI, because of the reduction of the cladding effective RI as the number of air holes are located in the cladding region that reduce the effective RI of the cladding of the PCF. 120
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3.5 Effect of ts and dc on sensor performance
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Finally, we analyze the effect of thickness of the side opening channel, ts and air-core diameter, dc on the loss profile, which is depicted in Fig. 8(a-b). We vary the thickness of the side opening channel from ± 1% to ± 5% from its optimum value (i.e., ts = 0.5 µm) for the analyte RI of 1.43. From Fig. 8(a), it is seen that for ts = 0.5 µm (keeping other parameters fixed), the loss depth is found 104.4 dB/cm for 1.432 µm wavelength, but, as expected, insignificant change of loss depth as well as resonance wavelength is found when we vary ts from ± 1% to ± 5% from the optimum value. In addition, loss intensity as a function of ts and wavelength is plotted in Fig. 8(c), showing the same information as sated above. Fig. 8(b) depicts the loss profile as a function of wavelength with varying air-core diameter, dc. As before, we vary dc from ±1% to ±5% from its optimum value. In addition, we plot the loss intensity profile as a function of air-core diameter and wavelength, which is shown in Fig. 8(d). Similar to Fig. 8(a,c), we found insignificant change of loss profile and the resonance wavelength with changing dc, which is expected because such a small variation of dc would not significantly change the index contrast between core and cladding.
4. Conclusions
In summary, we have proposed an air-core PCF based SPR sensor where the core is infiltrated with analyte through a side opening channel. One of the key findings of the proposed sensor is that it can be able to detect the RI of the analyte higher than that of the PCF background. In addition, this sensor is free from interference effect as there is no direct contact exists between the analyte and plasmonic material. Owing to the superior sensing performance, we believe that the proposed sensor can be used to detect highly active chemical and biological liquid samples.
References
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
rna
[2]
M. Liu, X. Yang, P. Shum, and H. Yuan, “High-sensitivity birefringent and single-layer coating photonic crystal fiber biosensor based on surface plasmon resonance,” Appl. Opt., vol. 57, no. 8, pp. 1883 ‒1886, 2018. A. Yasli and H. Ademgil, “Geometrical comparison of photonic crystal fiber-based surface plasmon resonance sensors, “Opt. Eng., vol. 57, no. 3, p. 030801, 2018. G. An et al., “Extra-broad photonic crystal fiber refractive index sensor based on surface plasmon resonance,” Plasmonic, vol. 12, no. 2, pp. 465 ‒ 471, 2017. 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 Photon. J., vol. 10, no. 1, pp. 1 ‒ 9, 2018. R. K. Gangwar and V. K. Singh, “Highly sensitive surface plasmon resonance based D-shaped photonic crystal fiber refractive index sensor,” Plasmonics, vol. 12, no. 5, pp. 1367‒1372, 2017. M. R. Hasan et al., “Spiral photonic crystal fiber-based dual-polarized surface plasmon resonance biosensor,” IEEE Sens. J., vol. 18, no. 1, pp. 133 ‒140, 2018. F. Wang, Z. Sun, C. Liu, T. Sun, and P. J. Chu, “A high-sensitivity photonic crystal fiber (PCF) based on the surface plasmon resonance (SPR) biosensor for detection of density alteration in non-physiological cells (DANCE),” Opto-Electron. Rev., vol. 26, no. 1, pp. 50 ‒ 56, 2018. F. Wang, Z. Sun, C. Liu, T. Sun, and P. K. Chu, “A highly sensitive dual-core photonic crystal fiber based on a surface plasmon resonance biosensor with silver-graphene layer,” Plasmonic, vol. 12, no. 6, pp. 1847‒1853, 2017. S. Das and V. K. Singh, "Refractive index sensor based on selectively liquid infiltrated birefringent photonic crystal fiber," Optik, vol. 201, p. 163489, 2020. Z. Fan, Z. Guo, X. Kong, and Z. Meng, "Photonic crystal fiber based wide-range of refractive index sensor with phase matching between core mode and metal defect mode," Optics Communications, p. 125233, 2020. A. A. Rifat, R. Ahmed, G. A. Mahdiraji, and F. M. Adikan, “Highly sensitive D-shaped photonic crystal fiber-based plasmonic biosensor in visible to near-IR,” IEEE Sens. J., vol. 17, no. 9, pp. 2776 ‒ 2783, 2017. A. K. Paul, A. K. Sarkar, M. A. B. S. Rahman and A. Khaleque, “Twin core photonic crystal fiber plasmonic refractive index sensor,” IEEE Sens. J., vol. 18, pp. 5761 – 5769, 2018. X. Yu et al., “A selectively coated photonic crystal fiber based surface plasmon resonance sensor,” J. Opt., vol. 12, no. 1, p. 015005, 2009. A. K. Paul, A. K. Sarkar, M. H. Islam, M. Morshed, “Dual core photonic crystal fiber based surface plasmon resonance biosensor”, Optik, vol. 170, pp. 400 – 408, 2018. Q. Xu et al., “A generic approach to desired metallic nanowires inside native porous alumina template via redox reaction,” J. Mater. Chem., vol. 21, no. 12, pp. 2397‒2402, 2009. M. B. Hossain, S. R. Islam, K. T. Hossain, L. F. Abdulrazak, M. N. Sakib, and I. Amiri, "High Sensetivity Hollow Core Circular Shaped PCF Surface Plasmonic Biosensor Employing Silver Coat: A Numerical Design and Analysis with External Sensing Approach," Results in Physics, p. 102909, 2020. C. Liu et al., “Symmetrical dual D-shape photonic crystal fibers for surface plasmon resonance sensing,” Opt. Exp., vol. 26, no. 7, pp. 9039-9049, 2018. T. Ahmed, A. K. Paul, M. S. Anower, and S. A. Razzak, "Surface plasmon resonance biosensor based on hexagonal lattice dual-core photonic crystal fiber," Applied Optics, vol. 58, no. 31, pp. 8416-8422, 2019. A. A. Rifat, et al., “Highly sensitive selectively coated photonic crystal fiber-based plasmonic sensor, “Opt. Lett., vol. 43, no. 4, pp. 891 ‒ 894, 2018. J. N. Dash and R. Jha, “Graphene-based birefringent photonic crystal fiber sensor using surface plasmon resonance,” IEEE Photonics. Technol. Lett., vol. 26, no. 11, pp. 1092 ‒ 1095, 2014. N. Luan and J. Yao, “High refractive index surface plasmon resonance sensor based on a silver wire filled hollow fiber,” IEEE Photon. J., vol. 8, no. 1, pp. 1 ‒ 9, 2016. N. Luan and J. Yao, “A hollow-core photonic crystal fiber-based SPR sensor with large detection range,” IEEE Photon. J., vol. 9, no. 3, pp. 1 ‒ 7, 2017. R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St. J. Russell, P. J. Roberts, and D. C. Allan, “Single-mode photonic band gap guidance of light in air,” Science, vol. 285, no. 5433, pp. 1537‒1539, 1999.
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G. S. Wiederhecker et al., “Field enhancement within an optical fibre with a subwavelength air core,” Nat. Photonics, vol. 1, pp. 115–118, 2007. 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 Photon. J., vol. 10, no. 5, p. 6803707, 2018.
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Credit Author Statement
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M. S. H conceived the idea of the sensor. A. K. P performed all the numerical simulations. A. K. P performed the data analysis, with assistance from M. S. H, S. M. R, and N. H. H. The
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manuscript was written by A. K. P with suggestions and additions from all authors.
PII: DOI: Reference:
S0030-4018(20)30171-1 https://doi.org/10.1016/j.optcom.2020.125556 OPTICS 125556
To appear in:
Optics Communications
Received date : 9 December 2019 Revised date : 15 February 2020 Accepted date : 17 February 2020 Please cite this article as: A.K. Paul, Md.S. Habib, N.H. Hai et al., An air-core photonic crystal fiber based plasmonic sensor for high refractive index sensing, Optics Communications (2020), doi: https://doi.org/10.1016/j.optcom.2020.125556. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
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An Air-Core Photonic Crystal Fiber Based Plasmonic Sensor for High Refractive Index Sensing Alok Kumar Paul,1 Md. Samiul Habib,1 Nguyen Hoang Hai,2 and S. M. Abdur Razzak,1 1
Department of Electrical & Electronic Engineering, Rajshahi University of Engineering & Technology, Kazla, Rajshahi-6204, Bangladesh (e-mail: [email protected]) 2
School of Electronics and Telecommunication, Hanoi University of Science and Technology, Hanoi 100000, Vietnam
Abstract: Most surface plasmon resonance (SPR) based photonic crystal fiber (PCF) sensors have been designed to detect the analyte refractive index (RI) ranging from 1.33‒1.41. In this paper, we propose a new approach to extend the detection range of the sensor, and it can be fabricated with the existing fabrication techniques. Contrary to the existing PCF based sensors, our designed sensor is based on an air-core PCF where analyte is placed to the core of the PCF through a vertical side opening channel and is able to detect RI of the analyte higher than that of the PCF background. We use a chemically stable plasmonic material (i.e., gold), where the plasmonic material and analyte are placed in such a way that there is no direct contact between them, thus reducing the interference effect. According to our simulation, we show that the spectral sensitivity and resolution are achieved as 11,700 nm/RIU and 8.55×10–6 RIU, respectively, for the analyte RI of 1.42. We believe that our proposed sensor can be used to detect highly active chemical and biological liquid samples.
Index Terms: Photonic crystal fiber, optical fiber sensors, surface plasmon resonance.
1. Introduction
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Sensors should be highly sensitive for the refractive index (RI) where the sensing region consists of bio-fluids and biomolecules that change their bulk RI. Due to the precise response for the small variation of external RI, surface plasmon resonance (SPR) sensors are now playing an important role for direct and real-time observations of bio-molecular interactions [1]. Among various SPR sensors, photonic crystal fiber (PCF) based sensors have gained increasing attention in the scientific community because of their excellent guiding properties, design freedom and also for the miniaturized size. Owing to the miniaturized size and design flexibility, they are easily to incorporate with the SPR and are broadly used as sensing scheme for its extremely sensitive behavior [2]. Over the past few years, a number of SPR sensors have been proposed by several groups for detecting the analytes ranging from 1.33-1.41 [3]–[10]. Existing PCF based SPR sensors can be divided into five categories e.g., internal and external metal coated based PCF sensors, nanowire based sensors, micro-fluidic slotted, and D-shaped configuration based sensors [11]. For example, in external metal coated PCF sensors, the free electrons of the metal are excited by the incident photon, which thus increase the confinement loss, and at particular condition, the loss is maximum known as resonance condition. If the analyte’s RI is changed, then resonance condition also changes, and this mechanism is used for detecting the various analytes by observing the red and blue shift at the resonance wavelength [12]. However, two major limitations have been observed for further enlargement of these sensors. Firstly, one of the major problems of PCF based sensors is the fabrication issue: Internal metal coated and nanowirebased PCF sensors are difficult to fabricate because it requires coating of the plasmonic material and filling of analyte to the micro-meter sized air-hole’s surface [13]. To metallize the PCFs, chemical vapor deposition (CVD) technique is the well-established method and used to anoint the inner walls of the air-holes with metal [14]. Besides, in case of metallic nanowire infiltration into the fiber holes, the molten metal is pumped through the fiber holes [15]. But, these processes are complex as the air-holes are in micro-meter scaled. In contrast, micro-fluidic slotted based PCF sensors contain metal in the outer surface, however, this process is not easier because these sensors require micro meter sized slots to create the channel. Among various configurations, external metal coated PCF sensor is easier to fabricate [14]. The major drawback of such sensors is that they are limited to a few number of air-hole rings that lead to high propagation loss. Secondly, most of the SPR sensors are limited to detect the high RI measurement of the bio-molecules: They cannot detect the analytes higher than the analytes RI of 1.41 [16]. As an example, Liu et al., have recently developed a biosensor that can detect the analytes RI up to 1.41 [17]. In addition, dual core hexagonal lattice high index PCF sensor with the detection range of 1.33-1.40 has been proposed recently [18]. For the satisfactory requirement of total internal reflection (TIR), the RI of core should be greater than that of cladding. So far, the existing sensors are designed to infiltrate the analyte of the selected channels, RI contrast is not maintained properly to meet the requirement of TIR [19]. However, the material coating and filling of analytes to the micro-meter sized fiber holes are also mandatory in these
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sensors that do not comply for real time sensing. Moreover, several groups have been proposed plasmonic sensors with various configurations, where analyte and plasmonic metal are placed outside the fiber structure to reduce the fabrication complexity [6], [12], [14], [20]. Though, the plasmonic material coating and analyte filling of the sensors provide an idea of real time sensing, these types of sensors are limited to detect the analytes whose RI is higher than that of 1.41. For example, if analyte is filled with RI of 1.43, effective RI of core will be lower than that of cladding, and hence no TIR mechanism is satisfied. Recently, hollowcore PCF based SPR sensors have been proposed by N. Luan et al,. [21], [22], where the analyte is placed to the fiber core to create the waveguide. As the analyte RI is greater than that of cladding, the TIR is satisfied and the light guides through core region of the PCF. However, in such hollow-core PCF sensors, plasmonic material and analyte are placed into the fiber core, resulting in an interference effect. To overcome the aforementioned problems, we propose an air-core PCF based plasmonic sensor, in which plasmonic material and the analyte are placed ~ 2.5 µm apart (this distance is a few wavelengths larger than the operating wavelength), which would reduce the interference effect. We show that the proposed sensor can be able to detect the analyte RI ranging from 1.42 to 1.54 (to the best of our knowledge this is the highest detection range using SPR), which cannot be realized with the conventional PCF based sensors. The proposed sensor is analyzed through wavelength sensitivity and amplitude sensitivity with the finite element method (FEM) based simulation tool.
2. Design Methodology
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Fig. 1 shows the sensors cross-sectional view with hexagonal lattice PCF in the transverse plane (i.e., x-y plane) with a vertical side opening channel. The diameter of the air-core is dc = 4.50 µm, thickness of side opening channel is ts = 0.50 µm and gold layer thickness is set to tg = 40 nm. The first ring and second ring are placed at 3.0 µm and 4.50 µm away from the center of the air-core, respectively, with diameter of air-holes of each rings is d = 1.0 µm. The proposed sensor can be fabricated in two steps: air-core PCF fabrication and creation of vertical side opening channel. To fabricate the aircore PCF, stack and draw method can be applied to form the PCF, as mentioned in [23]. In [23], the diameter of the fabricated PCF’s air-core was 14.80 µm. Moreover, an extremely sub-wavelength air-core with diameter ~ 110 nm was fabricated using stack and draw method [24]. Since the diameter of the air-core of our sensor is 4.50 µm, we believe that the proposed PCF can be fabricated using the standard stack and draw method. In the second step, the vertical opening channel can be fabricated using the focused ion beam milling and femtosecond laser micromachining [25]. Finally, the proposed structure can be manufactured by polishing two opposite side of the PCF with depositing gold film by CVD technique [17]. The permittivity of the gold is retrieved from the well-known Drude-Lorenz model [19]. To design the proposed sensor and get the best performance the following steps have been used: Step-1: Analyzing the existing SPR based PCF sensor. Step-2: Defining parameters, i.e. pitch, air-core diameter, air-hole diameter, thickness of side opening channel, gold layer thickness etc. Step-3: Determining the loss profile for different analyte RI. Step-4: Analyzing the sensor performance in terms of wavelength sensitivity, amplitude sensitivity, sensor resolution and so on. Step-5: If satisfactory, quit simulation. If not, then go to next step. Step-6: Change the design parameters. Step-7: Again, determining the loss profile for different analyte RI. Step-8: Analyzing the sensor performance in terms of wavelength sensitivity, amplitude sensitivity, sensor resolution and so on. Step-9: Go to Step-5.
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3. Results and Discussions 3.1 Coupling properties (c)
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Fig. 2. Optical field distribution of proposed sensor, core guided mode: (a) x- and (b) y-polarization; SPP mode: (c) x- and (d) y-polarization at 1.424 µm for analyte RI of 1.43.
The optical field distribution of the fundamental core mode and surface plasmon polariton (SPP) mode of the proposed sensor is shown in Fig. 2. Note, the fundamental core mode and SPP mode are plotted for both x- and y-polarization. As an example, the field distribution of the modes is shown at the resonance condition, and is calculated at 1.424 µm wavelength with analyte RI of 1.43. It can be seen from the Figs. 2 (a) and (b) that y-polarized core mode is stronger than the x-polarized mode, which is due to the fact that the plasmonic material is placed in the vertical direction, in our case in y-axis, see Fig. 1. Throughout this paper, we consider only the y-polarized mode to investigate the performance of the proposed sensor. Note, we set the diameter of the air-core such that the sensor always satisfies the operation of TIR either at high or low analyte RI. To understand the phase matching behavior, we calculate the dispersion relation between the fundamental core mode and SPP mode, which is depicted in Fig. 3. We have plotted the effective RI (neff) of both modes (core and SPP) for na = 1.43. It can be seen from the figure that real part of effective RI of both modes are matched at the wavelength of 1.424 µm, called resonance wavelength. At this wavelength the maximum energy is transferred to the SPP mode. As the imaginary part of the effective RI (neff) is responsible for the confinement loss, confinement loss will be maximum at the resonance wavelength, and can be calculated from the equation mentioned in [12].
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To realize the sensing mechanisms, the analyte RI is slightly changed from 1.42 to 1.43, we observe the changes in their real part of neff for the fundamental modes, and SPP modes and resonance peaks which are shown in Figs. 4 (a) and (b). From the figure it is seen that, the small change in the analyte RI changes the real part of neff for the core modes and SPP modes, hence, altering the position of the phase matching point. As a result, different core mode loss spectrums are obtained. Consequently, by detecting the changing of resonance wavelength and core mode loss spectra, the analyte changes can be detected. Moreover, amplitude interrogation method and wavelength interrogation method are the two well-known techniques for performance measuring approaches for SPR sensor [17].
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3.2 Wavelength sensitivity The difference of resonance wavelength with change in analyte RI is called wavelength sensitivity for SPR sensor. Basically, more amount of shift in resonance wavelength with a small change in analyte RI stands for high wavelength sensitivity which is highly desired. The wavelength sensitivity can be measured using wavelength interrogation method and can be calculated as [13], peak [nm/RIU] (1) S ( ) n a where, δλpeak and δna indicates resonance peak difference and analyte RI difference, respectively. The resonance peak can be calculated from the loss spectra for the corresponding analyte RI. Then, difference of resonance wavelength is gained from the loss curves of two successive analytes. The loss spectra as a function of wavelength for different analyte RI is shown in Figs. 5(a-b). From the figure it can be clearly stated that resonance wavelength changes its position with the change in analyte RI. For better understanding, we plot the loss intensity as a function of wavelength and analyte RI, as shown in Figs. 5(c-d). We plot the intensity in logarithmic scale (normalized), showing the important intensity characteristics. Note that each loss curve is normalized with its maximum value, and hence, the maximum intensity stands in each resonance wavelength for each analyte. In addition, the blue shift of the resonance wavelength occurs for higher analyte RI due to rise in index contrast between core and cladding as core RI increases for higher analyte RI. The resonance is occurred at the wavelength of 1.541 µm, 1.424 µm, 1.323 µm, 1.243 µm, 1.168 µm, 1.095 µm, and 1.041 µm for the analyte RI of 1.42-1.48 with increased rate of 0.01, respectively as shown in the Fig. 5(a). When analyte RI is changed from 1.42 to 1.43, the maximum resonance wavelength shift is observed of 117 nm. According to the Eq. 1., the calculated wavelength sensitivity is 11,700 nm/RIU for the analyte RI of 1.42. Similarly, the wavelength sensitivities are 10,100 nm/RIU, 8000 nm/RIU, 7500 nm/RIU, 7300 nm/RIU, and 5400 nm/RIU for the analyte RI of 1.43, 1.44, 1.45, 1.46, and 1.48, respectively. In Fig. 5(b), we show the loss spectrum for analyte RI ranging from 1.49 to 1.54. Note, as similar to Fig. 5(a), one can clearly see that the loss intensity decreases with increasing the analyte RI, which is due to the increment of index contrast between core and cladding. In Fig. 5(b), we see that resonance wavelength is occurred at 0.9918 µm, 0.9488 µm, 0.9136 µm, 0.8824 µm, 0.8569 µm, and 0.8353 µm for the analyte RI of 1.49 to 1.54 with wavelength sensitivity of 4920 nm/RIU, 4300 nm/RIU, 3520 nm/RIU,3120 nm/RIU, 2550 nm/RIU, 2160 nm/RIU, respectively. The resonance wavelengths and wavelength sensitivities as a function of analyte RI are summarized in Fig. 6. As expected, resonance wavelength is shifted to the lower wavelength with increasing analyte RI.
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Fig. 6. The resonance wavelengths and the wavelength sensitivities of the proposed sensor with different na.
3.3 Amplitude sensitivity Another analyte detection method is the amplitude interrogation method, the analyte RI (na) can be detected by observing the change in transmitted optical power at a fixed wavelength. The amplitude sensitivity is defined as [19],
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3.4 Sensor resolution Sensor resolution is very important as it provides how much the sensor can able to sense the change in analyte RI. The resolution for a PCF SPR RI sensor can be calculated as follows [20], R na min / peak [RIU]
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3.5 Effect of ts and dc on sensor performance
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Finally, we analyze the effect of thickness of the side opening channel, ts and air-core diameter, dc on the loss profile, which is depicted in Fig. 8(a-b). We vary the thickness of the side opening channel from ± 1% to ± 5% from its optimum value (i.e., ts = 0.5 µm) for the analyte RI of 1.43. From Fig. 8(a), it is seen that for ts = 0.5 µm (keeping other parameters fixed), the loss depth is found 104.4 dB/cm for 1.432 µm wavelength, but, as expected, insignificant change of loss depth as well as resonance wavelength is found when we vary ts from ± 1% to ± 5% from the optimum value. In addition, loss intensity as a function of ts and wavelength is plotted in Fig. 8(c), showing the same information as sated above. Fig. 8(b) depicts the loss profile as a function of wavelength with varying air-core diameter, dc. As before, we vary dc from ±1% to ±5% from its optimum value. In addition, we plot the loss intensity profile as a function of air-core diameter and wavelength, which is shown in Fig. 8(d). Similar to Fig. 8(a,c), we found insignificant change of loss profile and the resonance wavelength with changing dc, which is expected because such a small variation of dc would not significantly change the index contrast between core and cladding.
4. Conclusions
In summary, we have proposed an air-core PCF based SPR sensor where the core is infiltrated with analyte through a side opening channel. One of the key findings of the proposed sensor is that it can be able to detect the RI of the analyte higher than that of the PCF background. In addition, this sensor is free from interference effect as there is no direct contact exists between the analyte and plasmonic material. Owing to the superior sensing performance, we believe that the proposed sensor can be used to detect highly active chemical and biological liquid samples.
References
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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[2]
M. Liu, X. Yang, P. Shum, and H. Yuan, “High-sensitivity birefringent and single-layer coating photonic crystal fiber biosensor based on surface plasmon resonance,” Appl. Opt., vol. 57, no. 8, pp. 1883 ‒1886, 2018. A. Yasli and H. Ademgil, “Geometrical comparison of photonic crystal fiber-based surface plasmon resonance sensors, “Opt. Eng., vol. 57, no. 3, p. 030801, 2018. G. An et al., “Extra-broad photonic crystal fiber refractive index sensor based on surface plasmon resonance,” Plasmonic, vol. 12, no. 2, pp. 465 ‒ 471, 2017. 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 Photon. J., vol. 10, no. 1, pp. 1 ‒ 9, 2018. R. K. Gangwar and V. K. Singh, “Highly sensitive surface plasmon resonance based D-shaped photonic crystal fiber refractive index sensor,” Plasmonics, vol. 12, no. 5, pp. 1367‒1372, 2017. M. R. Hasan et al., “Spiral photonic crystal fiber-based dual-polarized surface plasmon resonance biosensor,” IEEE Sens. J., vol. 18, no. 1, pp. 133 ‒140, 2018. F. Wang, Z. Sun, C. Liu, T. Sun, and P. J. Chu, “A high-sensitivity photonic crystal fiber (PCF) based on the surface plasmon resonance (SPR) biosensor for detection of density alteration in non-physiological cells (DANCE),” Opto-Electron. Rev., vol. 26, no. 1, pp. 50 ‒ 56, 2018. F. Wang, Z. Sun, C. Liu, T. Sun, and P. K. Chu, “A highly sensitive dual-core photonic crystal fiber based on a surface plasmon resonance biosensor with silver-graphene layer,” Plasmonic, vol. 12, no. 6, pp. 1847‒1853, 2017. S. Das and V. K. Singh, "Refractive index sensor based on selectively liquid infiltrated birefringent photonic crystal fiber," Optik, vol. 201, p. 163489, 2020. Z. Fan, Z. Guo, X. Kong, and Z. Meng, "Photonic crystal fiber based wide-range of refractive index sensor with phase matching between core mode and metal defect mode," Optics Communications, p. 125233, 2020. A. A. Rifat, R. Ahmed, G. A. Mahdiraji, and F. M. Adikan, “Highly sensitive D-shaped photonic crystal fiber-based plasmonic biosensor in visible to near-IR,” IEEE Sens. J., vol. 17, no. 9, pp. 2776 ‒ 2783, 2017. A. K. Paul, A. K. Sarkar, M. A. B. S. Rahman and A. Khaleque, “Twin core photonic crystal fiber plasmonic refractive index sensor,” IEEE Sens. J., vol. 18, pp. 5761 – 5769, 2018. X. Yu et al., “A selectively coated photonic crystal fiber based surface plasmon resonance sensor,” J. Opt., vol. 12, no. 1, p. 015005, 2009. A. K. Paul, A. K. Sarkar, M. H. Islam, M. Morshed, “Dual core photonic crystal fiber based surface plasmon resonance biosensor”, Optik, vol. 170, pp. 400 – 408, 2018. Q. Xu et al., “A generic approach to desired metallic nanowires inside native porous alumina template via redox reaction,” J. Mater. Chem., vol. 21, no. 12, pp. 2397‒2402, 2009. M. B. Hossain, S. R. Islam, K. T. Hossain, L. F. Abdulrazak, M. N. Sakib, and I. Amiri, "High Sensetivity Hollow Core Circular Shaped PCF Surface Plasmonic Biosensor Employing Silver Coat: A Numerical Design and Analysis with External Sensing Approach," Results in Physics, p. 102909, 2020. C. Liu et al., “Symmetrical dual D-shape photonic crystal fibers for surface plasmon resonance sensing,” Opt. Exp., vol. 26, no. 7, pp. 9039-9049, 2018. T. Ahmed, A. K. Paul, M. S. Anower, and S. A. Razzak, "Surface plasmon resonance biosensor based on hexagonal lattice dual-core photonic crystal fiber," Applied Optics, vol. 58, no. 31, pp. 8416-8422, 2019. A. A. Rifat, et al., “Highly sensitive selectively coated photonic crystal fiber-based plasmonic sensor, “Opt. Lett., vol. 43, no. 4, pp. 891 ‒ 894, 2018. J. N. Dash and R. Jha, “Graphene-based birefringent photonic crystal fiber sensor using surface plasmon resonance,” IEEE Photonics. Technol. Lett., vol. 26, no. 11, pp. 1092 ‒ 1095, 2014. N. Luan and J. Yao, “High refractive index surface plasmon resonance sensor based on a silver wire filled hollow fiber,” IEEE Photon. J., vol. 8, no. 1, pp. 1 ‒ 9, 2016. N. Luan and J. Yao, “A hollow-core photonic crystal fiber-based SPR sensor with large detection range,” IEEE Photon. J., vol. 9, no. 3, pp. 1 ‒ 7, 2017. R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St. J. Russell, P. J. Roberts, and D. C. Allan, “Single-mode photonic band gap guidance of light in air,” Science, vol. 285, no. 5433, pp. 1537‒1539, 1999.
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G. S. Wiederhecker et al., “Field enhancement within an optical fibre with a subwavelength air core,” Nat. Photonics, vol. 1, pp. 115–118, 2007. 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 Photon. J., vol. 10, no. 5, p. 6803707, 2018.
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Credit Author Statement
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M. S. H conceived the idea of the sensor. A. K. P performed all the numerical simulations. A. K. P performed the data analysis, with assistance from M. S. H, S. M. R, and N. H. H. The
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manuscript was written by A. K. P with suggestions and additions from all authors.