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Abstract
An extremely highly sensitive photonic crystal fiber (PCF) based SPR or surface plasmon resonance biosensor is manifested in this article, in a cumulated form of circularly slotted spiral lattice structure (SLS). The numerical performance analysis of the sensor is critically interpreted using the finite element method (FEM), including a perfectly matched layer (PML) and scattering boundary conditions. Very well known chemically stable material gold is used as the plasmonic material and implanted inside the circular slots to lessen the fabrication challenge and oxidation problem. The prospective model shows the maximum value of sensitivity is 41,000 nm/RIU, employing the method of wavelength interrogation and a maximum value of sensitivity of 4242 RIU−1, using the method of amplitude interrogation. The proposed sensor has the capability of detecting analytes in a massive range of the refractive index 1.32 to 1.50 RIU (for wavelength 0.5 µm to 1.55 µm) with a highest sensor resolution of 2.44 × 10−6 RIU. It is highly capable of identifying biomolecules like benzene, carbon tetrachloride, ethyl alcohol, acetone, water, silicon oil, and glucose solution in water, fused silica, cornea, lens, liver cell, and intestinal mucosa of human, polylactic acid, vegetable oil, and glycerol, etc. The sensor exhibits high linearity by showing R2 value 0.97 with a maximum FOM of 683 RIU−1. For large detection range, immense sensitivity, high FOM, and low fabrication complexity, the illustrated sensor can be a supreme candidate in the realm of SPR biosensor.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the revolution of science and technology, specially, in the field of biomedical engineering and optics, the SPR Biosensor has occupied a magnificent impact. The term SPR stands for a phenomenon named ‘Surface Plasmon Resonance’ which occurs when beams of pole polarized light are incident on thin film metallic surfaces and releases free electrons. When the frequency of the incident light beam and the metal surface free electron frequency coincide then resonance is occurred and sharp resonance peak is created at specific wavelength for specific analyte RI. Therefore, SPR biosensor is one kind of sensor that detects unknown analyte on its surrounding by applying SPR phenomenon. SPR biosensor is applied in multi-purposes like gas sensing, environment monitoring, soil testing, DNA testing, antigen antibody detection, analyte detection, identifying pathogens, health monitoring, detection of biochemical samples etc [1–5]. During the last few decades SPR biosensors are being widely appreciated because of its capability of detecting small refractive index change over a huge range, specially, in medical diagnostics, antigen antibody interactions, sequencing of DNA, pharmaceuticals, identification of proteins etc [6].
In 1983, Liedberg first theoretically introduced the SPR sensor established on the basis of the prism coupling [7]. In this method, light penetrates through the prism from one edge and incident on the metal surface coated on the other edge. According to this concept, this prism coupling based SPR sensor was first visualized by Otto and Kritchman [8]. But the configuration being bulky and unable to perform remote sensing for its heavy and inflexible mechanical components, this sensor could not survive for long. Instead, PCF based SPR sensors have taken place in optical sensing, for their flexibility, remote sensing, high detection range, low propagation loss, stability, low cost and lower fabrication complexity. PCF stands for Photonic Crystal Fiber which is constructed by geometrically arranged air holes around the fiber core.
The sensing performance of a PCF based SPR sensor can be enhanced by varying its geometrical parameters [9] e.g. pitch, metal layer thickness, air hole diameter, PML layer, length of fiber, number of rings, arrangements of air holes etc. Plasmonic metal layers are one of the most important parameters for sensing. The free electron releasing from the metal layer depends on the type of the metal it has been used. Gold is one of the highly used plasmonic metals. Apart from this, silver, TiO2, aluminum, copper etc are also used in the previously proposed sensors. Silver exhibits an oxidization problem, yet it can be removed using an additional graphene layer [8].
The background material is another major part of the sensor. Fused silica is used for material in background to make the core for its low temperature sensitivity. The air holes are geometrically arranged around the core to make its clad. When transverse magnetic polarized light penetrates through the core and hits the thin metal film surface, free electrons are released from the energy of the incident light beams. At a particular wavelength, the free electrons frequencies and the polarized light beams coincides. Which results in a highest energy transmission from the core to the metal surface. This specific wavelength is known as Resonance Wavelength. Observing the resonance wavelength for an unknown analyte, the unrecognized analyte can be detected.
To date in literature, Sana Ghafari et al proposed a thermal sensor with temperature sensitivity of 160 (nm/K) [10]. Vafapour Zohreh et al proposed an optical metamaterial glucose sensor with sensitivity of 225 nm/RIU [11]. Wang, Qiao, et al represents all dielectric nano –metamaterials efficient coupling mechanism [12,13]. In 2020, E. S. Lari et al. proposed a nanostructure sandwich design based on the analysis of absorption for nonlinear optical liquid sensing with maximum sensitivity of 273 nm/RIU [14]. Z. Vafapour et al. proposed a semiconductor based far infrared biosensor using THz metamaterial in 2018 [15]. Mukherjee et al. expressed a detailed discussion on the effect of quantic linearity on self-phase modulation in multiple coupled quantum wells [16]. Zhou et al. published an article discussing on controllability of all-optical modulation speed in hybrid silicon-germanium devices [17]. Liao et al. proposed an alternative method on microwave electrometry in cold Rydberg atoms [18].
Various works on PCF-SPR biosensors had proposed in last year. In 2019, Rakibul et al. proposed a dual polarized sensor with gold coated around it [19]. Which shows maximum wavelength sensitivity of 13,000 nm/RIU and maximum amplitude sensitivity of 1189.46 RIU−1. The sensitivities are comparatively lower and the gold coating is expensive as we need much gold. Alok et al. proposed a hexagonal dual core with wavelength sensitivity of 16,000 nm/RIU and amplitude sensitivity of 2255 RIU−1, with Aluminum coated [20]. This configuration has the same drawbacks as previously mentioned. Sanfeng et al. in 2018, proposed a silver nano-continuous grating sensor which is very complex to fabricate the gratings [21]. In 2018, Liancheng et al. proposed a design of hollow fiber based SPR sensor [22]. But this type of sensor has fabrication complexities to make so many air holes and insert the plasmonic material around the inner air holes of the core.
Our proposed design is a spiral lattice structure with circularly tetra slotted. To control the penetration of light from the core to the metal surface, the arrangement of the air hole positions placed accurately so that maximum light is incident on the metal dielectric surface. Gold is used as a plasmonic material to remove oxidation problems. The sensor exhibits a wide detection range of 1.32–1.50 RIU with the highest sensitivity of 41,000 nm/RIU and 4241 RIU−1. For the circular slots, we need less gold coating and less analyte sample. Hence, fabrication cost and complexity is reduced. Fused silica is used as the background filled material because of its stability in variation of temperature. The sensor also exhibits high linearity and high figure of merit. For very large detection range, high sensing capability, high linearity and FOM, high fabrication tolerance, and low fabrication cost, the proposed design can be really a worthy candidate in the field of SPR biosensing.
2. Mathematical formulation
SPR phenomenon will occur when the core clad phase matching will happen. In our sensor fused silica is treated as material in the background and the refractive index of this material is presented by Sellmier equation of dispersion [23,24],
In our sensor plasmonic effect is generated by gold. Gold is selected because of its low relative loss in visible and near infrared ranges and high conductivity gold has excellent negative real permittivity properties. From the Drude-Lorentz model we can glean the dielectric constant of gold [25],
Here ɛAu indicates the permittivity property of gold, ɛ∞ is insinuated as high frequency dielectric constant marked as 5.9673, ω is expressed as circular frequency with the value of ω = 2πc∕λ, where c is the vacuum velocity of light, γD is the damping frequency, ωD is the plasma frequency. There, γD∕2π = 15.92 THz, ωD∕2π = 2113.6 THz, and the weighting factor Δɛ = 1.09. The oscillator strength of the Lorentz oscillators and spectral width are given by ΩL∕2π = 650.07 THz and ΓL∕2π = 104.86 THz, respectively. The confinement loss for a specific RI can be obtained from the Eq. given by [9],
Here, α denotes to the loss value in dB/cm, ko = 2π/λ is the number of wavelength and Im(neff) is for refractive index imaginary value and λ is the specific wavelength for which the simulation has to be operated. The wavelength sensitivity can be measured using the following equation, by the wavelength interrogation method [26],
Here, Δλpeak is the difference of resonance wavelength in nm, for two nearest RI and Δna is the variation of the analyte’s refractive index. The amplitude sensitivity can be obtained by the method of amplitude interrogation and can be measured by the equation in the following [22],
where, ∂(λ,na) is the two side by side RIs’ spectral loss difference and α(λ,na) is the spectral loss of the lower RI. The resolution of the sensor can be found by the mentioned equation, which is the indication of how much small variation in anlayte RI can be detected with the sensor [27],
In this equation, Here, Δλpeak is the difference of resonance wavelength in nm, for two nearest RI and Δna is the refractive index variation of the analyte. Another important parameter is the Figure of Merit (FOM) which represents the improved detection range. It is obtained by the following equation [28]
Here, S(nm/RIU) indicates linear slope of two resonance wavelengths and Full Width at Half Maxima is denoted by FWHM of a specific RI. There are several techniques for implanting the gold coating inside the slots. For the proposed sensor, Chemical Vapor Deposition (CVD) method is highly efficient which permits uniformly coating with minimized surface roughness. As an alternative, the recently proposed Atomic Gold Layer Deposition Method is more efficient for deposition of more uniform gold thin-film coating on the circular fiber surface.
3. Fundamental structural design
The structural design of our propounded sensor is performed by COMSOL Multiphysics software environment. The specific arrangement of the propounded PCF sensor is represented in Fig. 1(a). According to this figure the pictorial form, we say that the alignment of the air holes is furnished in a format of spiral lattice outline.
We give the arrangements of the air hole as six arms in the three air hole rings is depicted in Fig. 1(b). Dashed curved line indicates the spiral arm configuration. The distance between the center of the core to the center of the first ring air hole is called pitch Λ=1.8 µm. We use the core material as fused silica which indicates green color in Fig. 1(a) which is highly stable characteristics with the change of temperature and the radius of the fused silica core layer is 2.2 Λ.
For the first air hole ring the air hole numbers 1, 2, 3, 4, 5 and 6 are placed in hexagonal shape with 60° air gaps for each other. For the second ring the air hole number are 7, 8, 15, 16, 17 and 18 also in hexagonal shape with 60° each other but the four air hole number 15, 16, 17 and 18 are removed as to add slot circle with the diameter of 1.10 Λ at the position of 60°, 120°, 240° and 300° and the distance from the core center to circle center of the slot is 1.76 Λ for creating the sensing environment with the chemically stable plasmonic material gold layer with the thickness of 30 nm indicate as to pink color in Fig. 1(a). Finally, the third ring air hole number 9, 10, 11, 12, 13 and 14 are also in hexagonal shape with the distance of each other 60°. So we can say that the design is a spiral structure with six arms and the angle of adjoining air holes of the same arm θ=30°. The diameter of every air hole of the three rings is the same to 0.65 Λ. The distance between core to the center of the second and third ring are 1.4 Λ and 1.8 Λ respectively. Then the next analyte layer which is in circularly slotted form and the radius of the PML layer from the center of the core is 3 Λ. Finally, the last layer where we apply the scattering boundary condition and the radius of the layer from the core center is 3.55 Λ.
4. Simulation and results analysis
The proposed sensor works based on the interface of the pole polarized light beams of core-cladding evanescent field and the surface free electron of the metal thin film. When the frequency of polarized incident light beams and metal free electrons are coincided, there is a huge loss of energy transmission. At this point we observe a sharp peak on the loss curve. Observing the peaks in loss curves, the sensor performance is analyzed.
The core mode and the SPP mode electric field distributions are shown in Fig. 2. SPP is the abbreviation of surface plasmon polariton. When the pole polarized light beams are incident on the metal thin film on the circular slots of the spiral SPR sensor, the surface plasmon waves are generated. This mode is called SPP mode. The study and analysis are numerically performed using finite element method or FEM, applying perfectly matched layer or PML with scattering boundary conditions. Modal analysis has been carried out keeping the smallest meshes as possible. While applying the mesh configuration, the number of elements were 14700, vertex elements 108, boundary elements 1544 and the minimum quality element were 0.5789.
Fig. 2. Electric field distribution of the proposed sensor: (a) x- polarized core mode, (b) y- polarized core mode, SPP mode y polarized, for pitch 1.8 µm, thickness 30 nm, RI 1.43 and wavelength 0.74 µm.
4.1 Dispersion relation characteristics
The main theme of analyzing all the performance parameters is the resonance. Resonance is occurred when core guided pole polarized light beams and metal surface free electrons coincide in frequency with each other at a particular wavelength. This wavelength is known as resonance wavelength. At this time there is a maximum energy transmission from core to metal surface and we observe a high sharp peak in the confinement loss curve. The resonance curve is shown in Fig. 3. The intersecting point of the core mode real part and the surface plasmon polariton or SPP mode real part in the curve indicates the resonance point.
Fig. 3. The dispersion relation characteristics of the proposed spiral sensor, for Λ = 1.8 µm, tg = 30 nm, RI = 1.43.
4.2 Analyte variation analysis
As the performance parameter, we observed the analyte variation response of the proposed PCF sensor which is illustrated in Fig. 4(a). From the figure, it is obvious that the proposed sensor can clearly detect unknown analytes ranging from 1.32 to 1.50 RIU. From the figure we observe that the proposed sensor experiences red shifts and its loss peaks gradually increase as we increase the wavelength. It is because of the lower contrast in effective refractive index between core and clad. The highest loss peak was observed 1890 dB/cm for RI 1.50, where, pitch, Λ = 1.8 µm and tg = 30 nm. The wavelength sensitivity of the proposed sensor is obtained by Eq. (4). The maximum wavelength sensitivity of the proposed sensor is observed 41,000 nm/RIU for RI 1.44. The amplitude sensitivity of the sensor obtained from Eq. (5), and the sensitivity curve is shown in Fig. 4(b). From the figure we observe that as we increase the wavelength, the sensitivity also increases till RI 1.44. The maximum value of amplitude sensitivity was observed 4242 RIU−1.
Fig. 4. (a) The confinement loss curves for analyte variation 1.32–1.44 RIU and (b) extended version from 1.32 to 1.50 RIU, (c) amplitude sensitivity curve and (d) normalized intensity color plot for analyte variation 1.32–1.5 RI, Λ=1.8 µm, tg=30 nm.
4.3 Pitch variation analysis
The confinement loss curve due to the variation of pitch is illustrated in Fig. 5, for RI 1.44 and 1.45. From the below figure we observe that increasing in pitch leads the curves red shifts and their loss values decrease. It is because of the fact that increasing pitch reduces the confinement of light. As a result, their intensity also reduces. From Fig. 5(c) we observe that. the maximum sensitivity is obtained for pitch 1.8 um at 41000 nm/RIU. From simulation, we optimized the pitch value at 1.8 µm because of its exhibition of excellent sensing performance.
Fig. 5. (a) Pitch variation curve for RI = 1.44 and 1.45, (b) normalized intensity plot for RI 1.44, and (c) pitch vs wavelength sensitivity graph for RI 1.44. Here, tg = 30 nm.
4.4 Thickness variation test
Gold film layer thickness variation also plays a vital role as a performance parameter. The gold layer thickness variation is shown in Fig. 6, for thickness variation 30 nm to 45 nm we see that as increasing the thickness value, the loss value is also increasing and from Fig. 6(d), maximum wavelength sensitivity is obtained 41000 nm/RIU for 30 nm. Also from Fig. 6(c) the maximum value of amplitude sensitivity is found 4242 RIU−1 that is why, 30 nm has been taken for optimum thickness to analyze the rest of the parameters.
Fig. 6. (a) Thickness variation curve for RI 1.44 and 1.45, (b) normalized intensity plot for RI 1.44, (c) amplitude sensitivity for RI 1.44 and (d) thickness vs wavelength sensitivity for RI 1.44. here, Λ = 1.8 µm.
4.5 Fabrication tolerance test
Another important performance parameter is fabrication tolerance. Fabrication tolerance means the ability of a sensor to remain the stability for any intended or unintended variation in any fabrication parameters. We tested the tolerance for pitch variation and thickness variation for varying +2% and −2% of their optimum values, which are shown in Fig. 7. From the figure we see that for RI 1.44 and 1.45, varying in both pitch and thickness for +2% and for −2% of their optimum values, the loss characteristics curves are almost same. Which indicates that our sensor shows highly tolerable performance variation in any fabrication parameters.
Fig. 7. Tolerance test for +2% and −2% variation in (a) pitch and (b) thickness, here pitch, Λ = 1.8 µm, tg = 30 nm.
4.6 PML variation test
The sensor performance is greatly dependent on the PML layer. For calculating accurate simulation results scattering boundary condition must be needed to remove scattering from the outer surface of the sensor. PML performs this major task. We perform the thickness variation of PML and put on Fig. 8.
Fig. 8. (a) Effective RI and confinement loss vs PML thickness variation curve, (b) amplitude sensitivity vs PML thickness variation curve.
From the below Fig. 8(a) we see that the variation of PML thickness never shows a major impact on sensor performance. The effective RI and loss is almost same but at the thickness of 1 µm the loss is lowest at 164.10 dB/cm and also see that from Fig. 8(b) the amplitude sensitivity is highest 269.92 RIU−1 and 268.35 RIU−1 (y and x polarized respectively). So that at last, we can be optimized the PML layer thickness at 1 µm.
4.7 Linearity and Figure of Merits (FOM)
A good sensor shows high linearity. The linearity curve of the proposed spiral sensor is shown in Fig. 9(a). The R-square values and the fitting equations are shown inside the figure. Here, x represents the Refractive Index and y represents the resonance wavelength. From R-squared values given, we can observe that the proposed sensor clearly exhibits high polynomial fit of 4th order with R-square value of 0.97, which is really high with near unity and indicates good sensing performance.
FOM is very important performance parameter which sensor shows better sensing capability by having higher FOM value. Sometimes FOM also called the signal to noise ratio (SNR) which means more signals less ratio is the main goal but here sensitivity is the main factor. From the equation of FOM, we found that FOM indicates the ratio of sensitivity and full width half maxima (FWHM). So we can say that more sensitivity shows more FOM value. Our sensor shows highest wavelength sensitivity 41000 nm/RIU. So that we found that the highest FOM value 683.33 RIU−1 at RI 1.44. The variation curve of FOM value with respect to RI is shown in Fig. 9(b). We put the value of each RI FOM value in Table 1.
Table 1. Detailed performance analysis.
Table 1 represents the detailed performance analysis of the proposed PCF-SPR sensor in which we considered the confinement loss peaks (CL), wavelength sensitivity (WS), amplitude sensitivity (AS) and FOM.
The performance validation is manifested in Table 2, with comparison of some recent research activities in the field of biosensing. For this comparison, we highlighted some important parameters mentioned as the pathway of sensing, basic structural form, sensitivity and resolution, detection range and FOM. From the table it is obvious that our proposed sensor shows more improved performance than the previously proposed works.
Table 2. Performance comparison table.
4.8 Fabrication and practically set up process
Figure 10 represents the practically experimental set up by majorly four parts. Firstly we set an optical tunable light source and next these feed to the proposed sensor which was connected by the single mode fiber. The sensing analyte input path of the outer surface of the sensor and passed away by a pump to out path. The system is also monitored by the pump to change the sensing analyte. Then the next part optical spectral analyzer (OSA) which is the fundamental part by which we measured the interactivity of the sensing area and the analyte. Finally the last part the computer by which the sensing response of OSA is displayed. When the resonance is occurred at longer wavelength the response shows blue shift and the shorter wavelength the response is red shift and can be detected the analyte in the computer by analyzing the loss spectrum.
5. Conclusion
An externally sensing approach highly sensitive circularly slotted spiral lattice structure is proposed and all the performance parameters optimized accurately. Because of optimizing all the parameters, the highest value of wavelength sensitivity is obtained 41000 nm/RIU with the highest wavelength resolution 2.44×10−6 RIU and highest AS is 4241.82 RIU−1. The sensor is also used as the broad detection range of 1.32–1.50 RIU. As the sensor shows the great FOM value of 683 RIU−1, the highly adjusted linearity value with ± 2% fabrication tolerance this reason no changing effect on performance, the sensor is an extremely great candidate for sensing the bio samples and medical diagnosis.
Acknowledgments
Alhamdulillah, by the grace of Almighty Allah (SWT), We have done a research paper. we would like to gratefully and sincerely thank Md. Saiful Islam and Rifat Ahmmed, Rajshahi University of Engineering and Technology for their constant inspiration, patience, necessary guidance, continuous help, suggestions, technical support and most importantly, their friendly dealing during this research work. They encouraged me not only perform the research work but also to grow as an independent thinker.
Disclosures
The authors declare no conflicts of interest.
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