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Abstract
In this article, we design a microstructure based photonic crystal fiber (PCF) with external sensing surface plasmon resonance (SPR) sensor. The performance of the design is numerically evaluated incorporating the finite element method (FEM) with Perfectly Matched Layer (PML) boundary condition of scattering case. Modal analysis is performed using finer mesh analysis. At the optimized thickness (40nm) of chemically stable gold (Au) layer, the ever been maximum reported wavelength sensitivity (WS) and standard amplitude sensitivity (AS) are to 75,000 nm/RIU with wavelength resolution (WR) 1.33×10−6 RIU and 480 per RIU correspondingly. With a large detection range of 1.33 to 1.41 RIU, the sensor also exposed high polynomial fit (R2=0.99) as well as a high figure of merit (FOM) of 300 per RIU. Since very much high sensitivity, high detecting range and FOM, lower the cost of fabrication, the proposed design can be a pleasant competitor in detection of the analyte refractive index (RI). At the last, we can say that the proposed sensor would become a great candidate to detect biological and biochemical samples with fast and smart responses.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The SPR associating PCF sensor works on the basis of interaction between the incident light inside the PCF and the metal thin layer surface [12]. In the advancing world of science and technology, the PCF based SPR sensor is widely appreciated for its appliance in biomolecular sensing and detecting [3–10], temperature sensing [3], pollution sensing [4], environmental sensing and detecting [4], water testing [4], antigen-antibody interaction [5], medical diagnosis [6] etc. The theoretical concept of SPR was first introduced by Rithie et al. in 1950 and also the surface plasmon (SP) in 1957 [7]. Next in 1968, Kretschmann and Otto introduced the two major excitation processes of surface plasmon waves (SPW) that were the Attenuated Total Reflection (ATR) in prism coupler based and the diffraction grating [8–15]. But in a prism coupler based method the size of the structures is larger and having much more drawbacks.
The PCF is micro structured in size with circular glass fiber and the air holes inside it [15]. These air holes act as a lower density medium that causes total internal reflection inside the fiber [14]. When light beams penetrate through the core of the fiber they create an evanescent field of x polarized and y polarized light that penetrates to the layer of the metallic thin film [4,1618]. During the time of interactions between the polarized light of the pole to the metal film, they release free electrons from the surface of the metal and create surface plasmon waves [18]. In case of a particular wavelength, the plasmon electrons of the surface match with the incident light beam frequency of that corresponding wavelength and this causes a huge energy transfer from the light beam to free electrons of metal [19]. At that point we get a sharp loss peak in the resonance curve. This wavelength is called resonance wavelength [19]. The unknown analyte can be detected and sensed observing this loss peak. The core background is filled up by fused silica for its low temperature sensitivity. The plasmonic material is another issue for sensing performance [20]. Several metals like gold, silver, titanium di-oxide (TiO2), aluminium etc. are used as plasmonic material [16]. Although silver shows a very high resonance peak, it exhibits chemical instability due to its oxidation which can be prevented using graphene layer [21]. But this additional layer causes extra fabrication cost. So for the chemical stability and low fabrication cost, gold is mostly preferred plasmonic material [16,18,20]. Basically, the sensing approaches are two types those are the inside hollow core [19] and the outside external sensing approaches. In this sensor, we detect the sample by using outside detecting method.
Several SPR sensors were introduced previously with different outcomes in the literature. In recent, M. B. Hossain, et al. Ag coated hollow-core PCF sensor was reported that shows less WS of 21000 nm per RIU [17] with AS 2456 per RIU. Mahfuz et al. proposed an asymmetrical PCF based plasmonic sensor using the lower birefringence peak method that shows the WS 22000 nm per RIU [18]. S. Singh and Y. K. Prajapaty proposed a highly sensitive refractive index sensor based on D-shaped PCF with gold-graphene layers on the polished surface that shows the maximum WS value 33500 nm per RIU [20] and they also proposed a improved design TiO2/gold-graphene hybrid solid core SPR based PCF RI sensor for sensitivity enhancement that shows the WS value of 48900 nm per RIU with AS value 611.25 per RIU [21]. Those sensors reduced fabrication difficulties but then the model offered in this article.
Our proposed design exhibits a very high WS of 75,000 nm per RIU and standard AS of 480 per RIU. The sensor exhibits a high sensing range of 1.33 RIU to 1.41 RIU. The performance is evaluated numerically by combining FEM with PML following the boundary condition of the scattering case. Modal analysis is performed using finer mesh analysis. For very high sensitivity, high detection range, high linearity and FOM, less fabrication cost, the proposed sensor can be a great candidate in the recent research activities.
2. Arrangement of structural design
The schematic cross sectional outlook of the PCF microstructure based SPR design which is proposed in this article is sketched in Fig. 1.
In the very simple structure, the first ring is a hexagonal lattice and the second ring is a circular lattice with four missing air holes. These missing air holes are deployed to make a strong evanescent field for hitting the metal plasmonic layer strongly. In Fig. 1 defines the schematic 2D structure of the proposed sensor. In Fig. 2(a) shows the Stack preview of the sensor to be proposed and Fig. 2(b) represents the Stack view of the offered design. Figure 3(c) represents the 3D view of the proposed sensor. In phase identical points, the power is transmitted from fundamental core mode to fundamental SPP mode. As we proposed an asymmetric structure, a strong birefringence will occur through the detection process [19,22,23]. The centre to centre distance between two adjacent air holes is called pitch (Λ). The diameter of the first ring air hole is denoted by d1 which is 0.57 of pitch. Again the diameter of the air hole of the second ring is defined by d as 0.78 of pitch. The main background material is fused silica [11]. The RI of the silica material calculated by the Sellmier equation which is denoted by [24,25]:
Here, n is the effective RI of the fused silica function of wavelength (λ) which is measured in µm scale. Where B1, B2, B3, C1, C2 and C3 are the Sellmier equation constant those are taken from Ref. [25]. As gold (Au) is a chemically stable material used for external sensing layer which thickness (tg) is variable and developed thin film by chemical vapor deposition (CVD) technique [25]. The dielectric function of Au is received from Drude-Lorentz model [26,27]:
Fig. 2. (a) Stack preform design of the proposed sensor and (b) Stack view of the offered design and (c) 3D view of the proposed sensor.
Fig. 3. (a) x-polarized core mode, (b) y-polarized core mode,(c) SPP mode and (d) Matching relation of dispersion phase between the mode of the fundamental core and SPP at na=1.36 and tg=40 nm.
3. Simulation outcomes and performance analysis
The performance of the offered sensor is investigated by considering total number of elements 20,256 with number of vertex elements 128, number of boundary elements 1,628 and the quality of the element is 0.7988. To observe the sensitivity of a sensor, we need to obtain the confinement loss curves with respect to different parameters. The loss of the confinement curve is achieved using the following formula [30]:
Here, mode of the core of imaginary refractive index is expressed by “Im(neff)”, the number of waves is indicated by “ko”, where the operating wavelength is expressed by $\lambda $ as it is referred in [27].
Figure 3(a), (b), and (c) represent the x polarised core mode, y polarised core mode and the SPP mode respectively. The curve of confinement loss is appeared in Fig. 3(d) that represents the matching relation of dispersion phase between the mode of the fundamental core and SPP at na=1.36 and tg=40 nm.
To test sensor output, confinement loss is one of the keys to enhance the sensor performance which is calculated employing the Eq. (3). As it is seen from Fig. 4 the variation in the resonance peak loss is shifted to different wavelengths due to different analyte profiles. With increasing the analyte RI from 1.33-1.41, the loss depth also increased drastically.
So that as the analyte RI increases, the phase coordinating points shifted to greater wavelength. After obtaining loss curves, we calculated the sensitivity of the sensor to evaluate its performance. Therefore, it can be made a conclusion that the deviation of the analyte profile of RI is one of the significant ways in analyzing the output of the offered design which is measured by [28–]31:
Here, Δλpeak indicates the wavelength peak variation, the variance of the analyte RI is symbolized Δn Observing the shifting the peak of the resonance curve, we can achieve the sensitivity of performance in terms of wavelength by employing the interrogation method of wavelength. From the Fig. 4 it has been calculated that the maximum WS is 75,000 nm /RIU.
Another technique is the amplitude interrogation procedure to measure the sensor sensitivity. In this approach, the sensitivity of sensor in case of measuring the amplitude is given by [32]:
Here, α(λ,n) reveals the loss of propagation and α(λ,n) indicates the loss gap. As it is exhibited from Fig. 5 the maximum AS is reported to 480 per RIU in terms of wavelength and is plotted for analyte RI profile ranging between 1.33 to 1.40 RIU.
The sensor performances are greatly dependent on sensor thickness variation. So the optimization of gold layer thickness is a very important factor.
The gold layer thickness variation loss profile for 30nm, 40nm and 50nm is observed from Fig. 6(a) and Fig. 6 (b) is represented the amplitude sensitivities. From the Fig. 6(a) it is evident that as increasing thickness value the loss value is decreasing for both RI value 1.36 and 1.37. For the RI 1.36 the loss values are 13.31dB/cm, 6.71dB/cm and 3.39dB/cm at the 30nm, 40nm and 50nm respectively and also for RI 1.37 the loss values are 18.25 dB/cm, 8.60dB/cm and 4.28dB/cm at the 30nm, 40nm and 50nm respectively. So the corresponding WS values are found to be 4000nm/RIU, 5000nm/RIU and 4000nm/RIU for the 30nm, 40nm and 50nm respectively. Also from the Fig. 6(b) shows the optimum performance for the AS value. To sum up we select the gold layer thickness value as 40nm.
Fig. 6. (a) Gold layer thickness variation loss profile for 30 nm, 40 nm and 50 nm at RI 1.36 and 1.37, (b) AS curve for RI 1.36.
Pitch (Λ) means sensor centre to first hexagonal ring centre distance. It is a very important phenomenon. The sensor performance and structure is greatly dependent on this parameter. The loss characteristic curve of pitch variation is represented on Fig. 7(a) and amplitude sensitivity curve in Fig. 7(b). From the Fig. 7(a) we observe that as increasing pitch value the loss value is decreasing. Because during pitch value higher the air holes distance is far from the centre so that the more core evanescent mode light passes to the core mode. To sum up as less loss value and better performance for Λ = 2µm. So we select the pitch value as 2µm.
Fig. 7. (a) Representation of loss characteristic curve for pitch variation at 1.9µm and 2µm for RI 1.35 and 1.36 (b) Amplitude Sensitivity curve for pitch 1.9µm and 2µm.
The signal to noise ratio or SNR is another major factor to be investigated the superiority of the sensor which can be found with the figure of merit (FOM). The FOM is got commencing the following equation [33,34]:
Here, the FWHM indicates to “full width at half maximum”. The improved SNR results in a higher spectrum of detection of the design. From the Eq. (6), we observe that higher FOM can be obtained by increasing sensitivity and decreasing FWHM.
In Fig. 8(a), we take FOM with respect to RI (from 1.33 to 1.41) and visible that with the increasing analyte refractive index (na) the sensitivity is increasing and also the FWHM reduce as the narrow resonance peak is observed. Therefore, the maximum FOM is found 300 RIU−1 at RI na=1.41, when the WS is 75000 nm/RIU and FWHM is 250nm.
Fig. 8. (a) FOM characteristics representation. (b) Demonstration of the sensor's line fitting characteristics with varying RI from 1.33 to 1.41 RIU.
The index of a good sensor is a high resonance wavelength curve fitting characteristic. The curve fitting characteristics of this sensor is illustrated in Fig. 8(b) with analyte RI variation ranging from 1.33 to 1.41 RIU.
From Fig. 8(b) we can see that our offered design shows linearity fitting with R square value of 0.6513 and high polynomial fitting with R square value of 0.9988. Here, y indicates the wavelength of the resonance and x denotes the analyte's RI.
After fabrication of the proposed sensor is completed, the next process is the practically experimental set up where the Fig. 9 represents it. The sensor is connected with single mode fiber (SMF). Then the single mode light from the source is passing through the fiber.
Besides, the experimental sample is passing through the sensor analyte layer by a pump from input to output. Next on the interaction of the analyte layer is observed from the Optical Spectral Analyser (OSA) by the computer display. If the sensing response is observed as longer wavelength then it is called red shift phenomenon and another way to shorter wavelength is called the blue shift phenomenon.
From the Table 1 it is evident that the proposed sensor shows better results than the recently existing published works. In the viewpoints of higher WS which is the main criteria in the sensing points with proper sensing capability, reasonable AS, optimum FOM, highly adjusted to unity polynomial curve fitting characteristics and also the perfectly important range of liquid biochemical and biological sample, this sensor will become a great candidate to detect the bio sample with fast and smart response.
Table 1. Performance Comparison with the previously designed existing sensors.
4. Conclusion
In this article, a high wavelength sensitive microstructure PCF based SPR sensor is proposed, where the stable material gold external sensing method is used as a plasmonic material. The sensing output is analyzed using the FEM based approach and the maximum AS is 480 per RIU along with the maximum WS 75000 nm per RIU at the detection range in between 1.33 to 1.41 RIU. As the sensor is highly sensitive, the maximum FOM is found at 300 per RIU. All the performance outcomes are obtained by the simulation using COMSOL Multiphysics software where the numerical analysis is performed by applying PML for the boundary condition of the scattering case. In this analysis, the mesh size was kept as small as possible.
Acknowledgments
Alhamdulillah, by the grace of Almighty Allah (SWT), we have done a research paper. We would like to gratefully and sincerely thank Md. Shamim Anower 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.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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