Gold-coated photonic crystal fiber biosensor based on surface plasmon resonance: Design and analysis

Accepted Manuscript Gold-coated photonic crystal fiber biosensor based on surface plasmon resonance: Design and analysis

Sujan Chakma, Md Abdul Khalek, Bikash Kumar Paul, Kawsar Ahmed, Md Rabiul Hasan, Ali Newaz Bahar PII: DOI: Reference:

S2214-1804(17)30206-4 doi:10.1016/j.sbsr.2018.02.003 SBSR 223

To appear in:

Sensing and Bio-Sensing Research

Received date: Revised date: Accepted date:

26 November 2017 31 January 2018 7 February 2018

Please cite this article as: Sujan Chakma, Md Abdul Khalek, Bikash Kumar Paul, Kawsar Ahmed, Md Rabiul Hasan, Ali Newaz Bahar , Gold-coated photonic crystal fiber biosensor based on surface plasmon resonance: Design and analysis. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sbsr(2018), doi:10.1016/j.sbsr.2018.02.003

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ACCEPTED MANUSCRIPT

Gold-Coated Photonic Crystal Fiber Biosensor based on Surface Plasmon Resonance: Design and Analysis

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Sujan Chakma1,*, Md. Abdul Khalek1, Bikash Kumar Paul2,3, Kawsar Ahmed 1,2, Md. Rabiul Hasan4, Ali Newaz Bahar1 1

Group of Bio-photomatiχ, Bangladesh.

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Department of Information and Communication Technology (ICT), Mawlana Bhashani Science and Technology University (MBSTU), Santosh, Tangail-1902, Bangladesh 3

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Department of Software Engineering (SWE), Daffodil International University, Shukrabad, Dhaka-1207, Bangladesh 4

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Department of Electronics and Telecommunication Engineering and Technology, Rajshahi, Bangladesh

Engineering,

Rajshahi University of

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Abstract-The particularly sensitive circular lattice Photonic Crystal Fiber (PCF) based Surface Plasmon Resonance (SPR) sensor is proposed to gain high sensitivity for the detection of unknown analytes. In this model, two-layer PCF based on the SPR has been designed. A plasmonic chemically inactive material gold (Au) with thickness 35 nm is used to the outside of the PCF structure which exhibits negative real permittivity. A circular perfectly match layer (PML) outside the structure is applied to evaluate the performance of the sensor. The raised design has consisted of symmetric air-hole. Three small air-holes are used in second layer and center which help us to produce more evanescent field. Using the wavelength interrogation method the proposed model shows the maximum wavelength sensitivity of 9000 nm/RIU (Refractive Index Unit) and using the amplitude interrogation method it shows the maximum amplitude sensitivity of 318 RIU-1 with maximum sensor resolution 1.11×10-5 in the sensing range among analyte 1.34-1.37. Here the proposed model is investigated how phase matching points are varied with changing parameters as like diameter, PML, thickness of gold (Au), sensing layer and pitch. The obtained result reveals that the proposed model may be used in biochemical and biological analyte detection to find out the important application.

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Keywords—Surface Plasmon resonance; sensitivity; perfectly match layer; photonic crystal fiber; Bio-sensor.

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1. Introduction Nowadays, surface plasmons resonance (SPR) based sensors have became one of the hot topics to the researchers for their unique capabilities and widespread application in various fields of practical life. SPR sensors mainly used to analyze various kinds of sample such as water testing [1],

maintain food

quality,

bio-sensing,

medical diagnostics, gas detection, bio-imaging,

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environment monitoring, real time monitoring, organic chemical sensing, glucose monitoring,

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disease detection and so on [2-6] for its high sensitivity characteristics. The optical sensors [7-

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13], terahertz sensors [14], SPR sensors [2-6, 15] based applications are updating rapidly for the advancement of modern technology. The first observation about SPR has found through a

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theoretical way by Ritchie et al. in the 1950s [16]. On the basis of prism coupling Liedberg et al. in 1983 first introduced about SPR [17]. Usually, the prism is used to activate surface plasmons.

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There are some limitations to use prism based SPR sensing device such as; it provides a bulky size device with various kinds of optical and mechanical parts. Moreover, it is not suitable in

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remote sensing application [18].

R.C. Jorgenson first proposed about optical fiber based SPR sensor in 1993, where the gold film

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was used to coat the fiber core to reveal the plasmons response. The above limitation can be overcome using optical fiber in lieu of prism. The SPR based sensors are needed to reduce the

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technical cost and the size of sensor devices. PCF is also useful because of its various appealing characteristics e.g. controllable birefringence, high confinement, and single mode propagation [5-

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6, 16-19]. An evanescent field can be manipulated easily by using these characteristics. The evanescent field is used to control effective sensitive performance. The PCF based sensors also

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provide a remarkable design. Moreover, SPR sensors provide high sensitivity rather than fiber based sensors. It also provides low resonance peak than fiber based sensors [20-21]. From last few decades, researchers have tried to develop distinct structures. They have carried out unique outcomes in order to gain maximum sensitivity with high confinement loss and also to increase the performance of SPR sensors. A novel photonic crystal fiber biosensor structure using SPR is reported by Rifat et al. [22] where a gold layer with 40 nm thickness was used to coated the silica channel and gained maximum sensitivity of 1000 nm/RIU. But, Dash and Jha improved maximum wavelength sensitivity to 2000 nm/RIU and amplitude sensitivity to 80 RIU-1 by proposing SPR biosensor based on polymer PCF coated structure [20]. Recently, the article [23]

ACCEPTED MANUSCRIPT has improved the maximum wavelength sensitivity to 2200 nm/RIU and amplitude sensitivity to 266 RIU-1 by developing a new concept for evanescent sensing application compare to [20] that is shown in the table-1. In this paper, it is proposed that the maximum wavelength sensitivity and amplitude sensitivity are better than [23]. Here, we proposed a simple circular lattice PCF that consists of two air hole rings and a thin layer of gold placed outside the PCF structure. The proposed structure is capable to obtain maximum sensitivity including amplitude sensitivity and

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sensor resolution. To achieve the best sensing performance we also examine the effect of different

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gold thickness, pitch, diameter of circle, area of PML and chemical area. Moreover, through these

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analyses the proposed structure is capable to provide better performance than the others as shown in the table-1.

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2. Design and Numerical Method

Fig-1 represents the cross sectional view of the raised circular lattice PCF sensor. With two

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missing air holes in each ring it contains two layers air hole rings. In the first ring two air holes are used by improving 180⁰ anticlockwise rotation. In the second ring the air holes are arranged at

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a 30⁰ anticlockwise improving rotation. Two air holes of the second ring and one air hole placed

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in center are comparatively small. In this raised structure, the center-to-center distance between two adjoining air holes is defined by p, rc is the radius of the center air hole, r2 is the radius of the

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small air hole where rc is equal to r2 , r1 is the radius of the rest air holes and dg is the thickness of

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the gold layer.

Fig. 1 Designing views of the proposed circular lattice PCF sensor with p = 2 µm, r c = r2 = 0.2

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µm, r1 = 0.4 µm, and dg = 35 nm.

In this proposed sensor the background material is fused silica. Using Sellmeier equation [24] the refractive index of fused silica can be obtained, n2 (λ) = 1 +

𝐵1 λ2

λ2 −𝐶1

+

𝐵2 λ2

λ2 −𝐶2

+

𝐵3 λ2

λ2 −𝐶3

(1)

Where n is refractive index of fused silica that depends on wavelength and λ is the wavelength in µm. B1 , B2 , B3 , C1, C2 and C3 are the Sellmeir constants. The values of constants are respectively

ACCEPTED MANUSCRIPT 0.69616300, 0.407942600, 0.897479400, 0.00467914826, 0.0135120631, and 97.9340025 for fused silica.

In this design, we kept dg = 35 nm as optimum. Here, dg represents the fixed

thickness of the gold. To achieve the dielectric constant of the gold we have used the DrudeLorenz model and this model is distinguished by the following equation [25]: 𝜖𝐴𝑢 = 𝜖∝ −

𝜔2𝐷

− 𝜔(𝜔+𝑗𝛾𝐷 )

∆𝜖 . 𝛺2𝐿 (𝜔2 − 𝛺2𝐿 )+ 𝑗ΓL 𝜔

(2)

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Where, symbols have their usual meaning. The permittivity of gold is denoted by ϵ Au, ϵα is the permittivity at high frequency that has a value of 5.9673, the angular frequency is denoted by ω,

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the plasma frequency is denoted by ω D, the damping frequency is denoted by γD, where ω = 2πc⁄λ, ω D = 4227.2π THz, γD = 31.84π THz and weighting factor ∆ϵ = 1.09. The spectral width

Result Analysis and Discussion

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3.

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ΓL=209.72π THz and oscillator strength Ω L=1300.14π THz respectively.

The raised PCF SPR sensor supports fundamental mode as well as some higher order mode. Here, we consider the fundamental mode for further investigation. Generally, based on the

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evanescent field and on the geometrical parameters of the PCF, SPR sensor performs its task.

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These parameters must be selected by considering a way that will provide an easy interaction between the evanescent field and the metal surface. The proficient excitation of metal surface is

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used as a key factor of plasmonic phenomenon. The resonance can generate and the surface may excite by the incident field at a fixed wavelength [22]. Here, the purport of our raised structure is

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to increase the sensitivity through making a potential coupling within the core guided-mode and SPP mode. Missing air holes are considered in the raised structure because it occur birefringence

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and due to birefringence effective indices vary. The three small air-holes convey a meaningful impact on the phase matching act. The central air hole rc and the rest of two small air holes diameters have been fixed at 0.2 µm. If we set up the size of central air hole rc and the rest of two small air-holes smaller than 0.2 µm; it will conduct more confinement of light through core that will reduce the possibility of surface plasmon wave (SPW) generation effectively. On the contrary, a larger size of central air-hole rc and the rest of two small air holes reduce the effective index of the core guided mode. As a result it deteriorates the guidance along the core [26]. We set rc= r2 so that, an efficient interaction can be established easily towards the metallic layer with the sensing layer. Because, the larger value of r2 than 0.2 µm may prevent the interaction with

ACCEPTED MANUSCRIPT the sensing layer. Moreover, to reduce the light confinement in the metal – dielectric the value of rest air-holes are set at 0.4 µm. To gain the best performance of sensing the design parameters (p = 2 µm, rc = r2 = 0.2 µm, r1 = 0.4 µm and dg = 35 nm) are fixed entire the whole simulation process. Fig. 2(a)-2(c) represent the simulation of basic x-polarization, SPP mode of x – polarization

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and relation of radiation between the fundamental core-guided mode and SPP mode respectively.

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stronger electric field than Y – polarization near the metal surface.

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In our raised model we have analyzed the X- polarization of the core driven mode as it has

Fig. 2 (a) Surface mode of proposed structure (b) SPP mode of proposed structure (c) The relation of radiation between the fundamental core-guided mode and SPP mode with na = 1.37

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and dg = 35 nm.

Hence, the core X – polarization mode shows that the evanescent field can easily interact with

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the outward sensing layer compared to the core Y – polarization mode. Therein, the surface

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electrons of gold sharply coupled with X – polarization.

Fig. 2 (c) displays the loss spectrum of the basic core-guided mode of X – polarization, the

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surface plasmon polarization (SPP) mode and the refractive index neff of core guiding mode with analyte RI (na) of 1.37. For various operating wavelengths, the efficient refractive index of basic

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core-guided mode and SPP mode was taken from simulation software COMSOL 4.2. The dispersion curve of SPP mode obtained through plotting the efficient refractive indices as a

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function of wavelength. In Fig. 2(c) the bold blue line indicate the SPP curve. It’s proofing that, the actual part of the SPP mode and neff of the basic X – polarization core-guided mode intersect

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with each other at wavelength 0.71 µm. And this (0.71 µm) wavelength is known as resonance wavelength according to resonance or phase matching condition. This wavelength indicates that at this point from core guided mode to SPP mode maximum energy is transferred. In this raised structure the sensor’s performance evaluation of the confinement loss provides an important role whose parameters can be achieved by the following equation [27] 𝛼 = 8.686 × 𝑘0 . 𝐼𝑚[𝑛𝑒𝑓𝑓 ] × 104 dB/cm

(3)

Where, the number of free space is denoted by k 0 =2π/λ, operating wavelength is denoted by λ and the imaginary part of the effective refractive index denoted by Im(neff). A sharp peak is

ACCEPTED MANUSCRIPT obtained at the wavelength 0.71 µm. The peak loss varies for core-guided cladding. Due to the best performance of x – polarization than the y – polarization we considered only x – polarization for furthermore performance analyze. For the variation of gold layer thickness we observed that, the loss depth and amplitude sensitivity have a meaningful change. The consequent loss spectra have shown for different gold From the Figure we

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thickness layer for the values 1.36 and 1.37 of analyte RI in Fig. 3(a).

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observed that, loss depth varies with the increase of gold layer thickness. In addition, the peak loss shifts in the direction of a longer wavelength with the increasing of analyte. Fig. 3(b)

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describes the different amplitude sensitivity with the variation of gold thickness as 30 nm, 35 nm and 40 nm for analyte 1.36 and 1.37 respectively. We obtained amplitude sensitivity as 240, 318,

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288 RIU-1 for thickness 30 nm, 35 nm and 40 nm respectively from the raised sensor. For the thickness 35 nm we obtained the maximum value of amplitude sensitivity of 318 RIU-1 . Thus,

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we select the thickness 35 nm of gold layer as optimum thickness for furthermore test analysis.

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Fig. 3 (a) Relative confinement loss variation with different thickness of gold layer and (b)

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0.4 µm for alnalyte RI of 1.36.

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amplitude sensitivity for different thickness of gold layer with p = 2 µm, rc = r2 = 0.2 µm, r1 =

Fig. 4(a) Relative confinement loss variation with the increase analyte RI from 1.34 to 1.37 and

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dg = 35 nm.

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(b) amplitude sensitivity for different analyte RI with p = 2 µm, rc = r2 = 0.2 µm, r1 = 0.4 µm and

Fig. 4 (a) and (b) describe the shift of phase matching points towards the high resonance wavelength with the variation of na from 1.34 to 1.37. The values of amplitude sensitivity and loss depth differ with the short variation of analyte RI. But, it is almost a challenging issue to find out highest loss depth for the value of analyte larger than 1.37 through this raised structure. Moreover, the variation of analyte RI has also a significant impact to the neff of the SPP mode. Therein, the other high resonance wavelengths shift the phase matching point which is indicated as red. The values of resonance wavelengths are shifted from 0.62 µm to 0.56 µm, 0.56 µm to

ACCEPTED MANUSCRIPT 0.62 µm and 0.62 µm to o.71 µm due to the variation of analyte from 1.34 to 1.35, 1.35 to 1.36 and 1.36 to 1.37 respectively with the optimum value 35 nm of the gold layer thickness. From Fig. 4(a) we see that the loss depth increase with the increase of analyte na and the phase matching points shift both backwards and forwards of the high resonance wavelength. From Fig. 4(b) we observed that amplitude sensitivity differs with the change of analyte na as 1.35, 1.36 and 1.37. For the value of 1.36 and 1.37 of analyte na shows the maximum value of amplitude

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sensitivity. Comparatively for 1.34, 1.35, 1.36 and 1.37 values of analyte we found the maximum

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value of confinement loss 700.0445 dB/cm which obtained by using the equation (3) for analyte

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1.37 at wavelength 0.71 µm which is comparable with [17, 19, 20].

following formula [28] to compute the sensitivity, ∆λpeak ⁄∆n 𝑆λ (nm⁄RIU ) = a

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Sensitivity is used to measure the performance of PCF-based SPR sensor. We have used the

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(4)

Where, ∆λpeak is used to indicate the distinction of wavelength peak shifts and ∆na is used to

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indicate the difference of analyte refractive index RI. Here, in this raised fiber we evaluate ∆λ peak

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of 60 nm, 60 nm and 90 nm for the variation of RI 1.34-1.35, 1.35-1.36 and 1.36-1.37 with the value 0.01 of analyte ∆na .Therein, we obtained the wavelength sensitivities respectively 6000,

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6000, and 9000 nm/RIU. The obtained maximum sensitivity is comparable with preceding result [17, 19, 20]. A short change of analyte RI is possible to accurately detect by using the

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performance of sensor resolution. Using the following equation [29] we obtain the resolution of the raised structure:

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𝑅 (𝑅𝐼𝑈) = ∆𝑛𝑎 ∗ ∆λmin⁄∆λ

peak

(5)

Where, ∆na=0.01, ∆λmin =0.1 nm, and ∆λpeak = 90 nm; as a result we found a high value of sensor resolution as high as 1.11×10-5 . The phase detection method or the wavelength interrogation method is used to measure the sensitivity. Though these methods are cost effective but provide a complex process to measure the sensitivity. But this problem can be overcome by using amplitude interrogation method that

ACCEPTED MANUSCRIPT will measure the amplitude sensitivity at a fixed wavelength. According to the following equation [30] the amplitude sensitivity can be obtained: SA (λ)[RIU −1 ] = −

1

∂α(λ,na )

α(λ,na )

∂na

(6)

Here, α(λ, na) indicates the overall propagation loss at a specific refractive index RI of analyte and ∂α(λ, na) indicates the difference between the two loss spectra. The amplitude sensitivity for

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different analyte refractive index RI is described at Fig. 4(b). For different analyte RI values as

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1.35, 1.36 and 1.37 we have obtained maximum amplitude sensitivity 165, 291 and 318 RIU-1

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respectively. And we select analyte RI 1.37 and wavelength 0.73 µm due to its high amplitude

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sensitivity of 318 RIU-1 which is comparable with [20, 23, 31, 33] and is shown in table-1.

Fig. 5 (a) Relative confinement loss spectra for increasing pitch values from 1.5 µm to 2.5 μm;

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(b) amplitude sensitivity for different pitch values with an analyte RI of 1.35 and 1.36; The change of pitch of this raised structure also has a significant impact in confinement loss

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spectra and amplitude sensitivity. Fig 5 (a) and (b) describe the changing pitch values with the obtained confinement loss spectra and amplitude sensitivity. In this analysis we increase the

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pitch values from 1.5 µm to 2.0 µm and 2.0 µm to 2.5 µm with 0.5 µm distinctions with each other. Because, difference less than 0.5 µm of the pitch values are almost shows the same

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amplitude sensitivity. Therein, it is difficult to illustrate the variation of amplitude sensitivity. Thus, we observed the pitch values 1.5 µm, 2.0 µm and 2.5 µm to find out the variation of the

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amplitude sensitivity. Fig. 5(a) illustrates different confinement loss variation with the change of pitch values. The solid blue line, green line and red line indicate distinctly the highest

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confinement loss of the pitch value 1.5 µm, 2.0 µm and 2.5 µm respectively where we set analyte 1.36. We obtained highest confinement loss 817 dB/cm for pitch value 1.5 µm. We also obtained confinement loss 375 dB/cm and 175 dB/cm for the pitch value 2.0 µm and 2.5 µm respectively. From the Fig. 5(b) we observed that, the maximum value of amplitude sensitivity is 291.37 RIU-1 at pitch value 2.0 µm. Here the black line, the green line and the blue line represent amplitude sensitivity for pitch value 1.5 µm, 2.0 µm and 2.5 µm respectively. From above analysis we choose the pitch value 2.0 µm for furthermore analyze.

ACCEPTED MANUSCRIPT Fig. 6 (a) Relative confinement loss spectra for various diameter values of center air hole from 0.1 μm to 0.2 μm and without diameter; (b) amplitude sensitivity for various diameter values with analyte RI of 1.35 and 1.36; On the contrary, with the change of center diameter rc the raised structure shows almost similar amplitude sensitivity and confinement loss with slight variation which is negligible. The loss

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spectra and amplitude sensitivity is seemed to be similar which illustrated in Fig. 6 (a) and (b).

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Here, we observed three values of center hole diameter rc as 0.1 µm, 0.2 µm and without center. At each of these values of center diameter, the proposed structure shows maximum sensitivity

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291.39 RIU-1 . In the Fig. 6 (a) blue line, green line and red line indicate the confinement loss and in the Fig. 6 (b) black line, green line and blue line indicate amplitude sensitivity for the size of

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center diameter 0.1 µm, 0.2 µm and without center respectively where the value of analyte was 1.35 and 1.36. It’s almost hard to find out the each line distinctly from the Fig. 6 (a) and (b)

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because in each point they are showing similar values.

Fig. 7 (a) Relative confinement loss spectra for various chemical areas for 0.565 μm, 0.965 μm

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and 1.365 μm; (b) amplitude sensitivity for various chemical areas with analyte RI of 1.35 and

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1.36;

The changes of confinement loss and amplitude sensitivity are also negligible with the variations

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of chemical areas in this raised model. The confinement loss and amplitude sensitivity of changing chemical area are shown in Fig. 7 (a) and (b).When we change the chemical area 0.565

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μm to 1.365 μm the sensitivity varies slightly that’s not considerable. We also find out highest amplitude sensitivity 291 RIU-1 , 291 RIU-1 and 291 RIU-1 for chemical area 0.565 μm, 0.965 μm

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and 1.365 μm respectively where the value of analyte was 1.35 and 1.36. Therein, the Fig. 7 (a) and (b) shows the same line in the graph. A high linearity response of regression line indicates a good sensor. Fig. 8 shows the linear fitting of the resonant wavelength as a function of na. The linear fitting curve shows R2 value of 0.9584 which provides a better linearity. The regression equation for the linear line is defined by y = 4.4x – 5.337, where y indicates the resonance wavelength and x indicates the refractive index (RIU). Table 1 shows the comparison with amplitude sensitivity, wavelength sensitivity, and sensor resolution between the proposed sensor and existing sensors.

ACCEPTED MANUSCRIPT Fig 8. Regression line of the resonance wavelength as a function of analyte RI for P = 2 µm, r c= r2 = 0.2 µm, r1 = 0.4 µm.

4. Conclusion This paper numerically investigates the propagating characteristics of a highly sensitive PCF-

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based SPR sensor that can be applicable to detect unknown analytes. Scattering boundary

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condition is used to cover the thin gold layer and analyte layer in two-layer PCF based on SPR

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structure. All the parameters are tuned to obtain the lower confinement loss and higher amplitude sensitivity. Better performance of confinement loss and amplitude sensitivity are also shown

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through increasing or decreasing the value of pitch, air hole, size of diameter, size of chemical area and PML layer. As a result, the proposed structure shows maximum wavelength sensitivity

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of 9000 nm/RIU using wavelength interrogation method, maximum amplitude sensitivity of 318 RIU-1 using amplitude interrogation method and 1.11×10-5 is found as maximum resolution of the

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sensor. The proposed sensor is highly applicable in biomedical applications.

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Table 1 Performance analysis comparison of the proposed sensor with existing sensors in the recently published literature. Sensor resolution (wavelength inte.) (RIU)

Peak loss (dB/cm)

2000

5 × 10−5

2500

Ref. 22

118

1000

2.4 × 10−5

19.9

Ref. 23

266

2200

3.75 × 10−5

160

Ref. 31

47.77

3700

Ref. 32

—

2400

Ref. 33

72.47

2520

Ref. 34

—

6430

Ref. 35

—

7700

Ref. 36

—

3200

Proposed

318

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Wavelength sensitivity (nm/RIU)

Ref. 20

Maximum amplitude sensitivity (RIU-1 ) 80

—

—

—

3.97 × 10−5

60

—

—

1.3 × 10−5

107.11

3.12 × 10−5

400

1.11 × 10−5

700.05

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9000

—

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PCF Sensor

Disclosures

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The authors have no relevant financial interests in this article and no potential conflicts of interest to disclose.

Acknowledgement The authors are very grateful to those who participated in this research work. There is no financial support for this research work. Conflict of Interest This manuscript has not been published yet and not even under consideration for publication elsewhere. All the authors have read the manuscript and approved this for submission as well as no competing interests.

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