Optical Materials 102 (2020) 109800

0925-3467/© 2020 Elsevier B.V. All rights reserved.

Surface plasmon resonance sensor based on photonic crystal fiber with indium tin oxide film

Qiang Liu a, Jiudi Sun a, Yudan Sun a,b, Zonghuan Ren a, Chao Liu a,*, Jingwei Lv a, Famei Wang a, Liying Wang a, Wei Liu a, Tao Sun c,**, Paul K. Chu d

a School of Electronics Science, Northeast Petroleum University, Daqing, 163318, PR China b College of Mechanical and Electrical Engineering, Daqing Normal University, Daqing, 163712, PR China c Media Lab, Massachusetts Institute of Technology, Cambridge, MA02139, USA d Department of Physics, Department of Materials Science and Engineering, and Department of Biomedical Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

A R T I C L E I N F O

Keywords: Photonic crystal fiber Surface plasmon resonance Refractive index sensor Indium tin oxide

A B S T R A C T

A novel photonic crystal fiber (PCF) sensor composed of an indium tin oxide (ITO) coating on the outer surface based on surface plasmon resonance (PCF-SPR) is designed and investigated. The sensor consists of eight air holes arranged in a rectangular grid as the cladding, the ITO layer as the plasmonic materials, and an annular external channel. The finite element method (FEM) is used to study the sensing performance and effects of different geometric parameters. The sensor can be operated in the near-infrared region (1380–2260 nm) for the analyte refractive index range between 1.26 and 1.38. In addition, the sensor shows a maximum wavelength sensitivity of 35000 nm/RIU and amplitude sensitivity of 1120.73 RIU� 1. It has good wavelength and amplitude resolution reaching 2.86 � 10� 6 RIU and 8.92 � 10� 6 RIU, respectively and is suitable for accurate sensing applications in biomedicine and chemistry.

1. Introduction

In recent decades, there has been much interest in the design of artificial micro-nano structured devices such as optical couplers [1], beam-splitters [2], absorbers [3–5] lenses [6], logic gates [7], topolog-ical waveguide devices [8] and biosensors [9,10]. Among these optical devices, surface plasmon resonance (SPR) sensors which boast high sensitivity and high resolution in on-line and real-time monitoring are widely used in environmental monitoring, medical diagnostics, biochemical analysis, and food safety testing [11,12]. The SPR phe-nomenon can be described in terms of collective oscillations of the free electron plasma at the interface between the metal and dielectric me-dium [13–16]. Although the Kretschmann-Eather configuration was proposed in 1968 to generate SPR [17], it is quite bulky and costly and also has poor reliability [18]. In order to overcome the limitations of conventional SPR sensors and miniaturize the sensors, the SPR tech-nique has been combined with photonic crystal fibers (PCFs) [19]. Compared to conventional prism SPR sensors, PCF (PCF-SPR) sensors have many advantages such as the compact structure, label-free sensing,

fast response, and high sensitivity [20]. Moreover, the structure can be optimized by changing air hole arrangements and coating with different materials to enhance couple between the core guided mode and plas-monic mode to improve the performance [21]. In fact, PCF-SPR sensors have attracted much interest in medical diagnostics, solution measure-ments, and biochemical research [22].

Different kinds of PCF-SPR sensors have been reported by coating plasmonic materials onto the inner air holes or outer surface of the PCF [23]. However, it is difficult to add plasmonic materials to the inner wall of the air holes due to the tiny scale of the PCF and so coating plasmonic materials onto the surface of the optical fiber is more feasible [24]. The D-type PCF-SPR sensors require precise polishing which increases the difficulty of fabrication and reduces the mechanical strength of the fiber [25,26]. Therefore, the simplest method is to directly coat a plasmonic film onto the outer surface of the PCF. For example, a multi-hole PCF-SPR sensor with a gold film was proposed by A. A. Rifat [27], but the sensitivity was relatively low and the detection range was small. Liu et al. described a PCF-SPR sensor with an annular analyte channel but the sensitivity was only 7500 nm/RIU in the sensing range between 1.38

* Corresponding author. ** Corresponding author.

E-mail addresses: msm-liu@126.com (C. Liu), taosun@hotmail.com.hk (T. Sun).

Contents lists available at ScienceDirect

Optical Materials

journal homepage: http://www.elsevier.com/locate/optmat

https://doi.org/10.1016/j.optmat.2020.109800 Received 30 October 2019; Received in revised form 21 February 2020; Accepted 27 February 2020

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and 1.42 [28]. Recently, we proposed an eccentric core photonic quasi-crystal fiber (PQF) sensor based on SPR with a sensitivity of 21000 nm/RIU [29]. However, the complex quasicrystal structure makes it difficult to achieve in the fabrication process [30–32].

Herein, a novel PCF-SPR sensor with square lattice air holes and external coating is described and analyzed. An indium tin oxide (ITO) film is deposited onto the outer surface of the PCF by chemical vapor deposition (CVD) [33] and the sensor can operate in the near-infrared

region so that the evanescent field can penetrate more deeply. Furthermore, since the plasmonic surface contacts the analyte directly, the maximum wavelength sensitivity is 35000 nm/RIU and the ampli-tude sensitivity is 1120.73 RIU� 1 by optimizing the structure. The annular channel also facilitates analyte detection and the sensor is easy to implement.

Fig. 1. (a) Cross-sectional view of the PCF-SPR sensor and (b) General set-up for practical sensing.

Fig. 2. Electric field distribution for X and Y polarization: (a–d) Core-guided mode and SPP mode for na ¼ 1.34, respectively; (e–h) Core-guided mode and SPP mode for na ¼ 1.38, respectively; (i–j) Dispersion relation of the fundamental core mode, SPP mode, and confinement loss spectra for na ¼ 1.34.

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2. Modeling structure and theoretical modeling

The cross-sectional view of the PCF-SPR sensor is shown in Fig. 1(a). It comprises eight air holes arranged in a rectangular grid with silica as the background materials. The two small air holes with a diameter of d1 are placed on the left and right sides of the fiber core and are conducive to resonant coupling between the core guided mode and plasmonic mode. Four air holes with a diameter d2 are arranged at the vertices of the rectangle to obtain better light confinement and the large air holes with a diameter of d3 are placed on either side of the fiber core to reduce the mode field area of the core. The PCF can be fabricated by the stack- and-draw method [34] and the ITO film is coated onto the outer side of the PCF.

The optimized structural parameters are as follows: air hole diameter (d): d1 ¼ 2.0 μm, d2 ¼ 3.0 μm, and d3 ¼ 3.6 μm; center-to-center distance between two air holes (Λ): Λ1 ¼ 4.8 μm and Λ2 ¼ 4.0 μm; radius of the silica core rs ¼ 8.0 μm. The thickness of the plasmonic ITO is 50 nm and the outside of the ITO layer is the analytical layer. The dispersion rela-tionship of the fused silica as the cladding layer is calculated by Sell-meier equation [35]:

n2ðλÞ¼ 1þA1λ2

λ2 � B1þ

A2λ2

λ2 � B2þ

A3λ2

λ2 � B3(1)

where A1, A2, A3, B1, B2, and B3 are 0.696166300, 0.407942600, 0.897479400, 4.67914826 � 10-3 μm2, 1.35120631 � 10-2 μm2, and

Fig. 3. (a) Confinement loss spectra as the analyte RI is changed from 1.26 to 1.38; (b) Non-linear curve fitting of the resonant wavelength; (c) Amplitude sensitivity spectra for analyte RI from 1.26 to 1.38.

Table 1 Sensing performance of the sensor for different analyte RIs.

Analyte RI peak wavelength (nm)

Res. Peak Shift (nm)

Wavelength Sensitivity (nm/ RIU)

Amp. Sens. (RIU� 1)

Wavelength Resolution (RIU)

Amplitude Resolution (RIU)

1.26 1380 20 2000 38.795 5.0 � 10� 5 2.58 � 10� 4

1.27 1400 20 2000 42.972 5.0 � 10� 5 2.33 � 10� 4

1.28 1420 20 2000 48.168 5.0 � 10� 5 2.08 � 10� 4

1.29 1440 20 2000 54.612 5.0 � 10� 5 1.83 � 10� 4

1.30 1460 30 3000 62.839 3.33 � 10� 5 1.59 � 10� 4

1.31 1490 30 3000 73.570 3.33 � 10� 5 1.36 � 10� 4

1.32 1520 40 4000 88.327 2.5 � 10� 5 1.13 � 10� 4

1.33 1560 50 5000 109.023 2.0 � 10� 5 9.17 � 10� 5

1.34 1610 70 7000 142.440 1.43 � 10� 5 7.02 � 10� 5

1.35 1680 90 9000 201.095 1.11 � 10� 5 4.97 � 10� 5

1.36 1770 100 14000 338.313 7.14 � 10� 6 2.96 � 10� 5

1.37 1910 350 35000 1120.734 2.86 � 10� 6 8.92 � 10� 6

1.38 2260 N/A N/A N/A N/A N/A

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97.9340025 μm2, respectively. Here, λ is the incident light wavelength in vacuum. The wavelength dependence of the dielectric constant of the thin ITO film is calculated using the Drude model [36]:

εmðλÞ¼ ε∞ �λ2λc

λ2pðλc þ iλÞ

(2)

In this expression, λp¼5.649710–7 m and λc¼11.2107610–6 m are the plasmonic and collision wavelengths of ITO, respectively, ε∞¼3.80 is the dielectric constant for the infinite value of the frequency of ITO, and λ is the operating wavelength in μm.

Confinement loss is used to analyze the characteristics of the sensor and it can be evaluated by the following equation [24]:

αloss¼ 8:686�2πλ

Imðneff Þ � 104 ðdB=cmÞ (3)

where Im (neff) is the imaginary part of effective index of the funda-mental mode and λ is the operating wavelength in μm.

Note that the sensor sensitivity can be calculated by the wavelength interrogation method and amplitude interrogation method. The wave-length sensitivity can be expressed as follows [37]:

SλðλÞ¼Δλpeak

Δna ðnm=RIUÞ (4)

where Δλpeak is the peak wavelength shift and Δna is the variation of the analyte refractive index (RI).

The amplitude sensitivity of the proposed sensor is defined according

to the following equation [38].

SAðRIU� 1Þ¼ �1

αðλ; naÞ

∂αðλ; naÞ

∂na(5)

Here, α(λ,na) defines the confinement loss at the analyte RI of na, ∂α(λ,na) is the difference of the confinement loss due to two adjacent RI of two analytes.

Sensor resolution is another important parameter that determines the degree of detection with analyte RI variation and can be determined by the following equation [26,39]:

RðRIUÞ¼Δna � Δλmin=Δλpeak (6)

where, Δλpeak is the peak wavelength shift, Δna is the variation of the analyte RI and Δλmin is the spectrometer resolution and can be set to 0.1 nm.

The experimental setup for practical realization of RI sensing is described in Fig. 1(b). A wide-band light source can be used to launch light into the single mode fiber (SMF) and couple with the PCF-SPR sensor. An analyte flow channel at the outer layer facilitates IN and OUT of the analyte. Owing to the interactions of the ligand and analyte, different RIs can change the phase matching condition of the SPR mode and guided mode resulting in the loss peak shift. The transmission spectra are acquired and analyzed by an optical spectrum analyzer (OSA) and computer.

The sensor is numerically simulated by the finite element method using the COMSOL multiphysics software. A circular perfect matching

Table 2 Performance comparison of the sensor with recently reported sensors.

Structure Type Wavelength range (nm)

RI Range Wavelength sensitivity (nm/RIU)

Amplitude sensitivity (RIU� 1)

Wavelength resolution (RIU)

Amplitude resolution (RIU)

D-shaped PQF sensor [11] 1450–1650 1.415–1.427 34000 N/A N/A N/A Three-core PCF [24] 550–900 1.33–1.4 3435 N/A 2.91 � 10� 6 N/A D-Shaped PCF sensor [25] 1400–1900 1.31–1.37 11750 N/A 8.51 � 10� 6 N/A Hollow-core silver coated PCF

[26] 500–800 1.33–1.37 4200 300 2.38 � 10� 5 3.33 � 10� 5

External gold-coated PCF [27] 590–800 1.33–1.37 1000 118 1.0 � 10� 4 8.5 � 10� 5

External gold-coated PCF [28] 700–1100 1.38–1.42 4875 513 2.5 � 10� 5 1.95 � 10� 5

External ITO-coated PQF [29] 1400–2200 1.33–1.39 21100 N/A 4.739 � 10� 6 N/A U-shaped PQF sensor [35] 950–1350 1.42–1.436 33600 N/A N/A N/A Hollow-core PCF Based [38] 1078–2097 1.27–1.45 5653 590 N/A N/A D-Shaped PCF sensor [39] 1166–1995 1.18–1.36 20000 1054 5.0 � 10� 6 N/A Copper-Graphene coated PCF

sensor [43] 520–820 1.33–1.37 14600 140 5.0 � 10� 5 7.1 � 10� 5

Gold-coated spiral PCF sensor [44]

560–680 1.33–1.38 4600 420.4 2.17 � 10� 5 2.38 � 10� 5

External ITO-coated PCF-SPR sensor [This work]

1380–2260 1.26–1.38 35000 1120.73 2.86 � 10� 6 8.92 � 10� 6

Fig. 4. (a) Confinement loss spectra with varying ITO thicknesses and (b) Variation of wavelength sensitivity with different ITO thicknesses.

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layer (PML) is employed as the absorbing boundary condition and a free triangle mesh is adopted in the simulation [40]. The performance of the proposed sensor is studied and optimized.

3. Simulation results and analysis

Fig. 2 shows the real part of the effective refractive index, confine-ment loss, and field distribution of core-guided mode and SPP mode as a function of wavelength for the X and polarization modes. Fig. 2 (a)–(d) and (e)-(h) show the electric field distribution of the core guided mode and SPP modes for X and Y polarization at the resonant wavelength for na ¼ 1.38 and na ¼ 1.34, respectively. The core guided mode and SPP mode exhibit strong coupling and the coupling intensity for na ¼ 1.38 is larger than that for na ¼ 1.34. The coupling intensity can be described by the confinement loss and the peak loss appears at the intersection of the effective index of the core guided mode and SPP mode. It means that the phase matching condition is satisfied. The coupling intensity of x po-larization is much larger than that of y polarization due to the asym-metry of the structure and reaches the maximum at 1.615 μm and 1.505 μm, respectively. In the following calculation, the confinement loss of x- polarization is chosen to analyze the characteristics of the proposed sensor.

When there are slight changes in the analyte RI, the phase-matching point will shift. Fig. 3(a) shows the variation of the confinement loss spectra as the analyte RI is changed from 1.26 to 1.38. The relationship between the resonant wavelength of the loss peak versus RI is shown in Fig. 3(b). The resonant wavelength shows red-shift with increasing an-alyte RI and the loss peak also increases significantly. This is because the increasing analyte RI reduces the index contrast between the fiber core

and SPP mode and enhances the evanescent field resulting in stronger coupling [41]. Meanwhile, a larger analyte RI will increases the Re (neff) of the SPP mode, while the core mode is unchanged, leading to the phase matching point move to a longer wavelength [42]. The RI range of the sensor is chosen from 1.26 to 1.38, which is attributed to the fact that the confinement loss spectra only contain one peak in the determined RI range and a larger and smaller RIs will produce irregular confinement loss curve. The maximum wavelength sensitivity of the sensor is 35000 nm/RIU as the analyte RI changed from 1.37 to 1.38. The average wavelength sensitivity of 7333.33 nm/RIU for the RI range of 1.26–1.38. Moreover, the non-linear fitting curve of the resonant wavelength in Fig. 3(b) indicates a good continuous response. The polynomial fitting equation can be expressed as:

λ¼ 1320:82502þ 0:4307⋅eðx� 1:316660:00711 Þ þ 188:94343⋅eðx� 1:31666

0:05341 Þ (7)

The adjusted R2 of the non-linear curve fitting is 0.99966, providing the ability of accurate analyte detection.

The amplitude sensitivity of the sensor as the analyte RI is varied from 1.26 to 1.38 is calculated by Eq. (5) and the results are shown in Fig. 3(c). The amplitude sensitivity significantly increases to 1120.73 RIU� 1 when the RI of the analyte is 1.37. The sensor shows the maximum wavelength resolution and amplitude resolution of 2.86 �10� 6 RIU and 8.92 � 10� 6 RIU, respectively. Table 1 shows the changes of the sensor performance parameters such as the wavelength and amplitude sensitivity and wavelength and amplitude resolution with increasing analyte RI. The performance comparison of this sensor with recently reported ones is shown in Table 2. It is seen that the maximum wavelength sensitivity and amplitude sensitivity of the proposed sensor reach 35000 nm/RIU and 1120.73 RIU� 1, respectively. In addition, the

Fig. 5. (a) Confinement loss spectra for different air hole diameter d1; (b) Confinement loss spectra for different air hole diameter d2; (c) Confinement loss spectra for different air hole diameter d3.

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sensor can work in larger RI range of 1.26–1.38, indicating good sensing performance.

The plasmonic materials are the key component of the PCF-SPR sensor. The ITO layer thickness (tITO) has a large influence on the sensitivity and confinement loss. Fig. 4(a) shows the confinement loss spectrum with different ITO thickness (30–60 nm) for the analyte RI na ¼ 1.38. The peak of the confinement loss increases as tITO is changed from 30 to 50 nm. At the same time, the resonant wavelength red-shifts indicating that higher energy exchange occurs from the fundamental mode to the SPP mode and the evanescent wave penetrates the sensing layer. However, the loss peak decreases sharply when the ITO thickness is greater than 50 nm because the higher damping loss leads to an overall decrease in the mode loss [36]. The wavelength sensitivity increases first and then decreases with increasing thickness of the ITO layer from 30 to 60 nm by 10 nm [See Fig. 4(b)]. Therefore, tITO ¼ 50 nm is the optimal value by considering the wavelength sensitivity and confinement loss.

The PCF-SPR sensor is sensitive to structural parameters. The effects of the PCF-SPR structure on the confinement loss spectrum are investi-gated for an analyte RI of 1.38. Fig. 5(a)-(c) describe the variations of the confinement loss spectra for different air hole diameters of d1, d2 and d3. As shown in Fig. 5(a), the resonant wavelength red-shifts when d1 changes from 1.6 to 2.4 μm and the peak loss also changes. Because increasing the air hole d1 alters the phase-matching conditions, the resonant wavelength red-shifts. In the same way, Fig. 5(b) shows the effects of d2 on the confinement loss spectrum. The peak of the confinement loss spectrum increases gradually and then decreases with changing d2. Hence, d2 ¼ 3.0 μm is chosen as the optimal value. The air hole diameter d3 has significant influence on the sensing performance because it directly adjusts the core guide area. When d3 is gradually reduced, more light will concentrate on the fiber core but it becomes

difficult to excite the SPR. On the other hand, when the diameter of d3 is too large, the fundamental mode field will diverge. Accordingly the proper d3 is especially important. The confinement loss spectra for different d3 are shown in Fig. 5(c) and d3 ¼ 4.6 μm is the optimal parameter. Based on these considerations, the optimal d1, d2 and d3 are 2.0 μm, 3.0 μm and 4.6 μm, respectively.

The effects of the pitch distance on the performance of the PCF-SPR sensor is analyzed using the amplitude sensitivity of the confinement loss spectrum. Fig. 6(a) depicts the confinement loss spectra when the pitch (Λ1) is varied for na ¼ 1.37 and na ¼ 1.38. Increasing Λ1 causes blue-shift of the resonant wavelength and the peak loss also increases first but then decreases with increasing Λ1 from 4.7 μm to 5.0 μm for na ¼ 1.38. However, the resonant wavelength and peak loss show the monotonic relationship for na ¼ 1.37. A larger pitch increases the transmission path of the fundamental mode energy to the metal oxide layer and coupling between the core guided mode and SPP mode is enhanced. As the pitch is increased further, more energy is concentrated into the core to weaken resonant coupling between the core guided mode and SPP mode. As Λ1 is changed from 4.7 to 5.0 μm the amplitude sensitivity is shown in Fig. 6(b) and the corresponding maximum amplitude sensitivities are 6.01 RIU� 1, � 1120.73 RIU� 1, � 750.05 RIU� 1, � 506.67 RIU� 1, respectively. When Λ2 is changed from 3.9 μm to 4.2 μm, the result is similar to that with changing Λ1 as shown in Fig. 6(c) and (d). The amplitude sensitivities are � 21.90 RIU� 1, � 1120.73 RIU� 1, � 827.78 RIU� 1, and � 568.07 RIU� 1 for different Λ2. This clearly in-dicates that different pitches change the interaction with the sensing layer and the effective refractive index of the fundamental mode and SPP mode. This leads to the change in the phase matching points giving rise to different resonance intensities. All in all, by considering the peak loss and amplitude sensitivity, Λ1 ¼ 4.8 μm and Λ2 ¼ 4.0 μm are chosen

Fig. 6. (a) Confinement loss spectra and (b) Amplitude sensitivity with varying pitch size (Λ1); (c) Confinement loss spectra and (d) Amplitude sensitivity with varying pitch size (Λ2).

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as the optimal pitches for the PCF-SPR sensor.

4. Conclusion

A coated PCF sensor with ITO based on SPR is designed and analyzed. This sensor can be operated in the near-infrared region and the sensing performance is investigated by wavelength and amplitude interrogation methods. The sensor shows the maximum wavelength sensitivity of 35000 nm/RIU and maximum amplitude sensitivity of 1120.73 RIU� 1. In addition, the sensor has good wavelength resolution and amplitude resolution reaching 2.86 � 10� 6 RIU and 8.92 � 10� 6

RIU, respectively. The sensor has a broad refractive index scope ranging from 1.26 to 1.38 with high sensitivity and resolution and the perfor-mance of the sensor is better than those previously reported. Owing to the external sensing mechanism, it is easy to implement using existing technologies and the senor is suitable for accurate sensing in biomedi-cine and chemistry.

Funding

This work was jointly supported by the Natural Science Foundation of Heilongjiang Province [grant number E2017010], Youth Science Foundation of Northeast Petroleum University [grant number 15071120517], Provincial talent project [grant number ts26180221], Hong Kong Research Grants Council (RGC) General Research Funds (GRF) [grant number City U 11205617], as well as City University of Hong Kong Strategic Research Grant (SRG) [grant number 7005105].

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Qiang Liu: Conceptualization, Data curation, Writing - review & editing. Jiudi Sun: Conceptualization, Methodology, Software, Writing - original draft. Yudan Sun: Investigation. Zonghuan Ren: Formal analysis. Chao Liu: Funding acquisition, Validation. Jingwei Lv: Methodology, Supervision. Famei Wang: Formal analysis. Liying Wang: Software, Validation. Wei Liu: Validation. Tao Sun: Resources. Paul K. Chu: Project administration.

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