Surface plasmon resonance refractive sensor based on photonic crystal fiber with oval hollow core Junjie Lu a,d, Yan Li a,*, Yanhua Han a, Duo Deng a, Dezhi Zhu a, Mingjian Sun b, Zhongyi Guo c,youlin Zhang d,and Xianggui Kong d a Department of Optoelectronics Science, Harbin Institute of Technology at Weihai, Weihai, China b Department of Control Science and Engineering, Harbin Institute of Technology, Harbin 150001, China c School of Computer and Information, Hefei University of Technology, Hefei, China d State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Science, Changchun 130033, China *Corresponding author: liy@hit.edu.cn Abstract: A photonic crystal fiber (PCF) resonance sensor based on surface plasmon resonance (SPR) is proposed. An active plasmonic silver layer is placed outside the fiber to excite the surface plasmon polaritons. Graphene is used to protect silver from oxidation and enhance the performance of sensor. The proposed sensor is consist with the symmetrical circular air-holes and elliptical hollow core which is used to produce the more evanescent field for tuning the phase matching between the core guided mode and surface plasmon polaritons (SPP) mode. Surface plasmon along the metal surface is excited with the leaky Gaussian-like core guided mode. Numerical investigations of structure parameters and refractive index (RI) are analyzed by finite element method (FEM). Using wavelength interrogation method, the proposed sensor could provide a maximum wavelength sensitivity as high as 10000nm/RIU in sensing range of 1.33-1.39 with the RI resolution of 2 10-5 RIU. Due to the ultrahigh sensitivity and simple structure, the proposed sensor shows potential applications in developing a high-sensitivity, real time and fast-response SPR biosensor. 1. Introduction Surface plasmon resonance(spr) is the collective oscillation of free electron on metal-dielectric interface arising from incident electro-magnetic wave[1]. SPR based sensor has attracted much attention because of its high sensitive nature and wide range of applications such as environmental protection, medical diagnostics, biological analyte detection, organicchemical detection and food quality control[2]. Recent years, SPR sensors based on conventional optical fiber is considered as a possible way that it offers miniaturization, high degree of integration and remote sensing capabilities. Moreover, they are immune to electromagnetic interference, mechanically flexible and low loss in visible range. However, in these sensors it is 1
difficult to tune the sensitivity since conventional optical fiber has relevant few parameters to change [3]. Therefore, photonic crystal fiber has attracted a lot of attention owing to its unique optical properties such as single mode guidance, large mode area, design flexibilities and controllable birefringence[4-7]. By optimizing the parameters of PCF, biosensors with high sensitivity and wide sensing range can be realized. In order to improve the sensitivity of the PCF sensor, birefringent based PCF sensors have been proposed[8]. Further, in order to address the coating of the walls of PCF sensors, gold and silver have been widely used as SPR active metal. Gold is chemically stable and shows a larger shift of resonance wavelength, but it has several limitations such as large absorption coefficient contributing to broadening of resonance peak, formation of island in very thin layer[9]. On the contrary, silver shows a sharper resonance peak compared to gold thereby increasing the detection accuracy. But the drawback is that it is easy to oxidation when placing or using in humid environment or liquids, which leads to reduction in detection accuracy and increases the storage cost[10]. Hence, we introduce graphene to overcome this problem. Graphene has high surface to volume ratio so that it can improve the signal-to-noise ratio (SNR). Furthermore, due to high electron density of hexagonal ring, graphene is impervious to gas molecules such as oxygen and water vapor, which can protect silver from oxidation. Besides, graphene shows high absorption of molecules owing to π-π stacking, which is excellent in biochemistry sensing. Recently, several SPR sensors have been proposed by using graphene on the metal surface. For instance, based on graphene in a dielectric metal dielectric configuration, a low index dielectric mediated SPR sensor was designed[11], a liquid core PCF-SPR sensor with selectively filled analyte channels was proposed [10]. However, it is difficult to remove and re-filling the analyte in the liquid core, which makes it impossible for real-time and quick response. Besides, it is a timeconsuming task to coating material and filling analyte into the air holes since the air holes are in micron scale. Although, a D- shaped PCF-SPR sensor is convenient to clean and replace the analyte, the accurate polishing and etching effort to remove a predetermined part of PCF is still a challenge [12, 13]. In this paper, we propose a SPR sensor based on an elliptical air core PCF coated with silver and graphene. Graphene is used to protect silver from oxidation and enhance the performance of sensor. Numerical investigations of structure parameters and refractive index (RI) are analyzed by finite element method (FEM). Using wavelength interrogation method, the proposed sensor could provide a maximum wavelength sensitivity as high as 10000nm/RIU in sensing range of 1.33-1.39 with the RI resolution of 2 10-5 RIU. Due to the ultrahigh sensitivity and simple structure, the proposed sensor shows potential applications in developing a high-sensitivity, real time and fast-response SPR biosensor. 2
2. Design and analysis The proposed PCF-SPR sensor is shown in Fig.1. Both the core and cladding material of the proposed structure are made up of fused silica. The PCF has a center ellipse hole for generating the birefringence. The lattice pitch is Λ=2µm, the major and minor axis diameters of the center ellipse hole are a=0.3λ, b=0.15λ. The diameter of cladding holes is d=0.7λ. For generating the SPP mode, silver film was coated on the out wall of PCF and graphene was used to protect silver from oxidation. The thicknesses of silver and graphene are t Ag =40nm and t g =3nm. The electromagnetic mode of the sensor fiber is solved by finite element method (FEM) with a circular perfect matched layer (PML), which prevents reflection by absorbing the scattered light from the structure. The RI of fiber material is determined by Sellmeier equation [14]: 2 2 2 A1 A2 A3 2 2 2 B1 B2 B3 n( ) 1 (1) where A 1 =0.696166300, A 2 =0.407942600, A 3 =0.897479400, B 1 =4.67914826 10-3 µm 2, B 2 =1.35120631 10-2 µm 2 and B 3 =97.9340025µm 2. The RI of graphene can be obtained by the following equation [13]: n g ic 3 1 (2) 3 where C 1 =5.446µm -1 and λ is the vacuum wavelength. Fig.1. Schematic of proposed PCF-SPR sensor with hollow core. 3. Results and discussions 3
The confinement loss is calculated by using the imaginary part of the effective RI: (3) 4 =8.686 k0 Im( n eff ) 10 db/cm where k 0 = 2π/λ is the wavenumber with λ in micron and Im(n eff ) is the imaginary part of mode n eff. The Gaussian beam propagated along the z-direction and mode analysis is performed in the XY plane in this simulation. As shown in Fig. 2(a), there are five lines: the real parts of the effective RI for the x- and y-polarized resonant fundamental core mode, and the plasmonic mode and the loss of the x- and y-polarized resonant fundamental core mode. When the phase-matching condition is satisfied, the core mode line and the SPP mode line will cross at a point, where the effective index of the core-guided fundamental mode and SPP mode coincide for the analyte with RI 1.33. Consequently, a resonance occurs, which results in a loss peak at 600nm as shown in Fig. 2(a). The proposed sensor shows higher fundamental mode resonance loss peak using the y-component as compared to the x-component mode. Fig. 2(b) and (c) show the core guided fundamental mode for x- component and y-component at 560nm, respectively. Fig. 2(d) shows the plasmon mode when wavelength of incident light is 560 nm. Additionally, at the resonant peak of 600 nm, largest energy is transferred from core-guided fundamental mode to SPP mode, when both modes are strongly coupled as shown in Fig. 2(e). Fig.2 (a) Dispersion relations of the x- and y-polarized core mode and y-polarized core mode at analyte RI 1.33. (b) and (c) Electric field distribution of x-polarized core mode and y-polarized core mode at wavelength 560 nm. (d) Electric field distribution of y-polarized SPP mode at wavelength 560 nm. (e) Electric field distribution of y-polarized core mode at wavelength 600 nm. 4
Fig.3 shows the confinement loss spectra with varying structural parameters of the PCF. Figure 3(a) presents the loss spectra of the sensor with different pitch (Λ) value. With increasing of the pitch, the loss spectra shows slightly blue shift and confinement loss value decrease. The reason behind this is that when pitch value increase, the confinement of the perpendicular light becomes less so that less energy leaks to horizontal region. Hence, small pitch contributes to high confinement effect. From Fig. 3(b) we can see that the maximum loss is 186 db/cm for diameters of air holes d=0.9λ at the resonance wavelength 605 nm and the minimum loss equals to 101 db/cm at the resonance wavelength 590 nm. It means that the diameter of the air holes can affect the resonance wavelength. Fig.3(c) shows the variation of confinement loss with different eccentricity of center ellipse hole (ε) for Λ=2μm, d=0.7λ, b=0.15λ. There is no obvious resonance peak shift while resonance amplitude increases, implying that loss confinement can be tailored by varying the eccentricity of center ellipse air hole. Fig. 3(d) displays the dependence of loss spectra on minor axis diameter of center ellipse air hole. The minor axis diameter of center ellipse air hole is independent of the resonance wavelength. Fig.3. Variation of confinement loss as a function of wavelength with varying (a) pitch (Λ) for d=0.7λ, ε=2, b=0.15λ. (b) Diameters of air holes for Λ=2μm, ε=2, b=0.15λ. (c) Eccentricity of center ellipse hole (ε) for Λ=2μm, d=0.7λ, b=0.15λ. (d) Minor diameter of center hole (b) for Λ=2μm, d=0.7λ, ε=2. The thicknesses of silver and graphene also play a vital role in performance of PCF-SPR sensors. Form Fig. 4(a) we can see that the resonance peak damps obviously with increase of t Ag, which means more energy of core mode is wasted for overcoming the damping loss, thus, the interaction of core mode with silver layer decreases. Moreover, the resonance wavelength red shift and the full width at half maximum (FWHM) increases mean a worse of SNR. Therefore, the thickness of silver is set as 40nm for better performance of sensor. As shown is Fig. 4(b), we investigated the performance of the sensor via changing the thickness of graphene. The confinement loss decreases from 129.7dB/cm to 106.2dB/cm with resonance 5
wavelength shift from 600 nm to 675 nm. Increasing of graphene thickness leads to increasing of the effective RI at the metal-dielectric interface. Consequently, resonance condition is satisfied at a longer wavelength. Moreover, the imaginary part of dielectric constant of graphene leads to damping loss, which based on the same principles with silver. So the FWHM of resonance curves is broaden [13]. In practice, the optimized thickness of graphene is 3nm. Fig.4. Variation of confinement loss with varying (a) thickness of silver for t g =3nm, (b) thickness of graphene for t Ag =40 nm. SPR is extremely sensitive to the change of surrounding environment RI. Therefore, the transmission spectra was investigated by changing the RI of analyte. From Fig. 5(a) we can see that with increasing of the RI the resonance peak increases and the resonance wavelength is red shift. The change of RI could deeply affect the phase match point of core mode and SPP mode. In addition, the RI increase of analyte leads to the fact that the effective RI of SPP mode gets close to that of core mode, and more energy of core mode is transmitted to SPP mode. This phenomenon can be used to detect the RI of analyte by measuring the shift of resonance wavelength. The wavelength sensitivity of SPR sensor is calculated as follow [15] : S peak (3) n a where Δλ peak is the resonance wavelength shift and Δn a is the variation of analyte RI. Wavelength sensitivity of SPR sensor is shown in Fig. 5(b). With increasing of analyte RI, the resonance peak shows red shift and the correlation coefficient (R 2 ) is up to 0.99346. The sensitivity of this SPR sensor increases from 2500 nm/riu to 10000 nm/riu. Furthermore, the RI resolution is defined as [16]: R n (4) a where Δλ min is the wavelength resolution of optical spectral analyses(aq6370b, Yokogawa, Δλ min =0.02nm). So the theoretical RI resolution of corresponding SPR sensor is 2 10-5 RIU. Moreover, the concentration of glucose has a good linearity with RI [17]. Hence, by measuring the tiny change of glucose RI, we can detect the concentration in real time min / peak 6
without difficult operations. Nevertheless, benefit from the fiber sensor technology, only a small amount analyte is required for measurement. Thus, abovementioned sensor has a broad usage in monitoring the RI of various chemical analyte. Fig.5. (a) Variation of confinement loss as a function of wavelength with different analyte RI.(b) Wavelength sensitivity in the analyte RI range from 1.33 to 1.39 4. Conclusion A novel photonic crystal fiber biosensor based on surface plasmon resonance is demonstrated by keeping the mental layer and analyte layer outside the fiber. FEM has been used to analyte the performance of this structure. The graphene layer is used to inhibit silver from oxidation. As the phase condition of core mode and plasmonic mode match, the SPR sensing performance is increasing significantly. The wavelength sensitivity of the structure has been found to be as high as 10000 nm/riu along with a resolution of 2 10-5 RIU. Driving on the advantage of latest nanofabrication technique, the proposed structure can be utilized for environmental, biological and biochemical sensing applications. Acknowledgments This work was supported by the National Defense Foundation of China (Grant No. 0103200), the National Natural Science Foundation of China (Grant No. 11304064, 11304065 and No. 61371045), the Science and Technology Development Plan Project of Shandong Province (Grant No. 2015GGX103016, 2016GGX103032 and 2016JMRH0217), and the China Postdoctoral Science Foundation (Grant No. 2015M571413). We are also thankful to Innovative Practice Program for Changchun Institute of Optics, Fine Mechanics and Physics. References [1] A. A. Rifat, G. A. Mahdiraji, R. Ahmed, D. M. Chow, Y. M. Sua, Y. G. Shee, and F. R. M. Adikan, "Copper-Graphene- Based Photonic Crystal Fiber Plasmonic Biosensor," IEEE Photonics Journal 8, 1-8 (2016). [2] B. Lee, S. Roh, and J. Park, "Current status of micro- and nano-structured optical fiber sensors," Optical Fiber Technology 15, 209-221 (2009). 7
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