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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 11, JUNE 1, 2014
Graphene-Based Birefringent Photonic Crystal Fiber Sensor Using Surface Plasmon Resonance Jitendra Narayan Dash and Rajan Jha
Abstract— We propose a graphene-based photonic crystal fiber (PCF) sensor based on surface plasmon resonance. Graphene helps in prevention of oxidation of the silver layer used as a plasmonic active metal. The birefringent nature of the structure allows one component of the core guided mode to be more sensitive. Further, this structure does not need filling of the voids. The structural parameter of PCF and metal thickness has been optimized. The proposed sensor shows high amplitude sensitivity of 860 RIU−1 and has a resolution as high as 4 × 10−5 RIU. This reported performance is higher than bimetallic (gold on silver) configuration. Index Terms— Photonic crystal fiber, sensor, surface plasmon resonance (SPR), graphene.
I. I NTRODUCTION
S
URFACE plasmon is a transverse magnetically polarized electromagnetic wave due to the collective oscillation of free electrons at the metal dielectric interface. The oscillation occurs when certain resonance condition is satisfied [1]. SPR sensors usually involve a prism coated with metals like silver or gold [1]. The prism helps in resonant phase matching between incident wave and plasmonic wave at the interface of metal and dielectric. Recently, graphene has been used as a coating on metal surface for SPR based sensors due to its various advantages [2]. Graphene exhibits universal optical conductivity from visible to infrared frequency due to interband transition [3]. The high surface to volume ratio, broadband optical and plasmonic properties make it a suitable candidate to be used as a sensor and as a functional coating material for existing plasmonic devices [4]–[6]. The advantages of using graphene in SPR based sensors are increase of the adsorption of molecules due to π−π stacking [5]. Moreover, high electron density of hexagonal rings of graphene prevents atoms as small as helium to pass through its ring structure [6]. In the recent past several graphene based SPR sensors using conventional Kretschmann configuration have been reported [2], [5]–[7]. Graphene based gold coated SPR sensors showed a broader SPR curve as compared to silver coated graphene based sensors [2], [6]. Although silver has sharper
Manuscript received February 5, 2014; revised March 14, 2014; accepted March 27, 2014. Date of publication April 7, 2014; date of current version April 29, 2014. This work was supported by the Department of Science and Technology, India, under Project SR/FTP/PS-086/2010. The authors are with the Nanophotonics and Plasmonics Laboratory, School of Basic Sciences, IIT Bhubaneswar, Bhubaneshwar 751007, India (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2315233
resonance peak, the bulky arrangement of the Kretschmann set up along with no remote sensing option make it difficult to fabricate on large scale for online real time application. Miniaturization of sensing probe is possible by the usage of optical fiber. Utilizing the advantages of optical fiber and using graphene as coating, SPR based sensor has been very recently reported [8]. Here, in order to detect the sensing layer, the cladding of the fiber has been removed. Also the wavelength of operation is fixed and cannot be tuned as per requirement. In order to overcome these difficulties, photonic crystal fibers (PCF) can be used for SPR based sensor [9]–[12]. The advantages of PCF are design flexibility, single mode propagation, high confinement, propagation in air and solid core (no dopant required), controllable birefringence [12]. Further the effective index of the core guided mode can be tailored by changing the geometric structure which helps in phase matching of core guided mode and plasmonic mode in the wavelength range of interest. Taking the above advantages of graphene and PCF, we propose a graphene based PCF sensor by using silver as a SPR active metal. Most of the PCF based SPR sensors use gold or silver coating [9]–[11]. Gold is chemically stable and it shows larger shifts in resonance wavelength. However its absorption coefficient is larger which leads to broadening of resonance curve thereby reducing the performance in terms of detection accuracy [9]. As an alternative silver as SPR metal shows a sharper resonance peak compared to gold and hence shows higher detection accuracy, an important performance parameter [10]. But silver is prone to oxidation which reduces the detection accuracy [6]. So we have considered graphene layers on the silver coated PCF, as the hexagonal holes of graphene prevent oxygen molecule to pass through it thereby solving the oxidation problem. Also techniques are there to isolate single or few layers of graphene from graphite [13]. Moreover deposition of graphene around microfiber have been reported [14]. Further graphene can be grown on silver using the reported techniques [15]. In most of the previously reported structures, infiltration of sample is required which is difficult and cumbersome [11], [16]. In our case the sample can be kept around the outer surface of the proposed structure and the sensing is easier as the sensor can be used in a flow cell containing the analyte to be detected. II. S TRUCTURE AND T HEORETICAL M ODELLING The proposed sensor has a silica core with a small air hole at the center as shown in fig. 1. The central hole helps in
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DASH AND JHA: GRAPHENE-BASED BIREFRINGENT PCF SENSOR
Fig. 1.
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Cross section of the proposed PCF sensor.
Fig. 2. Variation of effective index of X and Y-component thereby showing birefringence behavior.
phase matching of core guided mode and plasmonic mode. Surrounding the central hole there are six holes arranged in a hexagonal manner in the first layer. The holes are separated by a distance . Out of these six holes, two holes are smaller than the others thereby introducing birefringence. This helps in effective interaction of light with the metal dielectric interface along a particular direction. The smaller outer holes in the second layer are used for better confinement of light in a particular direction. All the holes in this structure are air filled. As shown in fig. 1, the diameter of the first layer holes is d1 and the holes responsible for birefringence are db . The diameter of second layer holes is d2 while that of central hole is dc . A thin coating of silver followed by graphene layer is considered on the outer surface around the fiber and the analyte to be detected surrounds the structure. Finite element method (FEM) is used for calculation and analysis of the proposed sensor. The whole section of our proposed structure is divided into many triangular domains and the number of mesh elements is 191137. The refractive index of silver in the concerned structure has been taken from Palik [17]. Each single layer of graphene deposited on silver has thickness of 0.34 nm [5], [18]. The total thickness of graphene layer is calculated as 0.34 × L where L is number of graphene layers. The complex refractive index (R.I) of graphene in visible region can be obtained using the equation n g = 3 + i C1 λ/3 where C1 ≈ 5.446 μm−1 and λ is the vacuum wavelength [6], [18]. The refractive index of silica is determined by using Sellimeiers equation [9]. Here the symbols have their usual meaning. n 2 (λ) = 1 +
A1 λ2 A2 λ2 A3 λ2 + 2 + 2 − B1 λ − B2 λ − B3
λ2
(1)
III. R ESULTS AND D ISCUSSION We have optimized the structural parameters of PCF in order to increase the interaction of the modal field with the sensing layer (analyte). We optimized the diameter of central hole dc at 0.3. If the size of central hole becomes smaller than this, then it will lead to more confinement of light at the center of the core instead at the metal dielectric interface. Also large central hole leads to the reduction of effective index of core thereby deteriorating the guidance along the core. The diameter of holes in the first layer are fixed at an optimized value of 0.6 keeping in mind that larger holes leads to lesser confinement of light at the metal dielectric interface. The other
Fig. 3. Dispersion relations of core guided mode (black solid line), plasmonic mode (black dotted line) and imaginary part of core guided mode (red solid line). Inset (a) and (b) show the electric field profile of core and plasmonic modes respectively. The R.I of analyte is 1.330.
optimized structural parameters are = 2 micron, db = 0.4, d2 = 0.2. Taking the above optimized parameters into consideration, a section of our proposed structure is used for modal analysis due to its symmetrical nature. The simulation for modal analysis is done in XY plane while the light propagation is along the z direction. The variation of effective index of X and Y-component of the core guided mode with wavelength is shown in fig 2. The difference in the effective indices occurs due the birefringent nature of the structure. In the proposed structure we have analyzed the X- component of the core guided mode as it has higher modal loss than Y-component, due to the birefringent nature of our proposed structure. Resonance occurs when the real part of the effective index of core guided mode matches with that of plasmonic mode i.e mode at the metal dielectric interface. As can be seen from fig 3, this occurs at a wavelength of 510 nm where the confinement loss is maximum i.e. the value of for imaginary part of core guided mode is maximum at 510 nm. The inset (a) and (b) show the electric field profile of the core guided and plasmonic modes respectively. The value of confinement loss plays an important role in calculating the performance of any sensor. To study the sensor performance, analytes of different refractive indices (na ) are considered around the PCF coated with silver and graphene monolayer. The R.I of analyte is varied from 1.330 to 1.370. For a given R.I of the analyte, the modal loss is calculated using α(d B/cm) = 8.686 × (2π/λ)Im(n e f f ) × 104 [19] where λ is in μm. As can be seen from figure 4, the loss of the core guided mode increases with
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 11, JUNE 1, 2014
Fig. 4.
Variation of loss with refractive index of analyte.
Fig. 6.
Comparison of sensitivity of graphene and gold coated PCF.
Fig. 5.
Variation of loss with number of graphene layers.
Fig. 7.
Variation of sensitivity with thickness of silver.
R.I of the analyte (na ) and resonance peak is red shifted. The shift occurs due to the change in na which in turn affects the phase matching point between the core guided and plasmonic mode as per fig 3. As a result, the resonance condition is satisfied at different wavelength for different R.I of analyte. This mechanism can be effectively utilized for the detection of biochemicals. Further, the performance of the sensor is dependent on confinement loss which in turn depends upon the thickness of the metal and the number of graphene layers (L). In order to optimize the thickness of silver, we took mono layer of graphene on silver surface for our analysis and optimization. On varying the thickness of silver from 30 nm to 50 nm for a fixed RI of analyte, we found that the confinement loss decreases from 158.5 dB/cm at 30 nm to 35 dB/cm at 50 nm due to higher damping loss at higher metal thickness. However when we varied the number of graphene layers for constant silver thickness of 30 nm, the loss decreased with increase in number of graphene layers as shown in fig. 5. This is due to the fact that with the increase in the thickness of graphene, there will be more damping due to the presence of absorbing layers [7], [20]. So the field available for sensing layer decreases. This leads to the reduction in loss arising due to plasmonic resonance. Moreover, with increase in number of graphene layers, the resonance peak gets broadened as the damping is proportional to the thickness of the absorbing layer [21]. Keeping this variation of loss with thickness of silver and graphene, we have optimized the performance of the sensor by investigating the most important performance parameter i.e sensitivity based on amplitude interrogation (AI). In AI method, all measurements are carried out at a single wavelength. So this process is economical and no spectral manipulation is required. Considering P0 as the power launched to the fiber core mode, the power detected after
a length L of the sensor is P(L,λ,na )=P0 exp(-α(λ,na )L), where α(λ,na ) refers to the loss of fundamental core mode as a function of wavelength. Generally the length of sensor is taken as L=1/α(λ,na ) which in our case falls into subcentimeter range. The amplitude sensitivity of the sensor is defined as [10] S A (λ)(R I U −1 ) = −(∂α(λ, n a )/∂n a )/α(λ, n a )
(2)
In order to compare the sensitivity of our proposed sensor (graphene on Ag) with other sensors, we plotted the sensitivity for widely used bimetallic configuration (few nanometer of gold on silver) [21]. In both the cases the thickness of silver taken is 30 nm. As can be seen from figure 6, graphene coated silver is 18% more sensitive than that of bimetallic configuration. This is due to the fact that in case of bimetallic configuration, the gold metal layer on silver causes more damping. But in case of graphene damping is less and only few layers are sufficient to reduce the oxidation. To further study the performance of graphene on Ag we analyzed the sensitivity for different thicknesses of silver. Considering different thicknesses of silver, we found that the amplitude sensitivity decreased from 245 RIU−1 at 30 nm to 191 RIU−1 at 50 nm for analyte having refractive index of 1.330 as shown in figure 7. Further with increase in the thickness, the wavelength corresponding to maximum sensitivity shifts towards right owing to resonance condition. Considering the advantages of silver and taking the optimized thickness of 30 nm for high sensitivity, we have studied the variation of sensitivity with number of graphene layers L. With the increase in number of graphene layers, the sensitivity decreases from 245 RIU−1 for monolayer of graphene to 139 RIU−1 for four layers of graphene for analyte index of 1.330. Moreover the curve also broadens as can be seen
DASH AND JHA: GRAPHENE-BASED BIREFRINGENT PCF SENSOR
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and performance parameters have been optimized and sensitivity has been compared and found to be higher than widely used bimetallic configuration. The sensitivity of the sensor is found to be as high as 860 RIU−1 . Further the resolution of the sensor is found to be 4 × 10−5 RIU. R EFERENCES
Fig. 8.
Variation of amplitude sensitivity with number of graphene layers.
Fig. 9. Variation of power sensitivity (inset) and amplitude sensitivity with R.I of analyte.
from fig. 8. Damping of the surface plasmon waves increases due to the finite imaginary part of permittivity of graphene. With increase in the number of graphene layers, the surface plasmon waves accumulates more in the graphene layers and this leads to the broadening of the peak. Therefore, the slope of the resonance curve decreases thereby decreasing the sensitivity [20]. Also, we studied the variation in sensitivity with change in refractive index of the analytes. Keeping monolayer of graphene on silver, the variation of maximum amplitude sensitivity with higher refractive index of analyte is shown in fig. 9. Here the wavelength of operation is 540 nm for analyte of refractive index 1.330. However in order to get maximum sensitivity at higher analyte index as shown in figure 9, one may choose a source depending upon the fulfilment of resonance condition. Further, the inset of fig. 9 shows the variation of power sensitivity with different analyte index for a given wavelength. If the sensor can detect a 1% change in transmitted intensity, then the resolution of proposed sensor is 4 × 10−5 RIU for monolayer of graphene and 7 × 10−5 for four layers of graphene which is comparable to the commercial resolution. As synthesis of few layers of graphene has been achieved commercially, our proposed sensor can be used as an alternative to other gold or Ag based SPR sensors riding on the advantages of latest nanofabrication technique. IV. C ONCLUSION Graphene based PCF sensor using SPR has been proposed. Here, graphene helps in prevention of oxidation of silver layer used as a plasmon active metal. Numerical analysis of the proposed structure is carried out with FEM. All the structural
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