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Fano resonance in graphene-MoS2 heterostructure-based surface plasmon resonance biosensor and its potential applications
Optical Materials 66 (2017) 171e178
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fano resonance in graphene-MoS2 heterostructure-based surface plasmon resonance biosensor and its potential applications Gaige Zheng a, b, *, Xiujuan Zou a, Yunyun Chen a, b, Linhua Xu a, Weifeng Rao a, b a
School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, China Jiangsu Collaborative Innovation Center on Atmospheric Environment and Equipment Technology (CICAEET), Nanjing University of Information Science & Technology, Nanjing, 210044, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 November 2016 Received in revised form 14 January 2017 Accepted 3 February 2017
We propose a new configuration of surface plasmon resonance (SPR) sensor that is based on grapheneMoS2 hybrid structures for ultrasensitive detection of molecules. The present configuration is consisted of chalcogenide glass (2S2G) prism, Ag, coupling layer, guiding layer, graphene-MoS2 heterostructure and analyte. We perform numerical and analytical study of the impact of the thickness and refractive index (RI) of the coupling and guiding layer in a planar sensing structure within the Kretschmann configuration on the resonance properties of the excitation. Results of reflectivity calculations clearly demonstrate the sharp Fano-type resonance appears in the curve of SPR because of the coupling between surface plasmon polariton (SPP) and planar waveguide (PWG) modes. The properties of the Fano resonance (FR) strongly depend on the parameters of the structure. The calculated magnetic field profiles manifest that the hybrid nature of the electromagnetic (EM) modes excited in the present structure. The proposed system displays an enhancement factor of sensitivity by intensity more than 2 103-fold when compared to the SPR sensing scheme. © 2017 Elsevier B.V. All rights reserved.
Keywords: Surface plasmon resonance Optical sensor Graphene MoS2 Attenuated total reflection
1. Introduction Surface plasmon-based sensors have enabled bio-molecular detection with high speed and sensitivity, which is mainly because they eliminate time-consuming labeling process and reduce molecular binding disturbance compared to common fluorescent optical sensors [1e3]. Surface plasmon resonance (SPR) sensors generally utilize the resonance excitation of surface plasmon polariton (SPP) at the metal-dielectric interface [4,5]. Among different kinds of SPR sensors, the most common one is the Kretschmann's configuration which is extensively used for chemical/biomolecular sensing with high sensitivity [6]. In such configuration, a thin metal film is deposited on a prism and the other face of the metal is kept in contact with the sensing medium. When a transverse magnetic (TM)-polarized incident light passes the prism through the metal film into a dielectric media, an evanescent wave will occur, while the energy of the incident light is absorbed and surface plasmon waves (SPWs) will be excited at the interface [7].
* Corresponding author. School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, China. E-mail address: [email protected] (G. Zheng). http://dx.doi.org/10.1016/j.optmat.2017.02.001 0925-3467/© 2017 Elsevier B.V. All rights reserved.
The SPR condition takes place at a specific angle of incidence or wavelength, when the evanescent wave couples with the SPs on the surface of the metal layer. It is necessary to mention that the nature of the reflectance curve of attenuated total reflection (ATR) system dictates the performance of the SPR sensor. As such, the width of the SPR curve determines how precisely a sensor can detect the resonance angle and the shift. For high performance sensor, the shift in the resonance angle should be large, whereas the width of the SPR curve should be very small [8e10]. Different approaches for the improvement of the SPR sensor resolution have been proposed [11e13]. For example, high index coupling prism can be used to reduce the full width at half maxima (FWHM) of SPR curve thereby increasing the detection accuracy of the sensor [13]. Another way is choosing the metal layer correctly. Gold (Au) produces large SPR resonant angle shift while its response curve is relatively wider than that of silver (Ag). Au is usually preferred because it is more resistant to oxidation and corrosion in different environments. But, the drawback is that biomolecules adsorb poorly on gold, which limits the sensitivity of the conventional SPR biosensor. Aluminum (Al) not only exhibits narrower resonance curve compared to Au and Ag but also is relatively economical. However, Al is highly susceptible to oxidation, thereby deteriorating the sensor
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performance [14]. It has been shown that the use of bimetallic configuration (Au-Ag) increases the signal-to-noise ratio and decreases the width of the resonance curve, providing a significant improvement of the resolution while the sensitivity was comparable of that of conventional SPR [15e18]. To further improve the performance of biosensor, researchers have proposed and fabricated SPR based sensor coated with graphene layers [19e25]. Graphene is a single-atom thin planar sheet of sp2 carbon atoms perfectly arranged in a honeycomb lattice. Graphene and graphene oxide provide good support for biomolecule adsorption due to their large surface area and rich p conjugation structure, making them suitable dielectric top layers for SPR sensing [23]. However, graphene produces more damping in SPs due to large imaginary dielectric constant for higher graphene layers, and hence results in decreased performance [24]. More recently, ultra-thin layer of molybdenum disulfide (MoS2) that belongs to the transition-metal dichalcogenide (TMDC) semiconductor group is known as “beyond graphene” 2D nanocrystals material and they are widely used as solid lubricants due to its low friction property. Therefore, in order to enhance the performance of conventional SPR sensor and utilize the advantageous properties of graphene, a planar structure of prism-waveguide coupling system with graphene-MoS2 heterostructure that allows for coupling between SPP and WG mode is proposed. MoS2 layers are used for improving the light absorption in order to provide enough excitation energy or effective charge transfer, while monolayer graphene is acting as iorecognition component for capturing the target biomolecules through pi-stacking force. We show that sharp Fano line shapes appear in the ATR spectra when the structural parameters are appropriately chosen. The thickness of the coupling and guiding layer is optimized first with respect to sensitivity, full width at half maximum (FWHM), and minimum reflectance at 633 nm wavelength. Thereafter the variation of performance parameters sensitivity, detection accuracy, quality factor, and minimum reflectance is shown with respect to the regular change in the RI of the sensing layer. The results of the numerical calculations clearly exhibit sharp Fano-type resonances, and demonstrate that the sensor sensitivity by intensity can be enhanced by more than 2 103 orders of magnitudes compared to that of conventional SPR sensors. 2. Design consideration and theoretical model In our proposed new structure, a well-known Kretschmann configuration with multilayer thin films is employed. Multilayers are placed according to the following structure, i.e. 2S2G prism, Ag film, coupling layer, guiding layer, MoS2, monolayer graphene and sensing medium, as shown in Fig. 1. The wavelength of the incident light for the excitation of SPs is 632.8 nm. The TM-polarized light incidents from one lateral face of the prism, then reaches to its base and totally reflected out from the other lateral face, and finally can be collected and analyzed by a photodetector. 2.1. Refractive index of various layer components The first layer is 2S2G prism, and its refractive index (np) is calculated through the following relation [26]:
np ¼ 2:24047 þ
2:693 102 2
l
þ
8:08 103
l4
(1)
where l is the wavelength of incident light in micrometers. We choose 2S2G as the coupling prism due to its high refractive index. 2S2G also has shown potential in the fabrication of ultra-low-loss waveguides [27], and it has been widely used in sensing
technology [28]. In the calculation, the dielectric function of metal is described by the Drude model as follows:
εAg ðuÞ ¼ ε∞
u2p u2 þ igu
(2)
where ε∞ is the infinite frequency dielectric constant, up is the bulk plasma frequency, u is the angular frequency, and the collision frequency g which is related to the dissipation loss in the metal. These parameters are set as 6.0, 1.5 1016 rad/s and 7.73 1013 rad/ s, respectively [29]. The complex refractive index of monolayer MoS2 at 632.8 nm is obtained from the experimental measurement data by Castellanos-Gomez et al. [30] and the thickness of MoS2 layer is 0.65 nm [31]. The sixth layer in our model is monolayer graphene (with thickness of 0.34 nm) and its complex refractive index ng in the visible range is given as [32]:
ng ¼ 3:0 þ i
C1 l 3
(3)
where the constant C1 z 5.446 mm1 [33] and l is the wavelength of incident light in mm. The sensing medium for initial calibration is deionized (DI) water and its refractive index (ns) is determined by the following relation [34]:
n2s 1 ¼
2 4 X Ai l i¼1
l2 ti2
(4)
where the Sellmeier coefficients A1 ¼ 5.666959820 101, A2 ¼ 1.731900098 101, A3 ¼ 2.095951857 102, A4 ¼ 1.125228406 101, t1 ¼ 5.084151894 103, t2 ¼ 1.818488474 102, t3 ¼ 2.625439472 102, t4 ¼ 1.073842352 101 and l is the wavelength of incident light in mm. Eq. (4) is valid for wavelengths ranging from 0.182 to 1.129 mm.
2.2. Numerical formulation of reflectivity and field distributions To systematically investigate the reflectivity change in our graphene-MoS2 hybrid structure-based SPR sensing system, we employed the transfer matrix method (TMM) and Fresnel equations based on an N-layer model to perform a detailed analysis [13,19,20]. TMM method is a powerful tool in the analysis of light propagations through layered dielectric media. The central idea lies that electric or magnetic fields in one position can be related to those in other positions through a transfer matrix. Within the framework of the TMM [35,36], there are two kinds of matrices: one is the transmission matrix and the other is the propagation matrix. They connect the fields across an interface and the fields propagating over a distance within a homogeneous medium, respectively. For a stack of N dielectric layers shown in Fig. 1 (a), the transfer matrix can be obtained by transmission and propagation matrices across different interfaces and homogeneous dielectric media, respectively. In order to obtain the optical spectra, the equations which results from the ordinary boundary conditions that the fields satisfy at each interface need to be resolved. Since the system is uniform in the y direction, we can decompose the electromagnetic fields E(m), H(m) into two components and consider the TE and TM-polarized waves separately. And, the expressions of reflection and transmission coefficients for N-layer system (layer 0 corresponds to the prism and layer N to the analyte) is restated. For numerical calculations, it is assumed a planar multilayer slab sensor in Kretschmann configuration as schematically shown in Fig. 1 (c), where the
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Fig. 1. (a) Schematic of the multilayer structure consisting of Ag film, coupling layer, guiding layer, MoS2, graphene and the surrounding sensing layers placed over a SF11 prism. The thicknesses of the coupling and guiding layer are denoted by t1 and t2, respectively. (b) Schematic diagram of the graphene-MoS2 heterostructure. (c) Schematic of the coupling mechanism through the EM multilayer system.
origin of the coordinate system is located at the substrate interface.
other criterion FOM for the sensitivity by intensity that is given by
2.3. Performance parameters
FOMI ¼ max
In biochemical experiment, there are several criteria such as sensitivity, figure of merit (FOM), resolution, detection accuracy, reproducibility, cost, and device size, which can be used to evaluate the performance of a sensor. These parameters can be calculated with the help of the curve of incident angle versus reflectivity. To make qualitative and quantitative analysis of the structure sensing performance and also to make a comparison with other designed structure, bulk sensitivity and FOM of structure are two important criterions for designing structure sensor and will be discussed as follows. These two parameters are calculated with the help of the curve of incident angle versus reflectivity. The sensitivity for bulk sensing will be examined, where the changes in the angle-scan ATR resonance curve caused by the change in the refractive index of DI water is monitored. For comparison, a conventional SPR sensor is also considered. In general, the change in the resonance curve caused by a change in the refractive index Dn can be characterized either by an angular shift of the curve Dqres (sensing by angular modulation) or a change in the reflectance DR at a fixed angle (sensing by intensity modulation). According to previous papers on the performance of SPR sensors [37], the sensitivity by intensity is expressed by
where I represents the reflection intensity, DI is the intensity change caused by a small index change Dn. In order to get higher FOM value, the reflection spectra of the system should be achieved with high modulation depth and sharp line width simultaneously.
SI ðqÞ ¼ lim
Dn/0
DRðqÞ vRðqÞ ¼ vn Dn
q
(5)
The performance of the SPR sensor is affected by the resonance line width, typically represented by the full width at half maximum (FWHM). More specifically, in the experiment, the signal-to-noiseratio of the measurements is closely linked with the FWHM of the resonance mode: the narrower the line width, the smaller the signal-to-noise-ratio. Although bulk sensitivity is an important parameter which is usually considered in the design of a sensor, it is not the only factor. It is not appropriate that bulk sensitivity alone is considered as the evaluation criterion of the sensor. This can be avoided when one increases the bulk sensitivity by broadening the reflection spectrum (increasing the intrinsic loss) as an only criterion for the performance of structure sensor. So, to compare the sensitivities of different types of sensors it is necessary to use the
DIðqÞ ; I Dn
(6)
3. Results and discussion Optimization of the thicknesses of coupling and guiding layers is the crucial point in deciding the performance of the SPR sensor, which is done in the following steps. First, tc and tg are optimized for monolayer of graphene and MoS2. Then the refractive index of the guiding layer (ng) is optimized. The guiding layer can support planar waveguide (PWG) modes provided that ng is larger than those of the coupling and MoS2 layer. During the optimizing process, the shift in the resonance angle is observed from the reflectance curve, the variation of the sensitivity, FWHM and minimum reflectance (Rmin) are analyzed with respect to the change in sensing layer refractive index. Finally, the distributions of TM field with respect to distance normal to the layer interface are calculated and analyzed under different conditions. 3.1. Optimization of the thicknesses of coupling and guiding layers Influence of the thickness of coupling layer (tc) on the structure's reflectance under TM polarization is investigated and plotted in Fig. 2 (a), which illustrates the map of reflection spectra when tc is tuned from 0 to 1 mm when tg is fixed at 0.17 mm. Obviously, the linewidth of the resonance decreases with the increase of tc, where the corresponding Q-factor can be calculated as Q ¼ q∕Dq ¼ 46.00872/0.0018 ¼ 25560 (q is a resonant angle of the FR and Dq is defined as the difference of the angle at the reflection peak and dip) when tc ¼ 0.72 mm. Fig. 2 (b), 2 (c), and 2 (d) demonstrate the amplitude distributions of the Hy field corresponding to the three points denoted as “A,” “B,” and “C” in Fig. 2 (a), respectively. It is clear that for all three peaks, strong magnetic fields are concentrated within a certain region, with
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Fig. 2. (a) Contour plots of the reflection as a function of incident angle and thickness of coupling layer tc. Here, nc ¼ 1.46, ng ¼ 2, and tg ¼ 0.17 mm (b)e(d) Distributions of the magnetic field jHyj at the three points as shown in Fig. 2(a).
prominent differences in terms of the distribution profiles. As dip “A” corresponds to an angle of 44.3 deg, Fig. 2 (b) shows the Ag layer increases the field and shows a peak at the Ag-coupling layer interface and decays exponentially away from the interface, clearly indicating the excitation of the SPP mode. The magnetic field enhancement factor can be defined as the ratio of the square of the magnetic field amplitude to that of the incident light. The field enhancement factor at the Ag-coupling layer interface is 84. It can be easily seen from Fig. 2 (c) that the amplitude of the magnetic field is strong within the guiding layer and there is one field node (with the amplitude approaching a minimum value) throughout the whole device. The coupling between the SPP and WG modes is expected to take place with a strong magnetic field distribution in the guiding region (corresponding to point “B”). It is widely accepted that coupling between a narrow and a broad resonance leads to so-called Fano resonance (FR), characterized by an asymmetric line shape [37]. FR supported by nanostructures has attracted much interest and has been the subject of intensive theoretical and experimental studies [37e39]. It should be noted that the field enhancement factor at the guiding layer and Graphene-MoS2 interfaces is as high as 3481. Fig. 2 (d) shows that large magnetic field enhancements are generated at the Ag-coupling layer surface as well as the guiding layer (corresponding to point “C”). However, coupling between SPP and WGM has not been fully achieved, thus the field enhancement factor is smaller than 82. The giant field enhancement and light localization on the surface of the resonant elements of the Kretschmann configuration make it possible to realize strong interactions between light and the surrounding medium. In further, the effect of the thickness of guiding layer on the resonant characteristics of the structure is studied. Fig. 3 displays the reflection spectra with tg ranging from 0.11 to 0.21 mm in steps of 0.02 mm. To obtain the curves the thickness of the coupling layer was fixed at tc ¼ 0.72 mm. Sharp asymmetric resonances can appear at both the low- and high-angle side of the SPP ATR dip. In general, interaction between different electromagnetic modes is very difficult. However, the coupling between SPP and WGM becomes increasingly significant, if we increase the guiding layer thickness gradually. Under moderate tg, the SPP can couple effectively with the WGM, which is thought to be caused by the
overlap of the evanescent fields associated with these modes. It is widely accepted that coupling between a narrow and a broad resonance leads to so-called FR, characterized by an asymmetric line shape [40e43]. The appearance of the FR in Fig. 3 can be attributed to the interference between a broad SPP mode with a
Fig. 3. Reflection spectra with tg ranging from 0.11 to 0.21 mm (top to down). The red dash-dot lines indicate the position of SPP and FR. The other parameters are chosen as tc ¼ 0.72 mm, nc ¼ 1.46, ng ¼ 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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sharp WG mode located close to the SPP mode. Apparently, the reflection dip caused by the SPP and WG mode coupling moves to larger angle while the SPP resonance angle is kept nearly constant, as tg increases. Meanwhile, we note that the FWHM of resonant dip gradually increases as tg increases from 0.11 to 0.21 mm. The coupling between the SPP and WG mode is mediated by the overlap of their evanescent fields. Therefore, the coupling strength can be controlled by tg. 3.2. Optimization of the refractive index of guiding layer Since the FR in our multilayer structures arises from the coupling between the SPP and PWG modes, a change in the propagation constant of the PWG mode is thought to directly induce a shift of the FR. Therefore, the effect of guiding layer refractive index (ng) on the optical performance is also calculated. The added guiding layer is essential to obtain higher FOM. As shown in Fig. 4, the various refractive indices of the guiding layer have a distinct effect on the reflectance spectrum of the structure. Keeping all other parameters the same, one can see that higher ng value brings higher sensitivity with intensity. The FWHM and resonant angle of the FR spectra with different ng are also evaluated and shown in Fig. 4 (b). For ng ¼ 1.9, the TM PWG mode is tuned to the SPP mode and a sharp peak appears at the middle of the SPP dip (Fig. 4 (a)), the FR resonant angle locates at 44.412 , and the corresponding FWHM is 0.012 mm. The appearance of a sharp reflection peak in the SPP ATR dip implies an appearance of a sharp line of absorption suppression in a broad absorption band, which is the characteristic feature of plasmon induced transparency (PIT).
Fig. 5. Reflection spectra with nc ranging from 1.34 to 1.44 (top to down). The red dash-dot line indicates the position of FR. The other parameters are chosen as tc ¼ 0.72 mm, ng ¼ 2 and tg ¼ 0.17 mm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.3. Hybridization between different modes When two resonances occur where the k-vectors are matched, they will avoid crossing each other and form a hybridized mode [21]. As schematically shown in Fig. 1 (c), a metal-dielectric interface can support SPP modes, the guiding layer can support PWG modes. The Kretschmann configuration allows coupling between the SPP and PWG modes and makes it possible to observe the mode hybridization. In fact, when the SPP mode propagating along the interface between the metal and the coupling layer is excited by light incident on the prism, an evanescent EM field decaying exponentially away from the interface is generated. Then the tail of the SPP evanescent field can excite the PWG mode which is also accompanied by the evanescent fields in the coupling layer and graphene-MoS2 heterostructure. If the overlap between the evanescent fields of the SPP and PWG modes in the coupling layer is not negligible, the modes are influenced by each other and
consequently, coupling of the modes occurs (near-field coupling). The strength of the coupling is controlled by the degree of the overlap between the two evanescent fields. To understand the underlying physics of the resonance dips, we also investigate the dependence of the reflection spectrum on wavelength of incident light. The influence of RI of coupling layer (nc) on FR is illustrated in Fig. 5. It shows that the wavelength position of resonant dips can be tuned by varying nc, and FR dips have evident red shifts as nc increases. Moreover, the FWHM of resonant dip gradually increases with the increasing nc. The FWHM of FR dips for the RI of coupling layer, nc, of 1.34, 1.38, and 1.42 are 1 105 mm, 2 105 mm, and 1.5 104 mm, respectively. According to previous research [29], the Fano-like resonance is built from the interference between a continuum of radiative resonance (bright mode) and a non-radiative resonance (dark mode) and spatially overlaps. That is, this Fano-like resonance in the designed
Fig. 4. (a) Reflection for TM wave versus incident angle and ng. Here, nc ¼ 1.46, and tg ¼ 0.17 mm, tc ¼ 0.45 mm. (b) Resonant angle and the corresponding FWHM as a function of ng. The FWHM is plotted in logarithmic form.
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structure is the result of interference between a broad continuum resonance mode, which refers to SPP resonance modes, and a dark resonance mode, which provided by the guiding layer. Coupling between SPP and WG modes can be achieved when the optical constants of the materials and the thicknesses of the layers are properly chosen. In order to explain clearly the above Fano-like resonance, we fit the reflection spectra to the analytical model in Eq. (16) [44]:
2 þq þb a2 R¼ 2 2 2 2 2 2 u ua u us þ1 þ1 2Wa ua þ q 2Ws us þ q u2 u2a 2Wa ua
(7)
where u is the angular frequency of the incident light; ua and us are the resonant central frequencies of the Fano-like resonance and the superimposed pseudo-Lorentz resonance, respectively; Wa and Ws give an approximation of the spectral widths of both resonances in frequency units; q indicates the asymmetry parameter; b means the modulation damping parameter originating from intrinsic losses; and a manifests the maximum amplitude of the resonance. This analytical model allows us to distinguish different resonances in the spectra and extract respective resonant frequencies. In the realm of plasmonics, both symmetrical and asymmetrical structures are shown to exhibit pronounced Fano line shapes. The q value can be influenced by several factors such as geometrical configuration, refractive index of materials, temperature, and so on [45]. Fortunately, the reflection of dip to retrieve the underlying electromagnetic structure can be fit by using Eq. (7), and the involved results are expressed in Fig. 6. The involved parameters a, us, and Ws are used to reconstruct broad bright mode, while ua, Wa, q, and b are used to reconstruct narrow dark mode. They are chosen as a ¼ 0.958, us ¼ 1.7004 ev, ua ¼ 1.7040 ev, q ¼ 6.558, Ws ¼ 663.1 ev, Wa ¼ 388.7147 ev, q ¼ 1.02, b ¼ 0.0679, respectively.
of the reflected light at a fixed angle of incidence. In Fig. 7, sensors based on PWG, SPR, and PWG-coupled SPR are compared, which manifests that the FWHM for the PWG-coupled SPR sensor is narrower than those of PWG and SPR, implying that higher sensitivity can be expected. Fig. 7 (a) shows ATR spectra obtained for tc ¼ 0.72 mm assuming the TE-polarized incident light. The presence of the coupling layer with finite thickness modifies the TE WG mode, and the resonance dip is shifted to higher angles and becomes narrower as tc increases. The RI of DI water was increased by Dn ¼ 1.0 102. Fig. 7 (b) and (c) show the changes in the ATR spectra for the present Fano sensor and a conventional SPR, respectively. As for the conventional SPR sensor, a single layer of 34 nm-thick Ag layer deposited on top of the 2S2G prism is assumed and the RI of DI water is increased by Dn ¼ 1.0 102. As for the proposed Fano sensor, the structure with tc ¼ 0.72 mm, tg ¼ 0.21 mm and the RI was increased by Dn ¼ 1.0 105. The difference DR of the three curves, before and after the shift, are plotted. As can be seen in Fig. 7 (c), the SPR curve in the conventional SPR sensor is broad and has a small slope. Therefore, Dn as large as 1.0 102 is required to produce a change in the reflectance of DRmax ¼ 0.2226. Comparing to Fig. 7 (b), the FR curve is extremely narrow and steep, while Dn as small as 1.0 105 is sufficient to produce the change DRmax ¼ 0.4477. The ratio DRmax/Dn of the conventional SPR sensor and the Fano sensor are respectively 22.26 RIU-1 and 4.477 104 RIU-1, which suggests that the proposed Fano sensor offers an extremely high sensitivity by intensity relative to that of the conventional SPR sensor. Besides, the PWG-based sensor shows better performance than SPR, which can be seen from Fig. 7 (a) and (c). This can be attributed to it can support guided waves for the small thickness, when the dielectric layer (guiding layer) has a large RI. Although its index is much larger than the analyst index (DI water), being thin enough causes a greater fraction of the evanescent wave to be in the analyst. As the reflectance curve becomes sharper the FWHM will be
4. Performance of refractive-index sensor Since the Fano-resonant nanostructures offer the opportunities for achieving high-Q resonances that induce highly enhanced EM fields in the vicinities of the nanostructures, they have potentials for achieving high performances optical sensors. The SPP-PWG hybrid structure can be used as a sensor in exactly the same way as the conventional SPR sensor. In the case of bulk sensing, a medium such as water is placed on top of the WG layer and the change in the RI of the sensing medium is detected by monitoring either the shift of the ATR resonance curve or the change in the intensity
Fig. 6. Comparison of reflection spectra obtained by simulation results and analytical model.
Fig. 7. (a) The resonances in ATR angular spectra arising from the excitation of a TE WM for the structure. (b) Shift of the Fano lineshape for the structure with tc ¼ 0.72 mm, tg ¼ 0.21 mm, caused by an increase of the RI of DI water byDn ¼ 1.0 105. (c) Change in the ATR dip in a conventional SPR sensor consisting of a 34 nm-thick Ag film deposited onto a 2S2G prism, caused by an increase of the RI of DI water by 1.0 102.
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small and because of which detection accuracy will increase i.e. a slight change in the refractive index can be detected by the sensor. Monolayer graphene can act as a bio-recognition component to selectively detect targeting biomolecules through pi-stacking force. Since monolayer MoS2 has a higher light absorption rate than that of graphene, a stronger SPR excitation can be achieved by using the graphene-MoS2 hybrid structure as the sensing substrate [19]. 5. Conclusion A new configuration of SPR sensor that is based on grapheneMoS2 hybrid structures is proposed, and the resonance properties of the sensor in DI water environment for both TE and TM polarizations are studied. In the structure based on a guiding layer separated from an absorptive reflective film by a low-index coupling layer, the resonance excitations of the WG mode, SPP and FR have been considered. Sharp Fano line shapes appear in the ATR spectra when the structural parameters are appropriately chosen, which is identified by the coupling between the SPP and PWG modes. The width and height of resonance line shapes have been extracted from reflectivity spectra for a continuous interval of coupling layer thickness and different values of WG refractive index. The change in the refractive index induces the change in the propagation constant of the PWG mode, in further leading to the shift of the FR in the ATR spectra. Analyses of the shift based on the EM calculations allowed us to estimate the change in the refractive index. Based on our analysis, the possibility of achievement the extremely narrow deep resonances resulting in the giant sensitivity by intensity enhancement is discovered. The giant enhancement in the sensitivity by intensity of ~2 103-fold is higher than that of conventional SPR. In principle, our results may open a new avenue for realizing sensors with extremely high sensitivities, and enhancing spectroscopies with extremely high factors by riding on the advantages of two-dimensional materials like graphene and latest nanofabrication techniques. Acknowledgements This work is partially supported by the National Natural Science Foundation of China (Grant Nos. 61203211, 41675133, 41675154), the Six Major Talent Peak expert of Jiangsu province (2015-XXRJ014), the Natural Science Foundation of the Jiangsu Province (Grant No. BK20141483). References [1] J. Homola, S.S. Yee, G. Gauglitz, Surface plasmon resonance sensors: review, Sens. Actuators B Chem. 54 (1999) 3e15. [2] S.K. Srivastava, B.D. Gupta, Influence of ions on the surface plasmon resonance spectrum of a fiber optic refractive index sensor, Sens. Actuators B Chem. 156 (2) (2011) 559e562. [3] J. Homola, Present and future of surface plasmon resonance biosensors, Anal. Bioanal. Chem. 377 (2003) 528e539. [4] M. Kumar, A. Kumar, S.M. Tripathi, Optical waveguide biosensor based on modal interference between surface plasmon modes, Sens. Actuators B Chem. 211 (2015) 456e461. [5] A. Lahav, M. Auslender, I. Abdulhalim, Sensitivity enhancement of the guided wave surface-plasmon resonance sensors, Opt. Lett. 33 (21) (2008) 2539e2541. [6] F. Benkabou, M. Chikhi, Theoretical investigation of sensitivity enhancement in dielectric multilayer surface plasmon sensor, Phys. Status Solidi A 211 (3) (2014) 700e704. [7] S.A. Maier, Plasmonics: Fundamentals and Applications, Springer Science & Business Media, 2007. [8] R. Jha, A.K. Sharma, High-performance sensor based on surface plasmon resonance with chalcogenide prism and aluminum for detection in infrared, Opt. Lett. 34 (6) (2009) 749e751. [9] T. Srivastava, R. Jha, R. Das, High-performance bimetallic SPR sensor based on periodic-multilayer-waveguides, IEEE Phot. Technol. Lett. 23 (20) (2011) 1448e1450.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fano resonance in graphene-MoS2 heterostructure-based surface plasmon resonance biosensor and its potential applications Gaige Zheng a, b, *, Xiujuan Zou a, Yunyun Chen a, b, Linhua Xu a, Weifeng Rao a, b a
School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, China Jiangsu Collaborative Innovation Center on Atmospheric Environment and Equipment Technology (CICAEET), Nanjing University of Information Science & Technology, Nanjing, 210044, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 November 2016 Received in revised form 14 January 2017 Accepted 3 February 2017
We propose a new configuration of surface plasmon resonance (SPR) sensor that is based on grapheneMoS2 hybrid structures for ultrasensitive detection of molecules. The present configuration is consisted of chalcogenide glass (2S2G) prism, Ag, coupling layer, guiding layer, graphene-MoS2 heterostructure and analyte. We perform numerical and analytical study of the impact of the thickness and refractive index (RI) of the coupling and guiding layer in a planar sensing structure within the Kretschmann configuration on the resonance properties of the excitation. Results of reflectivity calculations clearly demonstrate the sharp Fano-type resonance appears in the curve of SPR because of the coupling between surface plasmon polariton (SPP) and planar waveguide (PWG) modes. The properties of the Fano resonance (FR) strongly depend on the parameters of the structure. The calculated magnetic field profiles manifest that the hybrid nature of the electromagnetic (EM) modes excited in the present structure. The proposed system displays an enhancement factor of sensitivity by intensity more than 2 103-fold when compared to the SPR sensing scheme. © 2017 Elsevier B.V. All rights reserved.
Keywords: Surface plasmon resonance Optical sensor Graphene MoS2 Attenuated total reflection
1. Introduction Surface plasmon-based sensors have enabled bio-molecular detection with high speed and sensitivity, which is mainly because they eliminate time-consuming labeling process and reduce molecular binding disturbance compared to common fluorescent optical sensors [1e3]. Surface plasmon resonance (SPR) sensors generally utilize the resonance excitation of surface plasmon polariton (SPP) at the metal-dielectric interface [4,5]. Among different kinds of SPR sensors, the most common one is the Kretschmann's configuration which is extensively used for chemical/biomolecular sensing with high sensitivity [6]. In such configuration, a thin metal film is deposited on a prism and the other face of the metal is kept in contact with the sensing medium. When a transverse magnetic (TM)-polarized incident light passes the prism through the metal film into a dielectric media, an evanescent wave will occur, while the energy of the incident light is absorbed and surface plasmon waves (SPWs) will be excited at the interface [7].
* Corresponding author. School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, China. E-mail address: [email protected] (G. Zheng). http://dx.doi.org/10.1016/j.optmat.2017.02.001 0925-3467/© 2017 Elsevier B.V. All rights reserved.
The SPR condition takes place at a specific angle of incidence or wavelength, when the evanescent wave couples with the SPs on the surface of the metal layer. It is necessary to mention that the nature of the reflectance curve of attenuated total reflection (ATR) system dictates the performance of the SPR sensor. As such, the width of the SPR curve determines how precisely a sensor can detect the resonance angle and the shift. For high performance sensor, the shift in the resonance angle should be large, whereas the width of the SPR curve should be very small [8e10]. Different approaches for the improvement of the SPR sensor resolution have been proposed [11e13]. For example, high index coupling prism can be used to reduce the full width at half maxima (FWHM) of SPR curve thereby increasing the detection accuracy of the sensor [13]. Another way is choosing the metal layer correctly. Gold (Au) produces large SPR resonant angle shift while its response curve is relatively wider than that of silver (Ag). Au is usually preferred because it is more resistant to oxidation and corrosion in different environments. But, the drawback is that biomolecules adsorb poorly on gold, which limits the sensitivity of the conventional SPR biosensor. Aluminum (Al) not only exhibits narrower resonance curve compared to Au and Ag but also is relatively economical. However, Al is highly susceptible to oxidation, thereby deteriorating the sensor
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performance [14]. It has been shown that the use of bimetallic configuration (Au-Ag) increases the signal-to-noise ratio and decreases the width of the resonance curve, providing a significant improvement of the resolution while the sensitivity was comparable of that of conventional SPR [15e18]. To further improve the performance of biosensor, researchers have proposed and fabricated SPR based sensor coated with graphene layers [19e25]. Graphene is a single-atom thin planar sheet of sp2 carbon atoms perfectly arranged in a honeycomb lattice. Graphene and graphene oxide provide good support for biomolecule adsorption due to their large surface area and rich p conjugation structure, making them suitable dielectric top layers for SPR sensing [23]. However, graphene produces more damping in SPs due to large imaginary dielectric constant for higher graphene layers, and hence results in decreased performance [24]. More recently, ultra-thin layer of molybdenum disulfide (MoS2) that belongs to the transition-metal dichalcogenide (TMDC) semiconductor group is known as “beyond graphene” 2D nanocrystals material and they are widely used as solid lubricants due to its low friction property. Therefore, in order to enhance the performance of conventional SPR sensor and utilize the advantageous properties of graphene, a planar structure of prism-waveguide coupling system with graphene-MoS2 heterostructure that allows for coupling between SPP and WG mode is proposed. MoS2 layers are used for improving the light absorption in order to provide enough excitation energy or effective charge transfer, while monolayer graphene is acting as iorecognition component for capturing the target biomolecules through pi-stacking force. We show that sharp Fano line shapes appear in the ATR spectra when the structural parameters are appropriately chosen. The thickness of the coupling and guiding layer is optimized first with respect to sensitivity, full width at half maximum (FWHM), and minimum reflectance at 633 nm wavelength. Thereafter the variation of performance parameters sensitivity, detection accuracy, quality factor, and minimum reflectance is shown with respect to the regular change in the RI of the sensing layer. The results of the numerical calculations clearly exhibit sharp Fano-type resonances, and demonstrate that the sensor sensitivity by intensity can be enhanced by more than 2 103 orders of magnitudes compared to that of conventional SPR sensors. 2. Design consideration and theoretical model In our proposed new structure, a well-known Kretschmann configuration with multilayer thin films is employed. Multilayers are placed according to the following structure, i.e. 2S2G prism, Ag film, coupling layer, guiding layer, MoS2, monolayer graphene and sensing medium, as shown in Fig. 1. The wavelength of the incident light for the excitation of SPs is 632.8 nm. The TM-polarized light incidents from one lateral face of the prism, then reaches to its base and totally reflected out from the other lateral face, and finally can be collected and analyzed by a photodetector. 2.1. Refractive index of various layer components The first layer is 2S2G prism, and its refractive index (np) is calculated through the following relation [26]:
np ¼ 2:24047 þ
2:693 102 2
l
þ
8:08 103
l4
(1)
where l is the wavelength of incident light in micrometers. We choose 2S2G as the coupling prism due to its high refractive index. 2S2G also has shown potential in the fabrication of ultra-low-loss waveguides [27], and it has been widely used in sensing
technology [28]. In the calculation, the dielectric function of metal is described by the Drude model as follows:
εAg ðuÞ ¼ ε∞
u2p u2 þ igu
(2)
where ε∞ is the infinite frequency dielectric constant, up is the bulk plasma frequency, u is the angular frequency, and the collision frequency g which is related to the dissipation loss in the metal. These parameters are set as 6.0, 1.5 1016 rad/s and 7.73 1013 rad/ s, respectively [29]. The complex refractive index of monolayer MoS2 at 632.8 nm is obtained from the experimental measurement data by Castellanos-Gomez et al. [30] and the thickness of MoS2 layer is 0.65 nm [31]. The sixth layer in our model is monolayer graphene (with thickness of 0.34 nm) and its complex refractive index ng in the visible range is given as [32]:
ng ¼ 3:0 þ i
C1 l 3
(3)
where the constant C1 z 5.446 mm1 [33] and l is the wavelength of incident light in mm. The sensing medium for initial calibration is deionized (DI) water and its refractive index (ns) is determined by the following relation [34]:
n2s 1 ¼
2 4 X Ai l i¼1
l2 ti2
(4)
where the Sellmeier coefficients A1 ¼ 5.666959820 101, A2 ¼ 1.731900098 101, A3 ¼ 2.095951857 102, A4 ¼ 1.125228406 101, t1 ¼ 5.084151894 103, t2 ¼ 1.818488474 102, t3 ¼ 2.625439472 102, t4 ¼ 1.073842352 101 and l is the wavelength of incident light in mm. Eq. (4) is valid for wavelengths ranging from 0.182 to 1.129 mm.
2.2. Numerical formulation of reflectivity and field distributions To systematically investigate the reflectivity change in our graphene-MoS2 hybrid structure-based SPR sensing system, we employed the transfer matrix method (TMM) and Fresnel equations based on an N-layer model to perform a detailed analysis [13,19,20]. TMM method is a powerful tool in the analysis of light propagations through layered dielectric media. The central idea lies that electric or magnetic fields in one position can be related to those in other positions through a transfer matrix. Within the framework of the TMM [35,36], there are two kinds of matrices: one is the transmission matrix and the other is the propagation matrix. They connect the fields across an interface and the fields propagating over a distance within a homogeneous medium, respectively. For a stack of N dielectric layers shown in Fig. 1 (a), the transfer matrix can be obtained by transmission and propagation matrices across different interfaces and homogeneous dielectric media, respectively. In order to obtain the optical spectra, the equations which results from the ordinary boundary conditions that the fields satisfy at each interface need to be resolved. Since the system is uniform in the y direction, we can decompose the electromagnetic fields E(m), H(m) into two components and consider the TE and TM-polarized waves separately. And, the expressions of reflection and transmission coefficients for N-layer system (layer 0 corresponds to the prism and layer N to the analyte) is restated. For numerical calculations, it is assumed a planar multilayer slab sensor in Kretschmann configuration as schematically shown in Fig. 1 (c), where the
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Fig. 1. (a) Schematic of the multilayer structure consisting of Ag film, coupling layer, guiding layer, MoS2, graphene and the surrounding sensing layers placed over a SF11 prism. The thicknesses of the coupling and guiding layer are denoted by t1 and t2, respectively. (b) Schematic diagram of the graphene-MoS2 heterostructure. (c) Schematic of the coupling mechanism through the EM multilayer system.
origin of the coordinate system is located at the substrate interface.
other criterion FOM for the sensitivity by intensity that is given by
2.3. Performance parameters
FOMI ¼ max
In biochemical experiment, there are several criteria such as sensitivity, figure of merit (FOM), resolution, detection accuracy, reproducibility, cost, and device size, which can be used to evaluate the performance of a sensor. These parameters can be calculated with the help of the curve of incident angle versus reflectivity. To make qualitative and quantitative analysis of the structure sensing performance and also to make a comparison with other designed structure, bulk sensitivity and FOM of structure are two important criterions for designing structure sensor and will be discussed as follows. These two parameters are calculated with the help of the curve of incident angle versus reflectivity. The sensitivity for bulk sensing will be examined, where the changes in the angle-scan ATR resonance curve caused by the change in the refractive index of DI water is monitored. For comparison, a conventional SPR sensor is also considered. In general, the change in the resonance curve caused by a change in the refractive index Dn can be characterized either by an angular shift of the curve Dqres (sensing by angular modulation) or a change in the reflectance DR at a fixed angle (sensing by intensity modulation). According to previous papers on the performance of SPR sensors [37], the sensitivity by intensity is expressed by
where I represents the reflection intensity, DI is the intensity change caused by a small index change Dn. In order to get higher FOM value, the reflection spectra of the system should be achieved with high modulation depth and sharp line width simultaneously.
SI ðqÞ ¼ lim
Dn/0
DRðqÞ vRðqÞ ¼ vn Dn
q
(5)
The performance of the SPR sensor is affected by the resonance line width, typically represented by the full width at half maximum (FWHM). More specifically, in the experiment, the signal-to-noiseratio of the measurements is closely linked with the FWHM of the resonance mode: the narrower the line width, the smaller the signal-to-noise-ratio. Although bulk sensitivity is an important parameter which is usually considered in the design of a sensor, it is not the only factor. It is not appropriate that bulk sensitivity alone is considered as the evaluation criterion of the sensor. This can be avoided when one increases the bulk sensitivity by broadening the reflection spectrum (increasing the intrinsic loss) as an only criterion for the performance of structure sensor. So, to compare the sensitivities of different types of sensors it is necessary to use the
DIðqÞ ; I Dn
(6)
3. Results and discussion Optimization of the thicknesses of coupling and guiding layers is the crucial point in deciding the performance of the SPR sensor, which is done in the following steps. First, tc and tg are optimized for monolayer of graphene and MoS2. Then the refractive index of the guiding layer (ng) is optimized. The guiding layer can support planar waveguide (PWG) modes provided that ng is larger than those of the coupling and MoS2 layer. During the optimizing process, the shift in the resonance angle is observed from the reflectance curve, the variation of the sensitivity, FWHM and minimum reflectance (Rmin) are analyzed with respect to the change in sensing layer refractive index. Finally, the distributions of TM field with respect to distance normal to the layer interface are calculated and analyzed under different conditions. 3.1. Optimization of the thicknesses of coupling and guiding layers Influence of the thickness of coupling layer (tc) on the structure's reflectance under TM polarization is investigated and plotted in Fig. 2 (a), which illustrates the map of reflection spectra when tc is tuned from 0 to 1 mm when tg is fixed at 0.17 mm. Obviously, the linewidth of the resonance decreases with the increase of tc, where the corresponding Q-factor can be calculated as Q ¼ q∕Dq ¼ 46.00872/0.0018 ¼ 25560 (q is a resonant angle of the FR and Dq is defined as the difference of the angle at the reflection peak and dip) when tc ¼ 0.72 mm. Fig. 2 (b), 2 (c), and 2 (d) demonstrate the amplitude distributions of the Hy field corresponding to the three points denoted as “A,” “B,” and “C” in Fig. 2 (a), respectively. It is clear that for all three peaks, strong magnetic fields are concentrated within a certain region, with
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Fig. 2. (a) Contour plots of the reflection as a function of incident angle and thickness of coupling layer tc. Here, nc ¼ 1.46, ng ¼ 2, and tg ¼ 0.17 mm (b)e(d) Distributions of the magnetic field jHyj at the three points as shown in Fig. 2(a).
prominent differences in terms of the distribution profiles. As dip “A” corresponds to an angle of 44.3 deg, Fig. 2 (b) shows the Ag layer increases the field and shows a peak at the Ag-coupling layer interface and decays exponentially away from the interface, clearly indicating the excitation of the SPP mode. The magnetic field enhancement factor can be defined as the ratio of the square of the magnetic field amplitude to that of the incident light. The field enhancement factor at the Ag-coupling layer interface is 84. It can be easily seen from Fig. 2 (c) that the amplitude of the magnetic field is strong within the guiding layer and there is one field node (with the amplitude approaching a minimum value) throughout the whole device. The coupling between the SPP and WG modes is expected to take place with a strong magnetic field distribution in the guiding region (corresponding to point “B”). It is widely accepted that coupling between a narrow and a broad resonance leads to so-called Fano resonance (FR), characterized by an asymmetric line shape [37]. FR supported by nanostructures has attracted much interest and has been the subject of intensive theoretical and experimental studies [37e39]. It should be noted that the field enhancement factor at the guiding layer and Graphene-MoS2 interfaces is as high as 3481. Fig. 2 (d) shows that large magnetic field enhancements are generated at the Ag-coupling layer surface as well as the guiding layer (corresponding to point “C”). However, coupling between SPP and WGM has not been fully achieved, thus the field enhancement factor is smaller than 82. The giant field enhancement and light localization on the surface of the resonant elements of the Kretschmann configuration make it possible to realize strong interactions between light and the surrounding medium. In further, the effect of the thickness of guiding layer on the resonant characteristics of the structure is studied. Fig. 3 displays the reflection spectra with tg ranging from 0.11 to 0.21 mm in steps of 0.02 mm. To obtain the curves the thickness of the coupling layer was fixed at tc ¼ 0.72 mm. Sharp asymmetric resonances can appear at both the low- and high-angle side of the SPP ATR dip. In general, interaction between different electromagnetic modes is very difficult. However, the coupling between SPP and WGM becomes increasingly significant, if we increase the guiding layer thickness gradually. Under moderate tg, the SPP can couple effectively with the WGM, which is thought to be caused by the
overlap of the evanescent fields associated with these modes. It is widely accepted that coupling between a narrow and a broad resonance leads to so-called FR, characterized by an asymmetric line shape [40e43]. The appearance of the FR in Fig. 3 can be attributed to the interference between a broad SPP mode with a
Fig. 3. Reflection spectra with tg ranging from 0.11 to 0.21 mm (top to down). The red dash-dot lines indicate the position of SPP and FR. The other parameters are chosen as tc ¼ 0.72 mm, nc ¼ 1.46, ng ¼ 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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sharp WG mode located close to the SPP mode. Apparently, the reflection dip caused by the SPP and WG mode coupling moves to larger angle while the SPP resonance angle is kept nearly constant, as tg increases. Meanwhile, we note that the FWHM of resonant dip gradually increases as tg increases from 0.11 to 0.21 mm. The coupling between the SPP and WG mode is mediated by the overlap of their evanescent fields. Therefore, the coupling strength can be controlled by tg. 3.2. Optimization of the refractive index of guiding layer Since the FR in our multilayer structures arises from the coupling between the SPP and PWG modes, a change in the propagation constant of the PWG mode is thought to directly induce a shift of the FR. Therefore, the effect of guiding layer refractive index (ng) on the optical performance is also calculated. The added guiding layer is essential to obtain higher FOM. As shown in Fig. 4, the various refractive indices of the guiding layer have a distinct effect on the reflectance spectrum of the structure. Keeping all other parameters the same, one can see that higher ng value brings higher sensitivity with intensity. The FWHM and resonant angle of the FR spectra with different ng are also evaluated and shown in Fig. 4 (b). For ng ¼ 1.9, the TM PWG mode is tuned to the SPP mode and a sharp peak appears at the middle of the SPP dip (Fig. 4 (a)), the FR resonant angle locates at 44.412 , and the corresponding FWHM is 0.012 mm. The appearance of a sharp reflection peak in the SPP ATR dip implies an appearance of a sharp line of absorption suppression in a broad absorption band, which is the characteristic feature of plasmon induced transparency (PIT).
Fig. 5. Reflection spectra with nc ranging from 1.34 to 1.44 (top to down). The red dash-dot line indicates the position of FR. The other parameters are chosen as tc ¼ 0.72 mm, ng ¼ 2 and tg ¼ 0.17 mm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.3. Hybridization between different modes When two resonances occur where the k-vectors are matched, they will avoid crossing each other and form a hybridized mode [21]. As schematically shown in Fig. 1 (c), a metal-dielectric interface can support SPP modes, the guiding layer can support PWG modes. The Kretschmann configuration allows coupling between the SPP and PWG modes and makes it possible to observe the mode hybridization. In fact, when the SPP mode propagating along the interface between the metal and the coupling layer is excited by light incident on the prism, an evanescent EM field decaying exponentially away from the interface is generated. Then the tail of the SPP evanescent field can excite the PWG mode which is also accompanied by the evanescent fields in the coupling layer and graphene-MoS2 heterostructure. If the overlap between the evanescent fields of the SPP and PWG modes in the coupling layer is not negligible, the modes are influenced by each other and
consequently, coupling of the modes occurs (near-field coupling). The strength of the coupling is controlled by the degree of the overlap between the two evanescent fields. To understand the underlying physics of the resonance dips, we also investigate the dependence of the reflection spectrum on wavelength of incident light. The influence of RI of coupling layer (nc) on FR is illustrated in Fig. 5. It shows that the wavelength position of resonant dips can be tuned by varying nc, and FR dips have evident red shifts as nc increases. Moreover, the FWHM of resonant dip gradually increases with the increasing nc. The FWHM of FR dips for the RI of coupling layer, nc, of 1.34, 1.38, and 1.42 are 1 105 mm, 2 105 mm, and 1.5 104 mm, respectively. According to previous research [29], the Fano-like resonance is built from the interference between a continuum of radiative resonance (bright mode) and a non-radiative resonance (dark mode) and spatially overlaps. That is, this Fano-like resonance in the designed
Fig. 4. (a) Reflection for TM wave versus incident angle and ng. Here, nc ¼ 1.46, and tg ¼ 0.17 mm, tc ¼ 0.45 mm. (b) Resonant angle and the corresponding FWHM as a function of ng. The FWHM is plotted in logarithmic form.
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structure is the result of interference between a broad continuum resonance mode, which refers to SPP resonance modes, and a dark resonance mode, which provided by the guiding layer. Coupling between SPP and WG modes can be achieved when the optical constants of the materials and the thicknesses of the layers are properly chosen. In order to explain clearly the above Fano-like resonance, we fit the reflection spectra to the analytical model in Eq. (16) [44]:
2 þq þb a2 R¼ 2 2 2 2 2 2 u ua u us þ1 þ1 2Wa ua þ q 2Ws us þ q u2 u2a 2Wa ua
(7)
where u is the angular frequency of the incident light; ua and us are the resonant central frequencies of the Fano-like resonance and the superimposed pseudo-Lorentz resonance, respectively; Wa and Ws give an approximation of the spectral widths of both resonances in frequency units; q indicates the asymmetry parameter; b means the modulation damping parameter originating from intrinsic losses; and a manifests the maximum amplitude of the resonance. This analytical model allows us to distinguish different resonances in the spectra and extract respective resonant frequencies. In the realm of plasmonics, both symmetrical and asymmetrical structures are shown to exhibit pronounced Fano line shapes. The q value can be influenced by several factors such as geometrical configuration, refractive index of materials, temperature, and so on [45]. Fortunately, the reflection of dip to retrieve the underlying electromagnetic structure can be fit by using Eq. (7), and the involved results are expressed in Fig. 6. The involved parameters a, us, and Ws are used to reconstruct broad bright mode, while ua, Wa, q, and b are used to reconstruct narrow dark mode. They are chosen as a ¼ 0.958, us ¼ 1.7004 ev, ua ¼ 1.7040 ev, q ¼ 6.558, Ws ¼ 663.1 ev, Wa ¼ 388.7147 ev, q ¼ 1.02, b ¼ 0.0679, respectively.
of the reflected light at a fixed angle of incidence. In Fig. 7, sensors based on PWG, SPR, and PWG-coupled SPR are compared, which manifests that the FWHM for the PWG-coupled SPR sensor is narrower than those of PWG and SPR, implying that higher sensitivity can be expected. Fig. 7 (a) shows ATR spectra obtained for tc ¼ 0.72 mm assuming the TE-polarized incident light. The presence of the coupling layer with finite thickness modifies the TE WG mode, and the resonance dip is shifted to higher angles and becomes narrower as tc increases. The RI of DI water was increased by Dn ¼ 1.0 102. Fig. 7 (b) and (c) show the changes in the ATR spectra for the present Fano sensor and a conventional SPR, respectively. As for the conventional SPR sensor, a single layer of 34 nm-thick Ag layer deposited on top of the 2S2G prism is assumed and the RI of DI water is increased by Dn ¼ 1.0 102. As for the proposed Fano sensor, the structure with tc ¼ 0.72 mm, tg ¼ 0.21 mm and the RI was increased by Dn ¼ 1.0 105. The difference DR of the three curves, before and after the shift, are plotted. As can be seen in Fig. 7 (c), the SPR curve in the conventional SPR sensor is broad and has a small slope. Therefore, Dn as large as 1.0 102 is required to produce a change in the reflectance of DRmax ¼ 0.2226. Comparing to Fig. 7 (b), the FR curve is extremely narrow and steep, while Dn as small as 1.0 105 is sufficient to produce the change DRmax ¼ 0.4477. The ratio DRmax/Dn of the conventional SPR sensor and the Fano sensor are respectively 22.26 RIU-1 and 4.477 104 RIU-1, which suggests that the proposed Fano sensor offers an extremely high sensitivity by intensity relative to that of the conventional SPR sensor. Besides, the PWG-based sensor shows better performance than SPR, which can be seen from Fig. 7 (a) and (c). This can be attributed to it can support guided waves for the small thickness, when the dielectric layer (guiding layer) has a large RI. Although its index is much larger than the analyst index (DI water), being thin enough causes a greater fraction of the evanescent wave to be in the analyst. As the reflectance curve becomes sharper the FWHM will be
4. Performance of refractive-index sensor Since the Fano-resonant nanostructures offer the opportunities for achieving high-Q resonances that induce highly enhanced EM fields in the vicinities of the nanostructures, they have potentials for achieving high performances optical sensors. The SPP-PWG hybrid structure can be used as a sensor in exactly the same way as the conventional SPR sensor. In the case of bulk sensing, a medium such as water is placed on top of the WG layer and the change in the RI of the sensing medium is detected by monitoring either the shift of the ATR resonance curve or the change in the intensity
Fig. 6. Comparison of reflection spectra obtained by simulation results and analytical model.
Fig. 7. (a) The resonances in ATR angular spectra arising from the excitation of a TE WM for the structure. (b) Shift of the Fano lineshape for the structure with tc ¼ 0.72 mm, tg ¼ 0.21 mm, caused by an increase of the RI of DI water byDn ¼ 1.0 105. (c) Change in the ATR dip in a conventional SPR sensor consisting of a 34 nm-thick Ag film deposited onto a 2S2G prism, caused by an increase of the RI of DI water by 1.0 102.
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small and because of which detection accuracy will increase i.e. a slight change in the refractive index can be detected by the sensor. Monolayer graphene can act as a bio-recognition component to selectively detect targeting biomolecules through pi-stacking force. Since monolayer MoS2 has a higher light absorption rate than that of graphene, a stronger SPR excitation can be achieved by using the graphene-MoS2 hybrid structure as the sensing substrate [19]. 5. Conclusion A new configuration of SPR sensor that is based on grapheneMoS2 hybrid structures is proposed, and the resonance properties of the sensor in DI water environment for both TE and TM polarizations are studied. In the structure based on a guiding layer separated from an absorptive reflective film by a low-index coupling layer, the resonance excitations of the WG mode, SPP and FR have been considered. Sharp Fano line shapes appear in the ATR spectra when the structural parameters are appropriately chosen, which is identified by the coupling between the SPP and PWG modes. The width and height of resonance line shapes have been extracted from reflectivity spectra for a continuous interval of coupling layer thickness and different values of WG refractive index. The change in the refractive index induces the change in the propagation constant of the PWG mode, in further leading to the shift of the FR in the ATR spectra. Analyses of the shift based on the EM calculations allowed us to estimate the change in the refractive index. Based on our analysis, the possibility of achievement the extremely narrow deep resonances resulting in the giant sensitivity by intensity enhancement is discovered. The giant enhancement in the sensitivity by intensity of ~2 103-fold is higher than that of conventional SPR. In principle, our results may open a new avenue for realizing sensors with extremely high sensitivities, and enhancing spectroscopies with extremely high factors by riding on the advantages of two-dimensional materials like graphene and latest nanofabrication techniques. Acknowledgements This work is partially supported by the National Natural Science Foundation of China (Grant Nos. 61203211, 41675133, 41675154), the Six Major Talent Peak expert of Jiangsu province (2015-XXRJ014), the Natural Science Foundation of the Jiangsu Province (Grant No. BK20141483). References [1] J. Homola, S.S. Yee, G. Gauglitz, Surface plasmon resonance sensors: review, Sens. Actuators B Chem. 54 (1999) 3e15. [2] S.K. Srivastava, B.D. Gupta, Influence of ions on the surface plasmon resonance spectrum of a fiber optic refractive index sensor, Sens. Actuators B Chem. 156 (2) (2011) 559e562. [3] J. Homola, Present and future of surface plasmon resonance biosensors, Anal. Bioanal. Chem. 377 (2003) 528e539. [4] M. Kumar, A. Kumar, S.M. Tripathi, Optical waveguide biosensor based on modal interference between surface plasmon modes, Sens. Actuators B Chem. 211 (2015) 456e461. [5] A. Lahav, M. Auslender, I. Abdulhalim, Sensitivity enhancement of the guided wave surface-plasmon resonance sensors, Opt. Lett. 33 (21) (2008) 2539e2541. [6] F. Benkabou, M. Chikhi, Theoretical investigation of sensitivity enhancement in dielectric multilayer surface plasmon sensor, Phys. Status Solidi A 211 (3) (2014) 700e704. [7] S.A. Maier, Plasmonics: Fundamentals and Applications, Springer Science & Business Media, 2007. [8] R. Jha, A.K. Sharma, High-performance sensor based on surface plasmon resonance with chalcogenide prism and aluminum for detection in infrared, Opt. Lett. 34 (6) (2009) 749e751. [9] T. Srivastava, R. Jha, R. Das, High-performance bimetallic SPR sensor based on periodic-multilayer-waveguides, IEEE Phot. Technol. Lett. 23 (20) (2011) 1448e1450.
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