- Email: [email protected]
High performance refractive index sensor with stacked two-layer resonant waveguide gratings
Results in Physics 12 (2019) 759–765
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
High performance refractive index sensor with stacked two-layer resonant waveguide gratings
T
⁎
X. Lua, G.G. Zhenga,b, , P. Zhoua a Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China b Jiangsu Collaborative Innovation Center on Atmospheric Environment and Equipment Technology (CICAEET), Nanjing University of Information Science & Technology, Nanjing 210044, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Guided-mode resonance Refractive index sensor Resonant grating
A refractive index (RI) sensor which contains a dielectric cavity composed of two parallel planar waveguide gratings (WGGs) is proposed. Reflection and transmittance through stacked gratings are studied after converting the diffraction problem into the two coupled resonances. The method of analysis is applied to estimate the performance when used as a biosensor. The sensor achieves its high-sensitive detection by tracking a narrow resonant reflection peak via a tunable wavelength light source, and the desired RI can be determined from the resonant wavelength shift. It will be shown that, in comparison with conventional single grating guided mode resonance (GMR) sensor with equal available area, extremely narrow resonance linewidth can be obtained by using stacked gratings. The sensitivity of the sensor can reach to 497.83 nm/RIU, and the value of the figure of merit (FOM) can reach up to 551. Because of its advantages such as high-sensitivity, label-free, small size and non-destructive, the sensor has an extremely broad application prospect in the field of biomedicine detection.
Introduction Optical biosensors that can detect analyte via the optical signals produced by the micro specificity reaction among biological molecules have become increasingly important as an effective detection and analysis tool for chemical, biological and medical applications [1,2]. It is not only competent for the biological test, but also for tasks such as unmarked immune detection and quantitative analysis [3,4]. By measuring the Refractive index (RI) of material, the optical properties, dispersion, concentration and other physical quantities can be acquired [5–7]. In the biological detection field, the samples are often made into solutions to obtain their various properties and parameters by detecting the small changes of the RI of the solutions. Based on this method, to meet the needs of low cost, high sensitivity, operability and label-free, various optical technologies have been proposed. These technologies include surface plasmon resonance (SPR) biosensors, ring resonator biosensors, micro-fiber sensor, GMR biosensors, etc [8–14]. In SPR sensors, a surface plasmon is excited at the interface between a metal film and a dielectric medium by incident light wave. The SPR phenomenon occured under phase-matching conditions [15]. Differing from SPR in concept and function, GMR is another resonance
phenomenon producing very sharp variations in the amplitudes of the electromagnetic fields. In recent years, the resonant waveguide grating (WGG)-based optical sensor has attracted great interest [16–21]. It exhibits a great application prospect in the bio-detection field. By varying the thickness or RI of a resonant WGG, its resonance frequency can be changed or tuned. This idea being applied to biosensors as the buildup of the attaching biolayer can be monitored in real time, without using chemical fluorescent tags, by following the corresponding resonance wavelength shift with a spectrometer. In this work, we demonstrate an RI sensor based on the effect of GMR in stacked resonant gratings. When the chamber of the sensor is flowing through different analyte liquids, the resonant reflection spectrum transforms depending on the RI of the liquid. Thus, the RI of the analyte liquid can be measured from the resonant wavelength. An important property of coupled GMR modes is that high energy density of resonant modes is localized within the chamber, which can yield high sensitivity to the changes of RI. The innovation point of this optical sensor is that the analyte liquid is not directly in contact with the laser and avoid reaction. That is, this sensor achieves non-destructive analysis [22,23]. In comparison with conventional single grating GMR sensor with an equal available area, extremely narrow resonance
⁎ Corresponding author at: Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China. E-mail address: [email protected] (G.G. Zheng).
https://doi.org/10.1016/j.rinp.2018.12.048 Received 15 November 2018; Received in revised form 10 December 2018; Accepted 10 December 2018 Available online 13 December 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 12 (2019) 759–765
X. Lu et al.
Fig. 1. (a) Schematic design structure of the resonance grating sensor. (b) The schematic of the single WGG structure. (c) The schematic reflectance spectrum of the single grating sensor of different RI.
the propagating diffraction orders are coupled. The small gap between WGGs causes that the evanescent coupling is the dominant one. Due to this coupling, the interaction of the leaky waves of the two waveguides results in the formation of two super-modes, one with the odd symmetry of the mode field spatial distribution and another with even symmetry. The leakage wave from the two gratings causes the indirect coupling through free-space propagation and will excite each other with phase retardation. Fig. 2 shows a general structure of two coupled identical WGGs where direct and indirect couplings both exist. The temporal change of the normalized mode amplitudes of the WGG
linewidth can be obtained by using stacked gratings. Consequently, this structure improves detection ability of the GMR biosensor particularly for small analyte detection.
Design and simulation The schematic structure of the proposed sensor is illustrated in Fig. 1(a). The structure of the sensor is composed of two symmetrical WGG structures and is sandwiched using two thin plates to create a sealed chamber. Each side of the grating waveguide (WG) structure consists of a grating layer and a WG layer, which is made from silicon (Si) and silicon dioxide (SiO2), respectively. The analyte liquid is passed through the chamber to avoid the changes of analyte’s properties and bioactivity by the direct contact with an incident laser. When the analyte liquid in the chamber has a significant difference in the RI, the transmission spectrum peak will have a different shift. For single WGG structure, when the sensor is irradiated by light, part of the incident illumination is diffracted into the WG layer, as shown in Fig. 1(b). When the part of diffracted light reaches the phase matching, it will cause multiple total reflections and form a guide mode. This guided mode leaks into the grating to form a leaky mode. The leaky mode coupled with the reflection light, resulting in a narrowband resonance reflectance peak, and the peak value can reach 100% theoretically [24]. When the incident light illuminate the single grating sensor, there is a shift in the wavelength of resonant reflection, and the magnitude of this shift is proportional to the refractive index of the analyte, as shown in Fig. 1(c). When two identical WGG structures are placed close to each other, in each WGG a leaky wave can be resonantly excited by the incident wave, the leaky waves interact and couple to each other [25,26]. Due to the periodicity of the structure, the evanescent diffraction orders and
Fig. 2. The general structure of the two WGGs with both evanescent and propagation wave couplings. 760
Results in Physics 12 (2019) 759–765
X. Lu et al.
structures, a1 and a2 can be described by [27]: da1 dt da2 dt
( = (jω
0
2 τ
−
) a − jμa + κs ) a − jμa + κs
2 τ
= π jω0 −
1
2
2
+1
1
+3
+ κs+2
+ κs+4
(1)
where ω0 is the resonance frequency and 1/τ is the decay rate of gratings in one direction, and μ is the direct coupling strength. s+i and s−i are the amplitudes of the incoming and outgoing waves, their coupling coefficient κ = e jφ 2/ τ , and we can get these following relations:
s−2 = s+1 − s−1 = s+2 − s−3 = s+4 − s−4 = s+3 −
κ ∗a1 κ ∗a1 κ ∗a2 κ ∗a2
(2)
So the phase retardation of θ from wave propagation gives
s+2 = e−jθs−3 = e−jθ (s+4 − κ ∗a2) s+3 = e−jθs−2 = e−jθ (s+1 − κ ∗a1)
(3)
Substituting Eqs. (3) to (1), we have da1 dt da2 dt
( = (jω
= jω0 −
2 τ
−
2 τ
0
) a − (jμ + ) a − (jμ +
)a )a
1
2 −jθ e τ
2
2 −jθ e τ
2
+ κs+1 + κe−jθs+4
1
+ κs+4 + κe−jθs+1
(4)
From the above equation, we can get the reflection coefficient, which is the ratio of the amplitudes of the incident and the reflected waves. When s+4 = 0, from Eqs. (2) and (4) the reflection coefficient is given by
r=
s−1 = s+1
−j
(ω − ω0) τ 2
(1 + e−j2θ ) + jμτe−jθ − 1 + e−j2θ
(ω − ω ) τ j 20 ⎡ ⎣
2
(
μτ
+ 1⎤ − j 2 + e−jθ ⎦
)
2
(5)
Fig. 3. (a) Calculated transmission efficiency of TM-polarized excitation at the RI of n = 1.333 and (b) as a function of the RI varied from 1.3 to 1.5.
And the transmission coefficient can be obtained by the reflection coefficient. When the direct evanescent coupling between the resonators is strong (μτ > > 1), the resonance peaks can be affected by μ, which may find many applications. When the two WGG structures are far away from each other, the evanescent coupling is negligible (μ ≈ 0). In this case, phase retardation contributes more to tuning. The tuning range is decided by the decay rate of the grating. In order to obtain a general insight of the influence of the light transmission with the different RI analytes, theoretical analysis based on the rigorous coupled-wave analysis (RCWA) is performed [28–34]. Fig. 3 shows the resonant transmission spectrum of the demonstrated grating sensor for a normal incident (θ = 0) transverse magnet (TM) polarized light. The parameters used in the simulation are n = 1.333, n2 = 3.5, n3 = 1.46, d = 1.55 μm, h = 0.47 μm (d is the height of chamber and h is the thickness of grating layer), grating filling factor of f = 0.64 and grating period of Λ = 0.645 μm. As seen in Fig. 3(a), the simulated structure supports three modes, which correspond to the resonance wavelengths at 1285 nm, 1453 nm and 1610 nm, respectively. And the resonance bandwidth (full width at half-maximum: FWHM) is about 2 nm, the transmittance of sideband is less than 0.01. When the RI in the grating chamber is changed, resonance shifts are obtained. Fig. 3(b) shows the calculated transmission with different chamber’s RI (n). The RI of the simulation ranges from 1.3 to 1.5, for this range covers most of the low concentration solution such as glycerol, aqueous, protein, cane sugar and so on. Based on the simulation, the general sensor characteristics are acquired. The resonance location and coupling strength of each mode are all affected by the change of chamber’s RI. As the RI is increased, the resonance peaks move to the longer wavelength. Fig. 4 shows the magnetic field distribution at the resonant wavelengths, a highly concentrated field within the waveguide region that can be observed. We set the intensity of the incident field E0 at 1, and the absolute values |E|/|E0| are the normalized
intensity of magnetic field. The maximum values of field strength at three resonant wavelengths have little difference, and both means there is an apparent field enhancement effect in the WG layer. Despite the magnetic field contributions of three order mode are similar, the spectral FWHM is larger at the two order mode in the longer wavelength regions, and the linear relationship between the RI and the transmittance peak is better at the shortest order mode over this RI range. Beside this, comparing with other two modes, more mode energy of the first order mode is transferred to the chamber of the sensor, resulting in the improvement of the sensing sensitivity. Therefore, the narrow spectral FWHM and near-perfect linear relationship of the first order mode are utilized for the sensitive RI measurement. Results and discussions Optimization of the grating sensor The quality of the sensor depends on how much the changes of RI can be detected. For the purpose of evaluating the quality of the sensor mathematically, the sensitivity (S) and FOM of the sensor are calculated:
S=
Δλ Δn
FOM =
(6)
S FWHM
(7)
In order to get more perfect characteristics of the sensor, considering both the larger peak value and higher FOM, i.e., the narrower FWHM, more cases are considered in our paper. Based on the above calculation, the spectral character of the first order mode is more 761
Results in Physics 12 (2019) 759–765
X. Lu et al.
Fig. 5. (a) Density plots of the transmittance as the function of grating period and incident light wavelength. (b) The peak transmittance and FWHM as the function of grating period at the range of 0.6–0.75 μm.
Fig. 4. Magnetic field distributions in the coupled grating at (a) λ = 1285 nm; (b) λ = 1453 nm; (c) λ = 1610 nm.
suitable to use in sensitive detection. We represent the transmittance with respect to the grating period, fill factor, and the height of grating layer and chamber. The better parameters are chosen considering the superior peak and the manufacturing technique. The first step is to determine the optimum grating period. For this purpose, reflection peak of the first order mode is calculated as a function of the grating period that varies from 0 to 1 μm, showing in Fig. 5. Fig. 5(b) shows that with the increasing of the grating period, the peak value of transmittance obviously increases and then flattens out after Λ = 0.63 μm. Accordingly, the spectral FWHM increases with the increasing of the peak. Hence the optimum value of theΛis determined to be 0.64 μm and then performs other parameters’ optimization. Fig. 6 shows the optical characterization of the first order mode as fill factor varies from 0.4 to 0.8. As can be seen from the figure, when the filling factor f is increased, the resonance wavelengths of the two order modes are red-shifted. And the first order mode corresponds to the position of f at 0.56–0.8, while the higher mode corresponds to the position of f at 0.4–0.56. When the f is above 0.56, the FWHM increased with the increasing of the wavelength. Weighed the larger peak value and narrower FWHM, the optimum value is determined to be 0.64. After considering the grating features, the height of chamber is optimized. As it is exhibited in the Fig. 7, in the range of 0.90–2.30 μm, when it ranges from 0.90 to 1.34, 1.35 to 1.79, 1.80 to 2.30, with the continuous increase of d, the value of transmittance peak wavelength
Fig. 6. Density plots of the transmittance as the function of fill factor and incident light wavelength.
shifts to longer mode wavelength region and new resonance peak occurs from short wavelengths periodically. And in each period, the peak value has a tendency that increases first and then decreases. So as to remain the suitable spectral peak width and transmittance peak value, the height of chamber is chosen with a value of 1.54 μm. Based on the above analysis, the influence of the change of the thickness of grating layer to the spectrum characteristic is simulated. The calculated results are shown in Fig. 8. As is observed visually, only the value of h at the range of 0.4–0.55 μm, the effect of GMR occur and sharp transmission spectrum is formed. In these ranges, the increase of h causes the resonant transmission peak increase first, then decreases and increases again, but FWHM decrease first, then increases steadily. The optimum thickness value is found to be 0.46 μm. Additionally, the structural thickness of the grating layer is selected in consideration of the further fabrication process. 762
Results in Physics 12 (2019) 759–765
X. Lu et al.
Sensing applications Fig. 9(a) shows the calculated transmission spectrum of the first order mode of the resonance grating sensor with optimized parameters of n2 = 3.5, n3 = 1.46, h = 1.54 μm, d = 0.46 μm, h3 = 0.5 μm, Λ = 0.64 μm, f = 0.64 and in TM mode at normal incidence. It displays the resonant transmission peak of the first order mode as a function of RI of analyte liquid in the chamber. The calculated transmittance peak in the demonstrated sensor clearly distinguishes the RI ranging from 1.333 to 1.500. In order to have a more intuitive analysis of the simulation results, the resonant peak wavelength of different RI is shown in Fig. 9(a). From the two figures, it’s seen that the FWHM is about 0.9–7 nm, and the transmittance peak value can up to 99%. And the increase of the RI results in a positive shift in wavelength of the resonant transmission. The relation of the RI against the resonant transmission peak wavelength is shown in Fig. 9(b), and it is clear that the peak wavelength has an almost perfect relationship with the RI. A linear fitting curve is used to obtain a more accurate description of this relationship. The mathematical equation of the fitted curve is:
λ res = 0.61195 + 0.49783n
(8)
And it indicates that the sensitivity of the sensor as high as 497.83 nm/ RIU. The value of FOM is 551 which can be calculated using Eq. (7). The results show that this sensor has great performance in sensing, and the sensitivity is higher than some of the other microfluidic sensor [35,36]. Based on the linear relationship between RI and peak wavelength, in a nomal laser incident, the RI of n can be obtained through the sensor’s transmission spectrum, and then get the information of the analyte liquid.
Fig. 7. (a) Density plots of the transmittance as the function of the height of chamber and incident light wavelength. (b) The peak transmittance and FWHM as the function of the height of chamber at the range of 0.8–2.4 μm.
Tolerance analysis In all the above analysis, one of the parameters of the sensor structure has been varied, keeping all others at fixed values for obtaining exclusive dependence of resonance peak on each parameter. Figs. 5–8 show that the resonance peak and FWHM are extremely sensitive to Λ, f, d and h of the structure. Since it can easily cause deviations of the resonance peak, FWHM and future performance, i.e. sensitivity and FOM, the fabrication error must be strictly controlled during the production process of the refractive index sensor. Here we evaluate the changes of S and FOM of the sensor with respect to Λ, f, d and h. Fig. 10(a) shows that the tolerance inΛhas effect on the S and FOM of the proposed sensor. The fabrication tolerance on ofΛgrating is ± 0.02 μm around 0.64 μm for achieving high sensing property (S is above 490.00 nm/RIU, FOM is above 500). Fig. 10(b) shows a slightly larger effect caused by the uncertainty in f, which is similar toΛtolerance. The fabrication tolerance on f is ± 0.02 μm around 0.64 μm for achieving high S and FOM. Fig. 10(c) is the effect on sensor performance caused by the tolerance in d, and a wide range of FOM can be achieved through tuning of the value of d. In order to keep a high sensing property of the sensor, the tolerance on d is ± 0.02 μm around 1.54 μm. As it can be seen in Fig. 10(d), h had a large effect on the value of S and FOM. The fabrication tolerance on h is ± 0.01 μm around 0.46 μm, which is half of the other parameters. Fortunately, h is one of the most easily controlled and measured parameters. In conclusion, as long as these parameters are maintained within an appropriate fabrication tolerance, high-performance refractive index sensor can be obtained.
Fig. 8. (a) Density plots of the transmittance of the sensor as the function of the thickness of grating layer and incident light wavelength. (b) The peak transmittance and FWHM as the function of thickness of grating layer at the range of 0.4–0.55 μm.
Conclusions For the purpose of improving the detection sensitivity of biological molecules such as enzymes, protein and the FOM of biosensor, we demonstrated a novel resonant grating RI sensor based on the theory of GMR. RCWA method is used to simulate the optimization of sensor 763
Results in Physics 12 (2019) 759–765
X. Lu et al.
Fig. 9. (a) Wavelengths and transmittance with different RI. (b) Peak wavelength of different RI and their linear fitting curve.
Fig. 10. S and FOM of different value of (a) Λ; (b) f; (c) d; (d) h.
expert of Jiangsu Province (2015-XXRJ-014, R2016L01), Jiangsu 333 High-Level Talent Cultivation Program (BRA2016425).
parameters to make the transmission spectrum peak have higher value and narrower width and get the results for different RI. At these optimized parameters, the sensor shows a near-perfect linear relationship between resonant transmission wavelength and RI, and the transmittance peak value can reach up to 99%, and the sensitivity is 497.83 nm/ RIU. The value of FOM reaches up to 551 which is very large and it means better detection accuracy of the sensor. The sensor can obtain the high-performance with a high refractive index sensitivity and FOM with ± 0.02 μm fabrication tolerance on optimized Λ, f, d, and ± 0.02 μm fabrication tolerance on optimized h. For its advantages that without direct contact between laser and analyte liquid, high sensitivity, label-free, small size, this exhibited sensor is suitable for detection of biological samples in the field of biochemistry analysis, biomedicine and so on.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rinp.2018.12.048. References [1] Borisov SM, Wolfeis OS. Optical biosensors. Chem Rev 2008;108(2):423–61. [2] Velasco-Garcia MN. Optical biosensors for probing at the cellular level: a review of recent progress and future prospects. Semin Cell Dev Biol 2009;20(1):27–33. [3] Wawro D, Tibuleac S, Magnusson R, Liu H. Optical fiber endface biosensor based on resonances in dielectric waveguide gratings. Proc SPIE 2000;3911:86–94. [4] Cooper MA. Optical biosensors in drug discovery. Nat Rev Drug Discov 2002;1(7):515–28. [5] Long F, Zhu A, Sheng J, He M, Shi H. Matrix effects on the microcystin-LR fluorescent immunoassay based on optical biosensor. Sensors 2009;9(4):3000–10. [6] Tanious FA, Nguyen B, Wilson WD. Biosensor-surface plasmon resonance methods for quantitative analysis of biomolecular interactions. Method Cell Biol 2008;84:53–7.
Acknowledgments This work is partly supported by the National Natural Science Foundation of China (Grant nos. 41675154), Six Major Talent Peak 764
Results in Physics 12 (2019) 759–765
X. Lu et al.
[21] Liu AJ, Hofmann W, Bimberg D. Integrated high-contrast-grating optical sensor using guided mode. IEEE J Quantum Elect 2015;51(1):6600108. [22] Chretiennot T, Dubuc D, Grenier K. Microwave-Based Microfluidic Sensor for nondestructive and quantitative glucose monitoring in aqueous solution. Sensors 2016;16(10):1733. [23] Rifai D, Abdalla AN, Ali K, Razali R. Giant Magnetoresistance Sensors: a review on structures and non-destructive eddy current testing applications. Sensors 2016;16(3):298. [24] Magnusson R, Wang SS. Transmission bandpass guided-mode resonance filters. Appl Opt 1996;34(35):8106–9. [25] Ko YH, Magnusson R. Flat-top bandpass filters enabled by cascaded resonant gratings. Opt Lett 2016;41(20):4704–7. [26] Ding Y, Magunusson R. MEMS tunable resonant leaky mode filters. IEEE Photon Technol Lett 2016;18(14):1479–81. [27] Yu Z, Chen H, Liu JJ, Jin XF, Li XJ, Hong Z. Guided mode resonance in planar metamaterials consisting of two ring resonators with different sizes. Chin Phys B 2017;26(7):077804. [28] Sharon A, Reseblatt D, Friesem AA. Resonant grating-waveguide structures for visible and near-infrared radiation. J Opt Soc Am A 1997;14(1):2985–93. [29] Wang SS, Magnusson R. Multilayer waveguide-grating filters. Appl Opt 1995;34(14):2414–20. [30] Ma JY, Fan YT. Guided mode resonance transmission filters working at the intersection region of the first and second leaky modes. Chin Phys Lett 2012;29(2):207–10. [31] Weiss T, Gippius NA, Gnanet G, Tikhodeev SG, Taubert R, Fu L, et al. Strong resonant mode coupling of Fabry-Perot and grating resonances in stacked two-layer systems. Photonic Nanostruct 2011;9(4):390–7. [32] Moharam MG, Gaylord TK. Rigorous coupled-wave analysis of planar-grating diffraction. J Opt Soc Am A 1981;71(7):811–8. [33] Karagodsky V, Sedgwick FG, Chang-Hasnain CJ. Theoretical analysis of subwavelength high contrast grating reflectors. Opt Express 2010;18(16):16973–88. [34] Karagodsky V, Chase C, Chang-Hasnain CJ. Matrix Fabry-Perot resonance mechanism in high-contrast gratings. Opt Lett 2011;36(9):1704–6. [35] Sahu S, Ali J, Yupapin PP, Singh G, Grattan KTV. High-Q and temperature stable photonic biosensor based on grating waveguides. Opt Quant Electron 2018;50:307. [36] Duval D, Guerrero ABG, Lecbuga LM. Silicon photonic based biosensors: the future of lab-on-a-chip diagnostic devices. IEEE Photon Soc News 2012;26(4):5–8.
[7] Diaz RP, Cancilla JC, Plechhkova NV, Matute G, Seddon KR, Torrecilla JS. Estimation of the refractive indices of imidazolium based ionic liquids using their polarisability values. Phys Chem Chem Phys 2014;16(1):128. [8] Sadrolhosseini AR, Naseri M, Halimah MK. Erratum: Polypyrrole chitosan cobalt ferrite nanoparticles composite layer for measuring the low concentration of fluorene using surface plasmon resonance. Chin Phys Lett 2017;34:057501. [9] Wang J, Ye Q, Deng Z, Zhou W, Mei J, Zhang C, et al. Application of matching liquid on the refractive index measurement of biotissue: a theoretical and experimental study. Opt Commun 2014;319(15):36–41. [10] Yuan J, Zhao C, Zhou Y, Yu X, Kang J, Wang J, et al. Reflective long period fiber grating based sensor with sagnac fiber loop mirror for simultaneous measurement of refractive index and temperature. Appl Opt 2014;53(29):1956–60. [11] Liu B, Jiang Y, Zhu X, Tang X, Shi Y. Hollow fiber surface plasmon resonance sensor for the detection of liquid with high refractive index. Opt Express 2013;21(26):32349–57. [12] Lu H, Fan YC, Dai SQ, Mao D, Xiao FJ, Li P, et al. Coupling-induced spectral splitting for plasmonic sensing with ultra-high figure of merit. Chin Phys B 2018;27(11):117302. [13] Sahu S, Ali J, Singh G. Refractive index biosensor using sidewall gratings in dualslot waveguide. Opt Commun 2017;402(1):408–12. [14] Wang L, Sang T, Li JL, Zhou JY, Wang B, Wang Y. High-sensitive transmission type of gas sensor based on guided-mode resonance in coupled gratings. J Mod Opt 2018;65(13):1601–8. [15] Homala J, Piliarik M. Surface Plasmon Resonance (SPR) Sensors. Springer Ser Chem Sens Biosens 2006;4:45–67. [16] Quaranta G, Basset G, Martin OJF, Gallinet B. Recent advances in resonant waveguide gratings. Laser Photonics Rev 2018;12(9):1800017. [17] Tu Z, Gao D, Zhang M, Zhang D. High-sensitivity complex refractive index sensing based on Fano resonance in the subwavelength grating waveguide micro-ring resonator. Opt Express 2017;25:20911–22. [18] Hu J, Liu X, Zhao J, Zou J. Investigation of Fano resonance in compound resonant waveguide gratings for optical sensing. Chin Opt Lett 2017;15:030502. [19] Boonruang S, Mohammed WS. Multiwavelength guided mode resonance sensor array. Appl Phys Express 2015;8(9):092004. [20] Zhou J, Sang T, Li J, Wang R, Wang L, Wang B, et al. Modal analysis on resonant excitation of two-dimensional waveguide grating filters. Opt Commun 2017;405(15):350–4.
765
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
High performance refractive index sensor with stacked two-layer resonant waveguide gratings
T
⁎
X. Lua, G.G. Zhenga,b, , P. Zhoua a Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China b Jiangsu Collaborative Innovation Center on Atmospheric Environment and Equipment Technology (CICAEET), Nanjing University of Information Science & Technology, Nanjing 210044, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Guided-mode resonance Refractive index sensor Resonant grating
A refractive index (RI) sensor which contains a dielectric cavity composed of two parallel planar waveguide gratings (WGGs) is proposed. Reflection and transmittance through stacked gratings are studied after converting the diffraction problem into the two coupled resonances. The method of analysis is applied to estimate the performance when used as a biosensor. The sensor achieves its high-sensitive detection by tracking a narrow resonant reflection peak via a tunable wavelength light source, and the desired RI can be determined from the resonant wavelength shift. It will be shown that, in comparison with conventional single grating guided mode resonance (GMR) sensor with equal available area, extremely narrow resonance linewidth can be obtained by using stacked gratings. The sensitivity of the sensor can reach to 497.83 nm/RIU, and the value of the figure of merit (FOM) can reach up to 551. Because of its advantages such as high-sensitivity, label-free, small size and non-destructive, the sensor has an extremely broad application prospect in the field of biomedicine detection.
Introduction Optical biosensors that can detect analyte via the optical signals produced by the micro specificity reaction among biological molecules have become increasingly important as an effective detection and analysis tool for chemical, biological and medical applications [1,2]. It is not only competent for the biological test, but also for tasks such as unmarked immune detection and quantitative analysis [3,4]. By measuring the Refractive index (RI) of material, the optical properties, dispersion, concentration and other physical quantities can be acquired [5–7]. In the biological detection field, the samples are often made into solutions to obtain their various properties and parameters by detecting the small changes of the RI of the solutions. Based on this method, to meet the needs of low cost, high sensitivity, operability and label-free, various optical technologies have been proposed. These technologies include surface plasmon resonance (SPR) biosensors, ring resonator biosensors, micro-fiber sensor, GMR biosensors, etc [8–14]. In SPR sensors, a surface plasmon is excited at the interface between a metal film and a dielectric medium by incident light wave. The SPR phenomenon occured under phase-matching conditions [15]. Differing from SPR in concept and function, GMR is another resonance
phenomenon producing very sharp variations in the amplitudes of the electromagnetic fields. In recent years, the resonant waveguide grating (WGG)-based optical sensor has attracted great interest [16–21]. It exhibits a great application prospect in the bio-detection field. By varying the thickness or RI of a resonant WGG, its resonance frequency can be changed or tuned. This idea being applied to biosensors as the buildup of the attaching biolayer can be monitored in real time, without using chemical fluorescent tags, by following the corresponding resonance wavelength shift with a spectrometer. In this work, we demonstrate an RI sensor based on the effect of GMR in stacked resonant gratings. When the chamber of the sensor is flowing through different analyte liquids, the resonant reflection spectrum transforms depending on the RI of the liquid. Thus, the RI of the analyte liquid can be measured from the resonant wavelength. An important property of coupled GMR modes is that high energy density of resonant modes is localized within the chamber, which can yield high sensitivity to the changes of RI. The innovation point of this optical sensor is that the analyte liquid is not directly in contact with the laser and avoid reaction. That is, this sensor achieves non-destructive analysis [22,23]. In comparison with conventional single grating GMR sensor with an equal available area, extremely narrow resonance
⁎ Corresponding author at: Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China. E-mail address: [email protected] (G.G. Zheng).
https://doi.org/10.1016/j.rinp.2018.12.048 Received 15 November 2018; Received in revised form 10 December 2018; Accepted 10 December 2018 Available online 13 December 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 12 (2019) 759–765
X. Lu et al.
Fig. 1. (a) Schematic design structure of the resonance grating sensor. (b) The schematic of the single WGG structure. (c) The schematic reflectance spectrum of the single grating sensor of different RI.
the propagating diffraction orders are coupled. The small gap between WGGs causes that the evanescent coupling is the dominant one. Due to this coupling, the interaction of the leaky waves of the two waveguides results in the formation of two super-modes, one with the odd symmetry of the mode field spatial distribution and another with even symmetry. The leakage wave from the two gratings causes the indirect coupling through free-space propagation and will excite each other with phase retardation. Fig. 2 shows a general structure of two coupled identical WGGs where direct and indirect couplings both exist. The temporal change of the normalized mode amplitudes of the WGG
linewidth can be obtained by using stacked gratings. Consequently, this structure improves detection ability of the GMR biosensor particularly for small analyte detection.
Design and simulation The schematic structure of the proposed sensor is illustrated in Fig. 1(a). The structure of the sensor is composed of two symmetrical WGG structures and is sandwiched using two thin plates to create a sealed chamber. Each side of the grating waveguide (WG) structure consists of a grating layer and a WG layer, which is made from silicon (Si) and silicon dioxide (SiO2), respectively. The analyte liquid is passed through the chamber to avoid the changes of analyte’s properties and bioactivity by the direct contact with an incident laser. When the analyte liquid in the chamber has a significant difference in the RI, the transmission spectrum peak will have a different shift. For single WGG structure, when the sensor is irradiated by light, part of the incident illumination is diffracted into the WG layer, as shown in Fig. 1(b). When the part of diffracted light reaches the phase matching, it will cause multiple total reflections and form a guide mode. This guided mode leaks into the grating to form a leaky mode. The leaky mode coupled with the reflection light, resulting in a narrowband resonance reflectance peak, and the peak value can reach 100% theoretically [24]. When the incident light illuminate the single grating sensor, there is a shift in the wavelength of resonant reflection, and the magnitude of this shift is proportional to the refractive index of the analyte, as shown in Fig. 1(c). When two identical WGG structures are placed close to each other, in each WGG a leaky wave can be resonantly excited by the incident wave, the leaky waves interact and couple to each other [25,26]. Due to the periodicity of the structure, the evanescent diffraction orders and
Fig. 2. The general structure of the two WGGs with both evanescent and propagation wave couplings. 760
Results in Physics 12 (2019) 759–765
X. Lu et al.
structures, a1 and a2 can be described by [27]: da1 dt da2 dt
( = (jω
0
2 τ
−
) a − jμa + κs ) a − jμa + κs
2 τ
= π jω0 −
1
2
2
+1
1
+3
+ κs+2
+ κs+4
(1)
where ω0 is the resonance frequency and 1/τ is the decay rate of gratings in one direction, and μ is the direct coupling strength. s+i and s−i are the amplitudes of the incoming and outgoing waves, their coupling coefficient κ = e jφ 2/ τ , and we can get these following relations:
s−2 = s+1 − s−1 = s+2 − s−3 = s+4 − s−4 = s+3 −
κ ∗a1 κ ∗a1 κ ∗a2 κ ∗a2
(2)
So the phase retardation of θ from wave propagation gives
s+2 = e−jθs−3 = e−jθ (s+4 − κ ∗a2) s+3 = e−jθs−2 = e−jθ (s+1 − κ ∗a1)
(3)
Substituting Eqs. (3) to (1), we have da1 dt da2 dt
( = (jω
= jω0 −
2 τ
−
2 τ
0
) a − (jμ + ) a − (jμ +
)a )a
1
2 −jθ e τ
2
2 −jθ e τ
2
+ κs+1 + κe−jθs+4
1
+ κs+4 + κe−jθs+1
(4)
From the above equation, we can get the reflection coefficient, which is the ratio of the amplitudes of the incident and the reflected waves. When s+4 = 0, from Eqs. (2) and (4) the reflection coefficient is given by
r=
s−1 = s+1
−j
(ω − ω0) τ 2
(1 + e−j2θ ) + jμτe−jθ − 1 + e−j2θ
(ω − ω ) τ j 20 ⎡ ⎣
2
(
μτ
+ 1⎤ − j 2 + e−jθ ⎦
)
2
(5)
Fig. 3. (a) Calculated transmission efficiency of TM-polarized excitation at the RI of n = 1.333 and (b) as a function of the RI varied from 1.3 to 1.5.
And the transmission coefficient can be obtained by the reflection coefficient. When the direct evanescent coupling between the resonators is strong (μτ > > 1), the resonance peaks can be affected by μ, which may find many applications. When the two WGG structures are far away from each other, the evanescent coupling is negligible (μ ≈ 0). In this case, phase retardation contributes more to tuning. The tuning range is decided by the decay rate of the grating. In order to obtain a general insight of the influence of the light transmission with the different RI analytes, theoretical analysis based on the rigorous coupled-wave analysis (RCWA) is performed [28–34]. Fig. 3 shows the resonant transmission spectrum of the demonstrated grating sensor for a normal incident (θ = 0) transverse magnet (TM) polarized light. The parameters used in the simulation are n = 1.333, n2 = 3.5, n3 = 1.46, d = 1.55 μm, h = 0.47 μm (d is the height of chamber and h is the thickness of grating layer), grating filling factor of f = 0.64 and grating period of Λ = 0.645 μm. As seen in Fig. 3(a), the simulated structure supports three modes, which correspond to the resonance wavelengths at 1285 nm, 1453 nm and 1610 nm, respectively. And the resonance bandwidth (full width at half-maximum: FWHM) is about 2 nm, the transmittance of sideband is less than 0.01. When the RI in the grating chamber is changed, resonance shifts are obtained. Fig. 3(b) shows the calculated transmission with different chamber’s RI (n). The RI of the simulation ranges from 1.3 to 1.5, for this range covers most of the low concentration solution such as glycerol, aqueous, protein, cane sugar and so on. Based on the simulation, the general sensor characteristics are acquired. The resonance location and coupling strength of each mode are all affected by the change of chamber’s RI. As the RI is increased, the resonance peaks move to the longer wavelength. Fig. 4 shows the magnetic field distribution at the resonant wavelengths, a highly concentrated field within the waveguide region that can be observed. We set the intensity of the incident field E0 at 1, and the absolute values |E|/|E0| are the normalized
intensity of magnetic field. The maximum values of field strength at three resonant wavelengths have little difference, and both means there is an apparent field enhancement effect in the WG layer. Despite the magnetic field contributions of three order mode are similar, the spectral FWHM is larger at the two order mode in the longer wavelength regions, and the linear relationship between the RI and the transmittance peak is better at the shortest order mode over this RI range. Beside this, comparing with other two modes, more mode energy of the first order mode is transferred to the chamber of the sensor, resulting in the improvement of the sensing sensitivity. Therefore, the narrow spectral FWHM and near-perfect linear relationship of the first order mode are utilized for the sensitive RI measurement. Results and discussions Optimization of the grating sensor The quality of the sensor depends on how much the changes of RI can be detected. For the purpose of evaluating the quality of the sensor mathematically, the sensitivity (S) and FOM of the sensor are calculated:
S=
Δλ Δn
FOM =
(6)
S FWHM
(7)
In order to get more perfect characteristics of the sensor, considering both the larger peak value and higher FOM, i.e., the narrower FWHM, more cases are considered in our paper. Based on the above calculation, the spectral character of the first order mode is more 761
Results in Physics 12 (2019) 759–765
X. Lu et al.
Fig. 5. (a) Density plots of the transmittance as the function of grating period and incident light wavelength. (b) The peak transmittance and FWHM as the function of grating period at the range of 0.6–0.75 μm.
Fig. 4. Magnetic field distributions in the coupled grating at (a) λ = 1285 nm; (b) λ = 1453 nm; (c) λ = 1610 nm.
suitable to use in sensitive detection. We represent the transmittance with respect to the grating period, fill factor, and the height of grating layer and chamber. The better parameters are chosen considering the superior peak and the manufacturing technique. The first step is to determine the optimum grating period. For this purpose, reflection peak of the first order mode is calculated as a function of the grating period that varies from 0 to 1 μm, showing in Fig. 5. Fig. 5(b) shows that with the increasing of the grating period, the peak value of transmittance obviously increases and then flattens out after Λ = 0.63 μm. Accordingly, the spectral FWHM increases with the increasing of the peak. Hence the optimum value of theΛis determined to be 0.64 μm and then performs other parameters’ optimization. Fig. 6 shows the optical characterization of the first order mode as fill factor varies from 0.4 to 0.8. As can be seen from the figure, when the filling factor f is increased, the resonance wavelengths of the two order modes are red-shifted. And the first order mode corresponds to the position of f at 0.56–0.8, while the higher mode corresponds to the position of f at 0.4–0.56. When the f is above 0.56, the FWHM increased with the increasing of the wavelength. Weighed the larger peak value and narrower FWHM, the optimum value is determined to be 0.64. After considering the grating features, the height of chamber is optimized. As it is exhibited in the Fig. 7, in the range of 0.90–2.30 μm, when it ranges from 0.90 to 1.34, 1.35 to 1.79, 1.80 to 2.30, with the continuous increase of d, the value of transmittance peak wavelength
Fig. 6. Density plots of the transmittance as the function of fill factor and incident light wavelength.
shifts to longer mode wavelength region and new resonance peak occurs from short wavelengths periodically. And in each period, the peak value has a tendency that increases first and then decreases. So as to remain the suitable spectral peak width and transmittance peak value, the height of chamber is chosen with a value of 1.54 μm. Based on the above analysis, the influence of the change of the thickness of grating layer to the spectrum characteristic is simulated. The calculated results are shown in Fig. 8. As is observed visually, only the value of h at the range of 0.4–0.55 μm, the effect of GMR occur and sharp transmission spectrum is formed. In these ranges, the increase of h causes the resonant transmission peak increase first, then decreases and increases again, but FWHM decrease first, then increases steadily. The optimum thickness value is found to be 0.46 μm. Additionally, the structural thickness of the grating layer is selected in consideration of the further fabrication process. 762
Results in Physics 12 (2019) 759–765
X. Lu et al.
Sensing applications Fig. 9(a) shows the calculated transmission spectrum of the first order mode of the resonance grating sensor with optimized parameters of n2 = 3.5, n3 = 1.46, h = 1.54 μm, d = 0.46 μm, h3 = 0.5 μm, Λ = 0.64 μm, f = 0.64 and in TM mode at normal incidence. It displays the resonant transmission peak of the first order mode as a function of RI of analyte liquid in the chamber. The calculated transmittance peak in the demonstrated sensor clearly distinguishes the RI ranging from 1.333 to 1.500. In order to have a more intuitive analysis of the simulation results, the resonant peak wavelength of different RI is shown in Fig. 9(a). From the two figures, it’s seen that the FWHM is about 0.9–7 nm, and the transmittance peak value can up to 99%. And the increase of the RI results in a positive shift in wavelength of the resonant transmission. The relation of the RI against the resonant transmission peak wavelength is shown in Fig. 9(b), and it is clear that the peak wavelength has an almost perfect relationship with the RI. A linear fitting curve is used to obtain a more accurate description of this relationship. The mathematical equation of the fitted curve is:
λ res = 0.61195 + 0.49783n
(8)
And it indicates that the sensitivity of the sensor as high as 497.83 nm/ RIU. The value of FOM is 551 which can be calculated using Eq. (7). The results show that this sensor has great performance in sensing, and the sensitivity is higher than some of the other microfluidic sensor [35,36]. Based on the linear relationship between RI and peak wavelength, in a nomal laser incident, the RI of n can be obtained through the sensor’s transmission spectrum, and then get the information of the analyte liquid.
Fig. 7. (a) Density plots of the transmittance as the function of the height of chamber and incident light wavelength. (b) The peak transmittance and FWHM as the function of the height of chamber at the range of 0.8–2.4 μm.
Tolerance analysis In all the above analysis, one of the parameters of the sensor structure has been varied, keeping all others at fixed values for obtaining exclusive dependence of resonance peak on each parameter. Figs. 5–8 show that the resonance peak and FWHM are extremely sensitive to Λ, f, d and h of the structure. Since it can easily cause deviations of the resonance peak, FWHM and future performance, i.e. sensitivity and FOM, the fabrication error must be strictly controlled during the production process of the refractive index sensor. Here we evaluate the changes of S and FOM of the sensor with respect to Λ, f, d and h. Fig. 10(a) shows that the tolerance inΛhas effect on the S and FOM of the proposed sensor. The fabrication tolerance on ofΛgrating is ± 0.02 μm around 0.64 μm for achieving high sensing property (S is above 490.00 nm/RIU, FOM is above 500). Fig. 10(b) shows a slightly larger effect caused by the uncertainty in f, which is similar toΛtolerance. The fabrication tolerance on f is ± 0.02 μm around 0.64 μm for achieving high S and FOM. Fig. 10(c) is the effect on sensor performance caused by the tolerance in d, and a wide range of FOM can be achieved through tuning of the value of d. In order to keep a high sensing property of the sensor, the tolerance on d is ± 0.02 μm around 1.54 μm. As it can be seen in Fig. 10(d), h had a large effect on the value of S and FOM. The fabrication tolerance on h is ± 0.01 μm around 0.46 μm, which is half of the other parameters. Fortunately, h is one of the most easily controlled and measured parameters. In conclusion, as long as these parameters are maintained within an appropriate fabrication tolerance, high-performance refractive index sensor can be obtained.
Fig. 8. (a) Density plots of the transmittance of the sensor as the function of the thickness of grating layer and incident light wavelength. (b) The peak transmittance and FWHM as the function of thickness of grating layer at the range of 0.4–0.55 μm.
Conclusions For the purpose of improving the detection sensitivity of biological molecules such as enzymes, protein and the FOM of biosensor, we demonstrated a novel resonant grating RI sensor based on the theory of GMR. RCWA method is used to simulate the optimization of sensor 763
Results in Physics 12 (2019) 759–765
X. Lu et al.
Fig. 9. (a) Wavelengths and transmittance with different RI. (b) Peak wavelength of different RI and their linear fitting curve.
Fig. 10. S and FOM of different value of (a) Λ; (b) f; (c) d; (d) h.
expert of Jiangsu Province (2015-XXRJ-014, R2016L01), Jiangsu 333 High-Level Talent Cultivation Program (BRA2016425).
parameters to make the transmission spectrum peak have higher value and narrower width and get the results for different RI. At these optimized parameters, the sensor shows a near-perfect linear relationship between resonant transmission wavelength and RI, and the transmittance peak value can reach up to 99%, and the sensitivity is 497.83 nm/ RIU. The value of FOM reaches up to 551 which is very large and it means better detection accuracy of the sensor. The sensor can obtain the high-performance with a high refractive index sensitivity and FOM with ± 0.02 μm fabrication tolerance on optimized Λ, f, d, and ± 0.02 μm fabrication tolerance on optimized h. For its advantages that without direct contact between laser and analyte liquid, high sensitivity, label-free, small size, this exhibited sensor is suitable for detection of biological samples in the field of biochemistry analysis, biomedicine and so on.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rinp.2018.12.048. References [1] Borisov SM, Wolfeis OS. Optical biosensors. Chem Rev 2008;108(2):423–61. [2] Velasco-Garcia MN. Optical biosensors for probing at the cellular level: a review of recent progress and future prospects. Semin Cell Dev Biol 2009;20(1):27–33. [3] Wawro D, Tibuleac S, Magnusson R, Liu H. Optical fiber endface biosensor based on resonances in dielectric waveguide gratings. Proc SPIE 2000;3911:86–94. [4] Cooper MA. Optical biosensors in drug discovery. Nat Rev Drug Discov 2002;1(7):515–28. [5] Long F, Zhu A, Sheng J, He M, Shi H. Matrix effects on the microcystin-LR fluorescent immunoassay based on optical biosensor. Sensors 2009;9(4):3000–10. [6] Tanious FA, Nguyen B, Wilson WD. Biosensor-surface plasmon resonance methods for quantitative analysis of biomolecular interactions. Method Cell Biol 2008;84:53–7.
Acknowledgments This work is partly supported by the National Natural Science Foundation of China (Grant nos. 41675154), Six Major Talent Peak 764
Results in Physics 12 (2019) 759–765
X. Lu et al.
[21] Liu AJ, Hofmann W, Bimberg D. Integrated high-contrast-grating optical sensor using guided mode. IEEE J Quantum Elect 2015;51(1):6600108. [22] Chretiennot T, Dubuc D, Grenier K. Microwave-Based Microfluidic Sensor for nondestructive and quantitative glucose monitoring in aqueous solution. Sensors 2016;16(10):1733. [23] Rifai D, Abdalla AN, Ali K, Razali R. Giant Magnetoresistance Sensors: a review on structures and non-destructive eddy current testing applications. Sensors 2016;16(3):298. [24] Magnusson R, Wang SS. Transmission bandpass guided-mode resonance filters. Appl Opt 1996;34(35):8106–9. [25] Ko YH, Magnusson R. Flat-top bandpass filters enabled by cascaded resonant gratings. Opt Lett 2016;41(20):4704–7. [26] Ding Y, Magunusson R. MEMS tunable resonant leaky mode filters. IEEE Photon Technol Lett 2016;18(14):1479–81. [27] Yu Z, Chen H, Liu JJ, Jin XF, Li XJ, Hong Z. Guided mode resonance in planar metamaterials consisting of two ring resonators with different sizes. Chin Phys B 2017;26(7):077804. [28] Sharon A, Reseblatt D, Friesem AA. Resonant grating-waveguide structures for visible and near-infrared radiation. J Opt Soc Am A 1997;14(1):2985–93. [29] Wang SS, Magnusson R. Multilayer waveguide-grating filters. Appl Opt 1995;34(14):2414–20. [30] Ma JY, Fan YT. Guided mode resonance transmission filters working at the intersection region of the first and second leaky modes. Chin Phys Lett 2012;29(2):207–10. [31] Weiss T, Gippius NA, Gnanet G, Tikhodeev SG, Taubert R, Fu L, et al. Strong resonant mode coupling of Fabry-Perot and grating resonances in stacked two-layer systems. Photonic Nanostruct 2011;9(4):390–7. [32] Moharam MG, Gaylord TK. Rigorous coupled-wave analysis of planar-grating diffraction. J Opt Soc Am A 1981;71(7):811–8. [33] Karagodsky V, Sedgwick FG, Chang-Hasnain CJ. Theoretical analysis of subwavelength high contrast grating reflectors. Opt Express 2010;18(16):16973–88. [34] Karagodsky V, Chase C, Chang-Hasnain CJ. Matrix Fabry-Perot resonance mechanism in high-contrast gratings. Opt Lett 2011;36(9):1704–6. [35] Sahu S, Ali J, Yupapin PP, Singh G, Grattan KTV. High-Q and temperature stable photonic biosensor based on grating waveguides. Opt Quant Electron 2018;50:307. [36] Duval D, Guerrero ABG, Lecbuga LM. Silicon photonic based biosensors: the future of lab-on-a-chip diagnostic devices. IEEE Photon Soc News 2012;26(4):5–8.
[7] Diaz RP, Cancilla JC, Plechhkova NV, Matute G, Seddon KR, Torrecilla JS. Estimation of the refractive indices of imidazolium based ionic liquids using their polarisability values. Phys Chem Chem Phys 2014;16(1):128. [8] Sadrolhosseini AR, Naseri M, Halimah MK. Erratum: Polypyrrole chitosan cobalt ferrite nanoparticles composite layer for measuring the low concentration of fluorene using surface plasmon resonance. Chin Phys Lett 2017;34:057501. [9] Wang J, Ye Q, Deng Z, Zhou W, Mei J, Zhang C, et al. Application of matching liquid on the refractive index measurement of biotissue: a theoretical and experimental study. Opt Commun 2014;319(15):36–41. [10] Yuan J, Zhao C, Zhou Y, Yu X, Kang J, Wang J, et al. Reflective long period fiber grating based sensor with sagnac fiber loop mirror for simultaneous measurement of refractive index and temperature. Appl Opt 2014;53(29):1956–60. [11] Liu B, Jiang Y, Zhu X, Tang X, Shi Y. Hollow fiber surface plasmon resonance sensor for the detection of liquid with high refractive index. Opt Express 2013;21(26):32349–57. [12] Lu H, Fan YC, Dai SQ, Mao D, Xiao FJ, Li P, et al. Coupling-induced spectral splitting for plasmonic sensing with ultra-high figure of merit. Chin Phys B 2018;27(11):117302. [13] Sahu S, Ali J, Singh G. Refractive index biosensor using sidewall gratings in dualslot waveguide. Opt Commun 2017;402(1):408–12. [14] Wang L, Sang T, Li JL, Zhou JY, Wang B, Wang Y. High-sensitive transmission type of gas sensor based on guided-mode resonance in coupled gratings. J Mod Opt 2018;65(13):1601–8. [15] Homala J, Piliarik M. Surface Plasmon Resonance (SPR) Sensors. Springer Ser Chem Sens Biosens 2006;4:45–67. [16] Quaranta G, Basset G, Martin OJF, Gallinet B. Recent advances in resonant waveguide gratings. Laser Photonics Rev 2018;12(9):1800017. [17] Tu Z, Gao D, Zhang M, Zhang D. High-sensitivity complex refractive index sensing based on Fano resonance in the subwavelength grating waveguide micro-ring resonator. Opt Express 2017;25:20911–22. [18] Hu J, Liu X, Zhao J, Zou J. Investigation of Fano resonance in compound resonant waveguide gratings for optical sensing. Chin Opt Lett 2017;15:030502. [19] Boonruang S, Mohammed WS. Multiwavelength guided mode resonance sensor array. Appl Phys Express 2015;8(9):092004. [20] Zhou J, Sang T, Li J, Wang R, Wang L, Wang B, et al. Modal analysis on resonant excitation of two-dimensional waveguide grating filters. Opt Commun 2017;405(15):350–4.
765