1.Engineering Research Center of Thin Film Optoelectronics Technology, Ministry of Education, Nankai University, Tianjin 300350, China 2.Tianjin Key Laboratory of Optoelectronic Sensor and Sensing Network Technology, Tianjin 300350, China 3.Department of Microelectronic Engineering, College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, China 4.College of Physics and Materials Science, Tianjin Normal University, Tianjin 300387, China 5.Institute of Modern Optics, College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, China 6.Tianjin Key Laboratory of Micro-scale Optical Information Science and Technology, Tianjin 300350, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 62075104, 61775105, 11674244)
Received Date:04 December 2020
Accepted Date:02 February 2021
Available Online:09 July 2021
Published Online:20 July 2021
Abstract:The metal-dielectric-metal multilayer structure sensor with a transparent top layer and an opaque bottom layer is proposed, which can provide a perfect narrow-band absorption resonance and is suitable for sensing refractive index change of the liquid to be measured in dielectric layer. The Fabry-Perot resonance analytical model that can accurately reproduce response spectrum and theoretically analyze the mechanism of the dielectric layer thickness to tune resonance wavelength and linewidth of response spectrum is constructed. Theoretical analysis shows that the resonance wavelength is directly proportional to the thickness of dielectric layer, and the full width at half maximum is inversely proportional to the thickness of dielectric layer. The analytical expressions for its resonance wavelength, quality factor, full width at half maximum and sensitivity are also given. When used for the refractive index sensing, the quality factor and figure of merit of the proposed multilayer structure based on the 8th order Fabry-Perot resonance are 2162.8 and 1648.1 RIU–1, respectively. However, due to the influence of the minimum resolution of the spectrometer, the conventional method of measuring resonance wavelength shift to achieve refractive index sensing has a high measurement limit. For the sensing of weaker refractive index perturbation, with the help of superposition of exceptional point degenerate state and tuning mechanism of Fabry-Perot resonance, in this paper proposed is a method of tunably sensing the liquid refractive index by measuring the increase of reflection coefficient or splitting of eigenvalue at a specific wavelength. Here, we take for example the metal-dielectric-metal multilayer structure sensor based on the 8th order Fabry-Perot resonance. According to the calculation results of Fabry-Perot model, when the change in refractive index of liquid to be measured is 10–4 RIU, the increase of forward reflection coefficient and the splitting of two eigenvalues of the scattering matrix are 0.319 and 1.1279, respectively. Keywords:metal-dielectric-metal resonator/ Fabry-Perot resonance model/ tunable sensing/ exceptional point
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2.1.数值仿真
MDM多层结构的示意图如1(a)所示, x方向和y方向是无限延伸的金属-介质-金属三层结构, 上、下金属层均沉积在折射率为1.5的玻璃衬底上, 上、下金属层之间是待测液体空腔, 可以通过控制上下金属层的距离改变待测液体空腔的厚度, 入射平面波垂直于各层界面入射(沿z轴负方向传播). 金属层银采用Drude模型, 折射率${n_{{\rm{silver}}}} $$ = \sqrt {{\varepsilon _\infty } - {{\omega _{\rm{p}}^2}}/{{\left( {\omega + {\rm{i}}\gamma \omega } \right)}}}$, 其中${\varepsilon _\infty } = 1$, 等离子体频率${\omega _{\rm{p}}} = 1.366 \times {10^{16}} \;{{{\rm{rad}}} / {\rm{s}}}$, 阻尼常数$\gamma = 3.07\; \times $$ {10^{13}}\; {{\rm{s}}^{ - 1}}$[23], 上下银层厚度分别为d1 = 41.54 nm和d2 = 150 nm. 待测液体的初始折射率n设为水的折射率1.312, 厚度d = 350 nm. 图 1 (a) MDM多层结构示意图, 箭头表示均匀平面波的传播方向, 红色、蓝色箭头对应前向、后向入射均匀平面波; (b) 前向反射率R、透射率T和吸收率A的光谱; (c) 不同玻璃层厚度dGlass对应的反射率谱, 蓝色实线表示玻璃厚度无限大时的反射率谱(即图(b)中的红色实线), 插图为上下玻璃层均为有限厚度时的结构示意图; (d) 双层增透膜结构的反射率谱, 插图表示位于空气、玻璃之间的双层增透膜结构; (e) 不同玻璃层厚度dGlass对应的反射率谱, 蓝色实线表示玻璃厚度无限大时的反射率谱, 插图为上下玻璃层均为有限厚度且玻璃表面镀双层增透膜的结构示意图 Figure1. (a) Schematic of MDM multilayer structure. The solid arrows indicate the propagation directions of plane waves. The red and blue arrows correspond to forward and backward travelling incident plane waves, respectively. (b) Spectra of forward reflectance R, transmittance T and absorptance A of the proposed structure. (c) Reflectance spectra for different thicknesses dGlass of the glass layer, where the blue solid curve corresponds to infinite glass thickness [i.e. the red solid curve in Fig. (b)]. The inset shows the structure diagram with upper and lower glass layers set to have a finite thickness. (d) Reflectance spectra of the double-layer antireflection coating. The inset shows the structure of the double-layer antireflection coating between the air and glass regions. (e) Reflectance spectra for different thicknesses dGlass of the glass layer, where the blue solid curve corresponds to infinite glass thickness. The inset shows the structure diagram where the upper and lower glass layers are both set to have a finite thickness and both glass surfaces are coated with the double-layer antireflection coating.
采用严格耦合波分析(rigorous coupled wave analysis, RCWA)[24]方法进行数值仿真, 由于结构沿x, y方向无变化, RCWA中只需选取0级谐波. 图1(b)是该MDM结构的反射率、透射率及吸收率光谱, 共振波长是1035.967 nm, 此时的反射率$R = 1.32 \times 1{0^{ - 8}}$, 透射率$T = 5.31 \times 1{0^{ - 5}}$, 吸收率$A = 1 - R - T$, 表现出近乎完美的窄带吸收特性. 考虑到实际结构的玻璃层厚度有限, 将上下玻璃层设置成相同的有限厚度dGlass (图1(c)插图), 平面波从顶部的空气区域入射. dGlass取值不同时仿真的反射率谱如图1(c)所示, 可以看到有限玻璃厚度与无限玻璃厚度对应的反射率曲线有很大的差别, 即玻璃厚度对结构特性存在较大影响. 针对该问题, 实际应用中可以通过在玻璃表面镀宽带增透膜[25,26], 使空气中选定波长范围的入射平面波全部透射到玻璃层, 实现与玻璃衬底厚度无限时(此时平面波从玻璃中入射)相同的效果. 为了证明上述方法具有可行性, 图1(d)插图设计了针对空气-玻璃界面的双层增透膜, 紧挨玻璃的介质层折射率为n1 = 1.7, 紧挨空气的介质层折射率为n2 = 1.38, 两层介质的光学厚度(即折射率与几何厚度的乘积)均为谐振波长的1/4(谐振波长取值1035.967 nm). 根据菲涅耳公式可知, 光垂直入射到空气-玻璃界面的反射率为0.04, 加入双层增透膜后得到的反射率谱如图1(d)所示, 在所选波长范围内反射率被极大地降低. 将图1(c)对应结构的上下玻璃表面均镀上双层增透膜, 如图1(e)插图所示. 此时dGlass取值不同时仿真的反射率谱如图1(e)所示, 可见加入增透膜后, 玻璃厚度变化时的反射率曲线与玻璃厚度无限时的反射率曲线符合很好. 因此, 通过在有限厚度的玻璃上镀增透膜, 能够实现与玻璃衬底厚度无限大时相同的效果. 为了便于分析, 下文的数值仿真和模型计算均基于玻璃衬底厚度无限的假设. 为了计算此传感器的灵敏度, 将中间层空腔中待测液体的折射率从1.312逐渐增加到1.352, 递增间隔为0.01. 图2(a)是5种不同折射率液体的反射率谱曲线, 随着被测液体折射率的增大, 共振峰发生红移. 图2(b)是共振波长随折射率n的变化曲线, 通过线性拟合可得灵敏度S = 789 nm/RIU. 图 2 (a) 不同折射率n对应的反射率谱线; (b) 谐振波长随被测液体折射率n变化的曲线 Figure2. (a) Reflectance spectra for different refractive indices n; (b) resonance wavelength plotted as a function of the refractive index n of the measured liquid.
22.2.Fabry-Perot模型 -->
2.2.Fabry-Perot模型
此结构的窄带吸收特性来自Fabry-Perot共振, 上、下不对称银层是形成Fabry-Perot共振腔的两个反射面, 共振时入射光能量被限制在待测液体空腔内. 下面通过建立Fabry-Perot共振解析模型, 解释谐振的形成机制并分析谐振的可调特性. 图3(a1)—(a4)是针对MDM结构构建的Fabry-Perot模型的参数定义, 对于本文的平面多层结构, a, b分别表示介质层中的下行和上行平面波模式系数, ${r_{{\rm{FP}}}}$, ${t_{{\rm{FP}}}}$分别表示平面波的反射和透射系数, 满足下列模式耦合方程: 图 3 Fabry-Perot模型 (a1) MDM结构中待求解的反射系数rFP, 透射系数tFP, 介质层中模式系数a, b的定义; (a2)—(a4) Fabry-Perot模型中双金属界面散射系数r1, t1, r2, t2, r3, t3的定义; (b1)—(b4) 对于r1, t1的Fabry-Perot模型, 中间金属层模式系数a', b'的定义, 以及单界面散射系数定义; (c1)—(c4) 对于r2, t2的Fabry-Perot模型, 相应的金属层模式系数和单界面散射系数定义; (d1)—(d4) 对于r3, t3的Fabry-Perot模型, 相应的金属层模式系数和单界面散射系数定义 Figure3. Fabry-Perot model: (a1) Definitions of reflection coefficient rFP, transmission coefficient tFP to be solved in MDM structure, and mode coefficients a, b in dielectric layer; (a2)–(a4) definitions of bimetal interface scattering coefficients r1, t1, r2, t2, r3, t3 in Fabry-Perot model; (b1)–(b4) Fabry-Perot model of r1 and t1, definitions of mode coefficients a', b' in intermediate metal layer, and single interface scattering coefficients; (c1)–(c4) Fabry-Perot model of r2 and t2, the corresponding metal layer mode coefficients and single interface scattering coefficients are defined; (d1)–(d4) Fabry-Perot model of r3 and t3, the corresponding metal layer mode coefficients and single interface scattering coefficients are defined.
考虑到银层折射率与近红外波长的缓变关系, 仅改变d时, 可近似认为${r_2}$, ${r_3}$是常数, 根据(4)式可知${\lambda _0}$与d呈线性关系. 图4(a)是不同介质层厚度的反射率谱曲线, d增大时${\lambda _0}$红移. 谐振波长与介质层厚度的关系曲线如图4(b)所示, 可见由反射率谱极小值点得到的谐振波长与方程(4)计算的结果一致. 图 4 (a) 不同介质层厚度d对应的反射率谱曲线, 点表示RCWA数值仿真结果, 实线表示Fabry-Perot模型计算结果; (b) 谐振波长随介质层厚度d的变化, 红色和蓝色实线表示Fabry-Perot模型和方程(4)的计算结果; (c) 不同谐振阶次m对应同一谐振波长1035.967 nm时(m越大则d越大), RCWA计算(点)、Fabry-Perot模型(实线)给出的反射率谱曲线; (d) Q, FOM, S随d的变化, 红色、蓝色和灰色实线代表由Fabry-Perot模型计算的反射率谱得到的Q, S, FOM, 粉色和青色虚线代表方程(5)和(6)计算的Q, S Figure4. (a) Reflectance as a function of wavelength for variable dielectric layer thicknesses d, dots represent RCWA numerical simulation results, solid lines represent Fabry-Perot analytical model calculation results; (b) resonance wavelength as a function of dielectric layer thickness d, red and blue solid lines represent the calculation results of Fabry-Perot model and Eq. (4); (c) reflectance spectrums given by RCWA calculation (dots) and Fabry-Perot model (solid lines), and different resonance orders m correspond to the same resonance wavelength 1035.967 nm (larger m is, larger d is); (d) Q, FOM, S as a function of d, red, blue and gray solid lines represent Q, S, FOM obtained from reflectance spectrum calculated by Fabry-Perot model, pink and cyan dotted lines represent Q, S calculated by Eqs. (5) and (6).