Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China
Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0303700) and the National Natural Science Foundation of China (Grant Nos. 11374150, 11274159).
Received Date:30 March 2019
Accepted Date:06 May 2019
Available Online:01 July 2019
Published Online:20 July 2019
Abstract:Surface-enhanced Raman scattering (SERS) makes the Raman signals, as fingerprints of different vibration modes of chemical bonds, significant in practical applications. Two main mechanisms, chemical and physical, are attributed to the SERS of molecules adsorbed on metals. The physical mechanism plays a major role in SERS, which is the focus of our paper. Recent SERS systems are mostly based on dimer structures, i.e. nanoparticle pairs, of noble metals. Large amplification of electrical field occurs in the gap of a dimer structure compared with a single nanoparticle. The above gap positions are called as " hot spots” of SERS. In addition, the reproducibility and reliability of SERS substrates are also important for practical applications. Here we use periodical subwavelength metallic structures to meet such needs, and develop other kinds of electrical field enhancement mechanisms. We present the electrical field enhancement of the band-edge mode of surface plasmon polariton, gap plasmon polariton mode, as well as their coupling mode. We choose one-dimensional subwavelength metallic structures to clarify the physical mechanism. Our purpose is to develop subwavelength metallic structures with even and intensive " hot spots”, serving as ultrasensitive solid-state SERS substrates with excellent reproducibility and reliability. Keywords:surface-enhanced Raman scattering/ surface plasmon polaritons/ subwavelength structures/ near-field optics
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2.1.SPP带边增强机制
区域1的局部最优结构参数是周期P = 475 nm, 宽度W = 273 nm. SPP带隙大小随深度D和占空比γ改变, 而带隙中心位置由周期P决定. 对γ = W/P = 273 nm/475 nm = 0.575, 通过反射率计算描绘的周期P = 445, 475, 505, 525 nm所对应的SPP能带结构如图2(a)所示. 可以看出, 随着周期变大, SPP带边位置会发生红移, 当周期P = 475 nm时, 上带边位置处在532 nm波长, 当周期P = 505 nm时, 532 nm位置处为带隙位置, 而当周期P = 525 nm时, 下带边位置处在532 nm波长. 在532 nm波长处, 随周期变化的垂直(0°)反射率计算结果如图2(b)所示, 反射率曲线中的两个谷, 分别对应着SPP的上带边和下带边. 反射率越低, 暗示着远场光能量向SPPs近场模式的转化越多, 因此场增强与反射率随周期变化的规律正好相反. 图2(c)是计算的532 nm波长的电场分布图, SPP上带边对应的电场强度最强, SPP带隙位置则最弱, 与SPP带边增强机制一致, 这是因为SPP群速度vg = dω/dk在带边位置最小, 根据电场强度大小反比于群速度大小的原理[17], 获得最大场增强, 而在带隙位置不存在SPP模式. 在SPP上带边位置, SERS增强因子达到峰值, 理论计算值达到6 × 106, 如图2(b)所示. 图 2 (a)计算的不同周期的SPP能带结构图, 其中白色虚线表示λ = 532 nm位置; (b)计算的不同周期结构在532 nm的反射率以及SERS增强因子; (c)对应(b)中四个不同周期值的电场分布图 Figure2. (a) The calculated bandgap structures for various periods but the same depth and duty ratio; (b) the calculated enhancement factor and reflectivity as a function of the period; (c) the calculated electric field distribution of structures with different periods corresponding to the positions A, B, C, and D in (b).
22.2.GPP增强机制 -->
2.2.GPP增强机制
从图1(b)来看, 区域2所对应的SERS增强因子比区域1的更大. 最优结构参数为周期P = 200 nm, 占空比γ = 0.9, 即槽非常窄, 此时槽宽约为20 nm, 这种情况下结构可以被归类为金属-介质-金属(metal-insulator-metal, MIM)结构[18]. MIM结构支持间隙等离子极化激元(gap plasmon polaritons, GPPs)模式的传播. 图3(a)是计算的这一结构的能带结构图, 为一条水平线, 对应于群速度vg = 0, 亦代表着场增强效应, 且GPP模式随着深度增加发生红移. 取周期P = 200 nm, 改变深度D和槽的宽度Wg = P –W, 可以计算出SERS增强因子随着深度D和槽的宽度Wg的变化关系, 如图3(b)所示. 图3(b)中出现若干个增强区域, 固定宽度为20 nm, 在每个区域内取一增强因子最大时对应的深度, 依次分别为35, 160以及290 nm, 计算的电场分布如图3(c), 可以看到, 这些不同的区域对应着槽中间GPP模式的不同阶次, 其中最低阶模式的场增强最显著. 图3(d)是周期P = 200 nm, 槽的宽度Wg = 20 nm时, 计算的SERS增强因子随着深度D变化的曲线, 最大增强因子达到2 × 107. 图 3 (a)计算的不同深度的GPP能带结构图; (b)周期P = 200 nm时, 计算的SERS增强因子随深度以及槽的宽度的变化图; (c)图(b)中三个位置处的电场分布图; (d)周期P = 200 nm, 槽的宽度Wg = 20 nm时, 计算的不同深度的SERS增强因子 Figure3. (a) The calculated bandgap structures for different depths but the same period and width; (b) the calculated enhancement factor as a function of depth and groove width, in which the period is set as 200 nm; (c) the electric field distribution of structures with different depths and widths, corresponding to the positions A, B, and C in (b); (d) the calculated enhancement factor as a function of the depth.
22.3.SPP和GPP耦合增强机制 -->
2.3.SPP和GPP耦合增强机制
基于上述两种不同模式的SERS增强机制, 与亚波长结构的结构参数紧密相关, 理论上可以通过相应的结构参数来独立调节这两个模式. 可以推知, 当这两种模式靠得很近时, 将会发生模式耦合效应, 一般来讲, 耦合后的模式其场增强效果会更佳. 为此, 我们设计加工了图4(a)的双槽结构, 也就是在一个周期内加工两个不同的槽, 其中窄槽提供GPP模式, 周期和占空比调节SPP的带边位置. 令两个槽的深度D一致, 两个槽中间的金属部分宽度和窄槽的宽度一致. 对于周期P、两个槽的宽度分别为W1和W2, 则占空比γ = 1-(W1 + W2)/P. 我们取周期P = 475 nm, W1 = 182 nm, W2 = 20 nm, 占空比γ = 273 nm/475 nm = 0.575. 改变槽的深度D, 则窄槽提供的GPP模式随之移动, 而因周期和占空比不变, SPP带边位置基本不发生移动. 图4(b)是不同深度D对应的能带结构图, 当深度D = 30 nm时, SPP带边和GPP模式在532 nm波长处发生耦合谐振现象. 图4(c)是计算的不同深度的SERS增强因子, 可以看出, 在深度D = 30 nm时, 两个模式发生耦合, SERS增强因子达到峰值. 图4(d)是不同深度下的电场分布图, 分别对应于图4(c)中的A, B和C点位置, 深度分别为10, 30和55 nm. 可以看出在深度为30 nm时, 模式耦合效应令电场显著增强. 图 4 (a)双槽结构示意图; (b)计算的不同深度的双槽结构的能带结构图; (c)计算的不同深度的SERS增强因子; (d)计算的不同深度下的电场分布图, 对应于(c)中的A, B和C点的参数 Figure4. (a) The cross-sectional sketch of the structure with double grooves; (b) the calculated bandgap structures for different depths with P = 475 nm, W1 = 182 nm and W2 = 20 nm; (c) the calculated enhancement factor as a function of the depth; (d) the calculated electric field distribution of different depth corresponding to the positions A, B, and C in (c).