Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 51972039), the Liaoning Revitalization Talents Program, China (Grant No. XLYC1902122), and the Fundamental Research Funds for the Central Universities (Grant Nos. DUT19LK19, DUT20YG132)
Received Date:24 October 2020
Accepted Date:20 November 2020
Available Online:25 May 2021
Published Online:20 May 2021
Abstract:Plasma nanostructures are of particular significance for serving as a substrate for spectroscopic detection and identification of individual molecules. By combining the excitation wavelength of the molecule with the resonance wavelength of the nanostructure, the sensitive single-molecule Raman detection can be achieved. A high and stable plasma substrate for coherent anti-Stokes Raman scattering(CARS) is very useful for developing the surface-enhanced coherent anti-Stokes Raman scattering (SECARS). In the plasma nanostructures, the strong coupling of plasmonic nanoparticles with an inter-particle gap smaller than the diameter of the individual nanoparticles results in the hybridization of the optical properties of these individual nanoparticles. There are also the charge transfer plasmons(CTP) appearing in conductive bridging nanoparticles. Their unique properties make linked nanosystems a suitable candidate for building artificial molecules, nanomotors, sensors, and other optoelectronic devices. In this work, we, starting from reality, theoretically design a new linked nanosystem SECARS substrate where Fano resonance can be generated by the plasmon hybridization (PH) model resonance and the charge transfer plasmon resonance. The introduction of charge transfer plasma improves the tunability of structural resonance. By adjusting the conductivity of the conductive junction, the wavelength of the charge transfer plasma resonance can be easily adjusted to change the wavelength position of the Fano resonance. The data obtained by numerical simulation of the Raman mode at 1557 cm–1 of L-tryptophan when a 1064 nm light source is used as the pump light show that this spatially symmetrical structure can generate multiple high-enhancement hot spots that do not depend on the polarization direction of the incident light. Ordinary CARS signal can generally be enhanced by 1012, and its maximum can reach 1014. Due to the ultrastrong field enhancement and insensitive-to-polarization, this method of using charge transfer plasma to design a substrate can be used in the practical substrate of SECARS and provide new ideas for designing other nonlinear optical processes such as four wave mixing and stimulated Raman scattering. Keywords:surface enhancement coherent anti-Stokes Raman scattering/ Raman scattering/ surface plasmon resonance
图1是基底的结构与参数示意图, 中间的圆盘的半径为r1, 外围圆盘的半径为r2, 中心圆盘与外围圆盘间的距离d, 所有的圆盘具有相同的厚度h1, 外围圆盘间的导电结宽度为l, 导电结与Y轴的夹角为$ \theta $, 导电结的厚度为h2, 导电结中心相对于中心圆盘原点的坐标为O(x, y), 整个基底结构所采用的材料为金. 图 1 基底的结构示意图 (a)与其对应的参数示意图(b) Figure1. Sketch of the structure(a) with the defined parameters and coordinate axis(b).
由于结构的对称性, 可以从图3看到各个角度的散射系数与Fano共振的波长位置完全相同. 图4(a)为在基底表面检测到的电场强度的空间分布, 在左右间隙中的热点具有最强的电场, 可以看到不同的三个波长处的电场增强具有相同的空间分布, 这为SECARS提供了有利条件. 图4(b)为计算出的GSECARS因子空间分布, 在θ = 0°的情况下, 在左右间隙处观察到两个高度局限的热点其GSECARS = 1.6 × 1014, 在此热点旁边也可以观察到存在一片强度不弱的增强区域. 当θ = 45°时, 在四个间隙处同时激发了多个具有相同幅度的热点, 其GSECARS = 5 × 1012, 不过由于能量的分散其最大强度将是各种偏振情况下最低的. 另外, 可以通过降低外盘与内盘之间的距离来进一步增强其热点处的GSECARS因子, 这已在类似系统中得到了证明[30,31]. 图 3 入射光不同偏振角度时的相同参数(l = 35 nm, h2 = 50 nm, O(x, y) = (100, 100), r1 = 63 nm, r2 = 97 nm, d = 10 nm)结构的散射系数, 偏振角度定义为入射光偏振方向与结构Y轴夹角 Figure3. . Scattering spectra for various excitation polarizations with the same parameters (l = 35 nm, h2 = 50 nm, O(x, y) = (100, 100), r1 = 63 nm, r2 = 97 nm, d = 10 nm), and the polarization angle is defined as the angle between the polarization direction and the Y-axis.
图 4 (a) 入射光偏振方向沿基底的Y轴方向时与入射光偏振方向与基底的Y轴的夹角为450时基底表面912 nm, 1064 nm, 1275 nm三个波长处与1064 nm时基底中心YZ横截面处的电场强度空间分布; (b) 当入射光偏振方向沿基底的Y轴方向夹角θ为0°, 15°, 30°, 45°时基底表面对应的增强GSECARS因子的对数空间分布图 Figure4. (a)The spatial distributions of enhanced electric-filed amplitude (|E/E0|) in the top surface plane of the structure at three characteristic wavelengths for two polarizations; (b) the corresponding SECARS map for various polarizations. From the top to bottom, the polarization angle θ equals to 0°, 15°, 30°, 45°, respectively.