局域表面等离激元诱导的三次谐波增强效应

本文研究了二维金三角形纳米颗粒阵列的三次谐波非线性光学效应. 金三 角纳米颗粒阵列是基于微球刻印技术制成. 采用反射式光学系统, 观察到该纳米结构在光谱分辨的飞秒激光照射下产生明显的三次谐波. 研究表明, 当激光抽运频率接近金纳米颗粒的局域表面等离激元共振位置时, 三次谐波信号得到增强; 三次谐波辐射方向满足动量匹配条件. 关键词： 微球刻印 三次谐波产生 表面等离激元     

金纳米柱阵列表面等离子体激元与J-聚集分子强耦合作用

采用脱模工艺制备了表面光滑的金纳米柱阵列结构,这种加工方法操作简单、可重复性强,可以将形貌相同的纳米柱阵列结构大批量复制到不同衬底上.采用这种金纳米柱阵列结构激发表面等离子体激元并实现了其与J-聚集染料分子的强耦合作用.在所制备的杂化耦合体系中,观测到了200 meV的拉比劈裂值.进一步通过调节金纳米柱阵列的周期来改变耦合强度,在反射谱中观测到了强耦合存在的标志性实验证据:耦合杂化态对应能量的反交叉曲线.这种简单并可以大批量制备的强耦合杂化体系对于促进强耦合作用在纳米等离子体器件中的广泛应用具有积极意义.     

金纳米环双体尺寸和耦合效应对表面等离子体共振特性的影响

采用离散偶极子近似方法系统地研究了金纳米环双体的消光光谱及其电场分布. 计算结果表明, 金纳米环双体在耦合作用下的共振消光峰对应着不同振动模式, 改变金纳米环双体的排列方式、 间距和尺寸大小, 其表面等离子体共振消光峰发生红移或蓝移. 因此可以通过对金纳米环双体结构参数和排列方式的设定, 调节其表面等离子体共振消光峰的位置. 电场分布表明, 水平排列的金纳米环双体较单个金纳米环产生更强的局部表面增强电场. 适当的小间距, 较大的内外半径的金纳米环水平阵列更适合做表面增强拉曼散射的衬底, 在生物分子检测等领域具有潜在的应用.     

传输矩阵法分析微环谐振器阵列传输特性

微环谐振器可用作未来高密度、超大规模集成光路的基本构件,其重要发展方向之一是多环化、阵列化,微环谐振器阵列近来成为研究的热点。基于定向耦合器、环形谐振腔、直波导腔的基本单元传输矩阵,建立了用于分析微环谐振器阵列传输特性的传输矩阵模型。讨论了列间距对传输特性的影响,并数值模拟了不同尺度的奇数行和偶数行情况下谐振器阵列的传输特性。结合此传输矩阵模型,讨论了通过改变微环谐振器阵列的尺度以及耦合系数以实现滤波特性改善的方案。最后数值研究了最小尺度微环谐振器阵列传输特性与腔间耦合系数的关系。     

椭圆金环结构的太赫兹透射增强

利用太赫兹时域光谱系统,测量了3种在硅衬底上淀积厚度为100nm金膜的椭圆环阵列结构后,并且分析了各样品在太赫兹波段的透射增强现象及产生机理.结果表明:在整个太赫兹波段,此组结构样品的透射系数均在0.68以上,产生明显的透射增强;当太赫兹波的偏振方向与椭圆环长轴之间的夹角为90°时,3个样品在1.67THz处的共振峰是由于短轴方向的电子形成偶极子振荡与入射太赫兹波进行耦合产生的;夹角为0°时,周期阵列样品无明显共振峰,而分形阵列样品的共振峰则不如斑图阵列样品的明显;样品结构的对称性越差,透射谱的信息越丰富.此外,分析相位差谱也验证了共振增强透射的存在.     

表面等离激元“热点”的可控激发及近场增强光谱学

金属纳米结构中特定表面等离激元模式的光学激发及其相互作用是发展高分辨、高灵敏、高精度界面光谱学的物理基础.本文综述了我们研究组近期在表面等离激元共振的光学激发、分类识别、近场增强及在界面光谱学中的应用等方面的进展,主要内容包括:1)利用时域有限差分法,分析了金属粒子-基底体系中SPR"热点"产生的物理机制及影响因素,讨论了电模式和磁模式下界面"热点"的可控激发及"热点"转移的影响因素; 2)利用粒子-金膜体系,实现了可见光频率的表面等离激元磁共振,并利用其形成的"热点"进行了表面增强拉曼光谱实验,获得了比常规电模式更高的拉曼增强; 3)通过界面SPR"热点"增强二次谐波的实验和理论研究,提出并实现了空间分辨率达到1 nm的等离激元增强二次谐波纳米尺; 4)讨论了针尖增强拉曼光谱及针尖增强荧光体系中的空间分辨率、定向发射等关键共性问题的解决方案.相关研究成果可为界面SPR"热点"的可控激发及进一步发展表面增强拉曼、针尖增强拉曼、表面等离激元增强二次谐波等各类高灵敏度,高空间分辨率的界面光谱学方法提供新的思路.     

基于表面等离子体的角动量探测器

为了精确辨别探测光的角动量,设计了一种基于表面等离子体的角动量探测器.该探测器体积小巧,结构简单,仅通过在一块金膜层上刻蚀设计好的纳米矩孔阵列就能实现对入射光角动量的探测.当入射光照射该探测器时,首先根据表面激发产生的表面等离子体波的耦合方向辨别入射光的偏振方向;之后通过测量出射端口的光强强度与耦合方向端口光强总强度之比,定量计算出-1到+1之间轨道角动量拓扑数的精确值.该微纳结构探测器可以精确识别入射光的角动量,为光学片上系统的设计与开发提供新思路.     

高数值孔径二次谐波显微术的分子取向探测

基于矢量衍射理论和格林函数法,利用坐标变换分析高数值孔径条件下二次谐波强度随胶原纤维取向的变化规律.研究结果表明,当胶原纤维垂直于z轴时,聚焦光场Ey、Ez分量的增强导致二次谐波相对强度极小值的上升；随着胶原纤维极角的改变,二次谐波强度的整体变化趋势发生变化,同时前向和后向二次谐波之间的强度变化也出现明显差异.     

基于二次耦合方法的高温超导谐振器弱耦合设计

本文分析了利用二次耦合实现谐振器之间弱耦合的方法,并采用ADS软件对该方法进行了建模验证,同时利用U型二次耦合结构实现了200MHz低频谐振器之间的可控弱耦合强度.该工作对于减小谐振器间距,实现滤波器的小型化设计具有重要指导意义.     

正交偏振双色激光场作用下高次谐波的椭偏特性研究

通过数值求解二维含时薛定谔方程，理论研究了正交偏振双色激光场作用下H2+分子高次谐波发射和孤立阿秒脉冲的产生. 数值结果显示，改变激光场的强度比，可以调控高次谐波强度以及谐波椭偏率. 结果表明随着场强比的增加，y方向的谐波强度不断提高. 当场强比为1:2.5时，x方向和y方向分别仅有奇次谐波和偶次谐波产生，并且偶次谐波的强度比奇次谐波强度高2-3个数量级. 此时，奇次谐波的椭偏率最大可以达到0.3，偶次谐波的椭偏率接近于0. 此外，通过改变分子的准直角可以控制谐波的椭偏率，基于这一现象，利用椭圆偏振高次谐波的平台区合成了椭偏率为0.05的椭圆偏振阿秒脉冲链.     

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研究了在三开口劈裂金属纳米环中,当入射场偏振方向不同时出现的多极局域表面等离激元共振现象及折射率传感特性。研究表明,当入射场偏振方向分别沿x 轴和y 轴时,在可见光-近红外区域分别激发起两个和三个明显的共振峰。通过改变缺口的张角,能够实现对共振峰位和强度的可控调整。共振峰位处劈裂纳米环的近场分布表明,LHA(左半弧)和DRHA(双右半弧)之间等离激元的杂化耦合是形成上述共振的原因。劈裂纳米环的多极共振非常适合折射率传感应用。当改变周围环境折射率,入射场沿x 轴偏振时,折射率敏感度的最大值可达到1365nm/RIU;入射场沿y 轴偏振时,折射率敏感度最大值可达2229nm/RIU。     

劈裂纳米环多极表面等离激元共振及折射率传感特性分析

研究了在三开口劈裂金属纳米环中,当入射场偏振方向不同时出现的多极局域表面等离激元共振现象及折射率传感特性。研究表明,当入射场偏振方向分别沿x 轴和y 轴时,在可见光-近红外区域分别激发起两个和三个明显的共振峰。通过改变缺口的张角,能够实现对共振峰位和强度的可控调整。共振峰位处劈裂纳米环的近场分布表明,LHA(左半弧)和DRHA(双右半弧)之间等离激元的杂化耦合是形成上述共振的原因。劈裂纳米环的多极共振非常适合折射率传感应用。当改变周围环境折射率,入射场沿x 轴偏振时,折射率敏感度的最大值可达到1365nm/RIU;入射场沿y 轴偏振时,折射率敏感度最大值可达2229nm/RIU。     

The Enhanced Second‐Harmonic Generation Based on Magnetic‐Lorentz‐Force Effect

Electromagnetic coupling generates a second‐harmonic signal via the magnetic component of Lorentz force of free electrons. In this work, the enhancement of artificial second‐harmonic generation (SHG) from plasmonic metamaterial with near‐field electromagnetic coupling is theoretically investigated by performing numerical simulations in both frequency and time domains. Simulation results demonstrate that a small gap between the near‐field electric dipole and magnetic resonators could increase the electromagnetic coupling and the SHG signal. In addition, the longer the near‐field electric dipole resonator, the stronger the SHG signal emits. By tuning the geometrical parameters, it has been verified that near‐field electromagnetic coupling results in enhancement of magnetic resonance, thereby improving the SHG signal by more than 3.4 times. This research paves a way toward the design of artificial nonlinearity with metamaterials.     

Magnetic monopole-like response in metamaterials

In this paper, we explored magnetic monopole-like responses in metamaterials. We designed a sub-wavelength metamolecule that is composed of two dielectric-spaced split-ring resonators. In response to incident waves, the induced magnetic field in the metamolecule resembles that of a two-dimensional magnetic monopole. The magnetic monopole-like response is resulted from electric resonance of the metamolecule, so an electric dipole is always attached. By combining two mirror-symmetric metamolecules with inward and outward radial magnetic fields, magnetic dipole-like responses can be produced just as an electric dipole is formed by separating two opposite-signed electric charges.     

Nanoparticle using parallel split rings and implementation of chain for creating Fano resonance with polarization independence for energy harvesting in mid-infrared

In optical devices, the polarization of the incident wave affects the Nano particle characteristics. Therefore, designing a polarization-independent device is significant in the process of designing optical structures. On the other hand, the concept of Fano resonance and dark mode has been utilized for achieving more energy enhancement. In this paper, we have developed a symmetrical Nano antenna by employing Fano resonance, which is independent of the incident wave polarization. The proposed Nano antenna is modified in mid infrared regime for biosensing and energy harvesting applications. The designed metamaterial antenna is made by Nano split ring resonators with etched capacitive gaps, which are utilized for concentrating energy. The introduced Nano antenna has a bright and dark mode with a weak enhancement of electric field. The effect of the incident wave polarization is investigated at wave incident angles between 0° and 45° to illustrate the independency of the polarization due to the symmetrical shape of the Nano antenna. In order to trigger the dark mode and enhance the electric field, a Nano chain is incorporated in the final structure. This arrangement has led to increasing of electric field drastically. Furthermore, the figure of merit has been calculated as an advantageous factor in sensing the surrounding materials with various refractive indices. Our findings illustrated that the chain arrangement has caused a peak in the linear form of the extinction cross section of the Nano antenna. This in turn has resulted in the appearance of Fano resonance with no impact on the resonance frequency that has been originally adjusted by capacitive gaps and inductive strips.     

Second-harmonic generation from complementary split-ring resonators

We present experiments on second-harmonic generation from arrays of magnetic split-ring resonators and arrays of complementary split-ring resonators. In both cases, the fundamental resonance is excited by the incident femtosecond laser pulses under normal incidence, leading to comparably strong second-harmonic signals. These findings are discussed in terms of Babinet's principle and in terms of a recently developed microscopic classical theory that leads to good agreement regarding the relative and the absolute nonlinear signal strengths. The hydrodynamic convective contribution is found to be the dominant source of second-harmonic generation--in contrast to a previous assignment [Science 313, 502 (2006)].     

Symmetric and antisymmetric modes of electromagnetic resonators

In this paper, we numerically study a new type of infrared resonator structure, whose unit cell consists of paired split-ring resonators (SRRs). At different resonant frequencies, the magnetic dipoles induced from the two SRRs within one unit cell can be parallel or antiparallel, which are defined as symmetric and antisymmetic modes, respectively. Detailed simulation indicates that the symmetric mode is due to magnetic coupling to resonators, in which the effective permeability could be negative. However, the antisymmetric mode originating from strong electric coupling may contribute to negative effective permittivity. Our new electromagnetic resonators with pronounced magnetic as well as electric responses could provide a new pathway to design negative index materials (NIMs) in the optical region. PACS 78.20.Ci; 73.20.Mf; 42.25.Bs     

基于人工表面等离激元探针实现太赫兹波的紧聚焦和场增强

为提高太赫兹近场显微成像技术的分辨率,设计了一款在Teflon探针的尖锥形表面镀上厚度渐变、具有相同占空比的超薄金属银制条带的探针,用于实现探针尖端处人工表面等离激元的激发和太赫兹波的亚波长聚焦.研究表明,对于频率为0.1 THz的入射波,厚度渐变镀银条带探针产生的紧聚焦光场的尺寸可稳定在20μm左右(λ/150),探针尖端处最大电场强度为入射电场强度的849倍.研究还发现,周期性金属条带的数目和入射电场的偏振方向可对探针尖端处产生的紧聚焦光斑的尺寸和电场强度等进行灵活有效的调控.     

Metallic nanocavity-enhanced second harmonic generation from a KNbO3 nonlinear nanowire

We propose and experimentally investigate a self-aligned metallic nanocavity that can enhance second harmonic signals from a single KNbO₃ nanowire. With pump beams in the spectral proximity of the metallic nanocavity resonance, second harmonic intensity is observed to be >1800 times stronger than that of a typical bare nanowire under the same illumination conditions. By studying spectral features and polarization characteristics, we confirm that the nonlinear enhancement originates from the locally intensified electric field of a surface plasmon-polariton mode. This simple and robust scheme represents a powerful platform to study single nanowire nonlinearity.     

偏振方向及结构间耦合作用对空心方形银纳米结构表面等离子体共振的影响

空心方形纳米结构能够激发更大面积的增强电场,故其可以作为衬底用于表面增强拉曼散射.应用离散偶极子近似算法研究了空心方形银纳米结构的消光光谱及其近场电场分布与入射光偏振方向之间的关系.研究表明,空心方形银纳米结构的表面等离子体共振峰不随入射光偏振方向的改变而移动,但是其表面增强电场分布却强烈地依赖于入射光的偏振方向.另外,还讨论了空心方形银纳米结构间的耦合作用对其表面等离子体共振模式的影响.结果发现,可以通过调节结构间的距离来改变结构间的耦合作用,同时改变了表面等离子体共振峰的位置.这些结果将为理解闭合纳米 关键词： 空心方形银纳米结构 表面等离子体 偏振 电场耦合