电子束诱导的纳米颗粒表面等离激元激发模拟（英文）

本文基于离散偶极子近似方法,发展出了一套计算任意纳米结构材料的电子能量损失谱的方法和程序.模拟了在单个银纳米颗粒附近不同入射位置下的电子能量损失谱,其计算结果与实验能谱非常吻合.虽然离散偶极子近似法早在很多年前就已经被应用到处理外加光场激发的情形,但这套方法才真正提供了一个研究电子束诱导金属纳米颗粒局域表面等离激元激发的计算工具.     

金纳米颗粒的有序制备及其光学特性

采用纳米球蚀刻技术在石英衬底上制备了不同高度的金纳米颗粒阵列.通过扫描电子显微镜对其表面形貌进行了观测,表明金纳米颗粒为有序分布的三棱柱结构.通过红外—紫外吸收光谱仪在190—900nm波长范围内对其光吸收特性进行了测量, 并成功观测到了金纳米颗粒表面等离子体振荡效应引起的光吸收峰,结果表明随着金纳米颗粒高度的增加,其吸收峰的位置向短波方向移动(蓝移).同时对金纳米颗粒的光吸收特性进行了基于离散偶极子近似的理论计算,并与实验结果进行了比较. 关键词： 纳米球蚀刻技术 金纳米颗粒 离散偶极子近似     

典型金属纳米结构的电子能量损失谱研究

利用格林函数推导出金属纳米结构电子能量损失谱的计算公式,基于时域有限差分方法对几种典型的结构进行建模仿真,数值模拟运动电荷和结构的距离、液晶环境材料对电子能量损失谱的调节作用.仿真结果表明:当增加电子与纳米结构的距离时,电子能量损失谱谱峰降低;当添加液晶材料或各向同性衬底材料时,电子能量损失谱的峰值发生明显红移,但液晶的光轴倾角改变对峰值的调制作用有限.通过计算电子能量损失谱研究金属纳米结构表面等离子激元共振特性,为高度复杂的等离子体激元纳米结构的设计提供了理论基础.     

电子能量损失谱探测银纳米棒与介质层强耦合的数值模拟

表面等离激元与量子发射体间的强耦合现象通常通过散射、吸收以及荧光等远场光谱探测方法进行研究.利用高度聚焦的电子束,电子能量损失谱能够实现亚纳米尺度的局域探测,可以更加有效地研究强耦合现象.本文在理论上分别模拟了银纳米棒、介质材料以及介质层包裹银纳米棒复合结构的电子能量损失谱.得到了可以与实验结果比拟的银纳米棒表面等离激元的电子能量损失谱.在上述复合结构的电子能量损失谱中观察到了谱峰的拉比劈裂,探究了银纳米棒尺寸对拉比劈裂的影响.分别在红外、可见光波段讨论了介质层的元激发与银纳米棒偶极辐射及高阶非辐射表面等离激元模式间的强耦合现象,从损失谱的空间分布成像角度探讨了强耦合引起的杂化等离激子(plexciton)的形成.本研究对强耦合现象的进一步实验和理论研究具有指导意义.     

石墨烯纳米片电子结构和量子输运特性第一原理研究

用基于密度泛函理论的原子紧束缚方法计算研究单层石墨烯纳米圆片和纳米带的电子结构,并结合第一原理和非平衡函数法计算量子输运特性.通过电子能态和轨道密度分布研究纳米碳原子层的电子成键状态,结合电子透射谱、电导和电子势分布分析电子散射与输运机制.石墨烯纳米带和纳米圆片分别呈现金属和半导体的能带特征,片层边缘上电极化分别沿垂直和切向方向,电子电导出现较大的差异,来源于石墨烯纳米圆片边缘的突出碳原子环对电子的强散射.石墨烯纳米带的电子透射谱表现为近似台阶式变化并在费米能级处存在弹道电导峰,而石墨烯纳米圆片的电子能带和透射谱在费米能级处开口并且因量子限制作用呈现更加离散的多条高态密度窄能带和尖锐谱峰.     

十字形银纳米结构的表面等离子体光子学性质

本文应用离散偶极子近似方法计算了十字形银纳米结构的消光光谱及其近场电场强度分布. 研究表明相比于单根纳米棒, 十字形纳米结构能够提供更强的表面电场; 由于相邻凸起间的电场耦合作用, 当入射光的偏振方向改变时, 在十字形纳米结构的侧表面总能激发出较强的电场.另外, 本文还系统地研究了十字形纳米结构的形貌参数对其表面等离子体共振峰的影响. 这些结果将会指导十字形纳米结构的制备, 以满足其在表面增强拉曼散射中的应用.     

微小颗粒的光散射数值模拟

简单介绍了以经典Mie理论为基础的光散射测量技术在颗粒直径和颗粒浓度测量中广泛的应用。分别以Mie理论和离散偶极子近似理论(DDA)为基础, 用数值计算方法分析了球型颗粒的光散射特性，给出了微小颗粒对平行入射光散射的强度函数和散射偏振度的数值计算方法。得到了强度函数和偏振度随相关物理参量变化的三维图，为微小颗粒散射研究提供了一种三维视图。计算结果表明：当尺度参量x＜4时，2种方法所得结果差异不大；随尺度参量增大，2种方法所得结果出现较大差异。与经典Mie理论相比，由于离散偶极子近似理论可以解决各种形状的颗粒散射问题，其应用前景更广泛。     

烟尘中电磁波传输特性的Monte Carlo模拟

根据等效颗粒模型和分形理论,运用Mie理论和离散偶极子近似方法分别计算了烟尘簇团粒子在波长为06328?μm的电磁波入射情况下的单次散射特性.根据辐射输运理论,利用Monte Carlo方法对电磁波在烟尘中的传输特性进行数值计算,给出烟尘中反射率和透射率随入射角和烟尘厚度变化的数值结果,并分析了入射角、烟尘粒子的密度和厚度对反射率和透射率的影响.这为研究电磁波在密集分布介质中的传输特性提供了一种方法. 关键词： 烟尘粒子 离散偶极子近似 传输特性 Monte Carlo模拟     

金纳米环结构的光学性质研究

采用离散偶极子近似方法(DDA)研究了两种不同形状的金纳米环结构的消光光谱及其近电场分布, 研究了等离子体消光峰的红移、蓝移现象及消光系数与结构参数之间的关系, 并与盘状的金纳米结构进行了比较. 在等离子体共振峰波长入射时, 金纳米环结构比金纳米盘结构产生更大的局域增强电场分布, 横截面为圆形的金纳米环结构比横截面为矩形的结构具有更大的局域增强电场分布, 更适合作为表面增强拉曼散射的衬底. 关键词： 离散偶极子近似 金纳米环 金纳米盘 光学性质     

金属纳米椭球结构表面的局域电场和吸附分子分布

过渡金属纳米结构表面吸附CO分子时会出现异常红外效应,这一现象可以用纳米结构表面吸附分子在外电场作用下产生局部凝聚从而相互作用能增加来解释.在前期研究的基础上,给出金属基底表面生长出的纳米颗粒为椭球状颗粒的理论计算结果.基于均匀外电场中金属纳米椭球颗粒按一定对称性排列的表面结构模型,用经典电磁学理论计算了纳米椭球颗粒表面附近的局域电场.在此基础上,将吸附的CO分子等效为偶极子,在考虑了偶极子与局域电场、偶极子之间以及偶极子与金属基底三种相互作用的情况下,用Monte-Carlo方法进行数值模拟,最后给出纳 关键词： 金属纳米结构表面 纳米椭球 吸附分子 局域电场     

Photoemission from multiply excited surface plasmons in Ag nanoparticles

Two-photon photoemission spectroscopy using femtosecond laser pulses is used to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation of this collective excitation leads to a two-orders-of-magnitude-enhanced two-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemission spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent multiple excitation of the surface plasmon in the Ag nanoparticles. This multiply excited plasmon mode can decay via the coupling to a single-particle excitation leading to the emission of an electron if its final state is located in the continuum. The surface plasmon in metallic nanoparticles is a model system to investigate collective excitations in multiphoton processes. Received: 26 June 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Electron spectroscopy of chemically synthesized ZnS clusters

Thin films of ionic compounds of ZnS clusters were measured by electron energy loss spectroscopy (EELS) and ultraviolet photoelectron spectroscopy (UPS). A size effect was observed in the valence plasmon energy measured by EELS from which the coherence length of the plasmon excitation can be estimated. The difference between the lowest excitations observed in UPS and EELS can be explained by the final state charging effect of a single cluster ion in UPS, which strongly depends upon the nominal charges of the clusters.     

Sidebands in the light absorption of driven metallic nanoparticles

The dynamics of the surface plasmon in laser-driven metallic nanoparticles is described by means of a master-equation formalism. Within the Markov approximation, the dynamics is studied for different regimes ranging from weak excitation in photoabsorption experiments to strong excitation in pump-probe spectroscopy. It is shown that two collective levels are sufficient to describe the dynamics of the surface plasmon. On this basis, we predict the appearance of sidebands in the absorption spectrum of the probe laser field in pump-probe experiments.     

Tuning the plasmon resonance of metallic tin nanocrystals in Si-based materials

The optical properties of metallic tin nanoparticles embedded in silicon-based host materials were studied. Thin films containing the nanoparticles were produced using RF magnetron sputtering followed by ex situ heat treatment. Transmission electron microscopy was used to determine the nanoparticle shape and size distribution; spherical, metallic tin nanoparticles were always found. The presence of a localized surface plasmon resonance in the nanoparticles was observed when SiO₂ and amorphous silicon were the host materials. Optical spectroscopy revealed that the localized surface plasmon resonance is at approximately 5.5 eV for tin nanoparticles in SiO₂, and at approximately 2.5 eV in amorphous silicon. The size of the tin nanoparticles in SiO₂ can be varied by changing the tin content of the films; this was used to tune the localized surface plasmon resonance.     

金属-介质-金属渔网超表面结构调控的荧光辐射：空间选择性激发和双增强

增强荧光辐射在生物成像、高灵敏探测、集成光源等方面都具有重要的应用价值.金属纳米颗粒的周围或者金属纳米结构的间隙都可以产生强的电磁场,相应的,这些结构附近的局域态密度也被极大地增强.虽然增强荧光辐射已经在多种金属纳米颗粒和颗粒对中被证明,但是利用金属纳米结构对荧光分子的吸收和辐射过程同时进行调制仍然是一个有挑战的问题.本文研究了金属-介质-金属超表面对荧光辐射的调控,其中局域表面等离激元(LSP)和磁等离激元(MPP)分別与于分子的吸收和辐射过程发中耦合相互作用.对于吸收过程,LSP的耦合作用使得可以通过旋转泵浦激光的偏振态来实现荧光分子的空间选择激发.此外,MPP模式的偏振依赖特性使得矩形渔网结构中的荧光分子的辐射波长和偏振态也受到调控.实验观测结果经过了时域有限差分模拟的验证.本文报道的纳米结构在光辐射器件和纳米尺度集成光源等方面都具有潜在的应用价值.     

Computing electron energy loss spectra with the Discontinuous Galerkin Time-Domain method

In this work, we demonstrate how to extract electron energy loss spectra of metallic nano-particles from time-domain computations. Specifically, we employ the Discontinuous Galerkin Time-Domain (DGTD) method in order to model the excitation of individual metallic nano-spheres and dimers of spheres by a tightly focussed electron beam. The resulting electromagnetic fields that emanate from the particles act back on the electrons and the accumulated effect determines the electrons’ total energy loss. We validate this approach by comparing with analytical results for single spheres. For dimers, we find that the electron beam allows for an efficient excitation of dark modes that are inaccessible for optical spectroscopy. In addition, our time-domain approach provides a basis for dealing with materials that exhibit a significant nonlinear response.     

Surface plasmon assisted photoemission from Au nanoparticles on graphite

The resonant multiple excitation of collective modes in metallic nanoparticles using ultrashort laser pulses leads to an enhanced multiphoton photoemission from the particles. This effect is here demonstrated for the surface-plasmon resonance of Au nanoparticles on graphite. The shape of the photoemission spectra is explained by multiphoton photo-assisted thermionic emission from the nanoparticles and resonant emission via the image-potential state on graphite. Tuning the photon energy between 1.7 eV and 3.2 eV allows the identification of an enhancement of the photoemission yield at 2.1±0.1-eV photon energy that is attributed to the resonant excitation of the surface plasmon in the Au nanoparticles. This identification of the surface-plasmon excitation in this energy range is also supported by electron energy loss spectroscopy. Received: 8 August 2001 / Revised version: 13 September 2001 / Published online: 10 October 2001     

Photoluminescence enhancement induced from silver nanoparticles in Tb3+ -doped glass ceramics

We experimentally verify that surface plasmon (SP) enhances the photoluminescence (PL) of visible light from Tb3+ -doped 60SiO2 -20Al2O3 -20CaF2 :0.3Tb 3+ , 20Yb 3+ glass ceramics by using electron beam lithography to fabricate silver nanoparticles on the surface of the glass ceramics. Numerical calculation for the SP enhancement spectroscopy is achieved by using the finite-difference time-domain algorithm. A PL enhancement of Tb3+ by as much as 1.6 times is observed. The PL enhancement is mainly due to the coupling of excitation from 7 F 6 to 5 D 4 transition dipole of Tb 3+ ion with SP mode induced from the silver nanoparticles.     

Shells on nanowires detected by analytical TEM

Nanostructures in the form of nanowires or filled nanotubes and nanoparticles covered by shells are of great interest in materials science. They allow the creation of new materials with tailored new properties. For the characterisation of these structures and their shells by means of analytical transmission electron microscopy (TEM), especially by energy dispersive X-ray spectroscopy (EDXS), and electron energy loss spectroscopy (EELS), the accurate analysis of linescan intensity profiles is necessary. A mathematical model is described, which is suitable for this analysis. It considers the finite electron beam size, the beam convergence, and the beam broadening within the specimen. It is shown that the beam size influences the measured result of core radius and shell thickness. On the other hand, the influence of the beam broadening within the specimen is negligible. At EELS, the specimen thickness must be smaller than the mean free path for inelastic scattering. Otherwise, artifacts of the signal profile of a nanowire can pretend a nanotube.     

Absorption Range and Energy Shift of Surface Plasmon in Au Monomer and Dimer

The resonance behaviors of local surface plasmon resonance in Au monomer and dimer are characterized systemically by electron energy loss spectroscopy in a scanning transmission electron microscope.The measured absorption range is about 20 nm larger than the physical size of the Au nanoparticles and the resonance peak energy shows a red shift when the electron beam passes off the nanoparticles.The Au dimer displays similar behaviors.Numerical simulation also reproduces those experimental results.