Superfocussing in a metal-coated tetrahedral tip by dimensional reduction of surface-to edge-plasmon modes

Metal-coated dielectric tetrahedral tips (T-tip) have long been considered to be interesting structures for the confinement of light to nanoscopic dimensions, and in particular as probes for scanning near-field optical microscopy. Numerical investigations using the Finite-Difference Time-Domain (FDTD) method are used to explore the operation of a T-tip in extraction mode. A dipole source in close proximity to the apex excites the tip, revealing the field evolution in the tip, the resulting edge and face modes on the metal-coated surfaces, and the coupling from these modes into highly directional radiation into the dielectric interior of the tip. These results are the starting point for illumination-mode numerical investigations by a Volume Integral equation method, which compute the field distribution that develops in a T-tip when a Gaussian beam is incident into the tip, and which show that a highly confined electric field is produced at the apex of the tip. The process of light confinement can be considered as a superfocussing effect, because the intensity of the tightly confined light spot is significantly higher than that of the focussed yet much wider incident beam. The mechanism of superfocussing can be considered as a dimensional reduction of surface plasmon modes, where an edge plasmon is the most important link between the waveguide-modes inside the tip and the confined near field at the apex.     

Plasmon resonance microsensor for droplet analysis

Microscale fiber tip sensors based on the plasmon resonance are reported. The fabrication process derived from our previous approach for manufacturing near-field scanning optical microscopy probes has been optimized for sensing applications. A typical tip sensor is a tapered fiber 400 microm in length, coated with a nanoporous thin silver film. The miniaturized geometry of the sensor allows detection in a single droplet of liquid solution (approximately 20 microl). The tip sensor is sensitive for refractive indices between 1.33 and 1.40 with a sensitivity of at least 3 x 10(-4) refractive index unit (RIU)/nm. The Raman scattering enhancement through these microsensors demonstrates the important role played by the localized plasmon resonance. The sensors' linear response covers a large region, interesting for biosensing in aqueous environments such as biomedical applications.     

Au表面等离子体激元激发的扫描探针电子能谱

结合扫描隧道显微镜（STM）与电子能谱仪是实现表面微区元素分析的途径之一．我们将环形电子能量分析器和三维扫描探针系统相结合,建立了一台扫描探针电子能谱仪(SPEES)．通过测量针尖近场发射束流激发的Au表面能量损失谱,我们用研究了Au原子的等离子体激元激发现象．进一步通过改变针尖－样品距离,我们研究了Au等离子体激元峰与弹性散射峰的强度比随针尖－样品距离变化的关系．研究结果发现该强度比与针尖－样品距离的关系并不是单调变化,而是在一个特定位置存在极大．     

Apertureless near-field laser nanotechnology

We consider apertureless near-field optics that provides subwavelength resolution. We study the enhancement of the electromagnetic field near nanospheres and under the tip of a scanning probe microscope using the finite difference time-domain (FDTD) method. We discuss the mechanisms of field enhancement connected with the system geometry (“lightning rod effect”) and resonance excitation of local plasmon eigenmodes for different materials of the tip and various geometrical parameters of the system. We describe the possible applications in nano-optics and nanotechnology. We present the experimental achievements in apertureless near-field nanolithography.     

用并行扫描方法开展的表面等离激元电子能谱成像

本文报道了高定向热解石墨上银纳米结构表面等离激元的电子能谱成像研究. 本文利用现有扫描探针电子能谱仪的角度色散特性,发展了一种多道测量模式. 通过沿一个方向扫描,可以并行获得银纳米结构表面等离激元电子能谱的二维分布. 实验确定的能谱成像空间分辨为80 nm左右.     

Adiabatic nano-focusing of plasmons by sharp metallic wedges

In this paper, we demonstrate the possibility of efficient adiabatic nano-focusing of plasmons by a sharp triangular metal wedge. The geometrical optics approach and the approximation of continuous electrodynamics are used for the analysis. In particular, it is demonstrated that both the phase and group velocities of an incident anti-symmetric (with respect to the magnetic field) plasmon tend to zero at the tip of the wedge, and the plasmon adiabatically slows down, eventually dissipating in the metal. Typically, the amplitude of the plasmon significantly increases near the wedge tip, but this increase is finite even in the absence of dissipation in the metal. The dependence of the local field enhancement near the tip on structural parameters, dissipation in the metal, angle of incidence, etc., is analyzed in this paper. It is also shown that an anti-symmetric film plasmon can effectively be guided by a triangular metal wedge, forming a wedge plasmon mode that is localized near the tip of the wedge and propagates along this tip. A new existence condition for these localized wedge plasmons is derived and discussed. PACS 78.67.-n; 68.37.Uv; 73.20.Mf     

STM tip-enhanced photoluminescence from porphyrin film

Tunneling electrons-induced molecular fluorescence in organic film is enhanced by the surface plasmons. The plasmon enhancement can be expected not only by the plasmons of the substrate but also by the noble metal tip of scanning tunneling microscope (STM). In this report we investigate the tip effect in photoluminescence of meso-tetrakis(3,5-di-tertiarybutyl-phenyl)porphyrin (H₂TBPP) film on indium tin oxide (ITO) combined with a STM. The experimental result shows the PL of molecules is enhanced by an Ag tip. This enhancement factor is evaluated larger than 2000.     

Plasmon resonance in sputtered gold films observed in scanning tunneling microscopy

Gold films grown on BaTiO₃ substrates, illuminated by a monochromatic beam in the visible range, were investigated using scanning tunneling microscopy (STM). Irregular variations in the tip displacements versus photon energy are observed at energies of the surface plasmon resonances (2.0 ± 0.1 and 2.7 ± 0.1 eV) of the rough gold film.     

Cloaked near-field scanning optical microscope tip for noninvasive near-field imaging

Near-field imaging is a well-established technique in biomedical measurements, since closer to the detail of interest it is possible to resolve subwavelength details otherwise unresolved by regular lenses. A near-field scanning optical microscope (NSOM) tip may indeed overcome the resolution limits of far-field optics, but its proximity inherently perturbs the measurement. Here, we apply the recent concept of a "cloaked sensor" to an NSOM device in collection mode, showing theoretically how a proper plasmonic cover applied to an NSOM tip may drastically improve its overall measurement capabilities.     

数值模拟探针诱导表面等离子体共振耦合纳米光刻

采用有损耗介质和色散介质的二维时域有限差分方法,数值模拟了以光波长514.5nm的p偏振基模高斯光束为入射光源,激发Kretschmann型表面等离子体共振,并通过探针的局域场增强效应实现纳米光刻的新方法——探针诱导表面等离子体共振耦合纳米光刻.分别就探针与记录层的间距以及探针针尖大小,模拟分析了不同情况下探针的局域场增强效应和记录层表面的相对电场强度振幅分布.结果表明,探针工作在接触模式时,探针的局域场增强效应最明显,记录层表面的相对电场强度振幅的对比度最大;当探针针尖距记录层5nm时,针尖下方记录层表面的相对电场强度振幅大于光刻临界值的分布宽度与针尖尺寸相近. 关键词： 纳米光刻 表面等离子体共振 时域有限差分方法     

Effect of multiple tips on light emission induced by STM from gold nanostructures

Light emission induced by scanning tunneling microscope on gold islands grown on MoS₂ surfaces has been investigated. Surface geometry and roughness show that different apexes of the same tip can modify the energy of photons emitted in the tunneling junction. Comparisons of topography and photon map are used to locate islands imaged twice and to represent approximately the tip shape used. Light emission spectroscopy on the same island with two apexes of the multiple tip reveals variations of emission properties according to the apex used, showing the importance of tip geometry in the emission process induced by tip induced plasmon modes.     

带金属尖四棱锥小孔光探针光斑近场分布数值模拟

采用时域有限差分法(FDTD)研究了带金属尖四棱锥小孔光探针光斑近场分布特性,分析了金属尖的长度、尖端半径及距离小孔位置等因素对近场电场分布的影响.讨论了该类光探针的电磁波通过等离子体激元方式从小孔传输到金属尖的机理.Taminiau等人用不同方法获得的计算结果在本文的计算结果中得到了印证.在此基础上,本文重点分析、讨论了Pohl等人在1984—1986年获得20—25nm高分辨率图像的原由,讨论了该实验系统的成像机理和类型的归属.计算结果表明:Pohl等人的实验系统成像机理已离开小孔径扫描近场光学显微镜类型,报告认为应归属金属尖散射扫描近场光学显微镜一类.本结果对设计性能优良的光探针具有参考意义.     

Spontaneous emission of an atom placed near a prolate nanospheroid

The frequency shift and linewidth variation of an atomic oscillator placed next to a prolate dielectric or metal spheroid are found within the framework of the classical approach. Both the frequency shift and linewidth are shown to be substantially dependent on the location of the atom and the form of the nanospheroid and capable of reaching very high values near the surface of the nanospheroid under plasmon (polariton) resonance conditions. The predictions are compared with those found for spherical and cylindrical geometries. The prolate spheroid is treated as a model of a needle tip in apertureless optical scanning microscopy. Effects of sharpness of interaction between the nanospheroid tip and atoms are considered. Received 2 January 2002 and Received in final form 3 April 2002 Published online 28 June 2002     

Scanning electro-optic microscope based on surface plasmon resonance

An optical scanning probe microscope, based on surface plasmon resonance (SPR) in the attenuated total reflection geometry, is shown to successfully image the polarization structures in self-assembled monolayers of hemicyanine adsorbed on a gold surface. Application of an ac field to the tip gives rise to the linear electro-optic effect in the monolayer just below the tip and the local change in the refractive index of the monolayer was detected via the ac component of the reflected light intensity, in which SPR-based detection intensifies the ac component. Polarization structures in a monolayer can be clearly imaged by this technique.     

Theory for photon emission from a scanning tunneling microscope

We have calculated the rate of light emission from a scanning tunneling microscope with an Ir tip probing a silver film. In the calculation we model the tip by a sphere. We find a considerable enhancement of the light emission compared with for example inverse photoemission experiments. This enhancement is explained as the result of an amplification of the electromagnetic field in the area below the microscope tip due to a localised interface plasmon. We estimate that one out of 10⁴ tunneling electrons will emit a photon in the visible range. Due to an electromagnetic decoupling of the sphere from the sample the enhanced emission is lost for photon energies above a certain value. We also find that the experimentally observed maximum in the light emission as a function of bias voltage is related to the behavior of tip-sample separation versus bias voltage.     

阴极荧光在表面等离激元研究领域的应用

表面等离激元以其独特的光学性质广泛应用于纳米尺度的局域电磁场增强、超高分辨成像及微弱光电探测.阴极荧光是电子与物质相互作用而产生的光学响应,利用电子束激发金属纳米结构能够实现局域等离激元共振,并在亚波长尺度实现对共振模式的调控,具有超高空间分辨的成像特点.阴极荧光探测通常结合扫描电子显微镜或透射电子显微镜而实现,目前己被应用于表面等离激元的探测及共振模式的分析.本文从阴极荧光物理机理出发,综述了单一金属纳米结构和金属耦合结构的等离激元共振模式阴极荧光研究进展,并总结了阴极荧光与角分辨、时间分辨以及电子能量损失谱等关键技术相结合的应用,进一步分析了其面临的关键问题,最后展望了阴极荧光等离激元研究方向.     

Nanofocusing of optical energy in tapered plasmonic waveguides

We predict theoretically that surface plasmon polaritons propagating toward the tip of a tapered plasmonic waveguide are slowed down and asymptotically stopped when they tend to the tip, never actually reaching it (the travel time to the tip is logarithmically divergent). This phenomenon causes accumulation of energy and giant local fields at the tip. There are various prospective applications in nano-optics and nanotechnology.     

Second-harmonic generation of semiconductor quantum dots studied by near-field optical microscopy

We present a theoretical study of optical second-harmonic generation(SHG) of symmetric semiconductor quantum dots (QDs) excited by the near field of the tip in a near-field scanning optical microscope. We show that the usual optical transition selection rules for the SH nonlinear interaction between the tip field and the QD are broken when the tip is scanned over the QD, because the tip field varies rapidly over the QD domain. It is also demonstrated that the tip-position dependence of the SH signal essentially maps the spatial distribution of the tip field.     

Tip models and force definitions in molecular dynamics simulations of scanning force microscopy

The definition of the time varying force on a tip with internal degrees of freedom in atomistic molecular dynamics (MD) simulations of scanning force microscopy experiments is discussed. We show that the static expression for the tip force is inadequate for calculating force fluctuations within the MD simulations and suggest a different method of calculating the tip force. By studying the size of tip force fluctuations for different tip models and various tip positions with respect to the surface, we demonstrate that the new method works equally well in both static and dynamic cases.     

Highly efficient second-harmonic nanosource for near-field optics and microscopy

A nanometric source of second-harmonic (SH) light with unprecedented efficiency is demonstrated; it exploits the grazing-incidence illumination of a metal tip, which is conventionally used for atomic force microscopy, by 25-fs laser pulses of a high-energy Ti:sapphire oscillator. Tip scanning around the beam focus shows that the SH generation is strongly localized at its apex. The polarization dependence of the SH light complies with the model of an on-axis nonlinear oscillating dipole.