A new positioning approach of fiber probe for near-field scanning optical microscope

A fiber probe-based positioning scan approach was established, whose precision can reach as high as 0.8 μm. And a set of modified microscope system was designed utilizing this approach, in which the scanning probe microscope (SPM) combined with an optical microscope was manipulated. The optical microscope and scanning probe can conveniently be switched through a switch panel. The observation period of samples can significantly be shortened. And more reliable images can be provided using this approach. Our design can effectively solve the inherent disadvantages of SPM technology, which makes SPM scan and image more reliably, conveniently, safely and rapidly.     

Direct observation of variations of optical properties in single quantum dots by using time-resolved near-field scanning optical microscope

We study the variations of optical properties of self-assembled In_0.5Ga_0.5As single quantum dots (QDs) in the spatial and time domains by combining a near-field scanning optical microscope with an ultrafast pulsed laser. Through the examinations of several tens of QDs, we find that the variations of photoluminescence (PL) intensity strongly depend on the condition of the initial carrier creation. The differences in quantum efficiency and those in the carrier flow rate into QDs cause the large distribution of PL intensity when the carriers are excited in the barrier layers. From the results of time-resolved PL decay measurements, we find that there are two types of QDs exhibiting quite different PL decay profiles.     

Dual mode near-field scanning optical microscopy for near-field imaging of surface plasmon polariton

In this paper, we theoretically and experimentally demonstrate that metal coated apertured probes are efficient near-field probes on surfaces with high reflectivity for the scattering as well as for the collection mode near-field scanning optical microscopy (NSOM). We show that a blunt apertured metal coated tip is very effective in suppressing image dipoles which affect strongly the signals scattered from frequently used sharp metal tips or gold nanoparticle attached probes. By using a simultaneous collection and scattering mode (dual mode) NSOM we measure the near-field images of surface plasmon polariton (SPP) launched from a slit. The collection mode measures propagating SPP along lateral distance in a long scan range with high signal-to-noise ratio, and the scattering mode measures the polarization resolved near-field of SPP. Comparisons of the measured data obtained in the dual mode enable to easily characterize SPP and to separate the measured near-field into the propagating SPP and the directly transmitted light.     

Sensitivity analysis of scanning near-field optical microscope probe

In this paper, we study the dynamic modes of a scanning near-field optical microscope (SNOM) which uses an optical fiber probe; and the sensitivity of flexural and axial vibration modes for the probe were derived and the closed-form expressions were obtained. According to the analysis, as expected each mode has a different sensitivity and the first mode is the most sensitive mode of flexural and axial vibration for the SNOM probe. The sensitivities of both flexural and axial modes are greater for a material surface that is compliant with the cantilever probe. As the contact stiffness increases, the high-order vibration modes are more sensitive than the lower-order modes. Furthermore, the axial contact stiffness has a significant effect on the sensitivity of the SNOM probe, and this should be noted when designing new cantilever probes.     

Nano-optical image and probe in a scanning near-field optical microscope

We study a nanometer-sized optical probe and image in a scanning near-field optical microscope (SNOM). We demonstrated the potential to observe 5-nm wide optical patterns using the SNOM. The probe profile was measured by using a knife-edge method and a modulated transfer function evaluation method. An aluminum covered and pipet-pulled fiber probe used here has two optical probes, one which has a large diameter of 350 nm and one which has a small diameter of around 10 nm.     

激光扫描声学显微镜光学系统的设计

半导体激光器具有半导体和固体器件的许多优点：结构紧凑、效率高、价格便宜、长寿命，在许多方面优于气体激光器，因此在一些测量领域有较好的应用前景。本文基于棱镜对光的一维压缩及新型半导体激光器，对激光扫描声学显微镜的光学系统进行了新的设计，给出了相关数据及具体技术指标，实验证实其光学性能满足要求。整个光学系统具有结构紧凑、可靠性高、使用方便等特点，现已用于激光扫描声学显微镜定型设计。     

Influence of the probe--sample interaction on scanning near-field optical microscopic images in the far field

We have studied the influence of probe--sample interaction in a scanning near-field optical microscopy (SNOM) in the far field by using samples with a step structure. For a sample with a step height of $\sim \lambda $/4, the SNOM image contrast between the two sides of the step changes periodically at different scan heights. For a step height of $\sim \lambda $/2, the image contrast remains approximately the same. The probe--sample interaction determines the SNOM image contrast here. The influence of different refractive indices of the sample has been also analysed by using a simple theoretical model.     

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.     

Quantitative model for the change of optical resonance in neural activity detection systems based on surface plasmon resonance

The mechanism of neural activity detection using the surface plasmon resonance (SPR) phenomenon was theoretically explored in this paper. Investigating the mechanism of SPR neural recordings has been difficult due to the complex relationship between different physiological and physical processes such as excitation of a nerve fiber and coherent charge fluctuations on the metal surface. This paper examines how these different processes may be connected by introducing a set of compartmental theoretical models that deal with the molecular scale phenomena; Poisson-Boltzmann (PB) equation, which was used to describe the ion concentration change under the time varying electrostatic potential, Drude-Lorentz electron model, which was used to describe electron dynamics under the time varying external forces, and a Fresnel's three-layered model, which expresses the reflectivity of the SPR system in terms of the dielectric constants. Each physical theoretical model was numerically analyzed using the finite element method (FEM) formulated for the PB equation and the Green's method formulated for the Drude-Lorentz electron equation. The model predicts that the ionic thermal force originating from the opening of the K⁺ ion channel is fundamental for modifying the dipole moment of the gold's free electron; thus, the reflectivity is changed in the SPR system. The discussion was done also on important attributes of the SPR signal such as biphasic fluctuation and the electrical noise-free characteristics.     

光磁共振实验中扫场的作用和影响

讨论了方波和三角波扫场对光抽运信号和磁共振信号的作用和影响,阐明了应如何选择方波和三角波扫场信号的大小和方向,才能得到理想的光抽运信号和磁共振信号的方法．     

A mechanism for cutting carbon nanotubes with a scanning tunneling microscope

We discuss the local cutting of single-walled carbon nanotubes by a voltage pulse to the tip of a scanning tunneling microscope. The tip voltage ( V) is the key physical quantity in the cutting process. After reviewing several possible physical mechanisms we conclude that the cutting process relies on the weakening of the carbon-carbon bonds through a combination of localized particle-hole excitations induced by inelastically tunneling electrons and elastic deformation due to the electric field between tip and sample. The carbon network releases part of the induced mechanical stress by forming topological defects that act as nucleation centers for the formation of dislocations that dynamically propagate towards bond-breaking. Received 6 April 2000     

Apertureless near field optical microscopy: a contribution to the understanding of the signal detected in the presence of a background field

Apertureless scanning near field optical microscopy techniques have become a common way of studying surface samples. By using a nano-probe that scatters the electromagnetic non-propagative waves emerging from a given sample, this microscopy provides optical images with a resolution beyond the diffraction limit. Despite a great diversity of works covering a wide variety of physical domains, the formation of the images obtained is not yet fully understood. The purpose of this letter is to assess the influence of the tip position and imposed oscillation of the tip in apertureless SNOM when a background field is added to the scattered near field. We propose a simple analytical model which enables us to account for the experimental results and explains how, depending on the experimental conditions, the near field signal can totally disappear or, on the contrary, be greatly enhanced.     

Visualization of space charge field effect on excitons in a GaAs quantum dot by near-field optical wavefunction mapping

Near-field photoluminescence (PL) imaging spectroscopy was used to investigate multi-exciton and charged-exciton states confined in a single GaAs interface fluctuation quantum dot. We determined the origin of peaks in the PL spectra by employing a wavefunction mapping technique. We observed distortion of the exciton wavefunction due to the electric field produced by an excess electron at a nearby confined state. Near-field wavefunction mapping was demonstrated to be a powerful tool for visualizing the local environment, which affects the emission properties of quantum dots.     

Laser nanotraps and nanotweezers for cold atoms: 3D gradient dipole force trap in the vicinity of scanning near-field optical microscope tip

Using a two-dipole model of an optical near-field of scanning near-field optical microscope tip, i.e., taking into account contributions of magnetic and electric dipoles, we propose and analyze a new type of 3D optical nanotrap found for certain relations between electric and magnetic dipoles. Electric field attains a minimum value in vacuum in the vicinity of the tip and hence such a trap is quite suitable for manipulations with cold atoms.     

Modelling of surface plasmon resonance sensor for detection of mass concentration of ethanol and methanol in a binary mixture

Modelling of a surface plasmon resonance sensor for detection of ethanol or methanol content in water is presented. The reflectivities of these sensors are derived using transfer matrix method and hence the corresponding sensitivity and detection accuracy are obtained in the presence of high index overlayer, like silicon and chalcogenide. It is observed that the resonant wavelength of surface plasmon resonance sensor is shifted toward higher wavelength with increase of refractive index and thickness of overlayer. Hence, by adjusting their values, the desired spectral operation range of this sensor is obtained. The sensitivity of this sensor with silicon overlayer is larger than the chalcogenide overlayer but at the same time detection accuracy is smaller. This sensor can be used to detect the mass concentration of ethanol, methanol and other similar materials. A method to obtain the percentage mass concentration of ethanol is discussed.     

A proposed method for generating and receiving narrow beams of ultrasound in the Fast Reactor liquid sodium environment

In the liquid sodium coolant of the Fast Reactor there are several measurements which may be performed using ultrasonic techniques. This paper gives a brief outline of one such measurement and then describes the design of an ultrasonic waveguide for the generation and detection of ultrasound in this hostile situation. The successful laboratory tests of the device are also reported.     

A Cartesian method for fitting the bathymetry and tracking the dynamic position of the shoreline in a three-dimensional, hydrodynamic model

This paper presents a Cartesian method for the simultaneous fitting of the bathymetry and shorelines in a three-dimensional, hydrodynamic model for free-surface flows. The model, named LESS3D (Lake & Estuarine Simulation System in Three Dimensions), solves flux-based finite difference equations in the Cartesian-coordinate system (x,y,z). It uses a bilinear bottom to fit the bottom topography and keeps track the dynamic position of the shoreline. The resulting computational cells are hybrid: interior cells are regular Cartesian grid cells with six rectangular faces, and boundary/bottom cells (at least one face is the water–solid interface) are unstructured cells whose faces are generally not rectangular. With the bilinear interpolation, the shape of a boundary/bottom cell can be determined at each time step. This allows the Cartesian coordinate model to accurately track the dynamic position of the shorelines. The method was tested with a laboratory experiment of a Tsunami runup case on a circular island. It was also tested for an estuary in Florida, USA. Both model applications demonstrated that the Cartesian method is quite robust. Because the present method does not require any coordinate transformation, it can be an attractive alternative to curvilinear grid model.     

A method for calculating the surface tension of a droplet in a lattice-gas model with long-range interaction

Since the droplet of the lattice-gas model with long-range interaction is not circular and is determined by the Wulff construction due to surface tension anisotropy, a calculation method of the surface tension of the droplet is proposed in this paper. The calculated surface tension is in good agreement with the surface tension measured by using the Laplace's formula in consideration of the surface thickness. Received 15 March 2000 and Received in final form 31 May 2000