Energy emission from evanescent wave and interference of opposite wave streams

A basic formula for the coefficient of energy emission from an evanescent electromagnetic wave at scattering by a dielectric structure is derived. The derived formula is interpreted in terms of interference of an incident evanescent wave with its reflection by the structure and applied to evanescent wave scattering by a 3D random medium.     

Pairs of opposite decaying evanescent waves in random media and tunneling radiative transfer

A tunneling mechanism of radiative transfer through a dielectric random medium is revealed applying technique of Dyson and Bethe-Salpeter equations for electromagnetic wave multiple scattering by medium inhomogeneities (scatterers) with near fields effects in scattered fields. The mechanism consists in existing inside of a random inhomogeneity a pair of virtually opposite decaying evanescent waves whose interference results in energy flux.     

Near-Field Fluorescence and Topography Characterization of a Single Nanometre Fluorophore by Apertureless Tip-Enhanced Scanning Near-Field Microscopy

Tip-enhanced near-field fluorescence and topography characterization of a single nanometre fluorophore is conducted by using an apertureless scanning near-field microscopy system. A fluorophore with size 80hm is mapped with a spatial resolution of 10hm. The corresponding near-field fluorescence data shows significant signal enhancement due to the apertureless tip-enhanced effect. With the nanometre spatial resolution capability and nanometre local tip-enhanced effect, the apertureless tip-enhanced scanning near-field microscopy may be further used to characterize a single molecule by realizing the local near-field spectrum assignment corresponding to topography at nanometre scale.     

Near-field investigation of exciton dynamics under semiconductor surface

The near-field images calculation method for the semiconductor surface with the excitons generated by strong focused laser pulse was proposed. Calculation was performed using Green function method in the frame of concept of local field. The main characteristic of the proposed approach is maximal usage of the analytical calculations. The near-field images for the Si surface were studied. Developed approach is universal and could be able to find with experimental data on time-resolved near-field microscopy some parameters of exciton such as diffusion constant, relaxation time, and surface state density.     

Factors affecting phase and height contrast of diblock copolymer PS-b-PEO thin films in dynamic force mode atomic force microscopy

Asymmetric PS-b-PEO block copolymer exhibits well-ordered cylindrical morphology with nanoscale domain sizes due to microphase separation. Since the PS and PEO blocks have large stiffness difference, this polymer system represents an ideal candidate for studies of the phase contrast behavior in atomic force microscopy (AFM). In this paper, PS-b-PEO films are investigated under different scanning conditions using two different atomic force microscopes. It is found that the phase contrast of the film can be well described in terms of energy dissipation, though the exact phase image may also depend on the scanning parameters (e.g., the repulsive versus attractive regimes) as well as the settings of the microscope. Height variation on sample surface does not have significant effect on phase contrast. However, in order to obtain true topography of the polymer film, care has to be taken to avoid damage to the sample by AFM. Under certain conditions, true topography can be obtained during the first scan in spite of the surface-damaging forces are used.     

Near-field scanning optical microscopy studies of thin film surfaces and interfaces

In this article, the results of the modeling of topography related artifacts appearing in near-field scanning optical microscopy measurements are presented. The results obtained for near-field scanning optical microscope operation in reflection mode with off-axis far field detector position are compared with experimental results. It is shown that the chosen numerical method - Finite Difference in Time Domain method (FDTD) - can be used for efficient modeling of main topography related artifact. It is also seen that the far field detector position can have large influence on the resulting reflection mode optical images.     

Fine-structured field emission images originating from coherently scattering of electrons within a multi-walled carbon nanotube

High resolution field emission image of a single multi-walled carbon nanotube was studied by field emission microscopy. The images contain patterns consisting of rather ordered bright fringes. We propose a model based on coherent electron scattering to explain the observed field emission image. The emitted electrons will undergo coherent scattering within the cap region of a multi-wall carbon nanotube, which may be viewed as elastic scattering by a polycrystalline structure with an infinite size. This study is helpful for understanding the physical mechanism of field emission of carbon nanotube.     

Diffusion and desorption of submonolayer platinum deposited on the multi-faceted surface of a tungsten micro-crystal

Diffusion and desorption of platinum on the tungsten micro-crystal in the form of the W(1 1 1) oriented emitter tip has been studied using the field electron microscopy (FEM) technique. Diffusion of small dose of platinum (average thickness about 0.18 geometrical ML after spreading) on the thermally clean W emitter tip was studied at temperatures 648-742 K. Average activation energy for diffusion E_diff was found to lie between 1.16 ± 0.08 eVand 1.30 ± 0.16 eV. During annealing at the diffusion temperatures Pt-induced faceting of the emitter surface was visible in the neighbourhood of the {1 1 1} pole. The layer equilibrated in the diffusion process was stable at temperatures up to 1100 K where reduction of the high voltage at a fixed emission current, characteristic of alloying of Pt with W, was detected. Submonolayer of platinum (Θ_Pt = 0.18 ML) started to desorb at tip temperature ≥1780 K. The measurements of average activation energy for desorption of ‘zero coverage’ Pt (0.03 ML ≤ Θ_Pt ≤ 0.06 ML) from the entire W emitter surface were carried out at temperatures 1990-2170 K and yield the value of E_des = 5.19 ± 0.22 eV to 5.33 ± 0.19 eV. The results are compared with data for diffusion of individual Pt atoms and small clusters and with data for adsorption of Pt atoms on a planar W(1 1 0) surface. In discussion the atomic surface structure of the substrate, modified by the strong interaction of Pt with the W micro-crystal, is also taken into account.     

Numerical analysis of the spectral response of an NSOM measurement

Near-field Scanning Optical Microscopy (NSOM) is a powerful tool for investigating optical field with resolution greater than the diffraction limit. In this work, we study the spectral response that would be obtained from an aperture NSOM system using numerical calculations. The sample used in this study is a bowtie nanoaperture that has been shown to produce concentrated and enhanced field. The near- and far-field distributions from a bowtie aperture are also calculated and compared with what would be obtainable from a NSOM system. The results demonstrate that it will be very difficult to resolve the true spectral content of the near-field using aperture NSOM. On the other hand, the far-field response may be used as a guide to the near-field spectrum.     

Simulation of the Propagation Property of Metal Wires Terahertz Waveguides

The propagation property of metal wires terahertz waveguides is studied and simulated under the framework of the Sommerfeld model. The group velocity dispersion, attenuation amplitude, transverse magnetic mode and propagating energy have been obtained by numerically solving the complex eigenvalue equation for the propagation constant. It is found that the group velocity dispersion and attenuation amplitude decrease with the increasing radii of metal wires. The energy power within the dielectric layer increase with the increase of radiation frequency.     

A Thin Liquid Film and Its Effects in an Atomic Force Microscopy Measurement

Recently, it has been observed that a liquid film spreading on a sample surface will significantly distort atomic force microscopy （AFM） measurements. In order to elaborate on the effect, we establish an equation governing the deformation of liquid film under its interaction with the AFM tip and substrate. A key issue is the critical liquid bump height yoc, at which the liquid film jumps to contact the AFM tip. It is found that there are three distinct regimes in the variation of yoc with film thickness H, depending on Hamaker constants of tip, sample and liquid. Noticeably, there is a characteristic thickness H^＊ physically defining what a thin film is; namely, once the film thickness H is the same order as H^＊, the effect of film thickness should be taken into account. The value of H^＊ is dependent on Hamaker constants and liquid surface tension as well as tip radius.     

Fabrication of Pure Silica Core Multimode Ultraviolet Optical Fibre Probes by Tube Etching

As a light wave-guide component for transmitting ultraviolet （UV） laser pulses, pure silica core UV fibre probes have attracted a great deal of attention in the near-field optical data storage and bio-medical studies. We fabricate UV fibre probes with tips in dimension of about 2-5μm and taper angle 16° by the tube etching method, using 40% HF acid as etching solution and xylene as overlayer. Probes produced have curvy configuration with smooth surface. The yield of fine probes is rather high and etching operation greatly simplified. With higher damage threshold, pure silica core multimode UV fibre probes can be coupled into more laser power. In addition, using UV light reduces the cutoff wavelength of the fibre probes, which is in favour of increasing the transmission efficiency of the probe. Furthermore, the larger tip dimension helps to enhance the light throughput either. The advances of fabrication technique of UV optical fibre probe may further support the studies of UV light data storage, pulsed laser biosurgery and UV photolithography.     

Inhomogeneity of a highly efficient InGaN based blue LED studied by three‐dimensional atom probe tomography

The InGaN based multiple quantum well (MQW) structure in a commercially available white light emitting diode (LED) was studied by transmission electron microscopy (TEM) and three‐dimensional atom probe tomography (APT). The average In mole fraction by three‐dimensional (3D) APT was found to be about 18% in the InGaN well which is consistent with the secondary ion mass spectrometry (SIMS) analysis. The In distribution in the InGaN well layer was analyzed by the iso curve mapping of 3D APT and found to be non‐uniform in the InGaN active layer. In clustering or In rich regions in the range of 2–3 nm size were found, in contrast to recent reports. Our results thus indicate that In clustering is essential for high‐brightness InGaN based LEDs. We have also observed a discontinuity in the range of 50–100. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancement of electron field emission by carbon coating on vertically aligned Si nanowires

Electron field emission properties of vertically aligned Si nanowires, synthesized by chemically etching p-type Si wafers with different etching times were investigated in detail. Fabrication of Si nanowires was confirmed by field emission scanning electron microscopic investigation. It was observed that a thin layer of amorphous carbon coating over the grown Si nanowires enhanced the field emission properties significantly.     

Adsorption of L-Alanine on Cu（111） Studied by Scanning Tunnelling Microscopy

The adsorption of L-alanine on Cu（111） surface is studied by means of scanning tunnelling microscopy under ultra-high vacuum conditions. The results show that the adsorbates are chemisorbed on the surface, and can form a two-dimensional gas phase, chain phase and solid phase, depending on deposition rate and amount. The adsorbed molecules can be imaged as individual protrusions and parallel chains in gas and chain phases respectively. It is also found that alanine can form （2 × 2） superstructure on Cu（111） and copper step facet to （110） directions in solid phase. On the basis of our scanning tunnelling microscopic images, a model is proposed for the Cu（111）（2 ×2）-alanine superstructure. In the model, we point out the close link between （110）-direction hydrogen bond chains with the same direction copper step faceting.     

Interaction of hydrogen with palladium surface: FIM and FEM studies

In the present paper, we focus on the geometrical and electronic changes in palladium surface structure which appeared during its interaction with hydrogen in the presence of an external electric field. The interaction process was examined by using the field ion microscopy (FIM) as well as the field emission microscopy (FEM) techniques. In order to study the geometrical changes in substrate surface structure, the distance distribution function (DDF) was constructed on the basis of FIM patterns of both a clean and hydrogen-covered palladium surface. The electronic changes were examined by the measurement of the total energy distribution (TED) of electrons emitted from the palladium tip surface. The most pronounce examples of such changes are an expansion of the equilibrium interatomic distance in palladium surface and a shift of the Fermi level of the metal. These changes may be explained among others by palladium hydrides formation. This process is the most efficient if the field strength exceeds 23 V/nm.     

Highly selective growth of vertically aligned double‐walled carbon nanotubes by a controlled heating method and their electric double‐layer capacitor properties

Vertically aligned double‐walled carbon nanotubes (DWCNTs) with the highest selectivity of 90% were synthesized by a controlled heating method and their electric double‐layer capacitor characteristics were evaluated. DWCNT arrays had a specific capacitance of 83 F/g, which is one of the highest values among CNT arrays in a nonaqueous solution and is almost equivalent to that for single‐walled CNT (SWCNT) arrays reported previously. At the same specific capacitance, DWCNTs with superior structural properties are more promising for practical capacitors than SWCNTs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hard disk magnetic domain nano-spatial resolution imaging by using a near-field scanning microwave microscope with an AFM probe tip

A near-field scanning microwave microscope (NSMM) incorporating an atomic force microscope (AFM) probe tip was used for the direct imaging of magnetic domains of a hard disk under an external magnetic field. We directly imaged the magnetic domain changes by measuring the change of reflection coefficient S₁₁ of the NSMM at an operating frequency near 4.4 GHz. Comparison was made to the magnetic force microscope (MFM) image. Using the AFM probe tip coupled to the tuning fork distance control system enabled nano-spatial resolution. The NSMM incorporating an AFM tip offers a reliable means for quantitative measurement of magnetic domains with nano-scale resolution and high sensitivity.     

Optical oscillation modes of plasmon particles observed in direct space by phase-contrast near-field microscopy

Light distributions near resonant metal nanoparticles are recorded by a scattering-type scanning near-field optical microscope (s-SNOM), for the first time with a sub-particle-size resolution (<10 nm) and with simultaneous amplitude and phase contrast. The images depict the optical oscillation patterns of single plasmon particles. Examples are presented of particles excited in dominantly dipolar and quadrupolar modes, and also of closely spaced particles sustaining a gap mode. The gap mode can provide enhanced optical fields in nanometric spots for non-linear and single-molecule spectroscopy applications. Received: 20 June 2001 / Revised version: 3 August 2001 / Published online: 19 September 2001