Surface electromagnetic radiation from a spatially modulated oscillating dipole layer

We present the calculation of surface electromagnetic radiation to all orders from a spatially modulated, oscillating dipole layer sandwiched between two dielectric media. The results properly include the macroscopic local field factors. In specular reflection direction, we show that the total radiation is partitioned into a term which is the zeroth-order counterpart to the higher-order diffraction and a term related only to the properties of two adjoining bulk media. The results unify the calculations by Shen and Heinz and McIntyre [1, 6, 7]     

Reflection-scanning near-field optical microscopy and spectroscopy of opaque samples

Opaque samples are imaged by Scanning Nearfield Optical Microscopy (SNOM) in reflection mode: A quartz glass fiber tip is used both to illuminate the sample and to collect light locally reflected from or emitted by the surface. The collected light is coupled out by a 2×2 fiber coupler and fed into a grating spectrometer for spectral analysis at each sampled point. The tip-sample distance is controlled by a shear-force feedback system. The simultaneous measurement of topography and optical signals allows an assessment of imaging artifacts, notably topography-induced intensity changes. It is demonstrated that an optical reflectance contrast not induced by topographic interference can be found on suitable samples. Local spectral analysis is shown in images of a photoluminescent layer.     

Diffusion of adsorbates on random alloy surfaces

In this work the diffusion of non-interacting adsorbates on a random AB alloy surface is considered. For this purpose a simple cubic (sc), body-centered cubic (bcc) or face-centered cubic (fcc) auxiliary metal lattice is introduced. The auxiliary lattice is truncated parallel to its (100) plane in such a way that the fourfold hollow positions of the metal surface form a regular net of adsorption sites with square symmetry. The adsorption energy of each adsorption site is determined by its own environment, i.e. by the numbers of direct A or B neighbors. The Monte-Carlo method has been utilized to simulate surface diffusion of adsorbates on such energetically heterogeneous alloy surfaces and to calculate the tracer, jump and chemical diffusion coefficients. The chemical diffusion coefficient was calculated via two different approaches: the fluctuation and the Kubo-Green method. The influence of energetical heterogeneities on the surface diffusion is largely pronounced at low temperatures and low surface coverages, where most of the adatoms are trapped by deep adsorption sites. It was found that at low temperatures the sequential occupation of the different types of adsorption sites can be observed. Received: 24 October 1997 / Accepted: 17 December 1997     

Study of Ge x Si1−x /Si superlattices by ellipsometry

Multiple-angle-of-incidence (MAI) ellipsometry is used to study Ge _x Si_1–x /Si superlattices fabricated by a RRH/VLP-CVD system. Based on the fundamental ellipsometric equations and properties of semiconductor superlattices (SL), various parameters of SL structure, such as the sublayer thickness, complex refractive index and composition, are achieved precisely and simultaneously. The results obtained from ellipsometry are well consistent with those from growth conditions and other characterization techniques.     

Mixing kinetics and write-once optical recording characteristics of Sb/Se bilayer films

Bilayer Sb/Se films are irradiated with 12 ns pulses from an ArF laser (extended areas) and from a focused Ar⁺ laser (micron-sized areas). Real-time reflectivity measurements are used to determine if the process occurs within the solid or liquid phase and the transformation time, in addition to measure the optical contrast and the medium sensitivity. Transmission electron microscopy is used to analyze the structure of the transformed areas and the medium resolution. The results show that mixing is initiated by preferential melting at the grain boundaries and an amorphous phase is produced upon irradiation at high energy densities. Finally, the characteristics of the mixing process in Sb/Se films as a write-once optical recording mechanism are discussed in terms of the sensitivity and resolution of the recorded spots and the time required for recording.     

A kinetic Monte Carlo investigation of island nucleation and growth in thin-film epitaxy in the presence of substrate-mediated interactions

Island nucleation and growth during thin-film epitaxy is typically described using mean-field rate equations, which can be solved to predict the density of stable islands as a function of the deposition rate and the diffusivity of an isolated adatom. Recent theoretical and experimental studies indicate that medium- and long-range interactions between adatoms may change the simple picture that nucleation theory provides, because the presence of these interactions invalidates some of its assumptions. In this work, we investigate the ramifications of medium-range, substrate-mediated interactions for aspects of island nucleation and growth. The interactions are quantified for Ag on a strained Ag (111) substrate using density-functional-theory calculations. We discuss our incorporation of these interactions into a kinetic Monte Carlo model to study thin-film epitaxy. The simulated thin-film growth is compared to predictions by standard nucleation theory. We discuss features of island nucleation and growth that are actuated by the presence of medium-range interactions. Received: 30 April 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Superhydrophobic properties of silver-coated films on copper surface by galvanic exchange reaction

Hydrophobic properties of thin nanostructured silver films produced by galvanic exchange reaction on a copper surface were studied after passivation with stearic acid. The morphology of the silver films was controlled by varying the concentration of silver nitrate in the solution. Water contact angle as high as 156° and contact angle hysteresis as low as 5° were achieved for samples obtained with initial silver ion concentration of 24.75 mM in the solution. However, a strong dependence of contact angle and contact angle hysteresis on the fractal-like morphology of the silver films was observed with the variation of silver ion concentration.     

Reliable Lateral Manipulation of Single Adatom on Metal fcc（lll） Surfaces with a Single-Atom Tip

Using molecular statistics simulations based on the embedded atom method potential, we investigate the reliability of the lateral manipulation of single Pt adatom on Pt（111） surface with a single-atom tip for different tip heights （tip-surface distance） and tip orientations. In the higher tip-height range, tip orientation has little influence on the reliability of the manipulation, and there is an optimal manipulation reliability in this range. In the lower tip- height range the reliability is sensitive to the tip orientation, suggesting that we can obtain a better manipulation reliability with a proper tip orientation. These results can also be extended to the lateral manipulation of Pd adatom on P d （111） surface.     

Reversible Vertical Manipulations of Single Pt Adatom on Pt（111） Surface with a Triple-Apex Tip

With a triple-apex tip, we investigate theoretically the vertical manipulation of single Pt adatom on the Pt（111） surface. The adatom adsorbed on the f cc site of the flat Pt（111） surface can be transferred vertically to the tip by adjusting the tip height properly. Moreover, based on the strong vertical trapping ability and the relatively weak lateral trapping ability of the tip, we propose a simple method to realize a reversible vertical manipulation of the Pt adatom from the highly coordinated sites, the kink and the step sites, of the stepped Pt（111） surface. All the vertical manipulations are completed using only the atomic force between the tip and the adatom, without the electric field.     

X-ray Bragg diffraction on periodic surface gratings

We report on the X-ray Bragg diffraction analysis of periodic surface gratings on (100) GaAs crystals. We show that the X-ray Bragg diffraction is a very powerful method to investigate ultrafine surface gratings and allow us to determine not only the grating period but, in particular, the width of the grooves and to characterize the quality of the sidewalls, which is not easily possible with optical methods. The X-ray diffraction on surface crystal gratings is analogue to the Fraunhofer diffraction of multiple slits or reflection gratings.     

On the theory of TE-polarized waves guided by a nonlinear three-layer structure

TE-polarized electromagnetic waves guided by a three-layer structure consisting of a film surrounded by semi-infinite media (all media are characterized by a Kerrlike dielectric function) are investigated by expressing the solution of the field equations in terms of Weierstrass' elliptic functions. Evaluation leads to a universal dispersion relation and its solutions and a universal expression for the power flow. Numerical results are presented for the effective wave number as a function of the intensity of the electric field at the lower surface of the nonlinear film, for various profiles of the field intensity, and for the power flow as a function of the effective wave number.     

Modulated optical reflectance measurements on amorphous silicon layers and detection of residual defects

Modulated optical reflectance measurements on amorphous silicon layers are presented and a simple theoretical model, which is in good agreement with the experiment, is proposed. Further, the correlation between defects remaining after recrystallization of the amorphous layers and the measured modulated optical reflectance is established. This measurement technique turns out to be useful for characterizing amorphous Si layers produced by ion implantation, for controlling the recrystallization of such layers, and for detecting residual defects.     

Phase diffractive optical gratings on glass substrates by laser ablation

A novel and rapid laser ablation method for the fabrication of phase diffractive gratings in low-cost glass substrates is presented. The structures are characterized in terms of their shape and physical parameters by confocal microscopy and SEM microscopy. The first-order diffraction efficiency at normal incidence under 632.8 nm wavelength is shown. The influence of the overlap factor on the period grating is studied.     

Design of Grazing-Incidence Broad-Band Multilayers for Hard X-Ray Reflectors

A new method of designing x-ray supermirrors with broad angular or energy response for use as coatings in x-ray optics is presented. The design is based on an analytical method with oversimplified analytical and semi-empirical formulae, and an extensive numerical method is used in the optimization design. A better initial multilayer is obtained with the former method and optimized with the latter method. In the optimization, a good design is achieved with much less computing time. In addition, the saturation effect due to the interfacial roughness in multilayer also emerges in the design of x-ray supermirrors with definite performances. The reflectivity of C/W x-ray supermirrors as a function of photon energy at the fixed grazing incident angle 0.5°is presented.     

Ellipsometric and reflectance studies of GaAs/AlAs superlattices

Multiple-angle-of-incidence ellipsometry at 632.8 nm and optical reflectance spectra are used to study GaAs/AlAs superlattices on GaAs substrates. The diagnostic potential of the two methods are compared to each other. Precise values of the total film thicknesses are obtained from the ellipsometric data. Significant disagreement between the measured and nominal thicknesses based on RHEED controlled MBE growth conditions is observed. Nonuniformity of a superlattice film is identified with the spatial resolution of about 1 mm². The thickness variations are accompanied by pronounced variations of the optical constants.     

Dynamic gratings in KDP induced by piezoelectric vibrations at optical frequencies

Optical interference fringes were observed in the beam spot of the He-Ne laser transmitted through KDP. The interference fringes originate from the dynamic gratings induced by the electromagnetic field of the incident light. Diffraction fringes appear only in certain propagation directions of light through KDP. This observation is different from that monitored in α-quartz.     

Design, Fabrication and Measurement of Ni/Ti Multilayer Used for Neutron Monochromator

Ni/Ti multilayers, which can be used for neutron monochromators, are designed, fabricated and measured. Firstly, their reflectivities are simulated based on the Nevot-Croce model. Reflectivities of two Ni/Ti multilayer mirrors with periods d = 10.3 nm （M1） and d = 7.8 nm （M2） are calculated. In the calculation, the reflectivity of the Ni/Ti multilayer is taken as a function of the gazing angle with different roughness factors δ =1.0 nm and = 1.5 nm. Secondly, these two multilayers are fabricated by the direct current magnetron sputtering technology. Thirdly their structures are characterized by small-angle x-ray diffraction. The roughness factors are fitted to be 0.68 nm and 1.16nm for M1 and M2, respectively. Finally their reflective performances are measured on the V14 neutron beam line at the Berlin Neutron Scattering Centre （BENSC）, Germany. The experimental data show that the grazing angle of the reflected neutron intensity peak increases, but the reflected neutron intensity decreases, with the decreasing periods of the multilayers.     

Fabrication of blazed gratings by area-coded effective medium structures

Area-coded effective medium structures (ACES) are a recently presented novel type of diffractive structure. Because of their higher stability compared to 2D binary blazed gratings, they have the potential of a broader use in micro-optics applications. The first fabrication with electron-beam lithography validate the theoretical model of blazed ACES. The measured diffraction efficiencies are in very good agreement with the values obtained from rigorous electromagnetic analysis.     

Mismatch dislocations caused by preferential sputtering of a platinum-nickel alloy surface

Preferential sputtering and recoil mixing of a Pt₂₅Ni₇₅(111) single crystal surface leads to platinum enrichment in the upper monolayers, thereby increasing the lattice constant in these layers. This results in subsurface lattice mismatch dislocations, which have been studied by scanning tunneling microscopy. While the subsurface dislocations are only visible as shallow ditches in STM topographs, the Burgers vectors of the dislocation system can be determined by means of atomically resolved images of dislocations reaching the surface. A comparison with simulations of lattice relaxation using embedded-atom potentials shows good agreement with STM data and further allows the determination of the thickness of the Pt enrichment. We have estimated the Pt concentration in these layers from the dislocation density and studied the annealing behaviour of the surface.     

Blended pastes of cement and lime: Pore structure and capillary porosity

Microstructure of blended pastes of lime and cement were studied in this paper. An increment of complexity of the microstructure was found when pastes increase their percentage in cement. Microstructural characteristics as porosity, morphology of the pores, pore size distribution and surface fractal dimension were evaluated in the different pastes studying the modification with the variation of composition. The capillary water absorption is also evaluated obtaining higher capillary coefficients values for the pastes with higher amounts of lime. The increase of complexity of the microstructure, due to the cement in the pastes, leads to slight deviations of the parallel tube model.