Beam Tilt Research Articles

Formation of a circularly polarized tilted beam using radiation cells of a crank‐line antenna

AbstractThe radiation cell of a crank‐line antenna is analyzed by using the method of moments. The maximum beam direction, axial ratio, and half‐power beamwidth (HPBW) are calculated as a function of the side lengths of the radiation cell. Calculations show that, when the side lengths are properly chosen, the radiation cell can radiate a tilted wide beam of circular polarization. An array consisting of four radiation cells is also analyzed. The effect of mutual coupling on the radiation characteristics is examined. It is found that the array antenna forms a fan beam with an HPBW of 65° in the elevation plane with a tilt angle of 46°. © 2001 Scripta Technica, Electron Comm Jpn Pt 1, 84(8): 23–30, 2001

Twist Reflector Evaluation using Network Analyser

Twist Reflector Antenna Systems are gaining importance because of their inherent advantages like light weight, non requirement of rotary joints and 2θ beam tilt for θ scan. This paper describes a theory for the calculation of scattered field from twist reflector in desired and orthogonal polarization. The experimentally measured electrical characteristics of the twist reflector have been presented and a comparison has been given with the theoretically expected results. The analysis gives an in depth understanding of the twist reflector in its application to antenna systems.

Alignment Error Envelopes for Single Particle Analysis

To determine the structure of a biological particle to high resolution by electron microscopy, image averaging is required to combine information from different views and to increase the signal-to-noise ratio. Starting from the number of noiseless views necessary to resolve features of a given size, four general factors are considered that increase the number of images actually needed: (1) the physics of electron scattering introduces shot noise, (2) thermal motion and particle inhomogeneity cause the scattered electrons to describe a mixture of structures, (3) the microscope system fails to usefully record all the information carried by the scattered electrons, and (4) image misalignment leads to information loss through incoherent averaging. The compound effect of factors 2–4 is approximated by the product of envelope functions. The problem of incoherent image averaging is developed in detail through derivation of five envelope functions that account for small errors in 11 “alignment” parameters describing particle location, orientation, defocus, magnification, and beam tilt. The analysis provides target error tolerances for single particle analysis to near-atomic (3.5 Å) resolution, and this prospect is shown to depend critically on image quality, defocus determination, and microscope alignment.

Exterior caustics produced in scattering of a diagonally incident plane wave by a circular cylinder: semiclassical scattering theory analysis.

We use the semiclassical limit of electromagnetic wave scattering theory to determine the properties of the exterior caustics of a diagonally incident plane wave scattered by an infinitely long homogeneous dielectric circular cylinder in both the near zone and the far zone. The transmission caustic has an exterior/interior cusp transition as the tilt angle of the incident beam is increased, and each of the rainbow caustics has a farzone rainbow/exterior cusp transition and an exterior/interior cusp transition as the incident beam tilt angle is increased. We experimentally observe and analyze both transitions of the first-order rainbow. We also compare the predictions of the semiclassical approximation with those of ray theory and exact electromagnetic wave scattering theory.

Dark spatial solitons in photopolymer films for optical interconnections

We demonstrate theoretically and experimentally that an initially Gaussian red light beam sent through a ?step phase mask and launched into a thin film of polymer poly(methyl methacrylate) doped with laser dye 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)4H- pyran known as DCM evolves into a spatial structure similar to a dark spatial soliton. This takes place due to the Kerr-type time-delayed nonlinearity associated with the mechanism of up-converted photobleaching of the dye-doped polymer. The proposed theoretical model is nonlocal in time and is based on the Shrodinger-type nonlinear equation for the amplitude of the propagating beam complemented by the rate equation for the light-induced decrease of the refractive index. The result of the structuring of the beam is the formation of a permanent pattern of the refractive index of the film that acts as a channel waveguide, trapping a weak Gaussian probe beam close to it. The probe beam can propagate along or against the soliton. We also demonstrate the tolerance of trapping to a possible shift or tilt of the probe beam with respect to the soliton. This makes the proposed approach potentially useful for interconnections between individual fibers, fiber ribbons, bundles, and multicore fibers as well as between fibers and planar integrated optical devices.

Beam tilt and angular dispersion in broad-bandwidth, nanosecond optical parametric oscillators

We show that the signal and idler beams generated by certain types of unseeded, nanosecond optical parametric oscillators are tilted and angularly dispersed and have anomalously large bandwidths. This effect is demonstrated in both laboratory measurements and a numerical model. We show how the optical cavity design influences the tilts and how they can be eliminated or minimized. We also determine the conditions necessary to injection seed these parametric oscillators.

Rutherford backscattering analysis on thin crystals modelled using a Bloch-wave formulation

The Lindhard model for the channeling of MeV ions in crystals is well established. Simulations usually use a Monte Carlo method, which utilises the classical ‘billiard ball’ theory of ions ‘bouncing’ between planes or tubes of charge in the crystal. We propose an alternative quantum mechanical approach, which is similar to that used for high-energy electron diffraction. The diffraction theory is based on a Bloch wave solution of the Schrödinger equation for an ion passing through the periodic crystal potential. The widely used universal screening function for ion–nucleus scattering is used to construct the crystal potential. Our formalism is used to simulate Rutherford backscattering (RBS) cross sections, measured as a function of beam tilt, for proton channeling in nickel. Sample temperature, dechanneling (absorption) and the convergence of the incident beam are taken into account in the calculations.

Imaging of symmetry-forbidden fringes with La 1− xCa xMnO 3 as example

It is shown (experimentally and theoretically) with La 1− x Ca x MnO 3 as example that symmetry forbidden fringes can be easily obtained in HREM images due to symmetry breaking in the diffraction process (crystal tilt) or in the non-linear imaging process (e.g. 2 and 3 fold astigmatism, coma, beam tilt, anisotropic information limit). Given the normal imaging conditions of HREM microscopes such forbidden fringes can hardly be prevented.

Digital television transmission parameters-analysis and discussion

Terrestrial digital (DTV) broadcasting is now underway in the major markets in the United States after the Federal Communications Commission (FCC) in several Reports and Orders set the standard on December 24, 1996, and subsequently released rules of operation and broadcaster channel allocations. Broadcasters are concerned with many in-band and out-of-band transmission parameters, including data signal quality, clock tolerance, radiated power tolerance, carrier phase noise, adjacent channel emissions, and precision frequency offset requirements. The FCC permits DTV power-level changes and/or transmitting antenna location and height and beam tilt in the context of the minimum interference levels. The Advanced Television Systems Committee (ATSC) has provided guidelines for broadcasters in the form of suggested compliance specifications, which are covered in this paper.

Development of direct broadcasting satellite reception system for a small car

To realize the DBS (Direct Broadcasting Satellite) reception system for a small car, we have developed an antenna system concentrating on “Miniaturization” and “Low-cost”. In this paper, an outline and the features of the developed antenna system are described. It can be mounted on a small car, and is a small, low-cost antenna whose design is simplified by adopting the planar beam tilt antenna and abolishing elevation tracking.

Image Periodicities Introduced by Three-Fold Astigmatism in HRTEM Images of α-Al2O3 and Related Materials

Abstract HRTEM images of α-Al2O3 oriented along [1100] often exhibit a modulation of the basal fringe intensity with a period that corresponds to twice the basal plane spacing. In the example shown in figure 1, note the strong basal fringe contrast in the A12O3 with a period of 4.33 Å. HRTEM image simulations for ideal imaging conditions fail to reproduce this doubling of the lattice fringe period. Instead, simulated images predict that the (0006) fringes (d0006=2.17 Å) should be of equal intensity (e.g., see Figure 3a). Incorrect beam and/or crystal tilt can strongly affect HRTEM image contrast. In particular, anomalous periodicities can arise under beam and crystal tilt conditions for which normally kinematically and dynamically forbidden diffracted beams contribute to the image. More recently, the importance of 3-fold astigmatism on HRTEM imaging has been recognized. As with beam tilt, this lens aberration produces asymmetrical phase shifts in the diffracted beams.

User Access to Lorentz Microscopy at the NCEM, LBNL, Berkeley.

A standard Philips CM200FEG electron microscope, without the special Lorentz lens, has been optimized for Lorentz imaging. The necessary field-free sample region is obtained by switching off the objective lens in the free lens mode. The limited range of magnification is compensated for by a post-column Gatan image filter (GIF) which magnifies by a factor of _ 20. Fresnel imaging is performed by defocusing with the diffraction lens. The use of low angle diffraction, in combination with the apertures located at the selected area aperture plane, allow Foucault imaging. The TEM analog of differential phase contrast (DPC) imaging has been implemented. This method makes it possible to obtain quantitave induction maps of the in-plane magnetization. TEM DPC is based on a series of Foucault images, recorded with different incremental beam tilts, which are processed to yield images equivalent to the quadrant signals obtained by the STEM DPC technique.

Residual wave aberrations in the first spherical aberration corrected transmission electron microscope

Here we demonstrate the optical properties of a spherical aberration corrected transmission electron microscope by means of beam tilt series. The high-resolution capabilities are characterized by the measured residual wave aberrations up to the fifth order. Limits for the wave aberration coefficients are determined. We compare the phase-contrast transfer function of the corrected versus the uncorrected objective lens with the help of diffractograms. The information limit and the improvement of the point resolution is discussed.

Simulation of a Radial Laser Scanner for Developing Mobile Robotic Algorithms

A Radial Laser Scanner is a device that provides distances to the surrounding objects by scanning the environment in a plane (usually parallel to the ground). This paper is concerned with the precise modeling and simulation of one of such as sensor, called the Explorer. Based on the calibration of this sensor, we propose a model where the sensor readings are a function of three independent parameters: a gaussian error noise (standard deviation), the true distance to the target and its orientation. By using this model we have developed a software that accurately simulates the data provided by the Explorer as it was working on a mobile robot navigating in a specified environment. Others parameters such as the tilt of the laser beam, the specular behavior of the surface and the mixed point effect are also taking into account when simulating. This simulator, that incorporates a powerful graphical interface, permits to develop and test new robotic navigation algorithms (basically for position estimation and map building) and has demonstrated to accurately reproduce the real sensor measurements

Exploring Spatial Resolution in Electron Back-Scattered Diffraction Experiments via Monte Carlo Simulation

A Monte Carlo model was used to simulate specimen-electron beam interactions relevant to electron back-scattered diffraction (EBSD). Electron trajectories were calculated for a variety of likely experimental conditions to examine the interaction volume of the incident electrons as well as that of the subset of incident electrons that emerge from the specimen, i.e., back-scattered electrons (BSEs). The spatial resolution of EBSD was investigated as functions of both materials properties, such as atomic number, atomic weight, and density, and experimental parameters, such as specimen thickness, tilt, and incident beam accelerating voltage. These simulations reveal that the achievable spatial resolution in EBSD is determined by these intrinsic and extrinsic parameters.

Accurate multislice theory for elastic electron scattering in transmission electron microscopy

Combining the concept of the slice-transmission-operator matrix with the transfer matrix technique, the complete Schrödinger equation for elastic electron scattering in solids is rigorously solved within the framework of the multislice scheme. This new approach extends the range in which the multislice theory is accurate. It is shown that the multislice theory can in principle be as accurate as the Bloch wave theory and the Green function theory in all cases of elastic electron scattering. It is pointed out that in some cases such as the calculation of high-order Laue zone (HOLZ) reflections, more accurate corrections should be made for the conventional high-energy approximation and therefore the conventional multislice method (CMS). A new multislice formula is developed for calculating the transmitted wave. This formula takes back-scattering effects into account and may therefore be accurate for large beam tilt. It is also shown that the new multislice approach keeps all the advantages of the CMS with respect to the Bloch wave method.

Bidimensional spatial solitary waves in quadratically nonlinear bulk media

We numerically investigate the excitation, propagation, and characteristic features of spatial solitonlike waves formed by three-wave mixing in a birefringent bulk medium in the presence of spatial walk-off. Effects such as angular steering, all-optical switching, and beam pointing stabilization are addressed in the cases of normal and tilted input beams.

Multislice Method for Large Beam Tilt with Application to HOLZ Effects in Triclinic and Monoclinic Crystals

Three existing multislice formulations (MS) that are claimed to be applicable for large beam tilt are introduced in a unified form and the validity of these formulations for large beam tilt is evaluated. It is shown that one formula, which is called MSSBT (the MS for small beam tilt), is valid only for tilt angles less than 6°, but the second one, which is called MSLBT (the MS for large beam tilt), can be used for tilt angles as large as 20°, while the third one yields results very close to the MSLBT results for tilt angles up to 12°. Simulations also show that the accuracy of the MSLBT is independent of the scattering power of the atoms. The reasons for the inaccuracy of these multislice formulations for beam tilt (MSBT) and the differences between the three MSBT formulations are discussed based on the complete Schrödinger equation, which includes back-scattering effects. For calculating the higher-order Laue-zone (HOLZ) effects from triclinic and monoclinic crystals, it is pointed out that the slices have to be cut parallel to the ab plane of the crystal and MSBT formulations should be used. The computational formulations for potentials of the crystal slices and the propagator are derived based on the transformations between the crystallographic coordinate system and the working coordinate system. Calculations of the HOLZ reflections for the monoclinic crystal Na2Ti3O7 [001] are carried out. It is shown that for triclinic and monoclinic (when the beam direction is along the c axis) crystals HOLZ effects dynamically influence not only the intensity but also the symmetry of zero-order Laue-zone (ZOLZ) diffraction patterns. Hence, in that case, the projection approximation for simulating the HRTEM images may not be used.

The phase spectrum-based measurement of the TEM parameters

The phase spectrum-based method proposed by Koster et al. (Ultramicroscopy 27 (1989) 251) has experimentally been demonstrated by a computerized 200 kV transmission electron microscope with a slow-scan CCD camera system, which has theoretical advantages compared to other methods utilizing diffractograms. Crucial for the practical implementation of the method has been the addition of a linear phase term in the mathematical model made of the phase contrast transfer function to represent image drift. Moreover, to improve the accuracy of the method for measuring the TEM parameters (defocus, astigmatism and beam tilt misalignment), the model for least squares fitting was used in its full mathematical form without approximation by the DSB imaging model, neglecting the SSB imaging as was done in the original proposal. Experimental results of the TEM parameter measurements, defocus, astigmatism and beam tilt misalignment, by the phase spectrum-based method are demonstrated.

Correction of three-fold astigmatism for ultra-high-resolution TEM

Due to the influence of three-fold astigmatism, the spatial resolution obtained on many modern high-resolution transmission electron microscopes is often not better than 0.2 nm. Although beam tilt can mask the effect of three-fold astigmatism when imaging periodic specimens, the correction of three-fold astigmatism is required for obtaining isotropic ultra-high resolution (≤0.15 nm). Three-fold astigmatism also hampers the correct alignment of the microscope, resulting in additional beam tilt and two-fold astigmatism, and consequently in a further loss of resolution. The amount of three-fold astigmatism of an instrument is found to be stable for periods well in excess of a year. After hardware correction through excitation of a magnetic hexapole in the objective lens stigmator unit of our Philips CM300ST FEG, we reduced the magnitude of the three-fold astigmatism from a value of 1.1 μm, for this particular instrument, to less than 0.1 μm, a value at which it allows at least 0.1-nm spatial resolution. Additionally, we show that the micrscope can now be properly aligned for ultra-high-resolution experiments using the standard manual alignment procedures.