光学衍射法细圆柱体直径测量的不确定度

为了提高光学衍射法细圆柱体直径测量精度,用递归法建立了更精确的改进模型。运用改进模型测量细圆柱体直径时,克服了传统测量模型由衍射角引起的原理误差,提高了测量精度。光源的波长误差、暗条纹位置误差和透镜焦距调整误差均会产生测量不确定度,使用改进模型时的计算值分别为0.024μm,1.920μm和0.529μm。细圆柱体直径测量实验结果表明,基于改进模型的光学衍射法的相对测量误差在0.5%以下,优于目前1%的水平;测量不确定度小于2μm,与理论分析相一致。     

Internal symmetries in conical diffraction from metallic gratings

Abstract

Previous investigations in conical diffraction have shown that for a metallic grating illuminated under conditions of resonant excitation of surface plasmons, the maximum efficiency of polarization conversion is obtained when the plane of incidence forms an angle φ = 45% with the main section of the grating. This fact has been understood as a consequence of the evident symmetry imposed by the incident wave and it has not been questioned up to now. Here we show that, apart from this highly symmetrical configuration, the polarization conversion efficiency has local maxima also in other cases which do not exhibit such an evident symmetry. We demonstrate that the occurrence of both the local maxima for the new configurations and the maximum corresponding to φ 45% are a consequence of internal symmetries imposed by the propagation direction of the resonantly excited surface plasmons.     

Polarization conversion in conical diffraction by metallic and dielectric subwavelength gratings

Subwavelength metallic and dielectric diffraction gratings which rotate the linear polarization of incident light by 90 degrees are examined. Using rigorous diffraction theory in total-internal-reflection configuration, it is shown that full conversion from incident transverse electric field to transverse magnetic zero-order field can be achieved with both dielectric and metallic elements, but dielectric gratings provide higher efficiency and are thus preferable. The fabrication aspects and constraints are discussed in detail and the behavior of the gratings over broad wavelength bands is presented.     

Modal method for conical diffraction on a rectangular slit metallic grating in a multilayer structure

The modal method is applied to the problem of conical diffraction on a rectangular slit metallic grating lying on an arbitrary multilayer medium. In the approximation of the surface impedance boundary condition on the grating walls, a single matrix equation is obtained, whose coefficients are expressed simply by the reflectivities on the different layers. A simple and comprehensive treatment is thus obtained for virtually any multilayer system. The method is illustrated for the case of a cavity formed by a planar metallic mirror and a grating, as well as the system formed by a doped layer with Drude susceptibility in a substrate below the grating. The method could be useful for the design of near- and far-infrared devices.     

Influence of conical projectile diameter on perpendicular impact of thin steel plate

A numerical study of conical projectiles for perpendicular impact on a thin steel plate is reported. The target material considered, Weldox 460 E steel, is frequently used for this kind of application and several results of experiments are available in the international literature to verify numerical simulations. The Johnson-Cook constitutive relation coupled with the Johnson-Cook failure criterion have been applied to analyse penetration of the target and also the failure process. The analysis has been focussed on the influence of the projectile diameter on the perforation process, assuming the same projectile mass. The aim was to preserve the same initial kinetic energy and identical nose angle. The goal is to estimate the ballistic limit, the residual velocity, the plastic work, and the temperature levels produced during the penetration process. The analysis has shown a linear increase of the ballistic limit with the projectile diameter.     

Fresnel diffraction from curved fiber snippets with application to fiber diameter measurement

Fiber curvature is examined for its effect on apparent measured fiber diameter in a double-diffraction-based instrument that is in widespread use in the wool industry. The development uses a two-dimensional Fresnel diffraction model. The magnitude of the effect is studied for 2-mm-long snippets of various diameters from 8 to 50 μm and with radii of curvature of 1 m (straight), 600 μm, 280 μm, 200 μm, and 160 μm. The two-dimensional Fresnel model gives rise to deeply nested vector integrations that make computations with a straightforward approach exceedingly time consuming and impractical. A number of simplifying techniques are used to facilitate and speed up the numerical computations, thereby permitting investigations to be carried out on a personal computer.     

Magnetic anisotropy of thin metallic film tapes deposited at oblique incidence

The origin of magnetic anisotropy of thin films deposited at oblique incidence is investigated. Electron micrographs of such films prepared at various incident angles demonstrate a characteristic particle structure. A model of the film developed on this basis is composed of identical elongated rotation ellipsoids whose long axes are oriented parallel to the beam direction. The energy densityFis calculated for all orientations of magnetization, taking into account the shape anisotropy of the particles and the interaction of their stray fields. Hence the anisotropic properties of the film are obtained. The comparison of calculated and experimental results verifies the assumption that anisotropy is caused preferably by effects of shape induced by technological processes. The parameters of films with optimal storage properties (maximum energy product) can be determined theoretically by the suggested method.     

Determination of Stress Tensors in thin textured copper films by grazing incidence diffraction

Ion-platted thin copper films were examined for residual stresses and texture by X-ray diffraction. The complete orientation distribution functions were determined and sharp (111)-fibre textures were found. The strains were measured by grazing incidence diffraction. The stress tensors were calculated using both texture-weighted elastic compliances and texture-independent X-ray elastic constants. The importance of the texture measurement for the stress tensor determination is discussed. The found stresses can be interpreted as thermally induced.     

Three-dimensional time-resolved optical tomography of a conical breast phantom

A 32-channel time-resolved imaging device for medical optical tomography has been employed to evaluate a scheme for imaging the human female breast. The fully automated instrument and the reconstruction procedure have been tested on a conical phantom with tissue-equivalent optical properties. The imaging protocol has been designed to obviate compression of the breast and the need for coupling fluids. Images are generated from experimental data with an iterative reconstruction algorithm that employs a three-dimensional (3D) finite-element diffusion-based forward model. Embedded regions with twice the background optical properties are revealed in separate 3D absorption and scattering images of the phantom. The implications for 3D time-resolved optical tomography of the breast are discussed.     

Photoacoustic determination of thermophysical properties of thin metallic plates using parameter estimation

The phase and the amplitude of the photoacoustic signal were measured as a function of chopping frequency for several kinds of widely used thin metallic plates (stainless steel 304, brass, aluminum, and copper) attached to plexiglass backing. The experimental data have been analyzed systematically by parameter estimation technique based on the two-layer model developed from Rosencwaig-Gersho (R-G) theory. Using this analysis, the values of thermal diffusivity and thermal effusivity of the materials have been determined.     

Gratings in a conical diffraction mounting for an extreme-ultraviolet time-delay-compensated monochromator

The conical diffraction mounting in which the direction of incident light belongs to a plane parallel to the direction of the grooves has the unique property of maintaining high diffraction efficiency, even in the extreme-ultraviolet (EUV) region. This property is useful for designing high-throughput time-delay-compensated monochromators for the spectral selection of ultrashort EUV pulses as the high-order harmonics generated by the interaction between an ultrashort laser pulse and a gas jet. The time compensation allows one to exploit the femtosecond scale duration of the harmonics both to have high intensity and to reach an unprecedented temporal resolution for pump and probe experiments. Because two gratings have to be used for time compensation, the high diffraction efficiency becomes an essential requirement, which can be fulfilled by the conical diffraction mounting. Measurements recently accomplished at the Bending Magnet for Emission Absorption and Reflectivity (BEAR) beam line (ELETTRA Synchrotron, Trieste, Italy) for three gratings in the 10-90 nm region are reported here that show a peak efficiency of as much as 0.7 in the first order. A model computing the electromagnetic propagation and the grating efficiency, implemented and tested with the experimental data, permits the study and design of rather complex systems operating in the conical mounting. Basic physical principles and mathematical aspects of the model are discussed here.     

Half-plane diffraction in a case of oblique incidence

Abstract

The classical problem of half-plane diffraction is examined in a particular case of oblique incidence. A scalar solution to the problem is proposed using boundary diffraction wave theory and some limitations of the solution are discussed.     

Image reconstruction of a buried perfectly conducting cylinder illuminated by transverse electric waves

This article presents a computational approach to the image reconstruction of a perfectly conducting cylinder illuminated by transverse electric waves. A perfectly conducting cylinder of unknown shape buried in one half‐space and scatters the incident wave from another half‐space where the scattered field is recorded. Based on the boundary condition and the measured scattered field, a set of nonlinear integral equations is derived, and the imaging problem is reformulated into an optimization problem. The steady state genetic algorithm is then employed to find out the global extreme solution of the cost function. Numerical results demonstrated that, even when the initial guess is far away from the exact one, good reconstruction can be obtained. In such a case, the gradient‐based methods often get trapped in a local extreme. In addition, the effect of different noise on the reconstruction is investigated. © 2006 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 15, 261–265, 2005     

Model of scattering by a perturbed thin cylinder

A model of wave scattering by a perturbed thin cylinder is constructed. The model is based on the theory of self-adjoint operator extensions. A method is found for choosing a model operator ensuring that the model solution is identical to the leading term of the asymptotic expansion, in terms of the small diameter of the cylinder, of the real scattering problem. Pis’ma Zh. Tekh. Fiz. 25, 42–45 (March 26, 1999)