不同入射角度下强流脉冲电子束能量沉积剖面和束流传输系数模拟计算

 强流脉冲电子束在材料中的能量沉积剖面、能量沉积系数和束流传输系数受其入射角的影响很大，理论计算了0.5~2.0MeV的电子束以不同的入射角在Al材料中的能量沉积剖面和能量沉积系数，并且还计算了0.4~1.4MeV电子束以不同入射角穿透不同厚度C靶的束流传输系数。计算结果表明，随着入射角的增大，靶材表面层单位质量中沉积的能量增大，电子在靶材料中穿透深度减小，能量沉积系数减小，相应的束流传输系数也减小；能量为0.5~2.0MeV的电子束当入射角在60°~70°时在材料表面层单位质量中沉积的能量较大。     

Exploring the Influence of a Focusing and Gaussian Profile Electron Beam in SEM Imaging through Monte Carlo Simulation

Gaussian profile is conventionally assumed as the probe shape of the incident electron beam in theoretical analysis of dimensional measurements by a scanning electron microscope (SEM). However, it is not suitable for samples with small and tiny structure. In this paper, a model of a focusing electron beam with finite width due to aberration was used in simulating the SEM image of gold particles/balls on a carbon substrate. An effective electron beam shape (EEBS) was displayed and was found that it deviates significantly from the Gaussian profile. The difference between images simulated by Monte Carlo method with ideal electron incident beam, electron beam focusing model and with ideal beam incident then convoluted by Gaussian profile were discussed in detail. Furthermore, the influence of external electric field effect: full extraction and no extraction for imaging were studied.     

无光栅频率分辨光学开关行迹分析

从无光栅频率分辨光学开关(GRENOUILLE)的测量原理出发,分析了不均匀的光束空间轮廓导致行迹畸变的原因,并通过数值方法模拟了GRENOUILLE内部存在的数据一致性校验机制对这种影响的抑制作用。结果表明:不均匀光束空间轮廓对GRENOUILLE测量结果的影响并不显著;实际测量中更应避免的是使用过小的光束口径,它将导致行迹不完整,从而显著影响时间波形的测量结果。     

Nonparaxial propagation of Hermite-Laguerre-Gaussian beams in uniaxial crystal orthogonal to the optical axis

Analytical expressions for the three components of the nonparaxial propagation of a Hermite-Laguerre-Gaussian （HLG） beam in uniaxial crystal orthogonal to the optical axis are derived. The intensity distribution of an HLG beam and its three components propagating in a uniaxial crystal orthogonal to the optical axis are demonstrated by numerical examples. Although the y and z components of an HLG beam in the incident plane are both equal to zero, they emerge upon propagation inside the uniaxial crystal. Moreover, the beam profile of the x component is relatively stable and the beam profiles of the y and z components have the same evolution law. If the ratio of the extraordinary refractive index to the ordinary refractive index is larger than unity, the beam profile of the HLG beam is elongated in the x direction and generally rotates clockwise. Otherwise, the beam profile of the HLG beam is elongated in the y direction and generally rotates anticlockwise. This research is beneficial to the optical trapping and nonlinear optics involved in the rotation of a beam profile.     

空气隙偏光镜对单模高斯光束光强分布影响的分析

根据光在格兰泰勒棱镜和格兰傅科棱镜空气隙胶合层中的干涉效应,分析了空气隙偏光棱镜对单模高斯光束光强分布的影响;结果表明:对于某一波长的入射光,当空气隙的厚度一定时,透射光强随光在空气隙介面上入射角的变化作周期性振荡;当入射角一定时,透射光强随空气隙厚度的变化作周期性变化;且透射高斯光束的形状也随光的入射角以及空气隙厚度的改变发生变化;且无论是透射光强的周期性振荡,还是透射高斯光束的形状的变化,格兰泰勒棱镜的影响均小于格兰傅科棱镜;这说明前者的综合性能优于后者。     

Elastic and inelastic scattering of slow positrons from a LiF(100) surface

The intensity profile for the elastic specular reflection of 5–100 eV positrons from a LiF(100) surface (ang1e of incidence 45°) has been measured using a simple time-of-flight spectrometer. The profile exhibits strong maxima below 25 eV and a smaller peak at 57 eV. Positron energy loss spectra have also been measured for a range of incident energies by retarding field analysis of the scattered beam. The mean energy loss appears to increase with increasing incident beam energy. Both the elastic and inelastic results are compared with similar data for slow-electron scattering obtained with the same apparatus.     

Femtosecond laser-induced blazed periodic grooves on metals

In this Letter, we generate laser-induced periodic surface structures (LIPSSs) on platinum following femtosecond laser pulse irradiation. For the first time to our knowledge, we study the morphological profile of LIPSSs over a broad incident angular range, and find that the morphological profile of LIPSSs depends significantly on the incident angle of the laser beam. We show that LIPSS grooves become more asymmetric at a larger incident angle, and the morphological profile of LIPSSs formed at an incident angle over 55° eventually resembles that of a blazed grating. Our study suggests that the formation of the blazed groove structures is attributed to the selective ablation of grooves through the asymmetric periodic surface heating following femtosecond pulse irradiation. The blazed grooves are useful for controlling the diffraction efficiency of LIPSSs.     

Generation of a hollow laser beam by a multimode fiber

A simple method to generate a hollow laser beam by multimode fiber is reported. A dark hollow laser beam is generated from a multimode fiber and the dependence of the output beam profile on the incident angle of laser beam is analyzed. The results show that this hollow laser beam can be used to trap and guide cold atoms.     

Spatial beam shaping by quartz crystal piano-convex lens

Based on the optical activity of quartz crystal, we proposed a scheme for shaping the spatial intensity distribution of a linearly polarized laser beam by utilizing a quartz crystal piano-convex lens in combination with a polarizer. The intensity profile of the shaped laser beam can be easily switched from one profile to another by controlling the polarization direction of the incident laser beam.     

Detection of local inhomogeneities in solids by Rayleigh angle reflection

A schlieren method is employed to observe changes in the reflection profile of an ultrasonic beam incident at the Rayleigh angle. These changes are used to determine the location of small elastic inhomogeneities of a flat solid sample.     

Iterative algorithm for the design of diffractive phase elements for laser beam shaping

An improved iterative algorithm for designing diffractive phase elements for laser beam shaping in free space is presented. The algorithm begins with the Gerchberg-Saxton approach to obtain a stable solution. This is followed by several new iterations, in which modified constraining functions are imposed in the Fourier domain while the phase distribution of each iteration remains unchanged. For super-Gaussian beam shaping suitable for inertial confinement fusion applications the mean-square errors of the amplitude and the intensity profile of the entire beam fitted to the corresponding parameters of the 12th-power super-Gaussian beam are approximately 0.035 and 9.75x10(-3), respectively. Approximately 97.4% of the incident energy is converged into the desired region.     

Transformation and shift of a Gaussian light beam upon reflection from a resonant medium

The specific features of the transformation and shift of a Gaussian beam reflected from a resonant absorbing medium are analyzed numerically. The study is based on the expansion of the field of the incident Gaussian beam into plane-wave components by using the Fourier integral transform. The calculations are performed for a frequency range covering the resonance region. The conditions for the splitting of the profile of the reflected beam and for its negative shift are established.     

激光入射角度对薄膜热场分布影响的数值分析

在多层介质薄膜中，激光的入射方式是影响薄膜抗损伤能力的关键因素之一.提出了一种模拟锥角高斯光入射多层介质薄膜后电场和热场分布的方法.该方法能够分析薄膜中高斯光各个角谱分量叠加形成的电场分布，进而得到由于薄膜本征吸收产生的热量沉积以及薄膜内部的温度场分布.针对中心波长为4.3 μm的中红外高反膜进行了分析，给出了高反膜系的温升峰值随激光入射角度和偏振态的变化.结果表明：对于s偏振光，斜入射时膜系的最高温升峰值高于垂直入射峰值，而p光的结果则相反.此种模拟方法克服了原有方法对激光入射角度的限制，较好地反映出斜入射情况下激光偏振态对薄膜损伤的影响. 关键词： 多层介质薄膜 高斯光 热过程 数值分析     

Automatic fringe analysis of the induced anisotropy of bent optical fibres

A new theoretical model considering the refraction of the incident light beam by the fibre is suggested to determine the refractive index profile of bent optical fibres. This new model (slabs model) considering the cross section of the bent optical fibre consists of large number of slabs. The slabs model bases on the refraction of the incident beam by the fibre. The refractive index profile of the optical fibre cladding before bending obtained using the automated Fizeau interferometer with the aid of suggested model is compared with other models such as, the homogenous model and the multilayer model to verify the ability of this slab model. The refractive index profile of the bent optical fibre cladding is investigated using this suggested model. In addition, the new model is used to obtain the induced birefringence and the guiding parameters. The bending radius is recommended to be greater than 7.1 mm for the used optical fibre. The consideration of the refraction increases the accuracy of the results.     

Peculiarities of light beam reflection from the boundary of a 1D-photonic crystal

Typical cases of lateral shift and deformation of the profile of two-dimensional light beams reflected from the surface of a one-dimensional photonic crystal are considered. We use harmonic modulation of the spatial dependence of dielectric permeability of a crystal, which permits one to study reflectance profiles analytically. The results of numerical calculations that were performed are presented for the cases when the angular spectrum of the incident beam is split.     

Stimulated Brillouin scattering of elliptical laser beam in collisionless plasma

This paper presents an investigation of self-focusing of elliptical laser beam in collisionless plasma and its effect on stimulated Brillouin scattering. The pump beam interacts with a pre-excited ion-acoustic wave leading to Brillouin back-scattered process. The transverse intensity gradient of a pump beam generates a ponderomotive force, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification in density effects the incident laser beam, ion-acoustic wave and back-scattered beam. Non-linear differential equations for the beam width parameters of pump laser beam, ion-acoustic wave and back-scattered beam are set up and solved numerically. It is observed from the analysis that the focusing of waves enhances the SBS back-reflectivity.     

An Optical Differentiator Based on a Three-Layer Structure with a W-Shaped Refractive Index Profile

An optical differentiator based on a three-layer resonant structure with a W-shaped refractive index profile is proposed. The differentiation operation is performed in reflection and is associated with the resonant excitation of an eigenmode of the structure localized in the central layer. The presented results of numerical simulations demonstrate the possibility of spatial differentiation of the transverse profile of an incident optical beam, temporal differentiation of the envelope of an incident optical beam, and simultaneous spatial and temporal differentiation with a high quality. The proposed structure can find applications in designing analog optical computing and optical information processing systems.     

Numerical and experimental analysis of the critically refracted longitudinal beam

The critically refracted longitudinal (L_CR) wave can be used in numerous non-destructive testing (NDT) applications, such as characterization of surface geometric aspects, subsurface defect detection and mostly for residual stress measurement. However, very few works characterize the associated ultrasonic beam. This paper deals with characterization of the L_CR beam profile both numerically and experimentally in order to optimize the incident angle choice in order to have sufficient energy in the experimental signal. The simulations are performed in time and frequency domains concerning solid elastic, homogenous and isotropic materials taking into account the liquid–solid interaction of the excitation by a water-coupled transducer. In the obtained results all components of the refracted acoustical field are demonstrated, as well as energy distributions of L_CR wave obtained with different incident angles.     

Beam displacement at diffractive structures under resonance conditions

A beam of light incident upon a diffractive structure can under resonance conditions be subject to a rather large lateral displacement. The effect is demonstrated with a Gaussian profile beam and two geometries based on a waveguide grating and a single-mode waveguide.     

一种强流脉冲电子束入射角等效计算方法

对电子束能量沉积剖面的变化规律进行了解释,给出了由能量损失函数和能量沉积效率函数之积表示的定性的能量沉积函数表达式。在此基础上,给出了一种关于能量沉积剖面的电子束等效入射角计算方法,分析了方法所带来的影响和适用的范围,采用蒙特卡罗方法对等效结果进行了验证。证明该方法优于入射角的算术平均,尤其在靶材浅层与混合束符合较好。