Nonlinear optical diffraction by standing acoustic waves in a GaAs film

Second-order nonlinear optical diffraction by standing acoustic waves in a crystalline plate is theoretically investigated. A detailed analysis of the polarization state of the second-harmonic light diffracted by both longitudinal and transversal acoustic waves is carried out. It is shown that longitudinal standing acoustic waves only allow p-polarized nonlinear optical diffraction, irrespective of the incoming state of polarization, whereas transversal standing acoustic waves allow all possible combinations of incoming and diffracted polarization states. Numerical estimates of the relative intensities of nonlinearly diffracted radiation peaks are made for a GaAs plate.     

Collinear diffraction of restricted light beams by nonmonochromatic sound

The collinear diffraction of Gaussian light beams by a two-frequency sound beam is investigated theoretically. A set of equations relating the amplitudes of transmitted and diffracted waves is derived in the case of third-order nonlinearity. The distributions of light intensities along an acousto-optical cell are calculated at different ratios between the radii of the light and sound beams. The transmission curves for detuned light waves and combination components of the diffracted waves are constructed at different ratios between the radii of the light and sound beams. The changes in the dynamic range of an acousto-optical device are calculated for different diffraction efficiencies.     

Polarization controllable spatial filter based on azo-dye-doped liquid-crystal film

This investigation establishes the feasibility of exploiting the surface-assisted photoalignment effect in dye-doped liquid-crystal (DDLC) films as spatial filters with controllable polarization in optical signal processing. The fabrication relies on the fact that the various intensities of the diffracted orders are responsible for various changes of the polarization state induced by the photoaligned DDLC film. Specific spatial orders in Fourier optical signal processing can be filtered by use of an analyzer placed behind the sample to control the polarization state of the diffracted orders. A simulation was performed, and the results agree closely with the experimental data.     

Amplitude modulation of diffracted light waves caused by adjacent ultrasonic beams of frequency ratio 1:n

A study has been made of the influence of the phase-difference of two ultrasonic beams, a fundamental and its nth harmonic, on the intensities or amplitudes of diffracted lightwaves. After showing the similarity between superposed and adjacent ultrasound in the Raman-Nath region ρ ? 1, simplified expressions for the intensities have been set up to draw conclusions concerning the symmetry of the diffracted pattern, the amplitude modulation, and the numerical calculation of the factor α_n, from the intensities in which α_n represents the ratio of the ultrasonic pressures. All results are illustrated in the case of a fundamental and its second or third harmonic.     

RHEED from stepped surfaces and its relation to RHEED intensity oscillations observed during MBE

Multiple scattering theory is used to calculate the intensities of reflection high energy electron diffraction from periodic arrays of surface steps. The intensities are found to depend strongly on the direction of the incident beam azimuth. When the incident beam azimuth is parallel to the step edges, both the specular and diffracted beam intensities are diminished with respect to the intensities from a flat surface. When the incident beam azimuth is perpendicular to the edges, the intensities of all the beams are of the same order of magnitude as for a flat surface but some of the peak heights are oscillatory functions of the number of atoms in the topmost layer. These peak intensity oscillations are very similar to the intensity oscillations observed during molecular beam epitaxial film growth.     

LEED-AES-TDS characterization of Sb overlayers on GaAs(110)

The GaAs(110)-Sb system is studied with Auger Electron Spectroscopy, Low Energy Electron Diffraction, Soft X-Ray Photoemission Spectroscopy and Thermal Desorption. Sb evaporated at room temperature forms a continuous film and a sharp interface with the substrate. For a coverage of one monolayer, LEED indicates a well ordered (1 × 1) structure which produces diffracted intensities very different from those measured from the clean substrate. Thermal desorption experiments show the particularly strong bonding between the first Sb monolayer and the substrate, confirming the large chemical stability observed during photoemission experiments. Two types of structures were considered for GaAs(110)-Sb(1 ML). Sb chains extending along the (110) direction, parallel and anti-parallel to the top layer Ga-As chains, and Sb dimers placed above the surface with one Sb bond to the surface Ga. The preliminary LEED analysis favors the later model.     

Atom-solid scattering: He + graphite basal plane

Theoretical studies of scattering of atoms by solid surfaces and calculations of atomic band structure of adsorbed atoms are presented. Inelastic effects on the intensities of diffracted beams are considered within the framework of the optical model potential. The atom-solid potential used for carrying out the numerical calculation is the sum of Lennard-Jones 12-6 pair potentials for the He-graphite system. Resonances with bound states of adsorbed atoms are shown to enhance the diffraction into some of the open channels and inelastic scattering. The degree of enhancement for a certain process depends on the coupling between that process and the resonant bound state and the coupling between the bound state and the incident beam. For first order bound state resonances, minima in the specular intensity may result from a large increase in some of the diffracted intensities or enhancement of inelastic scattering.     

Computations of relativistic LEED intensities and spin-polarisations for tungsten (001)

The effects of relativistic corrections in low-energy electron diffraction intensity calculations are assessed by reference to relativistic and non-relativistic computations for tungsten (001). Small changes are observed in the calculated intensities and band structures and a significant degree of spin-polarisation is predicted for the diffracted beams. The spin-polarisation profiles are more complicated than those observed in gas-phase experiments because of the influence of multiple scattering. The results suggest that LEED could provide a strong source of polarised electrons for use in scattering experiments. It is suggested that in future LEED experiments both spin-polarisation and intensity measurements should be performed on the diffracted electron beams.     

环形光束锥形衍射出射光场偏振特性及光场调控

环形光束的锥形衍射效应对于微粒的操控具有重要的应用价值.本文建立了环状高斯光束的锥形衍射模型,并基于Berry理论给出了线偏振态下环形光束锥形衍射出射光场的计算公式.理论仿真了环状光锥形衍射出射光场的偏振特性,得出环光锥形衍射出射光场的内、外亮环具有相互正交的偏振分布特性.搭建了线偏振态下环状高斯光锥形衍射的实验系统,实验验证了出射光场的偏振特性.针对环形光束锥形衍射出射光场具备的偏振特性,设计了一种组合偏振片,理论和实验研究了该组合偏振片对环光锥形衍射出射光场的调控.结果表明,随着组合偏振片方位角的变化,锥形衍射出射光场的内、外环强度发生周期性的变化.     

Height distribution on a rough plane and specularly diffracted light amplitude are Fourier transform pair

In this report we show that the amplitude of specularly diffracted light from a plane rough surface as a function of incident angle cosine is Fourier transform of the height distribution on the surface. Therefore, an even height distribution function, which is the case for many rough surfaces, can be obtained by measuring the specularly diffracted light intensities. Also, it is observed that for polychromatic illumination the spectrum of the specularly diffracted light is modified and the modification depends on roughness, incident angle, and wavelength. It is also shown that, for a fixed incident angle, the height distribution on the rough plane is Fourier transform of the spectral modifying function. Experimental studies on some surfaces of different roughnesses, prepared by grinding sheet-glasses by powders of different grain sizes, show that the corresponding height distributions are Gaussian and the rms heights obtained by the two approaches are quite consistent.     

A review of commonly used analytical and empirical formulae for predicting sound diffracted by a thin screen

This paper reviews some useful analytical and empirical formulae that are used for the calculation of sound diffracted by a barrier. A brief historic overview of the study of spherical waves diffracted by a rigid half plane is presented also. The physical principles of sound diffraction by a thin plane are explained. The mathematical expressions are cast in a convenient form for ease of numerical implementation. Accurate analytical solutions are expressed in terms of standard mathematical functions that can be computed readily. Among the approximate solutions quoted, quite a few empirical formulations are adequate for most engineering purposes. The information presented in this paper should also be helpful for those who are interested in the study of sound diffracted by a barrier in a room or in a long industrial space.     

Quantitative measurement of image intensity in transmission electron microscope images

We have made a thorough comparison of the ability of image simulations to predict the contrast in high-resolution electron microscope lattice images of GaAs. Simulations of the diffracted beam intensities from thickness fringes generally agreed with observations to within ∼20% over a range of GaAs thicknesses up to 150 nm. Likewise, simulations of lattice images agreed qualitatively with experimental lattice images over a range of defocus and sample thicknesses up to 20 nm. However, using the same parameters as for the diffracted beam intensities, lattice fringe amplitudes were calculated to be typically two to three times higher than observed experimentally.     

Spin-polarization in LEED: A comparison of theoretical predictions

Two distinct theories of spin-dependent low energy electron diffraction have been used to calculate scattered intensities and polarizations for the same model of the W(001) surface. The agreement between the results is excellent, demonstrating that calculations using either program should provide a reliable basis for the analysis of experimentally measured intensities and polarizations. The effects of using different surface barriers and of increasing the number of phase shifts and diffracted beams are also examined.     

Diffraction efficiency of sequentially stored gratings in reflection volume holograms

On the basis of the theory of coupled waves the amplitudes of the first-order diffracted waves and the diffraction efficiency is calculated for sequentially superposed phase gratings stored in dielectric reflection holograms. The investigations are restricted to the case of a fixed reference wave during recording and Bragg-angle incidence.By analogy to the transmission volume grating the coupling effects which appear between incident beams and diffracted waves give the possibility of building up beam-couplers and -splitters with certain coupling parameters determined by the recording process. Moreover, reflection gratings allow a favourable combination between coupling and reflecting properties and avoid the diffraction efficiency oscillations that are characteristic of transmission gratings. The performance of the reconstruction process with several waves simultaneously allows the tuning of the intensities of diffracted waves with dependence on amplitude or phase differences between incident beams.     

Moiré displacement measurement technique for a linear encoder

The light intensity diffracted from superimposed dual transmission gratings is sensitive to their relative lateral displacement but is also affected by changes in the air gap between them. Averaging the diffracted light intensities obtained in the gap range of a Fourier image distance, the resultant intensity was almost independent of the gap between the two gratings and had a period of half the pitch of the grating for the lateral displacement. This method can be used for a linear encoder in precision machining.     

Adiabatic three-wave volume hologram: large efficiency independent of grating strength and polarization

A transmission hologram with two volume gratings is considered in the regime of wave A diffracted into wave B via an intermediate weakly excited wave C. In analogy to the phenomenon of stimulated Raman adiabatic passage in nonlinear optics, this new scheme demonstrates diffraction efficiency with low sensitivity to the hologram's strength. A theory of three-wave adiabatic coupling has been developed and explored analytically. Numerical results show an example of a coupling profile that preserves high diffraction efficiency with almost no dependence on the hologram's strength, including the suppressed influence of polarization.     

柱状导体衍射级数

本文对于任意形状的光滑柱状理想导体的衍射提出一种级数解法。方法的原理与层变媒质的Bremmer级数相似:先以内接多面稜柱代替上述光滑柱体;将此稜柱产生的衍射场展为一个级数。级数之首项为几何光学场;级数之第二项为稜柱的所有各稜产生的元衍射场之和,其中每个元衍射场皆取Sommerfeld问题的解,即将该稜之两侧面视为半无限大的平面。上述每一元衍射场皆投射在其相邻稜上,并在相邻稜上发生衍射;这一衍射场随之又投射在下一个相邻稜上而发生衍射;依此类推。按此方式依次被各稜所衍射的场称为“主掠射元场”。级数之第三项即为这些主掠射元场之和。被某一稜A衍射而后又在相邻的稜B上衍射的某一元场,同样会回射到A上;然后以上述“主掠射”方式传递下去,这样的场称为“一次反射元场”。级数的第四项即为这些一次反射元场之和。依此类推。一般说来,级数之第m项(m>3)为m-3次反射元场之和。元场在任何一稜上的衍射皆取Sommerfeld解。当内接多面稜柱之面数趋向无穷,且每面之宽度趋向零时,多面稜柱即趋于光滑柱体,且级数每一项的求和变为一个积分。这时该级数总和之极限即为原问题之解。对级数之前三项单独进行了推导。对于一般的第m项(m>3),导出了一个递推公式。最后,对该级数之收敛条件进行了探讨。     

Controlling quantum systems by embedded dynamical decoupling schemes

A dynamical decoupling method is presented which is based on embedding a deterministic decoupling scheme into a stochastic one. This way it is possible to combine the advantages of both methods and to increase the suppression of undesired perturbations of quantum systems significantly even for long interaction times. As a first application the stabilization of a quantum memory is discussed which is perturbed by one- and two-qubit interactions.     

A fresh look at diffraction from a nonabsorbing perfect crystal in the Bragg-Laue configuration

The Bragg diffraction from a thick, nonabsorbing perfect crystal theory of dynamical diffraction. The intensities of two beams, one in the incident and the other in the diffracted direction, exciting the side face of the crystal are characterised. The implications of such beams for neutron diffraction experiments are indicated.     

掠入射凹面光栅谱仪高级次衍射光谱相对效率测量

 测量了波长2.847、3.373、4.027、9.873和10.24nm谱线的一级和高级次光谱强度。给出了以一级光谱强度为标准的高级次衍射光谱的相对效率,并对测量结果进行了分析、讨论。