X-ray focusing within a single-crystal waveguide under grazing-incidence diffraction conditions

X-ray-beam propagation through a parallel-plate waveguide is investigated with allowance for grazing-incidence Bragg diffraction at the crystalline structure of the plates. Expressions for the waveguide modes are derived. It is demonstrated that undamped waveguide modes exist at angles exceeding the critical value (total external-reflection angle), thereby making it possible to increase the angle of capture of radiation incident on the waveguide. The calculated output intensities of the waveguide are compared with the waveguide-mode intensities in absence of grazing-incidence diffraction in the waveguide material.     

浅海小掠射角的海底界面声反向散射模型的简化

在浅海,尤其是负梯度声速剖面和海面较为平静的浅海波导,海底界面反向散射是浅海混响的主要来源.经验散射模型只适用于分析浅海混响平均强度衰减特性,而基于物理机理建立的反向散射模型克服了这一缺陷,但同时也引入了其受地声模型约束的问题.本文结合了海底反射系数的三参数模型,对浅海远场海底反向散射模型进行了简化,以减少地声模型的输入参数.理论分析了海底反射系数的相移参数可以描述海底对声场的散射作用,无需任何海底地声参数的先验知识.通过对海底反向散射模型近似简化,结果表明在临界角附近和甚小掠射角范围内的海底粗糙界面反向散射模型的角度特性和强度特性受海底沉积层的影响不同:在临界角附近,海底反向散射的角度特性受海底反射系数的相移参数加权,而其散射系数则近似与相移参数无关;对于甚小掠射角,海底反向散射的角度特性近似与海底反射系数的相移参数无关,其散射系数则近似与相移参数的4次方成正比.     

Influence of parameters on light propagation dynamics in optically induced planar waveguide arrays

The diffraction and refraction of light beam in optical periodic structures can be determined by the photonic band-gap structures of spatial frequency. In this paper, by employing the equation governing the nonlinear light propagations in photorefractive crystals, we study the photonic band-gap structures, Bloch modes, and light transmission properties of optically induced planar waveguide arrays. The relationship between the photonic band-gap structures and the light diffraction characteristics is discussed in detail. Then the influence of the parameters of planar waveguide arrays on the band-gaps structures, Bloch modes, and linear light transmissions is analyzed. It is revealed that the linear light transmission properties of waveguide arrays are tightly related to the diffraction relationships determined by band-gap structures. And the Bloch modes corresponding to different transmission bands can be excited by different excitation schemes. Both the increases of the intensity and the period of the array writing beam will lead to the broadening of the forbidden gaps and the concentration of the energy of the Bloch modes to the high-index regions. Furthermore, the broadening of the forbidden gaps will lead to separation and transition between the Bloch modes of neighboring bands around the Bragg angle. Additionally, with the increase of the intensity of the array writing beams, the influences from light intensity will tend to be steady due to the saturation of the photorefractive effect. Supported by the Youth for Northwestern Polytechnical University (NPU) Teachers Scientific and Technological Innovation Foundation, the NPU Foundation for Fundamental Research, and the Doctorate Foundation of NPU (Grant No. CX200514)     

Holography with guided optical waves

This paper deals with “waveguide holograms” recorded in a layer of storage material covering a planar dielectric waveguide. Reference and read-out waves are guided modes of the waveguide. Their field in the storage medium is evanescent. A theory of the diffraction efficiencies of these waveguide holograms is presented fors-polarized wavefields. To calculate the hologram structure the attenuation of the reference wave caused by absorption in the light-sensitive storage material is taken into account. Analytical expressions for the local and the overall diffraction efficiencies and for the intensity profiles of the diffracted fields are derived. The dependence of these quantities on experimental parameters (the waveguide thickness, the mode numbers of the reference and read-out waves, and the angle of incidence of the plane object wave) is presented graphically, i.e., by computer plots. Grating couplers for integrated optics can be made by waveguide holography. We consider this application to be interesting because incoupling efficiencies for Gaussian beams of up to 96% can be achieved theoretically.     

ZnO薄膜表面和边缘的发光特性

研究了分子束外延方法生长的ZnO薄膜的光学特性,首先测量样品反射光谱,然后重点研究其光致发光光谱。实验中通过改变激发光的强度,以及采用波导配置和不同的实验几何配置等手段,观察到了ZnO薄膜中起源于激子-激子散射和电子-空穴等离子体复合发光的受激辐射,同时观察到两种配置下自发辐射谱中存在巨大的差异。通过对实验数据和理论计算结果的分析,初步认为造成了从ZnO薄膜表面和侧面得到的PL谱之间的显著区别原因有两种:一是非对称薄膜波导的吸收损耗造成的波长选择效应,二是薄膜波导对掠出射光的类似薄膜微腔的微腔效应。     

A theory of excitation of a planar semiconductor optical waveguide using a diffraction grating: Single-scattering approximation

The problem of excitation of a totally reflecting planar optical waveguide using a coupling diffraction grating in the form of a periodic relief of the waveguide-layer thickness is solved within the single-scattering approximation. The polariton mode in the presence of a quantum well near the waveguide is considered. Based on the developed concepts, the following experimental features of the dependence of the intensity of radiation conducted in the waveguide layer on the angle of incidence of the excitation beam on the coupling diffraction grating are interpreted: the dependence on the mode number, the interference effects in the presence of two coupling diffraction gratings, and the influence of the lower substrate boundary on the thermal behavior of the waveguide structure.     

阻抗劈绕射对破碎波后向散射特性的影响

海浪的破碎区会导致海面电磁散射特性发生很大改变,导致海尖峰现象的产生.本文结合阻抗劈结构模型分析了劈绕射对破碎波后向散射特性的影响.首先利用基尔霍夫近似求解破碎波的物理光学场;基于Maliuzhinets方法,从波动方程及精确阻抗边界条件出发,由谱函数的积分形式得到阻抗劈的一致性绕射系数,结合物理光学绕射系数导出阻抗劈等效边缘电磁流;利用边缘绕射场修正物理光学场,得到考虑劈绕射效应的破碎波散射总场.数值结果表明,阻抗劈的绕射场在Keller锥内出现HH极化大于VV极化的现象,因此计入绕射场的影响会使得破碎波生长到临近坍塌阶段时,小擦地角逆风观测出现总场的后向散射截面HH极化大于VV极化的现象,说明劈绕射是造成海尖峰现象产生的原因之一.     

Frequency control of surface plasmons with oscillating metal-insulator-metal waveguides

A single frequency of surface plasmon polaritons (SPPs) will be converted to many discrete frequencies as they are transmitted through a metal-insulator-metal waveguide with its core width undergoing a harmonic oscillation. The process of the frequency conversion shares many key properties with the light diffraction in discrete optical systems. By employing the conception of optical diffraction management, we can control the discrete frequencies of SPPs such as the intensity and spectrum width by changing the initial phase of the waveguide oscillation. The study bridges the spatial discrete diffraction and frequency transition of SPP modes. Theoretical analysis based on the effective index method and coupled mode theory is provided in detail.     

Application of obliquely interfering TE10 modes for electron energy gain

Two fundamental TE₁₀ modes are considered to interfere at a small angle θ and then propagate along the z-axis in an evacuated rectangular waveguide. The electron trajectory in the resultant field and the expressions for energy gain and the acceleration gradient are obtained, when the electron is injected along the z-axis. A 50 keV electron gains 718 keV energy in a 4.0 cm × 2.5 cm waveguide, when the microwave with intensity of 1 × 10¹⁰ W/cm² and frequency 5.577 GHz is used and the modes superpose at an angle of 10°; here the maximum acceleration gradient is obtained as 251 MeV/m. The energy gain and acceleration gradient are decreased with increasing width of the waveguide and microwave frequency. Higher gradient and larger energy gain are obtained for the higher microwave intensity, smaller angle of superposition and also when the electron is injected with larger initial energy.     

Peculiarities of the formation of the X-ray diffraction image of a surface acoustic wave in grazing geometry

The x-ray diffraction image of a Bluestein-Gulyaev surface acoustic wave (SAW) at grazing angles of incidence is considered in the noncoplanar symmetric Laue geometry. It is assumed that the SAW propagation direction makes a small angle with the diffraction vector. It is shown that small deviations from the Bragg angle (～0.01″) due to SAWs that do not lead to a change in the reflection coefficient result in the formation of diffraction satellites for both standing and traveling SAWs. The recorded x-ray diffraction image representing the time-averaged intensity distribution has characteristic profiles for even-and odd-order satellites.     

Peculiarities of spontaneous grating generation in light-sensitive waveguide layers during holographic grating formation

The generation of spontaneous (noise) gratings during the simultaneous formation of a holographic grating in thin (on the order of the cutoff thickness of the TE₀ waveguide mode) AgCl-Ag films by two laser beams with λ = 532 nm and polarization vectors that make an angle of 45° with the plane of incidence have been studied. The electron microscopy images and diffraction patterns have revealed a significant difference of the spontaneous-grating structure from the structure obtained under irradiation by one laser beam. The spontaneous gratings have a significant spread in the directions of their wave vector, and the diffraction pattern (recorded using a probe beam with λ = 337 nm) has the form of a bundle of diverging arcs that intersect at one point. This difference is caused by self-diffraction from the holographic grating, which is responsible for the main diffraction peaks with odd orders, the growth of spontaneous gratings because of the interference of the beams diffracted by the holographic grating with the waveguide TE₀ modes scattered in the film, and silver transport to the interference minima. The diffraction patterns are quantitatively analyzed and the period of the new (formed under two-beam irradiation) spontaneous gratings is calculated. The calculation results are in good agreement with the experimental data.     

硬X射线微束掠入射实验的控制和数据采集系统及其应用

在国内发展了硬X射线微束掠入射实验方法,并将此具有微米级高空间分辨率的方法应用于纳米厚度薄膜的微区分析。该实验方法对分析样品表面或薄膜在微小区域的不均匀组分、结构、厚度、粗糙度和表面元素化学价态等信息具有重要意义。基于X射线全反射原理,以微聚焦实验站的高通量、能量可调的单色微束X射线为基础,通过集成运动控制、光强探测、衍射和荧光探测,设计了掠入射实验方法的控制和数据采集系统。此系统采用分布式控制结构,并基于Experimental Physics and Industrial Control System （EPICS）环境设计SPEC控制软件。通过建立SPEC和EPICS的访问通道,实现SPEC软件对EPICS平台上设备的控制和数据获取。在所设计的控制和数据采集系统中,运动控制系统控制多维样品台电机的运动,实现定位样品位置和调节掠入射角;光强探测系统则监测样品出射光强度,通过样品台运动控制和光强探测的联控,实现样品台的扫描定位控制;通过衍射和荧光探测系统获取不同入射深度下样品的衍射峰强度和荧光计数。此外,为准确控制掠入射角角度,必须确定样品平面与X射线平行的零角度位置,对此给出一种自动定位零角度的方法,编写了该方法的控制算法,设计了相应的控制软件。零角度自动化定位的扫描结果表明,实验系统微区分析的空间分辨率达到2.8 μm,零角度定位精度小于±0.01°。利用该系统在上海光源微聚焦实验站首次实现了具有自动化准确控制零角度的微束掠入射X射线衍射和荧光同步表征的实验方法,实验中被测样品为10 nm Au/Cr/Si薄膜材料,Si基底最上层为10 nm厚的Au薄膜,其间为一层很薄的Cr粘附层。在不同掠入射角下测量样品的衍射信号,获取不同入射深度下样品的衍射峰强度,并实现在同一掠入射角下,同步采集样品的荧光计数信号,从而确定了样品表层的相结构信息以及荧光信号强度与入射角关系,实现了对纳米厚度薄膜在微小区域的相结构和组分分析。此外,通过该技术能够选取荧光计数最大值对应的入射角度,有助于提高后续发展的低浓度样品掠入射X射线吸收近边结构实验方法的信噪比。     

Asymmetric planar luminescent waveguide based on amorphous silicon carbide with polarized radiation in leaky modes

Plasma-enhanced chemical vapor deposition is used to fabricate asymmetric planar luminescent waveguides (APWs) based on amorphous silicon-carbide films with submicron thickness on quartz substrates. Narrow peaks of linearly (P and S) polarized radiation related to the emission in the APW leaky modes are detected in the APW emission spectra from the end surface of a substrate under excitation of photoluminescence. The dependence of the spectral positions of peaks on the angle at which the radiation is emitted from the end surface and the film thickness is analyzed. At grazing angles of emission, the radiation wavelength is almost independent of the angle. It is demonstrated that the difference between the wavelengths of the P- and S-polarized peaks in the PL spectra decreases with an increase in the waveguide thickness. The waveguide works as an optical microcavity for leaky modes. The amplitude of the S-polarized peak is higher than the amplitude of the P-polarized peak due to the fact that the Q factor for the S-polarized leaky modes is greater than the Q factor for the P-polarized leaky mode. The luminescent APWs can be used to generate optical beams with radial and azimuthal polarizations.     

Enhanced third harmonic reflection and diffraction in Silicon on Insulator photonic waveguides

Enhanced third harmonic (TH) generation from Silicon-On-Insulator (SOI) planar waveguides as well as SOI photonic crystal (PhC) slabs is studied in different angular configurations, both in the visible and infrared energy ranges. In the SOI planar waveguide, the multilayer structure causes the optical properties such as TH reflection to be different from those of bulk silicon samples. This behavior is well reproduced by calculations of TH reflectance.Measurements of third-harmonic reflection and diffraction from one-dimensional PhC slabs etched in the SOI waveguide are also reported. The angular positions of TH peaks at various diffraction orders agree well with those calculated from a nonlinear grating equation. Both reflection and diffraction processes contribute to enhanced TH generation efficiency in the PhC slabs.TH reflectance measurements performed on PhC slabs in the near infrared show a resonant interaction between the incident beam and the photonic structure, dependent on the angle of incidence. This leads to a nonlinear conversion efficiency which is strongly enhanced with respect to that of the SOI waveguide, due to the excitation of strong local fields associated with the presence of photonic modes in the PhC slab.     

十字结构银纳米线的表面等离极化激元分束特性

金属纳米线波导可以将光局域在亚波长尺度内传播, 在纳米光子集成回路方面有着重要的作用. 本文应用有限元方法, 研究了十字结构银纳米线的表面等离极化激元分束特性. 结果表明, 不同模式的表面等离极化激元在十字结构三个分支的输出依赖于端面的几何结构参数. 此外, 研究还发现由于不同模式表面等离极化激元叠加, 在十字结构的分支上出现了周期性电场分布.     

Analytical model of a corrugated optical waveguide

An analytical model of the diffraction energy exchange between the radiative and the waveguide modes in a planar optical waveguide corrugated by a waveguide grating with an arbitrary form of teeth is developed on the basis of the coupled-wave method. It is shown that the mechanism of the energy exchange between the modes is determined by the partial interaction of all components of the spatial frequency spectrum of the waveguide modes with the corresponding components of the spatial frequency spectrum of the grating. It is established that gratings with an asymmetric tooth profile providing a shift of the peak of the spatial frequency spectrum toward matching are characterized by a higher diffraction efficiency α; however, at small thicknesses δ of the waveguide grating, the efficiency is almost independent of the tooth profile. It is shown that gratings with a symmetric profile give on average a decreasing dependence α(δ), while gratings with an optimized asymmetric profile yield a monotonically increasing saturating dependence α(δ).     

Determining angles of incidence and heights of quantum dot faces by analyzing X-ray diffuse and specular scattering

Scattering of X rays by structures with multilayer ensembles of quantum dots MBE-grown in the In(Ga)As-GaAs system is studied by high-resolution grazing X-ray reflectometry. The peaks of the diffuse scattering intensity are discovered for the first time in structures with both vertically uncorrelated and vertically correlated quantum dots. It is shown that the position of the peak is totally determined by angle of inclination of the quantum dot pyramidal faces (the so-called blaze condition for diffraction gratings), which was theoretically predicted earlier. Comparison with the results of scattering simulation carried out by the technique of boundary integral equations indicates that a simple geometrical condition allows one to exactly determine the value of from the position of the intensity peak, the shape of which depends on many parameters. As follows from the theory and experiment, the width and height of the peaks for samples with vertically correlated quantum dots are larger than for those with uncorrelated dots. The roughness and interdiffusion of interfaces and the height of quantum dots are found from the position and amplitude of Bragg peaks. Thus, the conventional application of high-resolution grazing X-ray reflectometry is extended in this work to determination of the quantum dot geometry.     

Diffraction of fast atomic projectiles during grazing scattering from a LiF(001) surface

Light atoms and molecules with energies from 300 eV to 25 keV are scattered under a grazing angle of incidence from a LiF(001) surface. For impact of neutral projectiles along low index directions for strings of atoms in the surface plane we observe a defined pattern of intensity spots in the angular distribution of reflected particles which is consistently described using concepts of diffraction theory and specific features of grazing scattering of atoms from insulator surfaces. Experimental results for scattering of H, D, 3He, and 4He atoms as well as H2 and D2 molecules can be unequivocally referred to atom diffraction with de Broglie wavelengths as low as about 0.001 Angstroms.     

Guided-mode resonances in multimode planar periodic waveguides

<正>Using the effective medium theory and the waveguide eigenvalue equation,we design a multimode planar periodic waveguide.When a plane wave illuminates the grating at the designed angle and wavelength, more than one leaky modes are excited coincidentally.Then the reflection efficiency around this designed angle and wavelength is investigated using a rigorous coupled wave analysis formulation and a gap of reflection peaks is found.Electric field distributions reveal that this high reflectivity gap is due to the coupling between these two coincidentally excited leaky modes.     

全息明孤子的波导特性

研究了光折变晶体中的全息明孤子所产生的波导的特性，结果表明波导所有可能的导模数依赖于产生波导的全息明孤子光的强度.利用数值计算给出了在不同的孤子强度时波导的导模并考察了波导对导模和正弦余弦波的动态导引.