Fourier domain optical coherence tomography with a linear-in-wavenumber spectrometer

We describe Fourier domain optical coherence tomography equipped with a novel linear-in-wavenumber spectrometer. The presented device linearizes the spectral dispersion of the spectrometer in wavenumber using a specifically designed prism. The spectral linearity in wavenumber makes numerical interpolation into wavenumber unnecessary, reduces computing time, and furthermore results in improvement of the falloff of signal with image range inherent to frequency-domain optical coherence tomography imaging. Experiments demonstrate the improvement of the falloff and agree with the expected results from simulation.     

Comparison of high energy neutron-nucleus total cross sections with theory

A recent paper by Julius purports to show that previous estimates of the effect of inelastic screening on neutron-nucleus total cross sections are much too large. However, we show that most of this discrepancy is due to his uections for diffractive N¹ production which are much too small. The rest may be due to the assumptions in his model. We also emphasize that experimental support for the inelastic screening corrections does not rely on a specific choice of nuclear radii.     

Spectral-domain optical coherence tomography with a Fresnel spectrometer

We propose a novel spectral-domain optical coherence tomography (SD-OCT) equipped with a Fresnel spectrometer, which utilizes a Fresnel zone plate (FZP) as both dispersion and focusing optics and thus spreads the spectral interferogram evenly in wavenumber domain because of the proportional relation between the focal length of the FZP and the wavenumber. With no need of the conversion calculation from wavelength to wavenumber in conventional SD-OCT, this new design is favorable for fast imaging with high resolution. As only a FZP and CCD are used, the Fresnel spectrometer is simple and compact. It is experimentally shown that its performance is as good as that of numerical interpolation in conventional SD-OCT. Imaging of bio-tissue by Fresnel SD-OCT is also demonstrated.     

Measuring surface pressure with far field acoustics

This paper presents measurements of the wavenumber frequency spectrum of wall pressure fluctuations under a turbulent boundary layer made using sound radiated from hydrodynamically smooth ridges in the surface. The measurements also serve as a test of the scattering theory of roughness noise. The radiated sound reveals a cut through the full three-dimensional wavenumber frequency spectrum of the wall pressure at the wavenumber of the surface. Since ridges can be made with very small wavelengths, this technique can be used to probe the structure of the wall pressure spectrum on scales far smaller than those that can be reached using conventional wall-mounted transducers. Furthermore, the method reveals the wavenumber frequency spectrum directly, without the need for multi-point measurements or the spatial Fourier transforming of data. Measured spectra bear a close similarity to Corcos’ and Chase's model forms, and confirm the applicability of the theory of roughness noise and its prediction of roughness noise directivity.     

Hybrid natural low scale inflation

We discuss the phenomenological implications of Hybrid Natural Inflation models in which the inflaton is a pseudo-Goldstone boson but inflation is terminated by a second scalar field. A feature of the scheme is that the scale of breaking of the Goldstone symmetry can be lower than the Planck scale and so gravitational corrections are under control. We show that, for supersymmetric models, the scale of inflation can be chosen anywhere between the Lyth upper bound and a value close to the electroweak breaking scale. The observed density perturbations and spectral index are readily obtained by the choice of the free parameters. The tensor to scalar ratio and the spectral tilt are extremely small.     

Large scale spatio-temporal behaviour in surface growth

This paper presents new findings concerning the dynamics of the slow height variations in surfaces produced by the two-dimensional isotropic Kuramoto-Sivashinsky equation with an additional nonlinear term. In addition to the disordered cellular patterns of specific size evident at small scales, slow height variations of scale-free character become increasingly evident when the system size is increased. This paper focuses on the parameter range in which the kinetic roughening with eventual saturation in surface roughness and coarseness is obtained, and the statistical and dynamical properties of surfaces in the long-time stationary regime are investigated. The resulting long-range scaling properties of the saturated surface roughness consistent with the power-law shape of the surface spectrum at small wave numbers are obtained in a wider parameter range than previously reported. The temporal properties of these long-range height variations are investigated by analysing the time series of surface roughness fluctuations. The resulting power-spectral densities can be expressed as a generalized Lorentzian whose cut-off frequency varies with system size. The dependence of this lower cut-off frequency on the smallest wave number connects spatial and temporal properties and gives new insight into the surface evolution on large scales.     

偏振光谱BRDF建模与仿真

在偏振光条件下,物体的表面反射受到折射率、表面粗糙度、入射角等多种因素的影响。针对粗糙物体表面在不同波段光照下表现出不同的偏振反射特性,提出一种基于Kirchhof理论的偏振光谱BRDF模型。利用已知材质在不同波长下的复折射率,对其折射率和消光系数部分分别反演出的对应光谱模型,进而得到复折射率的光谱模型;同借鉴经典的表面粗糙度测量方法,结合菲涅耳反射公式,推导出表面粗糙度的光谱模型,将得到的复折射率和粗糙度光谱模型与BRDF模型相结合,推导出偏振光谱BRDF建模。模型分别在折射率随波长变化、粗糙度为常量,折射率、粗糙度均随波长变化以及原模型三种情况下进行仿真对比实验,并将所得到的数据与其他资料进行对比。其结果表明,该模型能够较为准确的反映物体表面的偏振反射特性,并且能够描述偏振度随波长变化趋势的光谱特征,能够为偏振遥感、物质分类等方面的应用提供可靠依据。     

Height and roughness distributions in thin films with Kardar-Parisi-Zhang scaling

We study height and roughness distributions of films grown with discrete Kardar-Parisi-Zhang (KPZ) models in a small time regime which is expected to parallel the typical experimental conditions. Those distributions are measured with square windows of sizes 8 ? r ? 128 gliding through a much larger surface. Results for models with weak finite-size corrections indicate that the absolute value of the skewness and the value of the kurtosis of height distributions converge to 0.2 ? ∣S∣ ? 0.3 and 0 ? Q ? 0.5, respectively. Despite the low accuracy of these results, they give additional support to a recent claim of KPZ scaling in oligomer films. However, there are significant finite-size effects in the scaled height distributions of models with large local slopes, such as ballistic deposition, which suggests that comparison of height distributions must not be used to rule out KPZ scaling. On the other hand, roughness distributions of the same models show good data collapse, with negligible dependence on time and window size. The estimates of skewness and kurtosis for roughness distributions are 1.7 ? S ? 2 and 3 ? Q ? 7. A stretched exponential tail was found, which seems to be a particular feature of KPZ systems in 2 + 1 dimensions. Moreover, the KPZ roughness distributions cannot be fitted by those of 1/f^α noise. This study suggests that the roughness distribution is the best option to test KPZ scaling in the growth regime, and provides quantitative data for future comparison with other models or experiments.     

光学元件表面粗糙度对第三代同步辐射光源空间相干性的影响

以单丝的同轴全息干涉花样为判别标准,通过计算机数值模拟,研究了Ｂｅ窗的表面粗糙度对硬Ｘ射线相干特性的影响。提出了Ｘ射线在粗糙表面反射的一个简单模型,以此为基础模拟研究了反射镜表面粗糙度的对硬Ｘ射线及软Ｘ射线空间相干特性的影响,模拟结果与实验结果相符。     

Surface correlations of hydrodynamic drag for transitionally rough engineering surfaces

Rough surfaces are usually characterised by a single equivalent sand-grain roughness height scale that typically needs to be determined from laboratory experiments. Recently, this method has been complemented by a direct numerical simulation approach, whereby representative surfaces can be scanned and the roughness effects computed over a range of Reynolds number. This development raises the prospect over the coming years of having enough data for different types of rough surfaces to be able to relate surface characteristics to roughness effects, such as the roughness function that quantifies the downward displacement of the logarithmic law of the wall. In the present contribution, we use simulation data for 17 irregular surfaces at the same friction Reynolds number, for which they are in the transitionally rough regime. All surfaces are scaled to the same physical roughness height. Mean streamwise velocity profiles show a wide range of roughness function values, while the velocity defect profiles show a good collapse. Profile peaks of the turbulent kinetic energy also vary depending on the surface. We then consider which surface properties are important and how new properties can be incorporated into an empirical model, the accuracy of which can then be tested. Optimised models with several roughness parameters are systematically developed for the roughness function and profile peak turbulent kinetic energy. In determining the roughness function, besides the known parameters of solidity (or frontal area ratio) and skewness, it is shown that the streamwise correlation length and the root-mean-square roughness height are also significant. The peak turbulent kinetic energy is determined by the skewness and root-mean-square roughness height, along with the mean forward-facing surface angle and spanwise effective slope. The results suggest feasibility of relating rough-wall flow properties (throughout the range from hydrodynamically smooth to fully rough) to surface parameters.     

Spontaneous coherent phase transition of polaritons in CdTe microcavities

We report on evidence for polariton condensation out of a reservoir of incoherent polaritons. Polariton population and first-order coherence are investigated by spectroscopic imaging of the far-field emission of a CdTe-based microcavity under nonresonant pumping. With increasing pumping power, stimulated emission with thresholdlike behavior and spectral narrowing is observed in the strong exciton-photon coupling regime. We show that it comes from a narrow ring in k space, exhibiting enhanced spatial and angular coherence at the stimulation onset.     

基底温度对中阶梯光栅厚铝膜质量的影响

大尺寸中阶梯光栅具有大孔径和极高的衍射级次,可以实现普通光栅难以达到的极高光谱分辨率。中阶梯光栅通常是利用刻划机在厚铝膜上刻划而成,所以制备大面积均匀性的高质量铝膜刻划基底是实现高性能大尺寸中阶梯光栅的关键因素。在较厚铝膜的制备工艺中,基底温度是至关重要的工艺参数。本文通过电子束热蒸发镀铝工艺在不同基底温度下制备了厚铝膜样品,并利用原子力显微镜、扫描电镜等手段从宏观和微观尺度详细分析了基底温度对铝膜质量的影响。铝膜平均晶粒尺寸从100℃时的264.34 nm增大到200℃时的384.97 nm和300℃时的596.35 nm,表面粗糙度Rq从100℃时的34.7 nm增长到200℃时的58.9 nm和300℃时的95.1 nm。结果表明,随着基底温度的升高表面粗糙度迅速增大,铝膜的表面质量严重退化。     

Silicon doped with gallium photoconductors: Effect of uniaxial stress and detector development

Conclusion The evolution of the spectral response of Si:Ga photoconductors with uniaxial stress is studied. The cut-off wavenumber is lowered by 14% at a stress of 0.6 GPa, in a spectral range where strong absorption by the lattice occurs. Unstressed Si:Ga detectors have been developed for the AROME instrument focal plane. This experiment was devoted to measure the diffuse near infrared emission of our Galaxy in order to estimate the power radiated in particular spectral bands characteristic to Polycyclic Aromatic Hydrocarbons. The recent transmediterranean balloon flight gave extensive data which are now being processed.     

Curvature effects in the composite model for low-grazing-angle rough-surface scatter

Abstract

The conventional composite surface model used for calculations of radar backscatter treats a large-scale surface component as an ensemble of tilted flat facets. This approach has limitations for low grazing angles. In the paper we develop a new composite model that takes into account the diffraction of an incident field on a curved, large-scale undulating surface. It is shown that, whereas corrections related to curvature effects are small for a steep incidence, those corrections for low-grazing-angle backscatter can be quite essential. Results of numerical simulations of radar backscattering cross sections for two polarizations are presented. Scattering from a surface with one-dimensionally varying roughness is considered. We have studied the role of undulating surface slopes and curvatures in the behaviour of the polarization ratio and found regimes in which the curvature influence on this ratio is more pronounced.     

基于界面信号的扫频光学相干层析成像系统相位矫正方法

由于扫频光源的采集触发信号和采样时钟信号存在时间上的随机延时,导致扫频光学相干层析成像(SS-OCT)系统干涉信号光谱的整体错移,进而引发OCT空间域信号的相位跳变,阻碍了基于相位信息的功能成像.为了获得稳定的相位,便于开展功能OCT的研究,提出了一种基于界面信号的数字相位矫正方法.对界面附近相邻A-line间同一深度的相位信号进行差分运算,计算得到相位跳变的A-line位置与光谱错移量(以像素为单位),然后在原始干涉信号上对齐光谱,重新傅里叶逆变换,得到矫正后的复信号.该数字矫正算法不会引入额外的相位噪声,可以实现OCT信噪比受限的相位探测.通过对反射镜、荧光板和小鼠脑皮层血流的多普勒成像验证了该方法的可行性.     

Electro-magnetic nucleon form factors and their spectral functions in soliton models

It is demonstrated that in simple soliton models essential features of the electro-magnetic nucleon form factors observed over three orders of magnitude in momentum transfert are naturally reproduced. The analysis shows that three basic ingredients are required: an extended object, partial coupling to vector mesons, and relativistic recoil corrections. We use for the extended object the standard skyrmion, one vector meson propagator for both isospin channels, and the relativistic boost to the Breit frame. Continuation to time-liket leads to quite stable results for the spectral functions in the regime from the 2- or 3-pion threshold to about two rho masses. Especially the onset of the continuous part of the spectral functions at threshold can be reliably determined and there are strong analogies to the results imposed on dispersion theoretic approaches by the unitarity constraint.     

Electro-magnetic nucleon form factors and their spectral functions in soliton models

It is demonstrated that in simple soliton models essential features of the electro-magnetic nucleon form factors observed over three orders of magnitude in momentum transfer t are naturally reproduced. The analysis shows that three basic ingredients are required: an extended object, partial coupling to vector mesons, and relativistic recoil corrections. We use for the extended object the standard skyrmion, one vector meson propagator for both isospin channels, and the relativistic boost to the Breit frame. Continuation to time-like t leads to quite stable results for the spectral functions in the regime from the 2- or 3-pion threshold to about two rho masses. Especially the onset of the continuous part of the spectral functions at threshold can be reliably determined and there are strong analogies to the results imposed on dispersion theoretic approaches by the unitarity constraint.     

Simple method for measurement of surface roughness using spectral interferometry

The phenomenon that correlation between two beams, in a two-beam interferometer, modifies the spectral characteristics of the light field, is used to estimate the average of the roughness of the surfaces. In the case of intensity interferometer, the path difference between the two arms of the interferometer should be less than the coherence length of the light and the light should be quasi-monochromatic. However, the advantage of spectral interferometer is that the light need not be quasi-monochromatic and the path difference could be much more than the coherence length of light. It is found that more the path difference between the two arms of the interferometer, accurate are the measurements of the roughness of the surfaces. This study shows that spectral interferometry enables one to measure average roughness of the surfaces in the subscales of optical wavelengths.