Edge enhancement of phase objects through complex media by using transmission-matrix-based spiral phase contrast imaging

The wavefront shaping based technique has been introduced to detect the edges of amplitude objects through complex media, but the extraction of the boundary information of invisible phase objects through complex media has not been demonstrated yet. Here, we present a phase contrast imaging technique to overcome the scattering, aiming to achieve the edge detection of the phase object through the complex media. An operator based on the experimentally measured transmission matrix is obtained by numerically adding a spiral phase in the Fourier domain. With the inverse of the filtered transmission matrix, we can directly reconstruct the edge enhanced images for both amplitude object and phase object beyond scattering. Experimentally, both digital and real objects are imaged, and the results verify that isotropic edge detection can be achieved with our technique. Our work could benefit the detection of invisible phase objects through complex media.     

Measuring the scattering parameters of tissues from quantitative phase imaging of thin slices

We employed the scattering-phase theorem recently derived in our laboratory to extract tissue scattering properties from quantitative phase images. Using tissue path-length maps that contain nanoscale information about the tissue, the scattering mean free path l(s) and anisotropy factor g were measured in a spatially resolved manner.     

高散射媒质中的光程可分割动态光散射研究

为研究散射光强度随光子在散射媒质中散射光程的变化,基于单散射理论与扩散波光谱理论,采用了低相干动态光散射装置对不同粒径大小的聚苯乙烯悬浮液进行研究。将测量得到的背散射光光场强度谱的线宽与相应的理论计算结果相比较发现,在短光程区域,考虑容器壁附近拖曳效应的影响后,对于不同粒径的颗粒,光程为约5倍粒子平均自由程的区域可看成为单散射区域;对于光程大于225 m的区域可看成为扩散光波区域。实验结果表明低相干动态光散射法可实现高散射媒质从单散射区域到低次散射再到扩散区域的全光程的可分割的光场强度谱测量。     

Measurement of optical constants for dense media by low-coherence dynamic light scattering

We perform the optical constants measurements for different absorption dense media by low-coherence dynamic light scattering (DLS) technique. The estimated particle size is used to calculate the scattering coefficient of particles suspended in dense media. The path-length resolved intensity distributions of light backscattered from the absorbing dense media are investigated experimentally by virtue of path-length resolved performance in low-coherence DLS measurements. The absorption coefficient can be obtained by applying the measured path-length resolved intensity distributions to the modified Lambert-Beer law. As a result, we proposed a new low-coherence DLS technique in simultaneous measurement of the scattering and absorption coefficients of absorbing dense media.     

Separation of changes in light scattering and chromophore concentrations during cortical spreading depression in rats

We used near-infrared spectroscopy to separate tissue scattering changes from changes in cerebral oxyhemoglobin and deoxyhemoglobin and the redox state of cytochrome- c -oxidase. A separate term of the transport scattering coefficient (micro(s)(?)) was included in a modified Lambert-Beer equation. It is shown by diffusion equation analysis that there is a simple relationship between the differential path-length factor (D(a)) and its scattering equivalent (D(s)) . The method was applied to cortical spreading depression (CSD) data recorded through the skulls of rats. Biphasic changes in micro(s)(?)of +/-0.1mm(-1)were observed during CSD's that spread with a velocity of ~5mm/min . The method proposed has the promise to permit monitoring of scattering changes noninvasively in humans during cortical activation or pathophysiological conditions.     

Initial studies of in vivo absorbing and scattering heterogeneity in near-infrared tomographic breast imaging

Simultaneously recovered absorption and scattering images that separate these optical property features within the female breast are demonstrated from frequency-domain measurements. A study of known absorbing and scattering objects is presented as a foundation for interpreting these in vivo images once the contrast space has been fully characterized. No measurable influence of absorbing-object contrast appears in the scattering images, whereas localized scattering contrast enhances the corresponding region within the absorption image by approximately 30% (e.g., a 2:1 scatterer also reconstructs as an approximately 1.3:1 absorber). Scattering and absorption images of a female volunteer with a 3.4-cm fibroadenoma show a clear 2:1 localized increase in absorption coefficient with little or no evidence of scattering enhancement in the lesion.     

Path-length distribution and path-length-resolved Doppler measurements of multiply scattered photons by use of low-coherence interferometry

We report first results of measurements by low-coherence Doppler interferometry of the path-length distribution of photons undergoing multiple scattering in a highly turbid medium. We use a Mach-Zehnder interferometer with multimode graded-index fibers and a superluminescent diode as the light source. The path-length distribution is obtained by recording of the heterodyne fluctuations that arise from the Brownian motion of particles in an Intralipid suspension as a function of the optical path length. The experimental path-length distribution is in good agreement with predictions of Monte Carlo simulations. In the heterodyne spectrum, an increase of the mean Doppler frequency with path length is observed.     

Images of crystalline alkanes obtained with scanning tunneling microscopy

Uncoated layered alkane (paraffin) objects with a thickness of up to a hundred nanometers have been imaged by scanning tunneling microscopy. This is surprising in view of the generally assumed excellent insulating nature of paraffins. Tilted layers were observed for n-alkanes of various chain lengths. The chemical nature of the imaged objects was demonstrated by the changes of structure observed near the respective melting temperature.     

Speckle diagnostics of multiple scattering media with the use of frequency-modulated laser radiation

A method for probing randomly inhomogeneous multiple scattering media with the use of frequency-modulated laser radiation is considered. The method is based on analysis of the dependence of the blinking index of time-averaged speckles formed upon scattering of the probing radiation in a medium on the frequency modulation depth of the probing radiation. In the case of a binary frequency modulation, the blinking index of the detected speckle-modulated radiation is determined by the cosine Fourier transform of the probability density of the optical path-length difference of partial components of the scattered field in the probed medium. The values of the probability density of the optical path-length difference reconstructed with the use of the proposed method from the measured blinking index of speckles for model scattering media (fluoroplastic layer and layer of TiO₂ particles on a glass substrate) are in a good agreement with the results of statistical simulation of the probing radiation transfer in multiple scattering media.     

Analysis of multiscale scattering and poroelastic attenuation in a real sedimentary rock sequence

Compressional waves in heterogeneous permeable media experience attenuation from both scattering and induced pore scale flow of the viscous saturating fluid. For a real, finely sampled sedimentary sequence consisting of 255 layers and covering 30 meters of depth, elastic and poroelastic computer models are applied to investigate the relative importance of scattering and fluid-flow attenuation. The computer models incorporate the known porosity, permeability, and elastic properties of the sand/shale sequence in a binary medium, plane layered structure. The modeled elastic scattering attenuation is well described by stochastic medium theory if two-length scale statistics are applied to reflect the relative thickness of the shale layers when compared to the sand layers. Under the poroelastic Biot/squirt flow model, fluid-flow attenuation from the moderate permeability (10(-14) m2) sands may be separated in the frequency domain from the attenuation due to the low permeability (5 x 10(-17) m2) shale layers. Based on these models, the overall attenuation is well approximated by the sum of the scattering attenuation from stochastic medium theory and the volume weighted average of the attenuations of the sequence member rocks. These results suggest that a high permeability network of sediments or fractures in a lower permeability host rock may have a distinct separable attenuation signature, even if the overall volume of high permeability material is low. Depending on the viscosity of the saturating fluid, the magnitude of the flow-based attenuation can dominate or be dominated by the scattering attenuation at typical sonic logging frequencies (approximately 10 kHz).     

Imaging of an Absorbing Object Embedded in a Dense Scattering Medium by Diffusing Light Topography

A new and simple imaging method for an absorbing object embedded in a dense scattering medium is proposed. The distinct characteristic is the positive usage of the diffusing light in the dense scattering medium to image the absorbing object. The principle is based on the equivalence between a probability distribution function of the path-length and a backscattered intensity distribution integrated spatially in the boundary plane between the medium and the air. The usefulness of the proposed method is experimentally confirmed for a modified pyramidal object painted black and thin tubes filled with black and red inks. The conditions under which the better image can be reconstructed are confirmed from the results of experiments and simulations. It is finally shown that our proposed method is capable of imaging the map of blood vessels distributed under the skin layers.     

Three-dimensional spin fluctuations in Na0.75CoO2

We report polarized- and unpolarized-neutron scattering measurements of magnetic excitations in single-crystal Na0.75CoO2. The data confirm ferromagnetic correlations within the cobalt layers and reveal antiferromagnetic correlations perpendicular to the layers, consistent with an A-type antiferromagnetic ordering. The magnetic modes propagating perpendicular to the layers are sharp, and reach a maximum energy of approximately 12 meV. From a minimal spin-wave model, containing only nearest-neighbor Heisenberg exchange interactions, we estimate the interlayer and intralayer exchange constants to be 12.2+/-0.5 meV and -6+/-2 meV, respectively. We conclude that the magnetic fluctuations in Na0.75CoO2 are highly three dimensional.     

四元裂解位相调制实现相干光通过散射介质聚焦

光在不均匀介质中传播会受到散射的干扰,在这些散射材料中,例如粉末、生物组织、亚波长颗粒对入射光多次散射使得出射光无法聚焦,从而在接收平面形成散斑.本文提出四元裂解位相调制方法对入射相干光场进行调制,使其通过散射介质聚焦.此方法利用入射光场全场调制,充分考虑光场单元之间的干涉作用,从整个空间光调制器的调制面开始,逐层进行四元裂解及位相优化.运用此方法在实验中实现了相干光的前向散射和后向散射有效聚焦,这为生物医学领域中通过散射介质成像提供了新的思路和方法.     

一种植物叶片生化成分光谱无损检测的光程长校正方法

利用可见/近红外光谱分析技术定量分析叶片生化参数含量时,由光散射效应和叶片厚度等引起的样本间光程长差异会影响校正模型的预测精度。文章提出了一种改进的扩展多元散射校正(EMSC)光程长方法,利用差谱矩阵的主成分代替物质纯谱,对实测光谱精确建模,根据实际需要,减去相应干扰因子,得到校正光谱。选择叶绿素含量基本相同,厚度不同的16个叶片样本以及叶绿素含量和厚度均不同的32个样本分别使用改进的EMSC方法校正,对处理前后光谱变异系数和主成分贡献率进行比较和分析,并比较了预处理前后的模型预测精度。结果表明,采用改进的EMSC预处理方法能够有效地校正光散射效应和叶片厚度差异导致的光程长差异引起的光谱误差,增强光谱数据对浓度的灵敏度,可提高预测模型的精度。     

Homodyne and heterodyne imaging through a scattering medium

We introduce a novel two-dimensional (2D) homodyne and heterodyne technique for imaging objects through or embedded in a scattering medium. Our imaging approach is based on heterodyning of light with different Doppler broadenings that is scattered from objects of two different textures or from an opaque object and a textured scattering medium. We report on the initial demonstration of pulling signals out of noise for an object hidden behind a scattering medium. Enhancements of signal-to-noise ratio of the order of 50 have been achieved by use of a 2D holographic phase-sensitive detector. We also discuss the experimental feasibility of this approach for objects embedded in a scattering medium.     

透过散射介质对直线运动目标的全光成像及追踪技术

光散射是限制光传输以及降低和破坏光学成像性能的主要因素,透过复杂散射介质对运动目标的全光成像是光学领域极具挑战性的技术之一.本文提出一种利用散斑差值自相关透过散射介质对运动目标进行实时追踪的方法.采用赝热光照明,基于光学记忆效应理论,通过对运动目标采集的两帧散斑做差值,然后做自相关运算,计算目标移动的距离,实现对目标的实时追踪,并且利用相位恢复算法进行简单处理就可以重建隐藏目标.对该方法进行了实验验证,成功地对隐藏的运动目标实现了成像与追踪.这种透过散射介质对运动目标的全光成像及实时追踪技术,在生物医学等领域具有重要应用潜力.     

Compensation of lens aberrations in digital holography

The possibility of compensating for the aberrations of an imaging lens in digital holography is outlined theoretically. The principle is demonstrated with a Mach-Zehnder arrangement and transilluminated objects. Diffraction-limited resolution can be obtained with a plano-convex lens. Amplitude and phase objects were imaged.     

基于石墨烯的太赫兹波散射可调谐超表面

设计了一个可调谐的太赫兹超表面,由在随机反射超表面基底中嵌入可偏置的双层石墨烯构成,可以实现对太赫兹波散射特性的动态调控.全波仿真试验结果证实了所预期的超表面散射可调性能.通过增大偏置电压提升石墨烯的费米能级,使得该超表面的太赫兹波散射样式从漫反射逐渐向镜面反射过渡,从而实现散射特性的连续调控,且该超表面具有对电磁波极化角度不敏感的特点.这些特性使得该超表面能很好地融合到变化的环境中,在太赫兹隐身方面具有潜在的应用价值.     

Memory effect evaluation based on transmission matrix calculation

The memory effect is a type of auto correlation observed in linear systems, which is widely used to control scattered light through thin scattering layers.We show that there exists a strong correlation among the optimized phase distributions of adjacent focal points in focusing through scattering media.The numeric simulation and experiment indicate that within the memory effect, the phase difference between the two adjacent focal points shows an optical phase fringe pattern, and the closer the adjacent focal points are, the wider the fringe pattern will be, corresponding to the tilting of a plane wave phase added onto the acquired optical phase distribution at the focal point.This effect can be utilized for achieving optimal phase distributions of focal point scanning without optical phase evaluation via the experiment, which has great potential application in imaging through the scattering medium.