Phase gradient imaging for positive contrast generation to superparamagnetic iron oxide nanoparticle-labeled targets in magnetic resonance imaging

Positive contrast imaging methods produce enhanced signal at large magnetic field gradient in magnetic resonance imaging. Several postprocessing algorithms, such as susceptibility gradient mapping and phase gradient mapping methods, have been applied for positive contrast generation to detect the cells targeted by superparamagnetic iron oxide nanoparticles. In the phase gradient mapping methods, smoothness condition has to be satisfied to keep the phase gradient unwrapped. Moreover, there has been no discussion about the truncation artifact associated with the algorithm of differentiation that is performed in k-space by the multiplication with frequency value. In this work, phase gradient methods are discussed by considering the wrapping problem when the smoothness condition is not satisfied. A region-growing unwrapping algorithm is used in the phase gradient image to solve the problem. In order to reduce the truncation artifact, a cosine function is multiplied in the k-space to eliminate the abrupt change at the boundaries. Simulation, phantom and in vivo experimental results demonstrate that the modified phase gradient mapping methods may produce improved positive contrast effects by reducing truncation or wrapping artifacts.     

用于大景深三维纳米分辨多分子追踪的衍射光学元件的设计制备和实验研究

发展能实时检测完整细胞内多个生物分子随时空变化的单分子探测和追踪技术, 对于研究生命过程的分子机理具有重要意义. 在变形光栅多阶成像和双螺旋点扩散函数成像方法的基础上, 基于波前编码的原理, 提出将二者优化结合, 获得全新的衍射光学元件, 该器件同时具有多重平面成像和双螺旋点扩散函数成像的功能, 旨在解决活细胞内单分子探测和追踪技术中的大景深探测难题. 设计和制备了该器件, 并基于该器件搭建了显微成像系统, 实验模拟证明该衍射光学元件同时可实现轴向12 μ的探测范围, 与理论设计结果相符合, 从而有效扩大了显微镜系统的景深, 证明了设计的可行性. 关键词： 多分子追踪 衍射光学元件 变形光栅 双螺旋点扩散函数     

Optimized phase pupil masks for extended depth of field

By properly designing a phase pupil mask to modulate or encode the optical images and then digitally restoring them, one can greatly extend the depth of field and improve image quality. The original works done by Dowski and Cathey introduce the use of a cubic phase pupil mask to extend the depth of field. The theoretical and experimental researches all verified its effectiveness. In this paper, we suggest the use of an exponential phase pupil mask to extend the depth of field. This phase mask has two variable parameters for designing to control the shape of the mask so as to modulate the wave-front more flexible. We employ an optimization procedure based on the Fisher information metric to obtain the optimum values of the parameters for the exponential and the cubic masks, respectively. A series of performance comparisons between these two optimized phase masks in extending the depth of field are then done. The results show that the exponential phase mask provide slight advantage to the cubic one in several aspects.     

Theoretical study on coupling effects of modulation depth between two photorefractive phase gratings with an external applied field

We used the perturbation expanding method to the hopping model and studied coupling effects of the modulation depth between two photorefractive phase gratings stored in one point with an external applied DC electric field . It has been found that the modulation depth of one of the two gratings seriously affects the spatial-charge field of the other grating.     

可调谐相位板空域频域联合分析

波前编码系统采用在传统光学系统中加入相位板来扩大光学系统的景深而避免传统景深延拓技术的不利影响. 由于相位板的参数不可调, 整个系统的景深延拓扩展率也不能动态可调. 采用两相位板组合的方法可以有效克服这一点. 本文首先从光线差的角度提出了两三次相位板组合下的光线像差分布以及点扩散函数尺寸的具体关系表达式, 直观体现了系统的光线结构, 指出了光线结构和点扩散函数尺寸受两三次相位板的面型和相对位移量的影响. 其次采用稳相法从空间域给出了系统点扩散函数表达式, 依据点扩散函数的振荡性质给出了有效带宽表达式, 提出了点扩散函数在像面的位置会随两相位板面型参数以及相对于光瞳中心的位移量而发生平移. 最后利用菲涅耳积分给出两三次相位板任意面型参数和相对位移组合下的准确光学传递函数. 在得到的调制传递函数中直观体现出了面型参数和相对位移量对调制传递函数和相位传递函数以及有效带宽的影响, 并说明了此系统相位传递函数的非线性性质. 通过空间域与频率域相结合的方法分析验证了传统的两三次相位板组合具有景深可调和带宽可调的性质, 为设计可调谐波前编码系统提供了理论依据.     

Theoretical study on coupling effects of modulation depth between two photorefractive phase gratings with an external applied field

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波前编码显微物镜景深扩展的研究

为了解决传统显微物镜景深与分辨率的矛盾,采用波前编码的方法,设计研究了波前编码的10倍显微物镜,结合传统光学设计软件,采用基于MTF一致性的相位板参数的优化方法,实现了在像面附近一定范围内系统的点扩散函数一致性;此外,还设计了扩展三次相位编码板的10倍显微物镜,比较了采用两种不同相位板系统的焦深扩展的效果,结果显示三次相位板的焦深扩展效果较好,可以将传统10倍显微物镜的焦深扩展15倍。成像模拟仿真结果显示滤波后的编码像在±15倍焦深范围内成像清晰,从而扩大了系统的景深。     

Physalis method for heterogeneous mixtures of dielectrics and conductors: Accurately simulating one million particles using a PC

Prosperetti’s seminal Physalis method, an Immersed Boundary/spectral method, had been used extensively to investigate fluid flows with suspended solid particles. Its underlying idea of creating a cage and using a spectral general analytical solution around a discontinuity in a surrounding field as a computational mechanism to enable the accommodation of physical and geometric discontinuities is a general concept, and can be applied to other problems of importance to physics, mechanics, and chemistry. In this paper we provide a foundation for the application of this approach to the determination of the distribution of electric charge in heterogeneous mixtures of dielectrics and conductors. The proposed Physalis method is remarkably accurate and efficient. In the method, a spectral analytical solution is used to tackle the discontinuity and thus the discontinuous boundary conditions at the interface of two media are satisfied exactly. Owing to the hybrid finite difference and spectral schemes, the method is spectrally accurate if the modes are not sufficiently resolved, while higher than second-order accurate if the modes are sufficiently resolved, for the solved potential field. Because of the features of the analytical solutions, the derivative quantities of importance, such as electric field, charge distribution, and force, have the same order of accuracy as the solved potential field during postprocessing. This is an important advantage of the Physalis method over other numerical methods involving interpolation, differentiation, and integration during postprocessing, which may significantly degrade the accuracy of the derivative quantities of importance. The analytical solutions enable the user to use relatively few mesh points to accurately represent the regions of discontinuity. In addition, the spectral convergence and a linear relationship between the cost of computer memory/computation and particle numbers results in a very efficient method. In the present paper, the accuracy of the method is numerically investigated by example computations using one dielectric particle, one isolated conductor particle, one conductor particle connected to an external source with imposed voltage, and two conductor/dielectric particles with strong interactions. The efficiency of the method is demonstrated with one million particles, which suggests that the method can be used for many important engineering applications of broad interest.     

Dynamics of a semiconductor laser with polarization-rotated feedback and its utilization for random bit generation

Chaotic semiconductor lasers have been proven attractive for fast random bit generation. To follow this strategy, simple robust systems and a systematic approach determining the required dynamical properties and most suitable conditions for this application are needed. We show that dynamics of a single mode laser with polarization-rotated feedback are optimal for random bit generation when characterized simultaneously by a broad power spectrum and low autocorrelation. We observe that successful random bit generation also is sensitive to digitization and postprocessing procedures. Applying the identified criteria, we achieve fast random bit generation rates (up to 4 Gbit/s) with minimal postprocessing.     

用于大景深单分子定位显微的多功能全息相位片的设计及数值模拟

发展具有大轴向定位范围的单分子定位技术对于实现厚样品的超分辨成像具有重要的价值.基于波前编码技术,将变形多值纯相位光栅与双螺旋点扩散函数相位片相结合,提出一种可以通过空间光调制器实现的具有高衍射效率的新型全息相位片的设计方法.这种全息相位片可以将样品内多个层面的分子信息以双螺旋的形式成像在同一个探测面的不同位置,在无需扫描的情况下提高双螺旋点扩散函数工程的轴向定位范围和分辨率,解决活细胞内单分子定位和示踪技术中的大景深探测难题.数值模拟表明,设计的5×5全息相位片可以将样品内25个层面上的分子信息以双螺旋的形式成像在同一探测面上的不同位置,相邻两个层面的间隔为0.5μm,实现了轴向12μm的探测范围,证明了设计的可行性.     

Simple calibration of a phase-based 3D imaging system based on uneven fringe projection

Phase-based fringe projection metrology systems have been widely used to obtain the shape of 3D objects. One vital step is calibration, which defines the relationship between the phase and depth data. Existing calibration methods are complicated because of the dependence of the relationship on the pixel position. In this Letter, a simple calibration procedure is introduced based on an uneven fringe projection technique, in which the relationship between phase and depth becomes independent of the pixel position and can be represented by a single polynomial function for all pixels. Therefore, given a set of discrete points with a known phase and depth in the measuring volume, the coefficient set of the polynomial function can be determined. A white plate having discrete markers with known separation is used to calibrate the 3D imaging system. Experimental results demonstrate that the proposed calibration method is simple to apply and can build up an accurate relationship between phase and depth data.     

Specific features of the electronic structure of fluorinated multiwalled carbon nanotubes in the near-surface region

The C 1s and F 1s X-ray photoelectron spectra of fluorinated multiwalled carbon nanotubes with different fluorine contents have been measured using the equipment of the Russian-German beamline at the BESSY storage ring by varying the energy of exciting photons. It has been established that two fluorocarbon phases in which the chemical bonding is characterized by a different electron transfer from carbon atoms to fluorine atoms are formed in the near-surface region of nanotubes with fluorine concentrations of 10–39 wt %. The content of the dominant first phase with a large electron transfer in nanotubes remains unchanged with an increase in the probing depth. This phase is identified as a bulk phase formed as a result of the covalent attachment of fluorine atoms to graphene layers of the graphite skeleton without its destruction. The second phase with a small electron transfer is a near-surface phase, because it is predominantly located within two or three upper graphene monolayers and its contribution considerably decreases with an increase in the probing depth of fluorinated multiwalled carbon nanotubes.     

连续相位板面形特性对远场分布的影响

惯性约束聚变(ICF)系统打靶对焦斑的大小及能量分布均匀性都有明确的要求,在连续相位板设计过程中要同时兼顾这两个方面。研究了连续相位板的面形与远场焦斑光强均匀性间的关系,从理论上推导了元件表面调制深度与焦斑尺寸的关系,并进行了定量计算。结果表明,焦斑半径随连续相位板的面形调制深度线性增加。鉴于连续相位板具有的随机特性,比较了不同相位板远场特性的区别,研究了面形调制深度对其光强峰谷值和均方根值的影响,结果表明不同设计的连续相位板,远场特性有较大的不同。当面形调制深度在5~15个波长时,焦斑具有较低的光强峰谷值和均方根值,连续相位板具有较好的束匀滑效果。     

Parametric resonance in the system of long Josephson junctions

The phase dynamics of the system of long Josephson junctions whose length exceeds the Josephson penetration depth has been studied. The possibility of the appearance of a longitudinal plasma wave and parametric resonance has been demonstrated. Both inductive and capacitive couplings between Josephson junctions have been taken into account in the calculations. The current-voltage characteristics, as well as time evolution of the spatial distribution of the electric charge in superconducting layers and the magnetic field, have been calculated in all Josephson junctions of the system. The coexistence of the longitudinal plasma wave and fluxon states has been observed in the region of parametric resonance beginning with a certain length of the Josephson junction. This indicates the appearance of a new unique collective excitation in the system of coupled Josephson junctions, namely, a composite state of the Josephson current, electric field, and vortex magnetic field.     

Imaging of trapped ions with a microfabricated optic for quantum information processing

Trapped ions are a leading system for realizing quantum information processing (QIP). Most of the technologies required for implementing large-scale trapped-ion QIP have been demonstrated, with one key exception: a massively parallel ion-photon interconnect. Arrays of microfabricated phase Fresnel lenses (PFL) are a promising interconnect solution that is readily integrated with ion trap arrays for large-scale QIP. Here we show the first imaging of trapped ions with a microfabricated in-vacuum PFL, demonstrating performance suitable for scalable QIP. A single ion fluorescence collection efficiency of 4.2±1.5% was observed. The depth of focus for the imaging system was 19.4±2.4 μm and the field of view was 140±20 μm. Our approach also provides an integrated solution for high-efficiency optical coupling in neutral atom and solid-state QIP architectures.     

Continuous wave terahertz phase imaging with three-step phase-shifting

We demonstrated a continuous wave terahertz (THz) phase imaging system which can present the depth information of an object's surface interior accurately and non-invasively. With the three-step phase-shifting method, the phase images of samples which can provide the relative optical depth profile of the sample's surface and interior have been achieved. Two samples with different thickness and materials were measured to demonstrate the phase imaging ability of the system. The results show that this method is effective and the measured values agree well with the actual ones. The longitudinal resolution of the system was also measured. To exhibit the advantages of the three-step phase shifting method, the multi-wavelength imaging method was also employed and the results are compared.     

DC electric-field-induced heterophase structure on the surface of a peierls metal

A theoretical study has been made of the formation of a heterostructure of alternating metallic and semiconducting phases in atomic chains of a Peierls-metal surface monatomic layer placed in an electrostatic field normal to the surface. It is shown that an increase in the electric field results in an increase in the critical temperature of the metal-semiconductor phase transition on the sample surface, an increase in the temperature interval within which the heterostructure exists, a decrease in its spatial period, and an increase in the depth of spatial modulation of the gap in the electronic spectrum.     

对数位相编码系统多色光成像特性研究

位相编码技术是在非相干光学系统的出瞳处加入位相板,使得图像传感器上得到的图像对景深不敏感,通过对图像进行解码而获得清晰图像,实现扩大光学系统景深.文章针对对数位相板编码系统,分析了系统与波长相关的两个因素:位相板的位相调制量和系统色差,通过对比在不同离焦量和色差情况下两种波长的点扩散函数(point spread fu...     

Numerical simulation of graphene in a magnetic field within the effective field theory

Monte Carlo simulation of graphene in an external magnetic field perpendicular to the plane of graphene has been reported. The calculations have been performed using the effective quantum field theory with a noncompact (3 + 1)-dimensional Abelian gauge field and (2 + 1)-dimensional Kogut-Susskind fermions. It has been revealed that the external magnetic field shifts the semimetal-insulator phase transition point toward higher dielectric constants of the substrate. The phase diagram of the semimetal-insulator phase transition has been plotted in the (dielectric constant of the substrate-magnetic field) plane.     

原子辅助光力系统中快慢光的量子调控

随着纳米科技以及半导体技术的迅猛发展,光力诱导透明、快慢光和光存储以及其他在光力系统中发现的量子光学和非线性光学效应成为人们目前研究的热点.本文将薄膜腔光力系统同被束缚在腔中的二能级冷原子系综相耦合,通过直接在薄膜振子上引入弱辅助驱动场来研究该原子辅助光力系统中原子和相位对量子相干性质及其快慢光的调控.经过分析发现,通过改变辅助驱动场的强度可直接实现对光力诱导透明窗口深度的调控,通过改变辅助场与探测场之间的相位差,可实现输出的探测场在"吸收"、"透明"和"增益"之间相互转换,进而对弱探测场进行动态调控实现光开关.与此同时,还发现系统的群延迟时间随相位差的改变呈周期性变化.通过调节相位差及原子数,不但可以改变群延迟时间,还可实现快慢光之间的相互转换.