Electron-holographic interference microscopy

Recent developments in electron interference microscopy are introduced based on holography principles. This technique has been given a major boost due to the development of a coherent field-emission electron beam. This has facilitated the measurement of the phase distribution of an electron beam transmitted through or reflected from an object to within 1/100 of the electron wavelength. Phase distribution can be displayed as a phase contour map in an electron micrograph. There the contour fringes directly and quantitatively indicate the thickness contours of a uniform sample, magnetic lines of force of a magnetic sample and equipotential lines of an electrostatic sample. Microscopic objects or fields have made their appearance in amplified interference micrographs through the use of this technique. Specific examples are the static and even dynamic observation of magnetic fluxons penetrating a superconductor and the quantitative measurement of specimen thickness or topography in atomic dimensions.     

A complement to the theory of Lohmann-type computer holograms

The introduction by Lohmann and co-workers of the detour phase method in realizing synthetic holograms has been an important step in the history of computer generated holography. In 1969, Brown and Lohmann proposed an improvement to their method: they used true phase and amplitude coding at the center of the diffracting aperture instead of periodically sampled phases and amplitudes. We present a mathematical analysis of this procedure and examine the approximation related to its practical realization.     

Image quality of microns-thick specimens in the ultra-high voltage electron microscope

Image quality of MeV transmission electrons is an important factor for both observation and electron tomography of microns-thick specimens with the high voltage electron microscope (HVEM) and the ultra-HVEM. In this work, we have investigated image quality of a tilted thick specimen by experiment and analysis. In a 3 MV ultra-HVEM, we obtained transmission electron images in amplitude contrast of 100 nm gold particles on the top surface of a tilted 5 μm thick amorphous epoxy-resin film. From line profiles of the images, we then measured and evaluated image blurring, contrast, and the signal-to-noise ratio (SNR) under different effective thicknesses of the tilted specimen and accelerating voltages of electrons. The variation of imaging blurring was consistent with the analysis based on multiple elastic scattering. When the effective thickness almost tripled, image blurring increased from ～3 to ～20 nm at the accelerating voltage of 3 MV. For the increase of accelerating voltage from 1 to 3 MV in the condition of the 14.6 μm effective thickness, due to the reduction of multiple scattering effects, image blurring decreased from ～54 to ～20 nm, and image contrast and SNR were both obviously enhanced by a factor of ～3 to preferable values. The specimen thickness was shown to influence image quality more than the accelerating voltage. Moreover, improvement on image quality of thick specimens due to increasing the accelerating voltage would become less when it was further increased from 2 to 3 MV in this work.     

Error analysis for a lens-less in-line digital holographic complex information security system based on double random phase encoding

We study the error tolerance properties of double random phase encoding implemented in the case of a lens-less whole (both amplitude and phase) information security system based on in-line digital holography. A generalized position-phase-shifting in-line digital holographic method is used as a tool for implementing our information security system. The effect on the decrypted image, due to additive and multiplicative noises in the retrieved complex encrypted image, is studied. The quality of the decrypted complex data is quantified by computing signal-to-noise ratio (SNR) and the mean square error (MSE). We also study the system tolerance due to the data loss and binarization of encrypted retrieved complex hologram. Results from numerical experiments are presented.     

Vibration Analysis by Phase Shifting Digital Holography

Phase-shifting digital holography has been used for the study of vibrating objects. The time-averaged complex amplitude of Fresnel diffracted field due to a vibrating object was obtained by using a three-step phase-shifting algorithm. Taking the inverse Fresnel transform of the complex amplitude resulted in an image of the object superimposed with Bessel fringes. The Bessel fringes are contour map of the vibration amplitude. By sinusoidally modulating the phase of the reference beam at the vibration frequency, the brightest fringe could be shifted to points of interest, thus extending the measurable range of vibration amplitude.     

球面参考光波数字全息的一些特点分析及实验

对以球面波作为参考光记录数字全息图的采样和再现分离条件进行了详细的分析。推导出了同时满足采样和再现分离条件时,CCD及物体的横向尺寸、物体及参考光源至CCD的距离之间需要满足的关系式,证明了完全散射物体上复振幅在记录面上的叠加可以等效为一个点光源在记录面上产生的效果,进行了相应的实验研究。结果表明：数字全息术中采用球面参考光波可以比采用平面参考光波更容易满足采样和分离条件,并可以记录到更多的物体信息;在物体至CCD的距离固定的情况下,无透镜傅里叶变换全息术是优化的记录光路结构;散射物体比衍射物体的数字全息图更容易满足采样和分离条件。     

用Stokes参量法实现数字同轴偏振全息的研究

提出了一种基于Stokes参量的数字同轴偏振全息方法.在实验中用一束线偏振光和一束椭圆偏振光作为参考光, 分别与物光进行干涉,通过拍摄在两个相互垂直方向上的全息图,计算出物光在这两个方向的振幅和相位信息, 从而得到物光的Stokes参量和物体的全偏振信息,实现对各向异性物体偏振态空间分布的图像重建. 实验结果表明,该方法可用于物体的全偏振特性的测量.这种方法在求出物光Stokes参量的同时, 也可消除零级像和共轭像的干扰,因此也可用于同轴或离轴全息.     

Analysis of Phase Measurement Errors in Electro-Optic Holography

This paper presents the results of an error analysis in electro-optics holography. These errors include phase measurement errors due to the linear phase shifter errors in static electro-optic holography, and phase measurement errors due to the errors in the vibrating bias amplitude and phase in dynamic electro-optic holography. Through the error analysis, we found that the phase shifting errors in static electro-optic holography are twice as large as those in the conventional 4-bucket phase shifting algorithm, and the phase shifting errors in dynamic electro-optic holography are similar to those in the 4-bucket phase shifting algorithm.     

SEM investigation of the dependence of magnetic domain structure on the thickness of cobalt monocrystals

The magnetic domain structure of cobalt monocrystal is observed by means of a scanning electron microscope (SEM). It is revealed by the so-called type-I magnetic contrast [1]. The dependence of magnetic domain width on the specimen is thickness is investigated and discussed. Digital image processing (image restoration, enhancement and analysis) is used on the images obtained directly from the SEM. The main reasons for the application of digital image processing are: poor resolution of type-I magnetic contrast due to the diffuseness of the leakage magnetic fields above the specimen surface, and complex character of magnetic domains. The resolution limit of type-I magnetic contrast in cobalt monocrystal is evaluated. Statistical distributions of magnetic domain width are also calculated and presented.     

Realization of Absolute‐Phase Unwrapping and Speckle Suppression in Laser Digital Holography

In this paper, an absolute‐phase unwrapping and speckle suppression approach to reconstruct a three‐dimensional (3‐D) image of an object with laser digital holography is described. This method offers three advantages to enhance the performance of the phase reconstruction technique. First, both speckle suppression and phase unwrapping are processed in the complex amplitude domain rather than in the single phase or amplitude domain. With this approach, the phase details of the object are better preserved upon phase reconstruction. Second, the proposed algorithm requires no threshold determination and thus achieves self‐adaptive speckle suppression and robust phase unwrapping, in contrast to other methods. Finally, an improved dual‐domain image denoising method is applied to further remove speckle‐remnant‐induced phase distortion. Ideal 3‐D phase reconstruction results are obtained both theoretically and experimentally for the first time.     

Image edge-enhancement in optical microscopy with a phase mismatched spiral phase plate

We present a spiral phase filtering system with a large tolerance for edge enhancement of both phase and amplitude objects in optical microscopy.The method is based on a Fourier 4-f spatial filtering system.A phase mismatched spiral phase plate (SPP) fabricated by electron beam lithography is employed as the radial Hilbert transform for image edge enhancement.Compared with holography,SPP is simple,economical,reliable,and easy to integrate.     

Width determination of SiO2-films in Si-based devices using low-loss EFTEM: image contrast as a function of sample thickness

Energy filtering transmission electron microscopy (EFTEM) has become one of the most efficient tools for specimen characterization at nanometer length scales. EFTEM imaging is most often carried out in the core-loss region but image intensity becomes more and more a limiting factor with decreasing feature size. Alternatively, it is possible to record EFTEM images in the low-loss region, where intensities are essentially higher and where in many cases the images contain material specific contrasts. In this paper we investigate the influence of the important parameters on the material contrast between silicon and silicon dioxide, e.g. specimen thickness, specimen orientation, energy-loss and energy selecting slit width. We show that sample thickness plays an important role and present two methods to calculate material contrast as a function of energy-loss and sample thicknesses. The first method uses spectra taken from both materials at different sample thickness by electron energy-loss spectroscopy, the second calculates contrast directly from a series of energy filtered images. From the results we determine the ideal acquisition parameters for the Si/SiO(2) system and demonstrate imaging at sufficient resolution below 2nm with a test sample of thin SiO(2) layers on Si.     

Optical Fourier techniques for medical image processing and phase contrast imaging

This paper briefly reviews the basics of optical Fourier techniques (OFT) and applications for medical image processing as well as phase contrast imaging of live biological specimens. Enhancement of microcalcifications in a mammogram for early diagnosis of breast cancer is the main focus. Various spatial filtering techniques such as conventional 4f filtering using a spatial mask, photoinduced polarization rotation in photosensitive materials, Fourier holography, and nonlinear transmission characteristics of optical materials are discussed for processing mammograms. We also reviewed how the intensity dependent refractive index can be exploited as a phase filter for phase contrast imaging with a coherent source. This novel approach represents a significant advance in phase contrast microscopy.     

Stokes参量数字全息法实现相位物体成像

为实现对相位物体的无损检测和成像,克服数字同轴全息相位物体成像技术在消除零级像和孪生像的干扰时存在的系列问题,提出一种基于Stokes参量的新的数字同轴全息技术。该方法区别于传统的利用干涉光场来记录原始像项的数字全息方法,通过测量物参光合成光束的Stokes参量来分别得到这两束光的振幅和相位差,从而准确、唯一地获得原始像项;再利用数字再现即可重构物光的振幅和相位信息。实验中对弱吸收的相位样品进行了测量,得到样品清晰的振幅和相位分布。结果表明,采用该方法对相位物体进行数字全息再现,可以克服传统同轴全息图中零级像和共轭像对相位物体信息的严重干扰,对于提取相位物体的振幅和相位信息是可行和有效的。     

Secure optical system that uses fully phase-based encryption and lithium niobate crystal as phase contrast filter for decryption

In this paper, the implementation of a secure optical system using fully phase encryption is described. A two-dimensional phase image obtained from an amplitude image is encrypted and decrypted by using a spatial light modulator working in phase mode. The fully phase encryption is achieved using double random phase encoding. The encrypted image is holographically recorded in a photorefractive crystal and is then decrypted by generating, through phase conjugation, the conjugate of the encrypted image. A lithium niobate crystal has been used as a phase contrast filter to change the decrypted phase image into an amplitude image, thus alleviating the need for alignment of the phase contrast set-up in the Fourier plane. Simulation results are provided to verify the proposed study. The mean square error between the primary image and decrypted image has been calculated to study the sensitivity of the system.     

电子束放射照相的特性与参数优化

短脉冲强激光产生的电子束具有源尺寸小、脉宽窄、准单能谱等特点, 在放射照相诊断中具有独特作用. 本文通过分析电子在材料中散射并采用蒙特卡罗方法数值模拟, 研究了100 keV到几百MeV能量电子束对有厚度起伏或存在界面的靶的透视, 并与质子、X射线束透视结果比较, 给出了电子束放射照相的特性与参数优化: 基于电子在材料中非弹性散射或能量损失, 选用能量使其射程与靶厚度接近的电子束来诊断靶厚度不均匀性; 基于电子在材料中的弹性散射, 选用射程超过靶厚度的电子束来诊断靶界面.     

An alternative technique for extraction of in-plane displacement from an image plane hologram

The combination of image plane holography and speckle photography for measurement of 3-D displacements has been reported earlier. It has been observed that when we interrogate such a hologram by an unexpanded laser beam, in addition to the formation of Young's fringes in the direct diffracted beam, very bright side lobes are also generated. If the specimen has undergone any in-plane displacement, high contrast fringes appear in the side lobes. We have demonstrated that these fringes can be used to obtain a precise quantitative in-plane displacement of the object.     

Hard X ray holographic diffraction imaging

We determine the absolute electron density of a lithographically grown nanostructure with 25 nm resolution by combining hard x-ray Fourier transform holography with iterative phase retrieval methods. While holography immediately reveals an unambiguous image of the object, we deploy in addition iterative phase retrieval algorithms for pushing the resolution close to the diffraction limit. The use of hard (8 keV) x rays eliminates practically all constraints on sample environment and enables a destruction-free investigation of relatively thick or buried samples, making holographic diffraction imaging a very attractive tool for materials science. We note that the technique is ideally suited for subpicosecond imaging that will become possible with the emerging hard x-ray free-electron lasers.     

Plasmonic particles of colloidal silver in high-resolution recording media

The optical properties of colloidal silver particles formed photographically in high-resolution silver halide photographic materials have been considered. The conditions that allow one to obtain exposed and developed light-sensitive silver halide particles in the form of colloidal particles of metallic silver having the properties of localized plasmons have been described. The results of the studies of the developed silver particles in traditional photographic materials for image holography and in nanoporous silver halide photographic materials for volume holography have been presented. The perspectives of using plasmonic silver nanoparticles produced photographically have been discussed.     

Selective edge enhancement in three-dimensional vortex imaging with incoherent light

We demonstrate a new imaging method enabling a selective edge contrast enhancement of three-dimensional amplitude objects with spatially incoherent light. The imaging process is achieved in a spiral modification of Fresnel incoherent correlation holography and uses a vortex impulse response function. The correlation recordings of the object are acquired in a one-way interferometer with the wavefront division carried out by a spatial light modulator. Two different methods based on applying a helical reference wave in the hologram recording and a digital spiral phase modulation in image reconstruction are proposed for edge enhancement of amplitude objects. Results of both isotropic and anisotropic spiral imaging are demonstrated in experiments using an LED as an incoherent source of light.