Quantitative phase imaging using actively stabilized phase-shifting low-coherence interferometry

We describe a quantitative phase-imaging interferometer in which phase shifting and noise cancellation are performed by an active feedback loop using a reference laser. Depth gating via low-coherence light allows phase measurement from weakly reflecting biological samples. We demonstrate phase images from a test structure and living cells.     

Enhanced quantitative X-ray phase-contrast images using Foucault differential filters

Enhanced quantitative X-ray phase-contrast(QXPC) imaging is implemented with a Foucault knife-edge array filter(FKAF), which is a real differential spatial filter. The intensities of Foucault differential filtering(FDF) are acquired according to the linear translation of the FKAF along the axes. The FDF using the FKAF scheme for obtaining the QXPC images is demonstrated by a stereoscopic rendering of the quantitative phase images of the tail fin of an anchovy containing soft and hard components in specimen. FDF is a noninterferometric quantitative phase-imaging method that depicts quantitative phase images and renders stereoscopic images.     

Spectral-domain optical coherence phase and multiphoton microscopy

We describe simultaneous quantitative phase contrast and multiphoton fluorescence imaging by combined spectral-domain optical coherence phase and multiphoton microscopy. The instrument employs two light sources for efficient optical coherence microscopic and multiphoton imaging and can generate structural and functional images of transparent specimens in the epidirection. Phase contrast imaging exhibits spatial and temporal phase stability in the subnanometer range. We also demonstrate the visualization of actin filaments in a fixed cell specimen, which is confirmed by simultaneous multiphoton fluorescence imaging.     

Multicontrast nonlinear optical microscopy with a compact and rapid pulse shaper

Homodyne detection can dramatically enhance measurement sensitivity for weak signals. In nonlinear optical microscopy it can make accessible a range of novel, intrinsic, contrast like nonlinear absorption and nonlinear phase contrast. Here a compact and rapid pulse shaper is developed, implemented, and demonstrated for homodyne detection in nonlinear microscopy with high-repetition rate mode-locked femtosecond lasers. With this method we generate two-photon absorption (TPA) and self-phase modulation images of gold nanostars in biological samples. Simultaneous imaging of two-photon luminescence and TPA also enables us to produce two-photon quantum yield images.     

Phase imaging without 2pi ambiguity by multiwavelength digital holography

We present a phase-imaging method with an axial range that can in principle be arbitrarily large compared to the wavelength and does not involve the usual phase unwrapping by detection of phase discontinuity. The method consists of the generation and combination of two phase maps in a digital holography system by use of two separate wavelengths. For example, we reconstructed the surface of a spherical mirror with approximately 10-nm axial resolution and an axial range of approximately 3 microm.     

Performance analysis of quantitative phase retrieval method in Zernike phase contrast X-ray microscopy

Since the invention of Zernike phase contrast method in 1930,it has been widely used in optical microscopy and more recently in X-ray microscopy.Considering the image contrast is a mixture of absorption and phase information,we recently have proposed and demonstrated a method for quantitative phase retrieval in Zernike phase contrast X-ray microscopy.In this contribution,we analyze the performance of this method at different photon energies.Intensity images of PMMA samples are simulated at 2.5 keV and 6.2 keV,respectively,and phase retrieval is performed using the proposed method.The results demonstrate that the proposed phase retrieval method is applicable over a wide energy range.For weakly absorbing features,the optimal photon energy is 2.5 keV,from the point of view of image contrast and accuracy of phase retrieval.On the other hand,in the case of strong absorption objects,a higher photon energy is preferred to reduce the error of phase retrieval.These results can be used as guidelines to perform quantitative phase retrieval in Zernike phase contrast X-ray microscopy with the proposed method.     

Spectral-domain optical coherence phase microscopy for quantitative phase-contrast imaging

We describe a novel microscopy technique for quantitative phase-contrast imaging of a transparent specimen. The technique is based on depth-resolved phase information provided by common path spectral-domain optical coherence tomography and can measure minute phase variations caused by changes in refractive index and thickness inside the specimen. We demonstrate subnanometer level path-length sensitivity and present images obtained on reflection from a known phase object and human epithelial cheek cells.     

Wide-field imaging through scattering media by scattered light fluorescence microscopy

To obtain images through scattering media, scattered light fluorescence (SLF) microscopy that utilizes the optical memory effect has been developed. However, the small field of view (FOV) of SLF microscopy limits its application. In this paper, we have introduced a re-modulation method to achieve wide-field imaging through scattering media by SLF microscopy. In the re-modulation method, to raster scan the focus across the object plane, the incident wavefront is re-modulated via a spatial light modulator (SLM) in the updated phase compensation calculated using the optimized iterative algorithm. Compared with the conventional optical memory effect method, the re-modulation method can greatly increase the FOV of a SLF microscope. With the phase compensation theoretically calculated, the process of updating the phase compensation of a high speed SLM is fast. The re-modulation method does not increase the imaging time. The re-modulation method is, therefore, expected to make SLF microscopy have much wider applications in biology, medicine and physiology.     

数字全息显微术中重建物场波前的相位校正

数字全息显微术克服了传统光学显微术无法直接提取样品相位信息的缺点,可以对活体细胞组织等相位型生物样品进行定量测量和有效观察。但在数字全息显微成像过程中,像场弯曲会对再现像相位分布的测量和观察产生影响。提出一种采用相位相减来校正数字全息再现像像场弯曲的方法。通过在样品加入前后两次拍摄全息图,并对数值重建像分别进行去包裹运算再令其相减,即可实现对像场弯曲的有效校正,对蝉翼和大蒜表皮细胞等相位型物体进行测量,并采用数值校正和相位相减两种方法对像场弯曲进行校正。与现有的数值校正方法相比,利用相位相减获得样品三维相位信息的方法更为简单、可靠,是校正像数字全息再现场弯曲的有效方法。     

Optical encryption and verification technique for information coding in multiple-wavelengths in Fresnel domain

We propose a new optical encryption and verification technique in which multiple frames of images recorded by different wavelengths can be synchronously encrypted into double pure phase masks (PMs). The height distributions of the two surface-relief PMs are designed by using an iterative phase-retrieval algorithm in combination with the conjugate-gradient method. Two PMs are located at the predesigned positions in the verification system. When a plane wave with a correct wavelength illuminates the optical verification system, the corresponding image is extracted to display on the output plane. Another incident plane wave with other different wavelength which was used in recording can produce another image. Only when all the frames of images recorded by different wavelengths are obtained, the user can then pass through the verification procedure. Compared with the previous encryption and verification technique, these predesigned wavelengths can be used as additional verification keys, which should significantly enhance the security of system. Numerical simulation shows that the proposed technique can successfully reconstruct the verification images for the authorized user having all the correct keys, but never for any unauthorized intruder.     

Energy structure and electromagnetically induced transparency of neutral donor in a multi-hilled GaAs quantum ribbon

The theoretical study of the energy spectrum and optical response of off-axis donor in a GaAs quantum ribbon with anisotropic rim height under the presence of constant crossed electric and magnetic fields is presented. The calculations are carried out within the effective mass and parabolic approximation, using an adiabatic approach combined with a diagonalization scheme. The rim height of the quantum ribbon has been modeled by including a phenomenological two-parametric function that accounts for realistic features of multi-hilled semiconductor nanostructure obtained from atomic force microscopy images. The first phenomenological parameter is introduced to control the number of quantum ribbon structural hills and the second one to deal with the height of the hills. It is shown that both the depth and the number of structural hills tend to substantially affect the Aharonov–Bohm oscillation pattern which can be quenched or restored by applying an electric field in an appropriate direction. The effect of the changes in the geometry and in the applied electric field onto the optical absorption and refractive index change are discussed. The phenomenon of electric-field-induced optical transparency in the system is particularly highlighted.     

Combined phase-sensitive acoustic microscopy and confocal laser scanning microscopy

Combined phase-sensitive acoustic microscopy (PSAM) at 1.2 GHz and confocal laser scanning microscopy (CLSM) in reflection and fluorescence has been implemented and applied to polymer blend films and fluorescently labeled fibroblasts and neuronal cells in order to explore the prospects and the various contrast mechanisms of this powerful technique. Topographic contrast is available for appropriate samples from CLSM in reflection and, with significantly higher precision, from the acoustic phase images. Material contrast can be gained from acoustic amplitude V(z) graphs. In the case of the biological cells investigated, the optical and acoustic images are very different and exhibit different features of the samples.     

Determination of bending-induced strain in optical fibers by use of quantitative phase imaging

Quantitative phase microscopy with polarized light has been used to determine the change in refractive index introduced into an optical fiber when the fiber is bent through a constant radius of curvature. By obtaining phase images for two orthogonal directions of the polarization of the incident light, one can infer the induced axial strain profile within the fiber. Radii of curvature from 1 to 8 cm were considered, and in each case excellent agreement, within lambda/100, was obtained between experimental results and theory.     

Spatial and temporal coherence effects in interference microscopy and full‐field optical coherence tomography

Low‐coherence optical microscopy or optical coherence microscopy uses light with short coherence length. The well‐known case is: “white‐light interferometry”, which became recently more known as: “optical coherence tomography”. However, when lenses and microscope objectives are used to create interferometric images, in what is known classically as “interference microscopy” or today as “full‐field optical coherence tomography” the spatial coherence starts to play a critical role. In this article the coherence effects in low‐coherence optical microscopy are reviewed. As this technology is becoming increasingly publicized due to its importance in three‐dimensional imaging, particularly of scattering biological media and optical metrology, the understanding of the fundamental physics behind it is essential. The interplay between longitudinal spatial coherence and temporal coherence and the effects associated with them are discussed in detail particularly when high numerical apertures are used. An important conclusion of this study is that a high‐contrast, high‐resolution system for imaging of multilayered samples is the one that uses narrowband illumination and high‐NA objectives with an index‐matching fluid. Such a system, when combined with frequency‐domain operation, can reveal nearly real‐time three‐dimensional images, and is thus competitive with confocal microscopy.     

Hilbert phase microscopy for investigating fast dynamics in transparent systems

We introduce Hilbert phase microscopy (HPM) as a novel optical technique for measuring high transverse resolution quantitative phase images associated with optically transparent objects. Because of its single-shot nature, HPM is suitable for investigating rapid phenomena that take place in transparent structures such as biological cells. The potential of this technique for studying biological systems is demonstrated with measurements of red blood cells, and its ability to quantify dynamic processes on a millisecond scale is exemplified with measurements of evaporating micrometer-sized water droplets.     

Understanding tapping-mode atomic force microscopy data on the surface of soft block copolymers

In this paper, we focus on better understanding tapping-mode atomic force microscopy (AFM) data of soft block copolymer materials with regard to: (1) phase attribution; (2) the relationship between topography and inside structure; (3) contrast-reversal artifacts; (4) the influence of annealing treatment on topography. The experiments were performed on the surface of poly(styrene–ethylene/butylene–styrene) (SEBS) triblock copolymer acting as a model system. First, by coupling AFM with transmission electron microscopy (TEM) measurements, the phase attribution for AFM images was determined. Secondly, by imaging an atomically flat SEBS surface as well as an AFM tip-scratched SEBS surface, it was confirmed that the contrast in AFM height images of soft block copolymers is not necessarily the result of surface topography but the result of lateral differences in tip-indentation depth between soft and hard microdomains. It was also found that there is an enlarging effect in AFM images on the domain size of block copolymers due to the tip-indention mechanism. Thirdly, based on the tip-indention mechanism, tentative explanations in some detail for the observed AFM artifacts (a reversal in phase image followed by another reversal in height image) as function of imaging parameters were given. Last, it was demonstrated that the commonly used annealing treatments in AFM sample preparation of block copolymers may in some cases lead to a dramatic topography change due to the unexpected order-to-order structure transition.     

利用光频梳提高台阶高度测量准确度的方法

提出一种利用光频梳和可调谐半导体激光器提高台阶高度测量准确度的方法. 通过将可调谐激光器锁定至光频梳,可对激光器的输出波长进行精确锁定与测量.基于可调合成波长链原理,利用锁定后的半导体激光器构建了一套台阶高度测量方案,该方案可消除合成波长误差对台阶高度测量不确定度的影响. 采用一台可调谐半导体激光器和光频梳进行了5000 s的连续锁定实验, 结果表明,锁定后的可调谐半导体激光器的频率稳定度达 1.8×10^-12.该方法的理论测量不确定度约为7.9 nm, 且测量结果可溯源至时间频率基准.     

Refractive-index profiling of optical fibers with axial symmetry by use of quantitative phase microscopy

The application of quantitative phase microscopy to refractive-index profiling of optical fibers is demonstrated. Phase images of axially symmetric optical fibers immersed in index-matching fluid are obtained, and the inverse Abel transform is used to obtain the radial refractive-index profile. This technique is straightforward, nondestructive, repeatable, and accurate. Excellent agreement, to within approximately 0.0005, between this method and the index profile obtained with a commercial profiler is obtained.     

Encrypted optical memory system using three-dimensional keys in the Fresnel domain

An encrypted optical memory system using double random phase codes in the Fresnel domain is proposed. In this system, two random phase codes and their positions form three-dimensional keys for encryption of images and are used as keys to recover the original data. The third dimension is the positions of the codes, which can have as many as three degrees of freedom. Original images encrypted by use of the two phase codes located in the Fresnel domain are stored holographically in a photorefractive material. We demonstrate in preliminary experiments encryption and decryption of optical memory in a LiNbO(3) :Fe photorefractive crystal by use of angular multiplexing.