Imaging Of Phase Objects Research Articles

Subwavelength imaging using a solid-immersion diffractive optical processor

Phase imaging is widely used in biomedical imaging, sensing, and material characterization, among other fields. However, direct imaging of phase objects with subwavelength resolution remains a challenge. Here, we demonstrate subwavelength imaging of phase and amplitude objects based on all-optical diffractive encoding and decoding. To resolve subwavelength features of an object, the diffractive imager uses a thin, high-index solid-immersion layer to transmit high-frequency information of the object to a spatially-optimized diffractive encoder, which converts/encodes high-frequency information of the input into low-frequency spatial modes for transmission through air. The subsequent diffractive decoder layers (in air) are jointly designed with the encoder using deep-learning-based optimization, and communicate with the encoder layer to create magnified images of input objects at its output, revealing subwavelength features that would otherwise be washed away due to diffraction limit. We demonstrate that this all-optical collaboration between a diffractive solid-immersion encoder and the following decoder layers in air can resolve subwavelength phase and amplitude features of input objects in a highly compact design. To experimentally demonstrate its proof-of-concept, we used terahertz radiation and developed a fabrication method for creating monolithic multi-layer diffractive processors. Through these monolithically fabricated diffractive encoder-decoder pairs, we demonstrated phase-to-intensity (P→I)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$({\\varvec{P}}\ o {\\varvec{I}})$$\\end{document} transformations and all-optically reconstructed subwavelength phase features of input objects (with linewidths of ~ λ/3.4, where λ is the illumination wavelength) by directly transforming them into magnified intensity features at the output. This solid-immersion-based diffractive imager, with its compact and cost-effective design, can find wide-ranging applications in bioimaging, endoscopy, sensing and materials characterization.

Annular illumination in 2D quantitative phase imaging: a systematic evaluation.

Quantitative phase imaging (QPI) is an invaluable microscopic technology for definitively imaging phase objects such as biological cells and optical fibers. Traditionally, the condenser lens in QPI produces disk illumination of the object. However, it has been realized by numerous investigators that annular illumination can produce higher-resolution images. Although this performance improvement is impressive and well documented, the evidence presented has invariably been qualitative in nature. Recently, a theoretical basis for annular illumination was presented by Bao et al. [Appl. Opt.58, 137 (2019)APOPAI0003-693510.1364/AO.58.000137]. In our current work, systematic experimental QPI measurements are made with a reference phase mask to rigorously document the performance of annular illumination. In both theory and experiment, three spatial-frequency regions are identified: low, mid, and high. The low spatial-frequency region response is very similar for disk and annular illumination, both theoretically and experimentally. Theoretically, the high spatial-frequency region response is predicted to be much better for the annular illumination compared to the disk illumination--and is experimentally confirmed. In addition, the mid-spatial-frequency region response is theoretically predicted to be less for annular illumination than for disk illumination. This theoretical degradation of the mid-spatial-frequency region is only slightly experimentally observed. This bonus, although not well understood, further elevates the performance of annular illumination over disk illumination.

Recurrent neural network reveals transparent objects through scattering media.

Scattering generally worsens the condition of inverse problems, with the severity depending on the statistics of the refractive index gradient and contrast. Removing scattering artifacts from images has attracted much work in the literature, including recently the use of static neural networks. S. Li et al. [Optica5(7), 803 (2018)10.1364/OPTICA.5.000803] trained a convolutional neural network to reveal amplitude objects hidden by a specific diffuser; whereas Y. Li et al. [Optica5(10), 1181 (2018)10.1364/OPTICA.5.001181] were able to deal with arbitrary diffusers, as long as certain statistical criteria were met. Here, we propose a novel dynamical machine learning approach for the case of imaging phase objects through arbitrary diffusers. The motivation is to strengthen the correlation among the patterns during the training and to reveal phase objects through scattering media. We utilize the on-axis rotation of a diffuser to impart dynamics and utilize multiple speckle measurements from different angles to form a sequence of images for training. Recurrent neural networks (RNN) embedded with the dynamics filter out useful information and discard the redundancies, thus quantitative phase information in presence of strong scattering. In other words, the RNN effectively averages out the effect of the dynamic random scattering media and learns more about the static pattern. The dynamical approach reveals transparent images behind the scattering media out of speckle correlation among adjacent measurements in a sequence. This method is also applicable to other imaging applications that involve any other spatiotemporal dynamics.

Coded aperture correlation holographic microscope for single-shot quantitative phase and amplitude imaging with extended field of view.

Recently, a method of recording holograms of coherently illuminated three-dimensional scene without two-wave interference was demonstrated. The method is an extension of the coded aperture correlation holography from incoherent to coherent illumination. Although this method is practical for some tasks, it is not capable of imaging phase objects, a capability that is an important benefit of coherent holography. The present work addresses this limitation by using the same type of coded phase masks in a modified Mach-Zehnder interferometer. We show that by several comparative parameters, the coded aperture-based phase imaging is superior to the equivalent open aperture-based method. As an additional merit of the coded aperture approach, a framework for increasing the system's field of view is formulated and demonstrated for both amplitude and phase objects. The combination of high sensitivity quantitative phase microscope with increased field of view in a single camera shot holographic apparatus, has immense potential to serve as the preferred tool for examination of transparent biological tissues.

Upconversion dark-field imaging with extended field of view at video frame rate.

Dark-field imaging is a well-known optical method for obtaining edge-enhanced images of objects containing steep gradients in either amplitude or phase. Edge enhancement is commonly achieved by using a small physical obstruction in the center of the Fourier plane of a 4f imaging setup. By blocking the low spatial frequencies in the center of the Fourier plane, only the higher spatial frequencies from the object reach the image plane. In this work, simultaneous optical image processing (i.e., dark-field imaging) and sum-frequency generation are performed by placing a periodically poled lithium niobate crystal in the Fourier plane of a 4f setup and using a 1575 nm upconversion pump beam with a dark core. We demonstrate upconversion dark-field (UDF) imaging in which edge-enhanced images are obtained at the upconverted wavelength ($\sim{630}\;{\rm nm}$∼630nm) as a result of infrared object illumination ($\sim 1\;\unicode{x00B5}{\rm m}$∼1µm). Furthermore, we experimentally confirm that UDF imaging can be extended from collinear to noncollinear interactions between the signal and pump. The presented system allows for adjustment of the spatial cutoff frequency while maximizing the power conversion efficiency. Simultaneous collinear and noncollinear upconversion imaging of phase objects is demonstrated using broadband 1 µm illumination with a spectral bandwidth of 6 nm-resulting in a UDF imaging system with an enhanced field of view at video frame rates.

Generalized interferometry.

When a parallel beam of light undergoes a discontinuous change in phase or in phase gradient in the central part of the beam, two wavefronts with a common border are formed. In this paper, we formulate the interference of two such wavefronts and show that conventional diffraction and interference are special cases of a generalized interference with amplitudes that are determined by the distances of the observation point from the wavefronts and their boundaries. This formulation permits us to reconstruct the interfering wavefronts at their original locations. Also, we modulate a phase function on the interference fringes near the common boundary, where the diffraction effect is significant, and show this kind of modulation provides a basis for quantitative imaging of phase objects in small scales. Furthermore, we evaluate the theoretical results by relevant experiments and elaborate on features and application potential of the interferometry introduced.

Pupil plane differential detection microscopy.

Differential interference contrast (DIC) microscopy is a powerful technique for imaging phase objects in transparent samples but does not work with scattering samples. This Letter, to the best of our knowledge, describes a new technique for obtaining DIC-like phase-gradient images in scattering media based on differential detection of forward-scattered light, using detectors arranged in a ring configuration around the microscope objective pupil or its conjugate pupil plane. This method, called pupil plane differential detection (P2D2) microscopy, does not need polarization optics or a confocal pinhole, yet produces images that are free of speckles and interference noises. We compared the P2D2 imaging technique with reflectance confocal microscopy and demonstrated P2D2 as a simple add-on to conventional laser scanning microscopes.

Fresnel diffraction from the edge of a transparent plate in the general case.

Fresnel diffraction from the edge of a transparent plate with arbitrary orientation of its surfaces is formulated in transmission mode. It is shown theoretically and evaluated experimentally that the thickness of a plate and the angles between its surfaces, near the edge, can be determined accurately. This allows us to construct a quantitative 3D image of the plate edge. It is also illustrated that the recorded diffraction pattern of a plate edge can be regarded as a hologram produced by the interference of two diffracted waves: one passing above the plate and the other transmitting through the plate near the edge. Thus, the hologram makes it possible to reconstruct the plate's edge as a 3D image, and the approach can be utilized in constructing 3D images of phase objects and in quantitative phase microscopy.

Efficient phase retrieval based on dark fringe extraction and phase pattern construction with a good anti-noise capability

This paper discusses the noisy phase retrieval problem: recovering a complex image signal with independent noise from quadratic measurements. Inspired by the dark fringes shown in the measured images of the array detector, a novel phase retrieval approach is proposed and demonstrated both theoretically and experimentally to apply the information of extracted dark fringes and its relationship with designed phase patterns. Here the dark fringes are extracted from the image plane, rather than the Fourier/focal plane, and the invalid extracted fringes can be tactfully bypassed to improve the anti-noise capability. A more accurate relative phase ratio between arbitrary two pixels is achieved by calculating the multiplicative ratios (or the sum of phase difference) on the path between them. Then the object phase image can be reconstructed precisely. Our approach is a good choice for retrieving high-quality phase images from noisy signals and has broad prospects in practical applications such as X-ray crystallography, diffractive imaging, and so on.

Experiments on sparsity assisted phase retrieval of phase objects

Iterative phase retrieval algorithms such as the Gerchberg–Saxton method and the Fienup hybrid input–output method are known to suffer from the twin image stagnation problem, particularly when the solution to be recovered is complex valued and has centrosymmetric support. Recently we showed that the twin image stagnation problem can be addressed using image sparsity ideas (Gaur et al 2015 J. Opt. Soc. Am. A 32 1922). In this work we test this sparsity assisted phase retrieval method with experimental single shot Fourier transform intensity data frames corresponding to phase objects displayed on a spatial light modulator. The standard iterative phase retrieval algorithms are combined with an image sparsity based penalty in an adaptive manner. Illustrations for both binary and continuous phase objects are provided. It is observed that image sparsity constraint has an important role to play in obtaining meaningful phase recovery without encountering the well-known stagnation problems. The results are valuable for enabling single shot coherent diffraction imaging of phase objects for applications involving illumination wavelengths over a wide range of electromagnetic spectrum.

Heralded phase-contrast imaging using an orbital angular momentum phase-filter

We utilise the position and orbital angular momentum (OAM) correlations between the signal and idler photons generated in the down-conversion process to obtain ghost images of a phase object. By using an OAM phase filter, which is non-local with respect to the object, the images exhibit isotropic edge-enhancement. This imaging technique is the first demonstration of a full-field, phase-contrast imaging system with non-local edge enhancement, and enables imaging of phase objects using significantly fewer photons than standard phase-contrast imaging techniques.

Coherent diffraction imaging of non-isolated object with apodized illumination.

Coherent diffraction imaging (CDI) is an established lensless imaging method widely used at the x-ray regime applicable to the imaging of non-periodic materials. Conventional CDI can practically image isolated objects only, which hinders the broader application of the method. We present the imaging of non-isolated objects by employing recently proposed "non-scanning" apodized-illumination CDI at an optical wavelength. We realized isolated apodized illumination with a specially designed optical configuration and succeeded in imaging phase objects as well as amplitude objects. The non-scanning nature of the method is important particularly in imaging live cells and tissues, where fast imaging is required for non-isolated objects, and is an advantage over ptychography. We believe that our result of phase contrast imaging at an optical wavelength can be extended to the quantitative phase imaging of cells and tissues. The method also provides the feasibility of the lensless single-shot imaging of extended objects with x-ray free-electron lasers.

Full-field interferometric confocal microscopy using a VCSEL array.

We present an interferometric confocal microscope using an array of 1200 vertical cavity surface emitting lasers (VCSELs) coupled to a multimode fiber. Spatial coherence gating provides ~18,000 continuous virtual pinholes, allowing an entire en face plane to be imaged in a snapshot. This approach maintains the same optical sectioning as a scanning confocal microscope without moving parts, while the high power of the VCSEL array (∼5 mW per laser) enables high-speed image acquisition with integration times as short as 100μs. Interferometric detection also recovers the phase of the image, enabling quantitative phase measurements and improving the contrast when imaging phase objects.

Total three-dimensional imaging of phase objects using defocusing microscopy: Application to red blood cells

We introduce Defocusing Microscopy (DM), a bright-field optical microscopy technique able to perform total three-dimensional (3D) imaging of transparent objects. By total 3D imaging, we mean the determination of the actual shapes of the upper and lower surfaces of a phase object. We propose a methodology using DM and apply it to red blood cells subject to different osmolality conditions: hypotonic, isotonic, and hypertonic solutions. For each situation, the shapes of the upper and lower cell surface-membranes (lipid bilayer/cytoskeleton) are completely recovered, displaying the deformation of red blood cell (RBC) surfaces due to adhesion on the glass-substrate. The axial resolution of our technique allowed us to image surface-membranes separated by distances as small as 300 nm. Finally, we determine the volume, surface area, sphericity index, and RBC refractive index for each osmotic condition.

Influence of the phase effect on gradient‐based and statistics‐based focus measures in bright field microscopy

Autofocusing is essential to high throughput microscopy and live cell imaging and requires reliable focus measures. Phase objects such as separated single Chinese hamster ovary cells are almost invisible at the optical focus position in bright field microscopy images. Because of the phase effect, defocused images of phase objects have more contrast. In this paper, we show that widely used focus measures exhibit an untypical behaviour for such images. In the case of homogeneous cells, that is, when most cells tend to lie in the same focal plane, both gradient-based and statistics-based focus measures tend to have a local minimum instead of a global maximum at the optical focus position. On the other hand, if images show inhomogeneous cells, gradient-based focus measures tend to yield typical focus curves, whereas statistics-based focus measures deliver curves similar to the case of homogeneous cells. These results were interpreted using the equation describing the phase effect and patch-wise analysis of the focus curves. Bioprocess engineering experts are also influenced by the phase effect. Forty-four focus positions selected by them led to the conclusion that they prefer to look at defocused images instead of those at the optical focus.

Direct Imaging of Phase Objects Enables Conventional Deconvolution in Bright Field Light Microscopy

In transmitted optical microscopy, absorption structure and phase structure of the specimen determine the three-dimensional intensity distribution of the image. The elementary impulse responses of the bright field microscope therefore consist of separate absorptive and phase components, precluding general application of linear, conventional deconvolution processing methods to improve image contrast and resolution. However, conventional deconvolution can be applied in the case of pure phase (or pure absorptive) objects if the corresponding phase (or absorptive) impulse responses of the microscope are known. In this work, we present direct measurements of the phase point- and line-spread functions of a high-aperture microscope operating in transmitted bright field. Polystyrene nanoparticles and microtubules (biological polymer filaments) serve as the pure phase point and line objects, respectively, that are imaged with high contrast and low noise using standard microscopy plus digital image processing. Our experimental results agree with a proposed model for the response functions, and confirm previous theoretical predictions. Finally, we use the measured phase point-spread function to apply conventional deconvolution on the bright field images of living, unstained bacteria, resulting in improved definition of cell boundaries and sub-cellular features. These developments demonstrate practical application of standard restoration methods to improve imaging of phase objects such as cells in transmitted light microscopy.

Entire 3-Dimensional Image of Red Blood Cells using Defocusing Microscopy

We present a simple bright-field optical microscopy technique, which we termed Defocusing Microscopy (DM) [1-3], able to actually reconstruct the 3-dimensional (3D) structure of phase objects. For this reconstruction it is only necessary to measure the object DM contrast at two different objective focal positions. We apply the developed method to red blood cells and obtain the shapes of the upper and lower surfaces, the last one deformed by the substrate. We reconstruct cells in isotonic, hypotonic conditions, as shown in the figure below. Additionally, we obtain the height fluctuations, with nanometer sensitivity, and the elastic modulii of each surface separately. For entire 3D imaging of phase objects in motion, one can use two cameras focused on two different positions. Therefore, DM technique can also be applied to obtain 3D reconstruction of cells flowing in micro-fluidic devices.[1] U. Agero, C.H. Monken, C. Ropert, R.T. Gazzinelli, and O.N. Mesquita, Phys. Rev. E 67, 051904 (2003).[2] L.G. Mesquita, U. Agero, and O.N. Mesquita, Applied Phys. Lett. 88, 133901 (2006).[3] G. Glionna, C.K. Oliveira; L.G. Siman, Moyses HW, D.M.U. Prado, C.H. Monken, and O.N. Mesquita, Applied Phys. Lett. 94, 193701 (2009).View Large Image | View Hi-Res Image | Download PowerPoint Slide

Stereo vision in spatial-light-modulator–based microscopy

We propose a technique for realizing stereoscopic microscopy. We employ a spatial-light-modulator-based microscope to record two images under different angles in one shot. We additionally investigate the possibilities of dynamic aberration correction. It is found that aberration correction is unavoidable because of the employed commercial liquid crystal on a silicon modulator. Also, imaging of phase objects and highly reflective specimens is experimentally investigated. For some of the specimens, an inversion of the recorded intensity is observed, which leads to problems when viewing the stereo pairs. We explain the origin of this effect and show that a reasonable visualization of microscopic three-dimensional objects can be achieved by simple image inversion.

Stokes参量数字全息法实现相位物体成像

Stokes参量数字全息法实现相位物体成像

Three-dimensional imaging of a phase object from a single sample orientation using an optical laser

Ankylography is a new 3D imaging technique, which, under certain circumstances, enables reconstruction of a 3D object from a single sample orientation. Here, we provide a matrix rank analysis to explain the principle of ankylography. We then present an ankylography experiment on a microscale phase object using an optical laser. Coherent diffraction patterns were acquired from the phase object using a planar CCD detector and were projected onto a spherical shell. The 3D structure of the object was directly reconstructed from the spherical diffraction pattern. This work may potentially open the door to a new method for 3D imaging of phase objects in the visible light region. Finally, the extension of ankylography to more complicated and larger objects is suggested.