High-resolution monochromatic x-ray imaging system based on spherically bent crystals

We have developed an improved x-ray imaging system based on spherically curved crystals. It is designed and used for diagnostics of targets ablatively accelerated by the Nike KrF laser. A spherically curved quartz crystal (d = .?, R = mm) has been used to produce monochromatic backlit images with the He-like Si resonance line (1865 eV) as the source of radiation. The spatial resolution of the x-ray optical system is 1.7 mum in selected places and 2-3 mum over a larger area. Time-resolved backlit monochromatic images of polystyrene planar targets driven by the Nike facility have been obtained with a spatial resolution of 2.5 mum in selected places and 5 mum over the focal spot of the Nike laser.     

Line focusing for soft x-ray laser-plasma lasing

A computational study of line-focus generation was done using a self-written ray-tracing code and compared to experimental data. Two line-focusing geometries were compared, i.e., either exploiting the sagittal astigmatism of a tilted spherical mirror or using the spherical aberration of an off-axis-illuminated spherical mirror. Line focusing by means of astigmatism or spherical aberration showed identical results as expected for the equivalence of the two frames of reference. The variation of the incidence angle on the target affects the line-focus length, which affects the amplification length such that as long as the irradiance is above the amplification threshold, it is advantageous to have a longer line focus. The amplification threshold is physically dependent on operating parameters and plasma-column conditions and in the present study addresses four possible cases.     

High-resolution imaging ellipsometer

We report on a novel imaging ellipsometer using a high-numerical-aperture (NA) objective lens capable of measuring a two-dimensional ellipsometric signal with high resolution. Two-dimensional ellipsometric imaging is made possible by spatial filtering at the pupil plane of the objective. A Richards-Wolf vectorial diffraction model and geometrical optics model are developed to simulate the system. The thickness profile of patterned polymethyl methacrylate is measured for calibration purposes. Our instrument has a sensitivity of 5 A and provides spatial resolution of approximately 0.5 microm with 632.8-nm illumination. Its capability of measuring refractive-index variations with high spatial resolution is also demonstrated.     

High-resolution ab initio three-dimensional x-ray diffraction microscopy

Coherent x-ray diffraction microscopy is a method of imaging nonperiodic isolated objects at resolutions limited, in principle, by only the wavelength and largest scattering angles recorded. We demonstrate x-ray diffraction imaging with high resolution in all three dimensions, as determined by a quantitative analysis of the reconstructed volume images. These images are retrieved from the three-dimensional diffraction data using no a priori knowledge about the shape or composition of the object, which has never before been demonstrated on a nonperiodic object. We also construct two-dimensional images of thick objects with greatly increased depth of focus (without loss of transverse spatial resolution). These methods can be used to image biological and materials science samples at high resolution with x-ray undulator radiation and establishes the techniques to be used in atomic-resolution ultrafast imaging at x-ray free-electron laser sources.     

Coherence-contrast x-ray imaging based on x-ray interferometry

Coherence-contrast x-ray imaging--which detects changes in the degree of coherence caused by the placement of a sample in an x-ray interferometer--was developed for biomedical applications. Because the technique's sensitivity depends on the density gradient in the sample, it is particularly suitable for observing biomedical samples with large density differences, such as samples that include both biological soft tissue and bone. A measurement principle and method of this technique are described, and a fine coherence-contrast image of a mouse leg is given as an example result.     

Simultaneous spatial and spectral imaging of lasing droplets

We present an experimental technique that allows the simultaneous spatial imaging and spectral analysis of falling droplets that exhibit lasing. Single droplet investigations serve as, among other purposes, a preliminary study for spray and combustion researchers. The described setup provides a valuable tool for the evaluation of microdroplet investigations with laser-spectroscopic techniques that rely on laser-induced fluorescence (LIF) or similar spectroscopical phenomena. The emphasis is that both spatial and spectral information are obtained from single-shot images of a falling droplet. Furthermore, combining spatial imaging and a spatially resolving optical multichannel analyzer makes a pointwise rastering of the droplets spectrum possible. This allows for the (almost) unambiguous determination of sources of influence on the spectrum of these droplets-such as geometrical distortion and lasing, nondissolved tracer lumps, and similar phenomena. Although the focus is on the experimental technique itself, we supplement detailed studies of lasing in falling microdroplets. These results were obtained with the aim of developing a system for measuring temperature distributions in droplets and sprays. In the light of these results the practice of calibrating a droplets spectrum by use of a bulk liquid sample needs to be critically reviewed.     

Time-resolved x-ray transmission grating spectrometer for studying laser-produced plasmas

The development of a new time-resolved x-ray spectrometer is reported in which a free-standing x-ray transmission grating is coupled to a soft x-ray streak camera. The instrument measures continuous x-ray spectra with 20-psec temporal resolution and moderate spectral resolution (deltalambda >/= 1 A) over a broad spectral range (0.1-5 keV) with high sensitivity and large information recording capacity. Its capabilities are well suited to investigation of laser-generated plasmas, and they nicely complement the characteristics of other time-resolved spectroscopic techniques presently in use. The transmission grating spectrometer has been used on a variety of laser-plasma experiments. We report the first measurements of the temporal variation of continuous low-energy x-ray spectra from laser-irradiated disk targets.     

High-resolution superconducting x-ray detectors with two aluminum trapping layers

Superconducting tunnel junctions coupled to superconducting absorbers may be used as high-resolution, high-efficiency x-ray spectrometers. We have tested three detectors with niobium x-ray absorbing layers coupled to aluminum layers that serve as quasiparticle traps. Two detectors differed only in barrier thickness. A third detector includes an extra absorbing layer. Here we present a comparison of detector performance. The best energy resolution measured was 36 eV full width at half maximum at 6 keV.     

High-resolution optical imaging of magnetic-domain structures

High-resolution optical techniques for imaging magnetic domains in ferromagnetic materials such as confocal microscopy and scanning near-field optical microscopy (SNOM) are reviewed. The imaging capabilities of different techniques and image formation are discussed in the case of in-plane as well as out-of-plane magnetic anisotropy in different illumination configurations. It is shown that the magnetooptical resolution of near-field measurements depends on the film thickness and is limited by the diffraction on magnetic domains throughout the film. For thin magnetic films, subwavelength resolution can be attained. In addition to well-known near-field magnetooptical effects (out-of plane magnetization sensitivity of linear near-field microscopy and in-plane magnetization sensitivity of nonlinear near-field measurements), linear SNOM imaging of in-plane magnetization in thin magnetic films as well as nonlinear imaging of out-of-plane domains has been demonstrated. Thus, linear and second-harmonic near-field imaging of both in-plane and out-of-plane oriented magnetic domains can be achieved in transparent and opaque specimens. This enables applications of SNOM for studies of all kinds of magnetic materials. High-resolution optical imaging is required for characterization of the micro-magnetic and magnetooptical properties of novel magnetic structures in order to adopt a bottom-up approach in the search for new materials with improved characteristics.     

High-resolution elasticity imaging for tissue engineering

An elasticity microscope provides high resolution images of tissue elasticity. With this instrument, it may be possible to monitor cell growth and tissue development in tissue engineering. To test this hypothesis, elasticity micrographs were obtained in two model systems commonly used for tissue engineering. In the first, strain images of a tissue-engineered smooth muscle sample clearly identified a several hundred micron thick cell layer from its supporting matrix. Because a one-dimensional mechanical model was appropriate for this system, strain images alone were sufficient to image the elastic properties. In contrast, a second system was investigated in which a simple one-dimensional mechanical model was inadequate. Uncultured collagen microspheres embedded in an otherwise homogeneous gel were imaged with the elasticity microscope. Strain images alone did not clearly depict the elastic properties of the hard spherical cell carriers. However, reconstructed elasticity images could differentiate the hard inclusion from the background gel. These results strongly suggest that the elasticity microscope may be a valuable tool for tissue engineering and other applications requiring the elastic properties of soft tissue at high spatial resolution (75 mum or less).     

Phase-space formulation for phase-contrast x-ray imaging

Phase-space formulation based on the Wigner distribution has been presented for analyzing phase-contrast image formation. Based on the statistical nature and affine canonical covariance of Wigner distributions in the phase space, we show that the partial coherence effects of incident x-ray wave field on image intensity are simply accounted for by a multiplication factor, which is the reduced complex degree of coherence of the incident x-ray wave field. We show especially that with the undulator sources one cannot obtain the phase-contrast intensity by summing over the contributions from all electron positions, since the van Cittert-Zernike theorem fails in general for undulators. We derive a comprehensive formula that quantifies the effects of partial spatial coherence, polychromatic spectrum, body attenuation, imaging-detector resolution, and radiation dose on phase-contrast visibility in clinical imaging. The results of our computer modeling and simulations show how the formula can provide design guidelines and optimal parameters for clinical x-ray phase-contrast imaging systems.     

High-resolution underwater acoustic imaging with lens-based systems

In recent years, several sonars designed for high-resolution, short-range underwater imaging have been developed. These imaging systems use an acoustic lens to focus the incoming waves on an array of transducers. In this article we describe three prototype systems that use a line-focus or a point-focus lens and operate at a frequency of 300 kHz or 3 MHz. The line-focus lens produces two-dimensional (2D) intensity images, while the point-focus lens produces 3D intensity images. We present sample images taken from moving and stationary platforms, and discuss the techniques used for processing the acoustic backscatter data to reconstruct and visualize the scene. The images, particularly those taken with a point-focus lens, show a remarkable degree of detail. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 377–385, 1997     

High-resolution terahertz reflective imaging and image restoration

We present a high-resolution terahertz (THz) reflective imaging system, operating at 2.52 THz, that employs a continuous-wave THz gas laser and a pyroelectric detector. The spatial resolution was evaluated from the system's modulation transfer function and tested by scanning a series of resolution targets. To further improve the image quality, Lucy-Richardson method was adopted to restore the scanning result. With the scanning spot profile measured using knife edge method, a satisfying restoration result can be obtained. Finally, the system's performance was observed by imaging some different test objects.     

High-resolution ultrasonic imaging using two-dimensional homomorphic filtering

A new method for two-dimensional deconvolution of medical ultrasonic images is presented. The spatial resolution of the deconvolved images is much higher compared to the common images of the fundamental and second harmonic. The deconvolution also results in a more distinct speckle pattern. Unlike the most published deconvolution algorithms for ultrasonic images, the presented technique can be implemented using currently available hardware in real-time imaging, with a rate up to 50 frames per second. This makes it attractive for application in the current ultrasound scanners. The algorithm is based on two-dimensional homomorphic deconvolution with simplified assumptions about the point spread function. Broadband radio frequency image data are deconvolved instead of common fundamental harmonic data. Thus, information of both the first and second harmonics is used. The method was validated on image data recorded from a tissue-mimicking phantom and on clinical image data.     

High-resolution phase-conjugate imaging in double-pumped phase conjugators

Phase-conjugate images with a resolution greater than 250 lines/mm are obtained through the use of a bridge, double-pumped phase conjugator. We demonstrate that this conjugator can carry out imageprocessing tasks, such as the addition and subtraction of complex spatial distributions, with a spatial resolution of >100 lines/mm. These results represent a significant improvement over previously reported resolutions obtained from photorefractive mutually pumped phase conjugators and approach the theoretical limit imposed by the grating spacing and cross talk.     

High-resolution, air-coupled ultrasonic imaging of thin materials

This paper describes the use of a focused air-coupled capacitance transducer combined with pulse compression techniques to form high-resolution images of thin materials in air. The focusing of the device is achieved by using an off-axis parabolic mirror. The lateral resolution of the focused transducer, operating over a bandwidth of 1.2 MHz, was found to be less than 0.5 mm. A combination of the focused transducer as a source and a planar receiver in through-transmission mode has been developed for the measurement of different features in paper products, with a lateral resolution in through-transmission imaging of /spl sim/0.4 mm. Images in air of thin samples such as bank notes, high-quality writing paper, stamps, and sealed joints were obtained without contact to the sample.     

Time-resolved ten-channel monochromatic imaging of inertial confinement fusion plasmas

We report on what we believe to be the first use of toroidally bent crystals to record two-dimensional, spatially resolved, monochromatic images of laser-produced fusion plasmas combined with a 34-ps fast x-ray framing camera. An array of five toroidal silicon (311) and five toroidal germanium (311) crystals was developed. The imaging properties of the geometries are checked by a ray-tracing program and are compared with experimental results. The total imaging system (crystal and detector) provides an experimentally measured spatial resolution better than 15 mum. Time histories for the hydrogenlike argon emission and the heliumlike argon emission of fusion pellets driven with the GEKKO XII glass laser system are presented.