Near-field Holography Research Articles

Solving the Phase Problem in X-Ray Near-Field Holography Beyond the Assumption of Weak Objects

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Reducing Rowland ghosts in diffraction gratings by dynamic exposure near-field holography.

Near-field holography (NFH) combined with electron beam lithography (EBL)-written phase masks is a promising method for the rapid realization of diffraction gratings with high resolution and high accuracy in line density distribution. We demonstrate a dynamic exposure method in which the grating substrate is shifted during pattern transfer. This reduces the effects of stitching errors, resulting in the decreased intensity of the optical stray light (i.e., Rowland ghosts). We demonstrate the intensity suppression of ghosts by 60%. This illustrates the potential for dynamic NFH to suppress undesirable periodic patterns from phase masks and alleviate the stitching errors induced by EBL.

Divide and update: towards single-shot object and probe retrieval for near-field holography.

We present a phase reconstruction scheme for X-ray near-field holographic imaging based on a separability constraint for probe and object. In order to achieve this, we have devised an algorithm which requires only two measurements - with and without an object in the beam. This scheme is advantageous if the standard flat-field correction fails and a full ptychographic dataset can not be acquired, since either object or probe are dynamic. The scheme is validated by numerical simulations and by a proof-of-concept experiment using highly focused undulator radiation of the beamline ID16a of the European Synchrotron Radiation Facility (ESRF).

The fluence-resolution relationship in holographic and coherent diffractive imaging.

This work presents a numerical study of the fluence-resolution behaviour for two coherent lensless X-ray imaging techniques. To this end the fluence-resolution relationship of inline near-field holography and far-field coherent diffractive imaging are compared in numerical experiments. To achieve this, the phase reconstruction is carried out using iterative phase-retrieval algorithms on simulated noisy data. Using the incident photon fluence on the specimen as the control parameter, the achievable resolution for two example phantoms (cell and bitmap) is studied. The results indicate the superior performance of holography compared with coherent diffractive imaging, for the same fluence and phase-reconstruction procedure.

表面等离激元的传播操控：从波束调制到近场全息

表面等离激元的传播操控：从波束调制到近场全息

Near-field holography enhanced with antireflection coatings—an improved method for fabricating diffraction gratings

Near-field holography (NFH), with its virtues of precise critical dimensions and high throughput, has a great potential for the realization of soft x-ray diffraction gratings. We show that NFH with reflections reduced by the integration of antireflective coatings (ARCs) simplifies the NFH process relative to that of setups using refractive index liquids. Based on the proposed NFH with ARCs, gold-coated laminar gratings were fabricated using NFH and subsequent ion beam etching. The efficiency angular spectrum shows that the stray light of the gratings is reduced one level of magnitude by the suppression of interface reflections during NFH.

Parallel Statistical Multiresolution Estimation for Image Reconstruction

We show that a careful parallelization of statistical multiresolution estimation (SMRE) improves the phase reconstruction in X-ray near-field holography. The central step in, and the computationally most expensive part of, SMRE methods is Dykstra's algorithm. It projects a given vector onto the intersection of convex sets. We discuss its implementation on NVIDIA's compute unified device architecture (CUDA). Compared to a CPU implementation parallelized with OpenMP, our CUDA implementation is up to one order of magnitude faster. Our results show that a careful parallelization of Dykstra's algorithm enables its use in large-scale statistical multiresolution analyses.

Near-Field Tomography and Holography of Low-Contrast Subsurface Objects

We develop a new method for electromagnetic computer tomography of distributed subsurface inhomogeneities and holography (i.e., retrieval of the form) of internally homogeneous subsurface objects. The method is based on solving the inverse problem of near-field scattering with allowance for the data of multifrequency measurements of the two-dimensional distribution of the scatteredfield complex amplitudes along the medium surface over the region of inhomogeneities.

Array processing methods for the determination of reflection properties of architectural surfaces

It has become popular in architectural acoustics to use microphone arrays, very often in spherical arrangements, to capture and analyze the sound field in rooms. This contribution will present a hemispherical receiver array, designed to obtain information about sound reflection from architectural surfaces, preferably in-situ. Similarly to previous studies, the analysis in the spherical harmonics (SH) domain is favored, with the additional challenge of data available on a hemisphere instead of a complete sphere. This problem is solved by obtaining orthonormal base functions on the hemisphere. As application cases, spherical beamforming for the determination of reflection factors will be presented as well as scattering near-field holography in order to determine the diffusion coefficient of small samples. Results from numerical and experimental case studies will be discussed.

Noise source identification with Beamforming in the pass-by of a car

Traffic noise is the most important environmental noise source. Standardized methods do not identify the main noise source or center an area of the car to do a more specific analysis. Other measurement techniques locate the noise source but have the problem of how to position the transducers to measure the noise sources during a pass-by. The best technique seems to be near-field holography (NAH), but it is only adequate at low frequency. In this paper the application of Beamforming for pass-by measurements has been considered. This technique allows not only to locate the main noise sources during the pass-by of a car, but also their characterization in terms of source strength. In order to test the accuracy of Beamforming in this kind of measurements, some pass-by measurements have been carried out, comparing the results obtained with other in the literature using different measurement techniques. Once the main noise sources are identified, a detailed analysis has been performed over sectors containing these noise sources, obtaining their acoustical characteristics, such as their spectra, sound pressure level, their dependence on the gear, the speed or the Strouhal number.

Reversible quasi-holographic line-scan processing for acoustic imaging and feature isolation of transient scattering

Transient acoustic scattering data from objects obtained using a one-dimensional line scan or two-dimensional raster scan can be processed via a linear quasi-holographic method [K. Baik, C. Dudley, and P. L. Marston, J. Acoust. Soc. Am. 130, 3838–3851 (2011)] in a way that is reversible, allowing isolation of spatially or temporally dependent features [T. M. Marston et al., in Proc. IEEE Oceans 2010]. Unlike nearfield holography the subsonic wavenumber components are suppressed in the processing. Backscattering data collected from a collocated source/receiver (monostatic scattering) and scattering involving a stationary source and mobile receiver (bistatic) may be processed in this manner. Distinct image features such as those due to edge diffraction, specular reflection, and elastic effects may be extracted in the image domain and then reverse processed to allow examination of those features in time and spectral domains. Multiple objects may also be isolated in this manner and clutter may be removed [D. J. Zartman, D. S. Plotnick, T. M. Marston, and P. L. Marston, Proceedings of Meetings on Acoustics 19, 055011 (2013) http://dx.doi.org/10.1121/1.4800881]. Experimental examples comparing extracted features with physical models will be discussed and demonstrations of signal enhancement in an at sea experiment, TREX13, will be shown. [Work supported by ONR.]

Measurement of thin films using very long acoustic wavelengths

A procedure for measuring material thickness by means of necessarily long acoustic wavelengths is examined. The approach utilizes a temporal phase lag caused by the impulse time of wave momentum transferred through a thin layer that is much denser than its surrounding medium. In air, it is predicted that solid or liquid layers below approximately 1/2000 of the acoustic wavelength will exhibit a phase shift with an arctangent functional dependence on thickness and layer density. The effect is verified for thin films on the scale of 10 μm using audible frequency sound (7 kHz). Soap films as thin as 100 nm are then measured using 40 kHz air ultrasound. The method's potential for imaging applications is demonstrated by combining the approach with near-field holography, resulting in reconstructions with sub-wavelength resolution in both the depth and lateral directions. Potential implications at very high and very low acoustic frequencies are discussed.

Green synthesis and characterization of size tunable silica-capped gold core–shell nanoparticles

Silica-coated gold nanoparticles (Au@SiO2) with controlled silica-shell thickness were prepared by a modified Stober’s method using 10-nm gold nanoparticles (AuNPs) as seeds. The AuNPs were silica-coated with a sol–gel reaction using tetraethylorthosilicate (TEOS) as a silica source and ammonia as a catalyst. An increase in TEOS concentration resulted in an increase in shell thickness. The NPs were characterized by transmission electron microscopy, selected area electron diffraction, energy-dispersive X-ray spectroscopy, scanning near-field ultrasound holography and scanning transmission electron microscopy. The method required no surface modification and the synthesized core shell nanoparticles can be used for various types of biological applications.

Surface plasmon resonance of centimeter-scale plasmonic crystal structures for sensor applications

Graphical abstractDisplay Omitted Highlight? We fabricate the centimeter-scale plasmonic crystal structures. We fabricate the centimeter-scale plasmonic crystal structures based on periodic arrays of gold nanowells and nanobumps on glass substrates. To generate plasmonic crystal structures, we use the near-field holography system with double-exposure in which the sample was rotated by 90?. The resonant characteristics of the plasmonic crystal structures are experimentally investigated.

The conception of structure-borne-sound-based near-field holography

A procedure for the projection of structure-borne sound fields is introduced, following the basic idea behind Near-Field Acoustic Holography. It is shown that the bending wave field of an infinite plate subjected to point force excitation can be reconstructed by means of Fourier transform-based wave field extrapolation. All required information on the wave field is obtained by vibrational response measurement in the near-field of the excitation, using a circular geometry with a limited number of positions. The procedure is verified both numerically and experimentally, to very satisfying results. The experiments indicate that the reconstruction at the source becomes erratic when standoff distances greater than half of the minimum wavelength under consideration are used. Reconstruction of the far-field plate response is found to be less restrictive.

Near-Field Electromagnetic Holography in Conductive Media

A new approach to the inversion of ill-posed boundary value (BV) problems is presented for an infinite conductive, homogeneous media. Our interest is to investigate the possibility of imaging underwater electromagnetic sources from remote electromagnetic sensor data when that data is measured coherently over a spatial sheet, commonly referred to as a hologram. Specifically, given independent holograms of two polarizations of the electric and/or magnetic fields on a cylindrical surface exterior to the electric and magnetic sources, we develop a frequency domain, back-projection (inverse) technique that reconstructs the complete electric and magnetic vector fields in the region between the BV (hologram) surface and the sources. Of particular interest is the Poynting vector that is constructed from the back-projected fields, providing the power per unit area radiated from the sources. We believe it may be of immense practical use in diagnosis of electromagnetic sources, such as underwater ship propulsors. Tikhonov regularization, developed here for the two component measured field, is used to stabilize the inversion. To investigate the accuracy and limitations of this new approach, we carry out a numerical experiment in which an array of either magnetic or electric dipole sources are excited in a frequency range of 1 to 1000 Hz in seawater. They are arranged 8 m apart in a line and generate coherent holograms of the axial components of the electric and magnetic fields on an imaginary cylindrical sheet of radius 30 m. Spatially random noise is added to these two holograms to simulate a relatively poor signal to noise ratio of 20 dB. Results show that we can successfully reconstruct the electric, magnetic and Poynting field vectors on the cylindrical sheet of 20 m radius (10 m closer to sources) with an accuracy of less than 30% for both magnetic dipole sources and electric dipole sources from 1 to 1000 Hz. The computations needed for this approach are easily carried out on a laptop computer and reconstructions of the complete field vectors are extremely fast, with a processing time in seconds.

Joint Patch Near-Field Holography

Traditional wave superposition method can't reconstruct the sound field precisely because of the absence of the equivalent source configuration information. A method named Joint Patch Near-field Holography (JPNAH) is proposed based on the statistically optimal near-field acoustical holography and the wave superposition method. The basic idea is that the fictitious sources' location essential to the wave superposition method can be decided by the statistically optimal near-field acoustical holography, and then complex sound field will be reconstructed by the wave superposition method with high efficiency and precision. Besides the reconstruction of the patch sound field, the JPNAH has many other virtues. Firstly, the JPNAH technique is not sensitive to the sound source outside the patch area. Secondly, compared to the NAH, the JPNAH performs better in the low frequency range. Finally, it is possible to achieve a real-time mapping of the complex sound field by using the JPNAH. Numerical simulations were performed based on a single-pulse-ball model; and also motor experiments were carried out in a semi-anechoic chamber. Both of numerical simulations and experiments show that the JPNAH can be used to reconstruct the sound field radiated from a complex sound source.

Method of superposition applied to patch near-field acoustic holography

The method of superposition may be applied to reconstruct the field on a partial surface on a radiating structure from measurements made on a nearby limited surface. Unlike conformal near-field holography, where the measurement surface surrounds the entire structure, in patch holography the measurement surface need only be approximately as large as the patch on the structure surface where the reconstruction is required. Using the method of superposition, the field on and near the measurement surface may be approximated by the field produced by a source distribution placed on a surface inside the structure. The source strengths are evaluated by applying boundary conditions on the measurement surface. The algorithm requires the inversion of the Green’s function matrix which may be ill-conditioned. Truncated singular value decomposition is used to invert it. The field on the structure surface is then approximated by the field produced by the source distribution. The algorithm is easier to implement than the boundary elements method because it does not require integrations over singular integrands and may be applied to flat or curved surfaces.

Recording of optical near fields in remote locations by near-field holography

We propose a method of near-field recording in a space that is quite apart from the original source (generator) of optical near fields. The method is based on the recently developed technique of near-field holography. Experiments based on our method have shown that near fields that originate from sub-diffraction-limit-sized objects can be stored in a photorefractive crystal 2 mm apart from the crystal surface, resulting in the retrieval of sub-diffraction-limit-sized spots. This means that our scheme can provide a method for multilayer (stackwise) near-field storage and, thus, contribute to a significant enhancement of the storage capacity of near-field optical memory.

Fabrication of high-spatial-frequency gratings through computer-generated near-field holography

We demonstrate an innovative method for fabrication of high-spatial-frequency grating structures. This technique makes use of the near-field diffraction patterns from computer-generated phase holograms for lithographic fabrication of grating structures with periods that are one half that of the phase hologram mask. Linear, rectilinear, and circular gratings were fabricated with this technique. Experimental results from gratings with periods to 0.5 mum and feature sizes to ~0.2 mum are presented.