Soft X-ray Holography Research Articles

Multi-reference lens-less Fourier-transform holography with a Greek-ladder sieve array

Lens-less Fourier-transform holography has been actively studied because of its simple optical structure and its single-shot recording. However, a low-contrast interferogram between the reference and object waves limits its signal to noise ratio. Here, multi-reference lens-less Fourier-transform holography with a Greek-ladder sieve array is proposed in the experiment and demonstrated effectively to improve the signal to noise ratio. The key technique in our proposed method is a Greek-ladder sieve array, which acts as not only a wave-front modulator but also a beam splitter. With advantages of the common path, single shot, and no need for a lens, this system has enormous potential in imaging and especially in extreme ultraviolet and soft X-ray holography.

Imaging Nanometer Phase Coexistence at Defects During the Insulator-Metal Phase Transformation in VO2 Thin Films by Resonant Soft X-ray Holography.

We use resonant soft X-ray holography to image the insulator-metal phase transition in vanadium dioxide with element and polarization specificity and nanometer spatial resolution. We observe that nanoscale inhomogeneity in the film results in spatial-dependent transition pathways between the insulating and metallic states. Additional nanoscale phases form in the vicinity of defects which are not apparent in the initial or final states of the system, which would be missed in area-integrated X-ray absorption measurements. These intermediate phases are vital to understand the phase transition in VO2, and our results demonstrate how resonant imaging can be used to understand the electronic properties of phase-separated correlated materials obtained by X-ray absorption.

The contribution of Diamond Light Source to the study of strongly correlated electron systems and complex magnetic structures.

We review some of the significant contributions to the field of strongly correlated materials and complex magnets, arising from experiments performed at the Diamond Light Source (Harwell Science and Innovation Campus, Didcot, UK) during the first few years of operation (2007-2014). We provide a comprehensive overview of Diamond research on topological insulators, multiferroics, complex oxides and magnetic nanostructures. Several experiments on ultrafast dynamics, magnetic imaging, photoemission electron microscopy, soft X-ray holography and resonant magnetic hard and soft X-ray scattering are described.

Automatable sample fabrication process for pump-probe X-ray holographic imaging

Soft X-ray holography is a recently developed imaging technique with sub-50 nm spatial resolution. Key advantages of this technique are magnetic and elemental sensitivity, compatibility with imaging at free electron laser facilities, and immunity to in-situ sample excitations and sample drift, which enables the reliable detection of relative changes between two images with a precision of a few nanometers. In X-ray holography, the main part of the experimental setup is integrated in the sample, which consequently requires a large number of fabrication steps. Here we present a generic design and an automatable fabrication process for samples suitable, and optimized for, efficient high resolution X-ray holographic dynamic imaging. The high efficiency of the design facilitates the acquisition of magnetic images in a few minutes and makes fully automatic image reconstruction possible.

Domain size in systems with canted magnetization

The equilibrium domain size in ferromagnetic single-layer films with canted magnetization, as observed in the spin-reorientation transition, is analytically described. It is demonstrated that for canted magnetization in the case of narrow domain walls the size of the magnetic domains depends solely on the second-order anisotropy constant and not on the first-order anisotropy. The single-layer model is extended to the application to multilayers. An approximation for the domain size of multilayers is developed and used for the analysis of data obtained via soft x-ray holography. Our analysis, including the canting angle, allows for the determination of the relative anisotropy constants to second order.

Exploring the limits of soft x-ray magnetic holography: Imaging magnetization reversal of buried interfaces (invited)

Only a very few experimental techniques can address the microscopic magnetization reversal behavior of the different magnetic layers in a multilayered system with element selectivity. We present an element-selective study of ferromagnetic (FM) [Co/Pt]n multilayers with perpendicular anisotropy exchange-coupled to antiferromagnetic (AFM) FeMn and IrMn films performed with a new experimental set-up developed for both soft x-ray spectroscopy and holography imaging purposes. The spectroscopy analysis allows the quantification of the unpinned (pinned) uncompensated AFM moments, providing direct evidence of its parallel (antiparallel) alignment with respect to the FM moments. The holography experiments give a direct view of both FM and uncompensated AFM magnetic structures, showing that they replicate to each other during magnetization reversal. Remarkably, we show magnetic images for effective thicknesses as small as one monolayer. Our results provide new microscopic insights into the exchange coupling phenomena and explore the sensitivity limits of these techniques. Future trends are also discussed.

Soft X-ray holographic microscopy of chromosomes with high aspect ratio pinholes

We used digital in-line soft X-ray holography (DIXH) in the Gabor geometry to image human chromosomes. The divergent wave front was generated by diffraction of synchrotron radiation from a high aspect ratio pinhole. As under our experimental conditions the achievable resolution depends on the pinhole radius, high aspect ratio holes with diameters in the 100 nm range were prepared by focused ion beam (FIB) milling. The central maximum of the obtained Airy pattern was used to image chromosomes prepared from metaphase HeLa cells at experimental resolutions of 370 ± 40 nm with soft X-rays at 260 eV photon energy provided by the BESSY II synchrotron radiation facility.

Imaging and quantifying perpendicular exchange biased systems by soft x-ray holography and spectroscopy

The magnetic properties of perpendicular exchange biased ferromagnetic (FM)/antiferromagnetic (AFM) [Pt/Co]n/IrMn systems were investigated by a new set-up combining element-selective soft x-ray holography and spectroscopy measurements. The holography experiments allow imaging the magnetization reversal of an exchange biased FM layer with an equivalent Co thickness below 5 nm in real space and in external magnetic fields, and provide direct evidence of its asymmetric nature. From the spectroscopy analysis, we have quantified the unpinned (pinned) uncompensated AFM moments, providing direct evidence of its parallel (antiparallel) alignment with respect to the FM moments.

Overcoming the field-of-view restrictions in soft x-ray holographic imaging

We present a new concept for imaging by soft x-ray holography. Microscopylike imaging capabilities were achieved by the separation of mask and sample. The use of two independent silicon nitride membranes, one for the field-of-view-defining mask and the reference beam, and the other for the sample, allows to image different areas on the sample. The movement of the field-of-view across the sample is realized by a piezomotor-driven sample stage that permits relative and stable positioning with nm-precision. We demonstrate the capabilities of the x-ray holographic microscopy (XHM) technique by showing images with 60 nm spatial resolution of an artificially structured 100 nm thick gold film with a lateral size of 19 × 4 μm2.

Magnetic soft x-ray holography study of focused ion beam-patterned Co/Pt multilayers

We report on Fourier transform holography (FTH) experiments on nanostructured Co/Pt multilayer films with 40 nm spatial imaging resolution. The films have been nanostructured by means of focused ion beam (FIB) milling. Applying the ion beam through the supporting membrane with controlled and homogeneous dosing allows for higher resolution magnetic structuring of the ion-sensitive film compared to direct FIB patterning. Nanostructured samples with magnetic stripes exposed to different ion doses and magnetic arrays with 200 nm lattice constant were successfully prepared and imaged by FTH. We present image-processing routines for artifact-free image reconstruction. With this, we could investigate the FIB-induced anisotropy modulation and the perpendicular domain structure in the nanostructured samples, showing how to control the domain size and configuration by applying the appropriate ion dose either homogeneously or concentrated in single spots.

Digital in-line soft x-ray holography with element contrast

Digital in-line soft x-ray holography (DIXH) was used to image immobilized polystyrene and iron oxide particles and to distinguish them based on their different x-ray absorption cross sections in the vicinity of the carbon K-absorption edge. The element-specific information from the resonant DIXH images was correlated with high-resolution scanning electron microscopy (SEM) pictures. We also present DIXH images of a cell nucleus and compare the contrast obtained for nuclear components with the appearance in optical microscopy.

Phase imaging of magnetic nanostructures using resonant soft x-ray holography

We demonstrate phase imaging by means of resonant soft x-ray holography. Our holographic phase-contrast method utilizes the strong energy-dependence of the refractive index at a characteristic x-ray absorption resonance. The general concept is shown by using a $\mathrm{Co}∕\mathrm{Pd}$ multilayer sample which exhibits random nanosized magnetic domains. By tuning below the Co $L$-edge resonance, our quantitative and spectroscopic phase method allows high-contrast imaging of nanoscale electronic and magnetic order while increasing the probing depth and decreasing the radiation dose by an order of magnitude. The complex refractive index is quantitatively obtained through the interference between resonant and nonresonant scattering.

Off-axis reconstruction of in-line holograms for twin-image elimination

In-line holography is an important experimental tool in particle-field analysis and soft X-ray holography. The sole disadvantage of in-line holography is the inherent twin-image problem. In this paper, a new method for twin-image elimination is proposed. This method is based on the principle of off-axis reconstruction of in-line holograms which is a new approach in the field of optical holography. Theoretical analysis and computer simulation, as well as initial experimental results, show that it is a simple and efficient method to eliminate the twin-image of in-line holograms.

Soft X-ray in-line holographic experiment

“Water window” soft X-ray has some advantages in the microimaging of biological specimens. Soft X-ray holography can give us three-dimensional images of biological cells in their natural condition. Quasi-monochromatic soft X-ray from the pdymonochromatic synchrotron radiation is obtained by a zone plate-pinhole linear monochromator at National Synchrotron Radiation Laboratory (NSRI,) at Hcfei. The pinhole is used to increase the temporal coherence of the soft X-ray. and also to increase the spatial coherence of it. An in-line holographic experiment has been finished with soft X-ray of main wavelength 4.5 nm and pinhole of diameter 0.03 mm. The magnified hologram is recowtructed with He-Ne laser. A smaller pinhole is wanted to be used to improve the cuherence of the soft X-ray so as to improve the quality of the hologram.

Image reconstruction from an in-line X-ray hologram with intensity distribution constraint

An image reconstruction method for in-line soft X-ray holography is presented. The image is reconstructed by a Fresnel phase-retrieval algorithm using a constraint on the intensity distribution. Experimental results show that the introduction of the intensity constraint is effective for eliminating artifacts such as a twin-image.

Subnanosecond in-line soft x-ray holography using germanium laser in the 20 nm wavelength region

Near- and far-field characteristics of a germanium soft x-ray laser at the wavelengths of 23.2–23.6 nm ( J = 2-1 transitions) and 19.6 nm ( J = 0-1 transition) are given for quantitative evaluation of the germanium laser for holographic applications. In-line Gabor holography of simple structured objects, such as thin opaque fibers and a standard electron microscope test specimen, is demonstrated. The holograms were recorded on PMMA (polymethyl methacrylate) photoresists and Ilford Q-plates. The holograms recorded on PMMA were retrieved with an atomic force microscope. The object image was reconstructed by numerical reconstruction with phase retrieval algorithm, resulting in a clear, ghost-free image with sharp edges which infer sub- μ m resolution. Future directions for x-ray laser holography are briefly discussed.

Subnanosecond In-Line Soft X-Ray Holography Using Germanium Laser in the 20 nm Wavelength Region

Near- and far-field characteristics of a germanium soft x-ray laser at the wavelengths of 23.2-23.6 nm (J = 2-1 transitions) and 19.6 nm (J = 0-1 transition) are given for quantitative evaluation of the germanium laser for holographic applications. In-line Gabor holography of simple structured objects, such as thin opaque fibers and a standard electron microscope test specimen, is demonstrated. The holograms were recorded on PMMA (polymethyl methacrylate) photoresists and llford Q-plates. The holograms recorded on PMMA were retrieved with an atomic force microscope. The object image was reconstructed by numerical reconstruction with phase retrieval algorithm, resulting in a clear, ghost-free image with sharp edges which infer sub-μm resolution. Future directions for x-ray laser holography are briefly discussed.

Signal to Noise Ratio in Soft X-ray Holography

Abstract Holographic imaging is analysed in the context of its application to soft X-ray microscopy. A partially coherent analysis is used to describe the hologram. The results are then used to calculate the signal-to-noise ratio for a complex holographic image and it is found that the noise depends only on the number of photons transmitted by the object. The results are expressed in terms of the complex refractive index of the object and thus make no assumption about its detailed structure.

Wavelength choice for soft x-ray laser holography of biological samples

The choice of an optimal wavelength for soft x-ray holography is discussed, based on a description of scattering by biological structures within an aqueous environment. We conclude that wavelengths slightly longer than the 43.7-A carbon K-edge provide a good trade off between minimizing the necessary source power and the dose absorbed by the sample and maximizing the penetrability of the x-rays through wet samples. This differs from the previous notion that wavelengths within the water window (between 23.2 A and 43.7 A) would be the best for holography. The problem of motion resulting from the absorption of x rays during a short exposure is described. The possibility of using ultrashort exposures in order to capture the image before motion can compromise the resolution is explored. The impact of these calculations on the question of the feasibility of using an x-ray laser for holography of biological structures is discussed.

Characteristics of optical components for soft x-ray microscopy and x-ray holography using an undulator radiation optical system (abstract)

X-ray Fresnel zone plates and transmission gratings, have been fabricated using fine-focused electron-beam lithography and Ta-on-SiN x-ray mask fabrication technique. The optical components for x-ray microscopy and x-ray holography were evaluated in an undulator radiation system. Nanometer lithography using fine-focused electron-beam writing machines is now regarded as an important technology to create new devices on very small structures such as quantum wires or boxes and new scientific probe elements such as x-ray optical components. A new fabrication process of Ta-on-SiN x-ray mask1 for x-ray optical components has been established. Tantalum is used for the x-ray absorber, because of having approximately the same absorption coefficient as that of gold. A thin SiN film, which has a high transparency for soft x rays, acts as a membrane supporting the Ta-absorbing patterns. The fabricated zone plates have an outermost zone width of 0.25 μm with up to 2 mm diameter. The pitch width of the transmission grating is 0.4 μm with up to 1.0 mm2 area. Adopting a highly brilliant and highly coherent undulator2 synchrotron radiation, we have been developing its optical system to apply to x-ray microscopy and holography. An x-ray microscope system3 consists of two kinds of zone plates with different dimensions for imaging and focusing undulator radiation. The resolving power of this microscope system was evaluated by using resolution test charts, which have the same structure as x-ray optical components. A resolution limit of 0.3 μm was obtained, which is very close to Rayleigh’s limit of 0.25 μm for the outer-most zone width. We constructed a divided wave-front interferometer with x-ray transmission gratings, and estimated the dispersion of undulator radiation to be 1/20. The authors thank S. Aoki of Tsukuba University for his useful discussion on x-ray optics. The authors are also grateful for the help from T. Tamamura and H. Yoshihara, NTT for fabrication of x-ray components, and from Y. Toyoshima of the Photon Factory in undulator experiments, respectively.