Advances in microscopy techniques based on x-rays have opened unprecedented opportunities in terms of spatial resolution, combined with chemical and morphology sensitivity, to analyze solid, soft and liquid matter. The advent of ultrabright third and fourth generation photon sources and the continuous development of x-ray optics and detectors has pushed the limits of imaging and spectroscopic analysis to structures as small as a few tens of nanometers. Specific interactions of x-rays with matter provide elemental and chemical sensitivity that have made x-ray spectromicroscopy techniques a very attractive tool, complementary to other microscopies, for characterization in all actual research fields. The x-ray penetration power meets the demand to examine samples too thick for electron microscopes implementing 3D imaging and recently also 4D imaging which adds time resolution as well. Implementation of a variety of phase contrast techniques enhances the structural sensitivity, especially for the hard x-ray regime. Implementation of lensless or diffraction imaging helps to enhance the lateral resolution of x-ray imaging to the wavelength dependent diffraction limit.

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Transmission and emission x-ray microscopy: operation modes, contrast mechanisms and applications.

http://xrm.phys.northwestern.edu/research/pdf_papers/2009/mcdermott_tcb_2009.pdf

Soft X-ray tomography and cryogenic light microscopy: the cool combination in cellular imaging

Principles and Applications of Zone Plate X-Ray Microscopes

Serial diffraction of proteins requires an injection method to deliver analyte molecules—preferably uncharged, fully hydrated, spatially oriented, and with high flux—into a focused probe beam of electrons or X-rays that is only a few tens of microns in diameter. This work examines conventional Rayleigh sources and electrospray-assisted Rayleigh sources as to their suitability for this task. A comparison is made and conclusions drawn on the basis of time-resolved optical images of the droplet streams produced by these sources. Straight-line periodic streams of monodisperse droplets were generated with both sources, achieving droplet diameters of 4 and 1 micrometer, respectively, for the conventional and electrospray-assisted versions. Shrinkage of droplets by evaporation is discussed and quantified. It is shown experimentally that proteins pass undamaged through a conventional Rayleigh droplet source.

Droplet streams for serial crystallography of proteins

An advanced type of diffractive optical element is presented that combines the concept of the photon sieve with an off-axis, off-normal incidence reflection geometry. Compared to transmission optical elements, the signal-to-background ratio is significantly increased by separating the first from other diffraction orders without drastically reducing the size of the smallest diffractive element. The reflection photon sieve produces sharp foci at maximum contrast and offers the advantages of effective heat dissipation and a large working space above the focal plane. Experimental results for a device working at a photon energy of 100 eV are presented and compared to theory.

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Focusing light with a reflection photon sieve.

The G . ottingen transmission X-ray microscope at the low emittance electron storage ring BESSY II uses the concept of dynamical aperture synthesis (Reynolds, DeVelis, Parrent, Thomson (Eds.), The New Physical Optics Notebook, SPIE, 1990, pp. 536–548) for the object illumination. The concept is well suited as a condenser, as it can match any required numerical aperture of the TXM objective. Furthermore, a novel off-axis transmission zone-plate monochromator is included, which can generate a monochromaticity of several thousand in the object illumination. # 2001 Elsevier Science B.V. All rights reserved.

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A rotating condenser and off-axis zone plate monochromator for the TXM at the undulator U41 at BESSY II

The Gottingen transmission X-ray microscope at the low emittance electron storage ring BESSY II will use the concept of dynamical aperture synthesis [1] for the condenser-monochromator. The concept is well suited as a condenser, as it can match the undulator U41 to the TXM objective and has many other advantages, too. It can use an off-axis transmission zone plate with comparatively wide zones of low aspect ratio, which therefore can be produced with almost theoretical efficiency. It will deliver a monochromaticity of 1000 to 3000. As the numerical aperture of any existing micro objective zone plate can be matched, the achievable resolution increases to the theoretical limit. Phase contrast imaging is possible with annular phase plates of extremely small width, a fast switching from amplitude contrast to phase contrast is possible. Stereo imaging with arbitrary stereo axis will be possible without tilting the object.

The condenser-monochromator with dynamical aperture synthesis for the TXM at an undulator beamline at BESSY II

Zone plates with high spatial resolution and high diffraction efficiency are required as objectives for X-ray microscopes. In the energy region of 2–8 keV gold is a well suited material for highly efficient zone plates. In this work a process for Ni zone plates is extended for the fabrication of sub-100 nm gold zone structures. This tri-level process is based on pattern generation with electron beam lithography followed by dry etching to transfer the zone pattern into a highly cross-linked polymer which serves as galvanoform. After the electrodeposition of a thin intermediate layer of Ni, the galvanoform is filled with gold by electroplating. With this process zone plates with outermost zone widths of 70 nm and zone heights of about 500 nm, yielding a first-order diffraction efficiency of 10% at a photon energy of 4.1 keV, were manufactured.

Electroplated gold zone plates as X-ray objectives for photon energies of 2–8 keV

A zone plate for hard X-rays with outermost zone width as small as 30 nm and thickness ∼14 μm fabricated by sputtered-sliced technology was evaluated. With the brilliant X-ray beam from the undulator at the SPring-8 the minimum focal spot size attained was ∼0.8 μm at the energy of 18.6 keV. The intrinsic diffraction efficiency was determined to be ∼12%.

G. Schmahl

Papers

A rotating condenser and off-axis zone plate monochromator for the TXM at the undulator U41 at BESSY II

The condenser-monochromator with dynamical aperture synthesis for the TXM at an undulator beamline at BESSY II

Electroplated gold zone plates as X-ray objectives for photon energies of 2–8 keV

Characterization of the sputtered-sliced zone plate for high energy X-rays