P. Charalambous

Off-axis zone-plate monochromator for high-power undulator radiation

We report the design and construction of an off-axis zone-plate monochromator for diffraction-imaging experiments at beam line 9.0.1 at the Advanced Light Source (ALS) synchrotron-radiation facility at Berkeley USA. The device is based on an off-axis zone plate which can be conveniently inserted into or retracted from the beam. We discuss design issues such as the efficiency and spectral purity of the system and the technique for designing heat-tolerant windows for soft x-ray undulator beams. The monochromator functions successfully and good-quality diffractions patterns are being made with the beam it delivers.

Scanning soft X-ray imaging at 10 nm resolution

Abstract The potential imaging and selected-area near-edge X-ray absorption fine structure spectroscopic (NEXAFS) performance of a new scanning probe X-ray microscope (SPXM) for soft X-rays from synchrotron radiation is examined by computer modelling; an optical design for the microscope is also given. Acceptable dwell times per pixel in image collection are predicted. The microscope is expected to have a spatial resolution for “water-window” X-ray wavelengths (2.3–4.4 nm) of about 10 nm for specimens up to 200 nm thick. The central component of the optical system is a tube collimator forming a probe a few wavelengths in diameter above a scanned specimen. The collimator is illuminated at near-normal incidence by a Fresnel zoneplate condenser, and its exit aperture is positioned a few wavelengths above the specimen. Physical understanding is gained by a two-dimensional (2D) waveguide approach, and by calculations using the full-vector theory of Maxwells equations. The calculations, because of computational limitations, are carried out mainly in 2D. The vector results agree well with a multislice scalar calculation in 2D which is then applied to 3D to describe the 3D probe imaging and to estimate the energy throughput.

Fabrication of high-resolution x-ray diffractive optics at King's College London

The fabrication of high resolution x-ray diffractive optics, and Fresnel zone plates (ZPs) in particular, is a very demanding multifaceted technological task. The commissioning of more (and brighter) synchrotron radiation sources, has increased the number of x-ray imaging beam lines world wide. The availability of cheaper and more effective laboratory x-ray sources, has further increased the number of laboratories involved in x-ray imaging. The result is an ever increasing demand for x-ray optics with a very wide range of specifications, reflecting the particular type of x-ray imaging performed at different laboratories. We have been involved in all aspects of high resolution nanofabrication for a number of years, and we have explored many different methods of lithography, which, although unorthodox, open up possibilities, and increase our flexibility for the fabrication of different diffractive optical elements, as well as other types of nanostructures. The availability of brighter x-ray sources, means that the diffraction efficiency of the ZPs is becoming of secondary importance, a trend which will continue in the future. Resolution, however, is important and will always remain so. Resolution is directly related to the accuracy af pattern generation, as well as the ability to draw fine lines. This is the area towards which we have directed most of our efforts so far.

Optical source model for the 23.2-23.6 nm radiation from the multielement germanium soft X-ray laser

Abstract Distributions of source intensity in two dimensions (designated the source model), averaged over a single laser pulse, based on experimental measurements of spatial coherence, are considered for radiation from the unresolved 23.2/23.6 nm spectral lines from the germanium collisional X-ray laser.The model derives from measurements of the visibility of Young slit interference fringes determined by a method based on the Wiener–Khinchin theorem. Output from amplifiers comprising three and four target elements have similar coherence properties in directions within the horizontal plane corresponding to strong plasma refraction effects and fitting the coherence data shows source dimensions (FWHM) are ∼26 μm (horizontal), significantly smaller than expected by direct imaging, and ∼125 μm (vertical: equivalent to the height of the driver excitation).

Development and first applications of an imaging microscope using the Vulcan x-ray laser

An imaging microscope, comprising a Schwarzschild condenser and zone plate optical arrangement, has been established on the Vulcan Nd-glass laser system at the Rutherford Appleton Laboratory (RAL). Images of simple test structures have been taken in x-ray transmission using doublet x-ray laser radiation at 23.2 nm and 23.6 nm from collisionally pumped Ne-like germanium. Image resolutions of about 0.15 micrometers have been measured. The results are intended as a proof of principle and demonstrate both that images can be taken successfully using the Vulcan x-ray laser, and of specimen regions which are destroyed on passage of the x-ray beam.

The Measurement of MTFs in X‐ray Microscopy Using Diffractograms

A novel method to characterize the optical performance of a high‐resolution transmission x‐ray microscope is presented. It makes use of test patterns that consist of random arrays of sub‐resolution holes in a thin metal film, and so approximate to white‐noise input signals for the microscope. The test patterns have been fabricated by electron‐beam lithography at length scales appropriate for the resolution available in x‐ray microscopy, so that diffractograms produced from the image data can be directly interpreted in terms of the contrast transfer function of the optical system. Results of this method are shown for both brightfield and differential phase contrast imaging.

Combined near-field STXM with cylindrical collimator and AFM

The SNXM [1] was first installed at the European Synchrotron Research Facility in November 1998. It has been designed for water-window operation at a spatial resolution of about 10 nm and in its final form will comprise a Zone Plate focusing X-ray onto a cylindrical collimator 10–20 nm in diameter, made by drilling an AFM tip, with its exist aperture within a few nm of the specimen surface. The operation of the microscope may be loosely defined to be in near-field in analogy with the optical SNOM e.g. [2]. Point to point resolution equal to the collimator diameter is expected for specimens up to 200 nm thick. The collimator to surface separation is also monitored by the AFM scanning tip. Simultaneous signals are available from X-ray transmission and surface topography. A progress report is given limited by the current availability of high energy X-rays (3–6 Kev).