X-ray optics and interactions of x-rays with matter

X-Ray Coherence and Imaging

This project aims to develop techniques for extracting the maximum possible information conveyed in optical fields. Partially incoherent fields contain far greater information capacity compared to fully coherent fields (e.g. lasers). A fundamental limitation of current imaging techniques is the lack of a technique that is capable of extracting the enormous amount of information carried in partially optical wavefields.

This research program will apply techniques such as Phase Space Tomography [Opt. Lett. 30 204-206 (2005); JOSA A 22 1691 (2005); Phys. Rev. Lett. 98, 224801 (2007)] to develop state-of-the-art X-ray imaging techniques in which all the information about an object encoded in a partially incoherent wavefield can be decoded or reconstructed.

Figure 1: Complete reconstruction of the phase-space correlation function of an X-ray wavefield defined by a 1D single slit.

X-ray interferometry

Various forms of interferometry-based techniques can probe both amplitude and phase changes in wavefields in a very sensitive, precise and powerful manner. This project aims to develop 'non-destructive' interferometric imaging techniques for quantitative studies of phase systems with significant improvement in sensitivity and precision.

Figure 2: Two-dimensional interference intensity distribution of an X-ray beam defined by four square pinholes. Profiles and positions of the interference peaks are very sensitive to, and can be used to reconstruct the complex refractive index profile of an object inserted in one of the pinholes.

Elemental Contrast Full-Field Imaging

In many frontier areas of research it is the distribution of a particular element in the sample which is of crucial interest. This project aims to develop the elemental contrast full-field imaging method recently proposed [Phys. Rev. A 78 13839 (2008)]. Rather than comparing the images measured above and below an absorption edge in conventional absorption contrast technique, this method enhances the phase effect due to a particular element by taking multiple-wavelength measurements in the vicinity of its absorption edge.

Figure 3: (Left) An intensity image of a 2 component sample. (Right) Reconstruction of the projected thickness of a single element of the compound sample using this technique.

The method can be incorporated to various techniques of X-ray full-field imaging and therefore promises a wide range of applications.

Elemental contrast tomography

This project aims to explore the application of Elemental Contrast Full-Field Imaging to X-ray tomography. The development of this combined technique provides a unique tool to achieve 3D elemental contrast of compound samples and therefore promises great uses in many important research areas including manufacturing, material sciences, mining industry and cellulous studies. 

Interaction of x-rays with matter (photo-absorption, scattering, fluorescence) 

This project involves critical study of atom-photon interactions by accurate determination of the complex atomic form factors. Photon-atom interaction cross-sections are important in many fields of fundamental and applied physics. As many uncritical applications are well established, researchers and users outside the field have assumed that experiment and theory have converged with no further critical goals in this area. This assumption is seriously flawed for all elements in many energy regions.

We have developed a novel experimental technique called the X-ray Extended Range Technique (XERT) for accurately determining these cross-sections. The project will apply XERT in the investigation of the angular dependence of X-ray scattering which probes wavefunction distributions, bonding, shake-up and shake-down processes and which is at the forefront of much modern atomic physics.

Figure 4: Discrepancies in the total attenuation coefficient of silicon between our work using the XERT (solid circles), other experiments (symbols) and theories (lines). The significance of the results was discussed in Phys. Rev. Letts., 90, 257401 (2003).