You are invited to our next Imaging and Optical Physics Seminar, to be held on May 11th at 1pm on Zoom.
Our two speakers are Joanne Etheridge from the Monash Centre for Electron Microscopy, speaking on "Imaging with symmetry at the picometre scale' and Timur Gureyev from the University of Melbourne, speaking on "Differential Holographic Tomography for atomic-resolution electron imaging". Please find abstracts for their talks below.
To connect on the day, please click this URL:
Or, open Zoom and enter meeting ID: 827 0500 8748 and passcode: 794464
Presenter: Joanne Etheridge
Imaging with symmetry at the picometre scale
An “image” is a representation of an object, providing information about its form and structure. In many imaging systems, image contrast is generated when the object alters the wavelength, amplitude and/or phase of the probing radiation. In this talk, I will discuss a different approach to image formation where contrast is generated when the object alters the symmetry of the probing radiation. We will consider the case when the probe is a spherical electron wave-field focussed to a point smaller than an atom and is raster-scanned across the object - a scenario that can be executed in a scanning transmission electron microscope. Due to the strength of the Coulomb interaction between the probing electron and the specimen, and the consequent multiple, dynamical scattering, these images are acutely sensitive to changes in specimen symmetry at the picometre scale. This provides a different mechanism for imaging atomic-scale features in condensed matter, particularly where symmetry or symmetry-breaking plays a significant role.
Presenter: Timur Gureyev
Differential Holographic Tomography for atomic-resolution electron imaging
It has been argued that in atomic-resolution transmission electron microscopy (TEM) of sparse weakly scattering structures, such as biological molecules, multiple electron scattering tends to have only a small effect on the defocused images and on the subsequent CT reconstruction from such images. At the same time, at the spatial resolution approaching one angstrom or finer, the role of in-molecule Fresnel diffraction becomes significant due to the intrinsically shallow depth of focus, which leads to non-negligible curvature of the Ewald sphere. This implies that suitable 3D reconstruction methods can be based on the 1st Born or the 1st Rytov approximation to the Fresnel diffraction. One such method, termed here Differential Holographic Tomography, has been recently developed and successfully tested on numerical models of several biological molecules. It has been shown that the method is capable of accurate reconstruction of the locations of atoms in a molecule from TEM images collected at a small number of random orientations of the molecule. Possible applications to cryogenic electron microscopy and other areas will be discussed.