Novel applications of vortex beams and spiral phase plates in transmission electron microscopy
23 September 2016
UAntwerpen, Campus Groenenborger, U0.24 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger
Organization / co-organization:
Department of Physics
PhD defence Roeland Juchtmans - Faculty of Science, Department of Physics
Being experimentally demonstrated only six years ago, electron vortex beams belong to a new and exciting field in transmission electron microscopy. Their properties and potential applications have only just become subject to investigation and much remains to be discovered.
Our research originated from the observation of the helical character of vortex beams and the question whether this can be used to detect the chirality of crystals with screw axis symmetry. We present analytical calculations that give a positive answer to this question and experimental results showing a proof of concept. However, we also find that this technique only applies for crystals with odd screw axis symmetry.
Looking for a way to measure the chirality of all screw axes, we subsequently look at spiral phase plate (SPP) imaging with an annular aperture, where a SPP adds a vortex like phase to the scattered wave in diffraction space. Based on the symmetry of the scattering process, we will show how the chirality indeed can be seen from easy to interpret features in these images.
SPP imaging in general, is getting more and more attention as an edge enhancement technique to visualize low contrast objects. After using this setup to look at the chirality of crystals, we study SPP imaging by describing the scattered wave in a cylindrical symmetric base of orbital angular momentum eigenstates, or vortex states. We will show how SPP images directly reveal the OAM-components of the exit wave and how this can be used to detect the topological charge of vortex beams or the magnetization state of a magnetic needle.
Finally, we wonder, since SPP images accentuate the edges of weakly interacting samples, whether they are somehow related to the gradient of the exit wave. We will tackle this problem analytically and compare our findings with numerical simulations, showing how, in some specific cases, the gradient indeed can be seen from individual SPP images. Moreover, we will learn how SPP images can be used to separate the magnetic contribution to the phase shift from the electrostatic one and how they might open up new pathways to exit wave reconstruction.