The creation and quantification of electron vortex beams, towards their application

Date: 27 September 2016

Venue: UAntwerpen, Campus Groenenborger, V0.09 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger)

Time: 4:00 PM

Organization / co-organization: Department of Physics

PhD candidate: Laura Clark

Principal investigator: Johan Verbeeck

Short description: PhD defence Laura Clark - Faculty of Science- Department of Physics


Electron vortex beams are a free electron state, which propagate along an axis, while possessing a non-zero value of orbital angular momentum (OAM) around that axis. In a circularly symmetric setup, they also must possess a 1D phase singularity along this axis, and can be described as Ψ = A(r)exp(ikz)exp(ilφ), where l  is a non-zero integer.

Such beams were only recently introduced into transmission electron microscopy, and have several, interesting, potential applications.

The earliest methods to produce electron vortex beams had certain limitations - either being of low intensity, not being isolated in the sample plane, or relying on a method that is difficult to reproduce. In this thesis, a method avoiding these limitations is demonstrated, through deliberate misaligning of the aberration corrector (a set of multipolar electromagnetic lenses, available on some transmission electron microscopes, as a tool to reduce the spherical aberration inherent in standard electromagnetic lenses). It is found that this method can produce well defined electron vortex beams of reasonable intensity, but with the drawback of reduced purity of OAM-modes.

Following this new method of electron vortex beam production, it is then important to develop practical methods of OAM measurement such that beam–sample OAM transfer can be detected. In this thesis, expanding on early methods to measure the topological charge of electron vortex beams, an experimental technique to enable a semi-quantitative measurement of the OAM spectrum of an electron vortex beam is demonstrated.

Vortices of topological charge |l | > 1 are unstable, and will typically decompose upon perturbation into a constellation of |l | = 1. The arrangement of these vortices, and how these vortices reach this state is non-trivial and symmetry-dependant. In this thesis, the effects of apertures of several different symmetries on electron vortex beams, and vortex propagation are investigated.

A pressing need for the investigation of the propagation of phase-structured electron waves is known, due to their potential for use in enhancing the contrast of weak-phase objects, by use of phase plates. In transmission electron microscopy, many important classes of materials are weak phase objects (including a vast range of biological materials), while also being beam-sensitive. In this thesis, a method to produce high-contrast images at low electron dose, using a spiral phase plate is investigated.