Electron vortex beams: production and propgation

Abstract

Electron vortex beams are electron beams which carry orbital angular momentum. They can be produced within an electron microscope by illuminating an appropriate holographic binary foil mask with a coherent electron beam.

In this work, the design of the holographic masks is investigated and found to be highly adaptable in producing a desired set of electron vortex beams through simple mask alterations. A number of mask parameters have been studied in order to determine what vortex beam properties can be produced experimentally.

The vortices form as the beam propagates beyond the mask, and exist in the far–field. The propagation of the beams has been studied in the near–field of the mask to display the vortex formation process, and studied in the far–field of the mask to predict the behaviour of the beams in the image plane of the electron microscope. This propagation series has been qualitatively compared with experimental data.

Finally, the effects of applying variously shaped apertures on the vortex beams was studied. From the produced simulations it is predicted that non–cylindrically symmetric apertures will cause a splitting of high–order vortex beams into lower orders. The splitting behaviour is dependent on the order of orbital angular momentum, and as such, is expected to be useful as a measurement tool. This is analogous to established behaviour reported in optical vortex beam literature.