Electron vortex beams

Electron beams have long been used to analyse materials - for example in electron microscopes. Their rotation usually plays no role in this. Classically, an electron stream in a vacuum does not carry any orbital angular momentum. Quantum mechanically, however, you have to think of electrons as a wave-like stream - and it can rotate all around its direction of propagation, similar to the air stream in a tornado. Vortex beams of light have been used in optics for some time (for example, as optical tweezers to manipulate small particles).  Vortex beams of electrons also offer many new possibilities for controlling nanoparticles or measuring angular momentum-related quantities.

The generation of vortex electron beams was made possible with the help of a grid-like mask cut out of a platinum foil. Similar to how light beams are bent when they are sent through a fine grid, the electron beam also behaves when it passes through the platinum mask. The shape of this mask, which measures only a few millionths of a metre, was specially calculated to convert an incident plane electron wave into vortex beams. Thus, a dextrorotatory and a levorotatory vortex beam is formed behind the grid, and an ordinary electron beam without rotation in the middle. If you irradiate a material with the electrons, which in turn influences the angular momentum of the electrons, and then send the electrons through the customised platinum mask, either the dextrorotatory or the levorotatory vortex beam is more intense afterwards. This makes it possible to study angular momentum processes in nanomaterials much more precisely than was previously feasible.

New possibilities for generating electron vortex beams using magnetic fields are currently being researched.


Portrait of Stefan Löffler


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