Electron beams have been used to analyze materials for a while, such as in electron microscopes. For the most part, the beams' rotation does not affect this analysis because in classical physics, an electron current in a vacuum does not have any orbital angular momentum. But in quantum mechanics, the electrons must be envisaged as a wavelike current, which can rotate as a whole about its propagation direction, similar to the air flow in a tornado.
Vortex light beams have also been used in optics, like optical tweezers for manipulating small particles. Vortex beams made from electrons also offer many new possibilities for managing nanoparticles or measuring angular momentum-related parameters. However, there were previously no really efficient methods of producing them.
In the future, these electron beams could be used in a targeted way to set tiny wheels in motion on a microscopic motor or the magnetic field of the rotating electrons could be used in the tiniest length scales. Even applications in data transfer (quantum cryptography) and quantum computers are feasible.
"When I was working on an idea of how these beams could be technically produced, it emerged that colleagues from Antwerp had had the same idea", explains Prof. Peter Schattschneider of the Institute for Solid State Physics and University Service Centre for Electron Microscopy at Vienna University of Technology (TU Vienna). "We therefore decided to pursue the project together: Antwerp had progressed further with the production and Vienna came up with a suggestion for the first application."
The production of vortex electron beams was made possible with the help of a grid-like screen cut from platinum foil. When it passes through the platinum screen, the electron beam is diffracted in a similar way to light beams when they pass through a fine grid. The shape of this screen, which measures only a few millionths of a metre, was specifically calculated so that a flat incident electron wave is converted into vortex beams. Right-rotating and left-rotating vortex beams are thus formed behind the grid and in the middle there is a conventional electron beam that does not rotate.
A flat wave (left) meets the specially shaped grid screen, which converts the electron beam into right-rotating and left-rotating vortex beams (top and bottom), and a middle beam that does not rotate. Similar to in a tornado, the rotation of the electron current is low internally. Credit: TU Vienna
If the electrons are used to irradiate a material which for its part also influences the angular momentum of the electrons, and if the electrons are subsequently directed through the made-to-measure platinum screen, then, after this, either the right-rotating or the left-rotating vortex beam will be more intense.
"This enables us to investigate processes affected by angular momentum in nanomaterials much more precisely than was previously possible," explains Schattschneider.
Citation: J. Verbeeck, H. Tian, P. Schattschneider, 'Production and application of electron vortex beams', Nature 467, 301–304 (16 September 2010) doi:10.1038/nature09366
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