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T. Schachinger, P. Hartel, P. Lu, S. Löffler, M. Obermair, M. Dries, D. Gerthens, R. Dunin-Borkowski, P. Schattschneider:
"Experimental realization of a 𝜋/2 vortex mode converter for electrons using a spherical aberration corrector";
Ultramicroscopy, 229 (2021), S. 1 - 6.

In light optics, beams with orbital angular momentum (OAM) can be produced by employing a properly-tuned
two-cylinder-lens arrangement, also called 𝜋/2 mode converter. It is not possible to convey this concept directly
to the beam in an electron microscope due to the non-existence of cylinder lenses in commercial transmission
electron microscopes (TEMs). A viable work-around are readily-available electron optical elements in the form
of quadrupole lenses. In a proof-of-principle experiment in 2012, it has been shown that a single quadrupole
in combination with a Hilbert phase-plate produces a spatially-confined, transient vortex mode.
Here, an analogue to an optical 𝜋/2 mode converter is realized by repurposing a CEOS DCOR probe
corrector in an aberration corrected TEM in a way that it resembles a dual cylinder lens using two quadrupoles.
In order to verify the presence of OAM in the output beam, a fork dislocation grating is used as an OAM
analyser. The possibility to use magnetic quadrupole fields instead of, e.g., prefabricated fork dislocation
gratings to produce electron beams carrying OAM enhances the beam brightness by almost an order of
magnitude and delivers switchable high-mode purity vortex beams without unwanted side-bands.

http://dx.doi.org/10.1016/j.ultramic.2021.113340