Experimental Realisation of a \pi/2 Vortex Mode Converter for Electrons Using a Spherical Aberration Corrector

TL;DR

This paper demonstrates an experimental method to generate electron vortex beams with orbital angular momentum using a modified aberration corrector in a transmission electron microscope, improving beam brightness and mode purity.

Contribution

It introduces a novel approach to create electron vortex beams by repurposing a TEM aberration corrector as a a/2 mode converter, enabling switchable high-purity OAM beams.

Findings

Successful realization of a a/2 vortex mode converter in TEM.

Enhanced beam brightness by nearly an order of magnitude.

Generation of switchable, high-purity electron vortex beams.

Abstract

In light optics, beams with orbital angular momentum (OAM) can be produced by employing a properly-tuned two-cylinder-lens arrangement, also called $\pi$/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 microscope (TEM). 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 $\pi$/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.