Abstract
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.
Highlights
Using electron vortex beams, a multitude of new analyses and techniques have been demonstrated, including nanoparticle rotation [1, 2], chiral crystal structure discrimination [3] and free electron Landau state observations [4,5]
The intensity difference between the two main lobes primarily stems from the slight phase shift deviation of 0.11 π, from the inevitable absorption in the ZAC film, which was measured to be of the order of 23 % and to a certain extent from alignment errors of the Hilbert phaseplate centre rim
The DCOR in the PICO transmission electron microscopes (TEMs) has been reconfigured to a vortex mode converter
Summary
A multitude of new analyses and techniques have been demonstrated, including nanoparticle rotation [1, 2], chiral crystal structure discrimination [3] and free electron Landau state observations [4,5]. In the last few years substantial progress has been made in the field of dynamic electron wavefront engineering in the transmission electron microscope (TEM) [12,13], holding promise for quasi instantaneous and (nearly) arbitrary wavefront shaping. It opens up the way for measurement schemes where adopting the beams’ geometry and phase structure to the sample characteristics and the question at hand significantly enhances the information gained from the sample [14,15]. Optical phase plates, working either with pulsed or continuous-wave laser excitation, are definitely an interesting and fruitful new approach but they suffer from e.g. the limited wavelength of the laser light and the need for costly and complicated external equipment [16]
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