80 research outputs found
Measuring the Orbital Angular Momentum of Electron Beams
The recent demonstration of electron vortex beams has opened up the new
possibility of studying orbital angular momentum (OAM) in the interaction
between electron beams and matter. To this aim, methods to analyze the OAM of
an electron beam are fundamentally important and a necessary next step. We
demonstrate the measurement of electron beam OAM through a variety of
techniques. The use of forked holographic masks, diffraction from geometric
apertures, diffraction from a knife-edge and the application of an astigmatic
lens are all experimentally demonstrated. The viability and limitations of each
are discussed with supporting numerical simulations.Comment: 5 pages, 4 figurs
HAADF-STEM block-scanning strategy for local measurement of strain at the nanoscale
Lattice strain measurement of nanoscale semiconductor devices is crucial for
the semiconductor industry as strain substantially improves the electrical
performance of transistors. High resolution scanning transmission electron
microscopy (HR-STEM) imaging is an excellent tool that provides spatial
resolution at the atomic scale and strain information by applying Geometric
Phase Analysis or image fitting procedures. However, HR-STEM images regularly
suffer from scanning distortions and sample drift during image acquisition. In
this paper, we propose a new scanning strategy that drastically reduces
artefacts due to drift and scanning distortion, along with extending the field
of view. The method allows flexible tuning of the spatial resolution and
decouples the choice of field of view from the need for local atomic
resolution. It consists of the acquisition of a series of independent small
subimages containing an atomic resolution image of the local lattice. All
subimages are then analysed individually for strain by fitting a nonlinear
model to the lattice images. The obtained experimental strain maps are
quantitatively benchmarked against the Bessel diffraction technique. We
demonstrate that the proposed scanning strategy approaches the performance of
the diffraction technique while having the advantage that it does not require
specialized diffraction cameras
Prospects for versatile phase manipulation in the TEM: beyond aberration correction
In this paper we explore the desirability of a transmission electron
microscope in which the phase of the electron wave can be freely controlled. We
discuss different existing methods to manipulate the phase of the electron wave
and their limitations. We show how with the help of current techniques the
electron wave can already be crafted into specific classes of waves each having
their own peculiar properties. Assuming a versatile phase modulation device is
feasible, we explore possible benefits and methods that could come into
existence borrowing from light optics where so-called spatial light modulators
provide programmable phase plates for quite some time now. We demonstrate that
a fully controllable phase plate building on Harald Rose's legacy in aberration
correction and electron optics in general would open an exciting field of
research and applications.Comment: 9 pages, 4 figures, special Ultramicroscopy issue for PICO2015
conferenc
Using electron vortex beams to determine chirality of crystals in transmission electron microscopy
We investigate electron vortex beams elastically scattered on chiral
crystals. After deriving a general expression for the scattering amplitude of a
vortex electron, we study its diffraction on point scatterers arranged on a
helix. We derive a relation between the handedness of the helix and the
topological charge of the electron vortex on one hand, and the symmetry of the
Higher Order Laue Zones in the diffraction pattern on the other for
kinematically and dynamically scattered electrons. We then extend this to atoms
arranged on a helix as found in crystals which belong to chiral space groups
and propose a new method to determine the handedness of such crystals by
looking at the symmetry of the diffraction pattern. Contrary to alternative
methods, our technique does not require multiple scattering which makes it
possible to also investigate extremely thin samples in which multiple
scattering is suppressed. In order to verify the model, elastic scattering
simulations are performed and an experimental demonstration on
MnSbO is given where we find the sample to belong to the right
handed variant of its enantiomorphic pair. This demonstrates the usefulness of
electron vortex beams to reveal the chirality of crystals in a transmission
electron microscope and provides the required theoretical basis for further
developments in this field
Asymmetry and non-dispersivity in the Aharonov-Bohm effect
Decades ago, Aharonov and Bohm showed that electrons are affected by
electromagnetic potentials in the absence of forces due to fields. Zeilinger's
theorem describes this absence of classical force in quantum terms as the
"dispersionless" nature of the Aharonov-Bohm effect. Shelankov predicted the
presence of a quantum "force" for the same Aharonov-Bohm physical system as
elucidated by Berry. Here, we report an experiment designed to test Shelankov's
prediction and we provide a theoretical analysis that is intended to elucidate
the relation between Shelankov's prediction and Zeilinger's theorem. The
experiment consists of the Aharonov--Bohm physical system; free electrons pass
a magnetized nanorod and far--field electron diffraction is observed. The
diffraction pattern is asymmetric confirming one of Shelankov's predictions and
giving indirect experimental evidence for the presence of a quantum "force".
Our theoretical analysis shows that Zeilinger's theorem and Shelankov's result
are both special cases of one theorem.Comment: 16 pages, 5 figure
Shaping electron beams for the generation of innovative measurements in the (S)TEM
In TEM, a typical goal consists of making a small electron probe in the
sample plane in order to obtain high spatial resolution in scanning
transmission electron microscopy. In order to do so, the phase of the electron
wave is corrected to resemble a spherical wave compensating for aberrations in
the magnetic lenses. In this contribution we discuss the advantage of changing
the phase of an electron wave in a specific way in order to obtain
fundamentally different electron probes opening up new application in the
(S)TEM. We focus on electron vortex states as a specific family of waves with
an azimuthal phase signature and discuss their properties, production and
applications. The concepts presented here are rather general and also different
classes of probes can be obtained in a similar fashion showing that electron
probes can be tuned to optimise a specific measurement or interaction
Electron Bessel beam diffraction for precise and accurate nanoscale strain mapping
Strain has a strong effect on the properties of materials and the performance
of electronic devices. Their ever shrinking size translates into a constant
demand for accurate and precise measurement methods with very high spatial
resolution. In this regard, transmission electron microscopes are key
instruments thanks to their ability to map strain with sub-nanometer
resolution. Here we present a novel method to measure strain at the nanometer
scale based on the diffraction of electron Bessel beams. We demonstrate that
our method offers a strain sensitivity better than and an
accuracy of , competing with, or outperforming, the best
existing methods with a simple and easy to use experimental setup.Comment: This article may be downloaded for personal use only. Any other use
requires prior permission of the author and AIP Publishing. This article
appeared in Appl. Phys. Lett. 114, 243501 (2019) and may be found at
https://aip.scitation.org/doi/abs/10.1063/1.5096245 Data available at:
https://doi.org/10.5281/zenodo.2566137 and code available at:
https://bitbucket.org/lutosensis/tem-thesis/sr
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