185 research outputs found
Analysis of optical OAM mode conversion using elastic vortex wave in graded-index optical fiber
We theoretically analyze acousto-optic (AO) mode conversion between optical orbital angular momentum (OAM) modes using an elastic vortex wave (EVW) carrying OAM in a squared graded index (GI) fiber. The AO mode conversion from the fundamental mode to the higher order optical OAM mode is a useful technology to generate optical OAM modes in the GI fiber. This paper clarify the contribution of each component of the dielectric constant perturbation caused by the EVW to the AO mode conversion
Theory and applications of free-electron vortex states
Both classical and quantum waves can form vortices: with helical phase fronts
and azimuthal current densities. These features determine the intrinsic orbital
angular momentum carried by localized vortex states. In the past 25 years,
optical vortex beams have become an inherent part of modern optics, with many
remarkable achievements and applications. In the past decade, it has been
realized and demonstrated that such vortex beams or wavepackets can also appear
in free electron waves, in particular, in electron microscopy. Interest in
free-electron vortex states quickly spread over different areas of physics:
from basic aspects of quantum mechanics, via applications for fine probing of
matter (including individual atoms), to high-energy particle collision and
radiation processes. Here we provide a comprehensive review of theoretical and
experimental studies in this emerging field of research. We describe the main
properties of electron vortex states, experimental achievements and possible
applications within transmission electron microscopy, as well as the possible
role of vortex electrons in relativistic and high-energy processes. We aim to
provide a balanced description including a pedagogical introduction, solid
theoretical basis, and a wide range of practical details. Special attention is
paid to translate theoretical insights into suggestions for future experiments,
in electron microscopy and beyond, in any situation where free electrons occur.Comment: 87 pages, 34 figure
Rutherford scattering of electron vortices
By considering a cylindrically symmetric generalization of a plane wave, the
first Born approximation of screened Coulomb scattering unfolds two new
dimensions in the scattering problem: transverse momentum and orbital angular
momentum of the incoming beam. In this paper, the elastic Coulomb scattering
amplitude is calculated analytically for incoming Bessel beams. This reveals
novel features occurring for wide angle scattering when the incoming beam is
correctly prepared. The result successfully generalizes the well known
Rutherford formula, incorporating transverse and orbital angular momentum into
the formalism.Comment: 9 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
Photon Self-Induced Spin to Orbital Conversion in TGG crystal at high laser power
In this paper, we present experimental evidence of a newly discovered
third-order nonlinear optical process Self-Induced Spin-to-Orbital Conversion
(SISTOC) of the photon angular momentum. This effect is the physical mechanism
at the origin of the depolarization of very intense laser beams propagating in
isotropic materials. The SISTOC process, like self-focusing, is triggered by
laser heating leading to a radial temperature gradient in the medium. In this
work we tested the occurrence of SISTOC in a terbium gallium garnet (TGG) rod
for an impinging laser power of about 100~W. To study the SISTOC process we
used different techniques: polarization analysis, interferometry and tomography
of the photon orbital angular momentum. Our results confirm, in particular,
that the apparent depolarization of the beam is due to the occurrence of
maximal entanglement between the spin and orbital angular momentum of the
photons undergoing the SISTOC process. This explanation of the true nature of
the depolarization mechanism could be of some help in finding novel methods to
reduce or to compensate for this usually unwanted depolarization effect in all
cases where very high laser power and good beam quality are required.Comment: 6 pages, 10 figures, submitte
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
Roadmap on structured light
Structured light refers to the generation and application of custom light fields. As the tools and technology to create and detect structured light have evolved, steadily the applications have begun to emerge. This roadmap touches on the key fields within structured light from the perspective of experts in those areas, providing insight into the current state and the challenges their respective fields face. Collectively the roadmap outlines the venerable nature of structured light research and the exciting prospects for the future that are yet to be realized.Peer ReviewedPostprint (published version
Extension of Friedel's law to Vortex Beam Diffraction
Friedel's law states that the modulus of the Fourier transform of real
functions is centrosymmetric, while the phase is antisymmetric. As a
consequence of this, elastic scattering of plane wave photons or electrons
within the first-order Born-approximation as well as Fraunhofer diffraction on
any aperture, is bound to result in centrosymmetric diffraction patterns.
Friedel's law, however, does not apply for vortex beams, and centrosymmetry in
general is not present in their diffraction patterns. In this work we extend
Friedel's law for vortex beams by showing that the diffraction patterns of
vortex beams with opposite topological charge, scattered on the same two
dimensional potential, always are centrosymmetric to one another, regardless of
the symmetry of the scattering object. We verify our statement by means of
numerical simulations and experimental data. Our research provides deeper
understanding in vortex beam diffraction and can be used to design new
experiments to measure the topological charge of vortex beams with diffraction
gratings, or study general vortex beam diffraction.Comment: 7 pages, 3 figure
Measuring the orbital angular momentum spectrum of an electron beam
Electron waves that carry orbital angular momentum (OAM) are characterized by a quantized and unbounded magnetic dipole moment parallel to their propagation direction. When interacting with magnetic materials, the wavefunctions of such electrons are inherently modified. Such variations therefore motivate the need to analyse electron wavefunctions, especially their wavefronts, to obtain information regarding the material’s structure. Here, we propose, design and demonstrate the performance of a device based on nanoscale holograms for measuring an electron’s OAM components by spatially separating them. We sort pure and superposed OAM states of electrons with OAM values of between −10 and 10. We employ the device to analyse the OAM spectrum of electrons that have been affected by a micron-scale magnetic dipole, thus establishing that our sorter can be an instrument for nanoscale magnetic spectroscopy
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