6,663 research outputs found
How fast is a twisted photon?
Recent measurements have highlighted that spatially shaped photons travel
slower than c, the speed of monochromatic, plane waves in vacuum. Here we
investigate the intrinsic delay introduced by `twisting' a photon, i.e. by
introducing orbital angular momentum (OAM). In order to do this we use a
Hong-Ou-Mandel interferometer to measure the change in delay of single photons
when we introduce OAM on a ring-shaped beam that is imaged through a focusing
telescope. Our findings show that when all other parameters are held constant
the addition of OAM reduces the delay (accelerates) with respect to the same
beam with no OAM. We support our results using a theoretical method to
calculate the group velocity and gain an intuitive understanding of the
measured OAM acceleration by considering a geometrical ray-tracing approach.Comment: 5 pages, 4 figure
Optical Polarization M\"obius Strips and Points of Purely Transverse Spin Density
Tightly focused light beams can exhibit electric fields spinning around any
axis including the one transverse to the beams' propagation direction. At
certain focal positions, the corresponding local polarization ellipse can
degenerate into a perfect circle, representing a point of circular
polarization, or C-point. We consider the most fundamental case of a linearly
polarized Gaussian beam, where - upon tight focusing - those C-points created
by transversely spinning fields can form the center of 3D optical polarization
topologies when choosing the plane of observation appropriately. Due to the
high symmetry of the focal field, these polarization topologies exhibit non
trivial structures similar to M\"obius strips. We use a direct physical measure
to find C-points with an arbitrarily oriented spinning axis of the electric
field and experimentally investigate the fully three-dimensional polarization
topologies surrounding these C-points by exploiting an amplitude and phase
reconstruction technique.Comment: 5 pages, 3 figures; additional supplementary materials with 4 pages,
3 figure
Decoherence of Anyonic Charge in Interferometry Measurements
We examine interferometric measurements of the topological charge of
(non-Abelian) anyons. The target's topological charge is measured from its
effect on the interference of probe particles sent through the interferometer.
We find that superpositions of distinct anyonic charges a and a' in the target
decohere (exponentially in the number of probes particles used) when the probes
have nontrivial monodromy with the charges that may be fused with a to give a'.Comment: 5 pages, 1 figure; v2: reference added, example added, clarifying
changes made to conform to the version published in PR
Spin-to-Orbital Angular Momentum Conversion and Spin-Polarization Filtering in Electron Beams
We propose the design of a space-variant Wien filter for electron beams that
induces a spin half-turn and converts the corresponding spin angular momentum
variation into orbital angular momentum of the beam itself by exploiting a
geometrical phase arising in the spin manipulation. When applied to a spatially
coherent input spin-polarized electron beam, such a device can generate an
electron vortex beam, carrying orbital angular momentum. When applied to an
unpolarized input beam, the proposed device, in combination with a suitable
diffraction element, can act as a very effective spin-polarization filter. The
same approach can also be applied to neutron or atom beams.Comment: 9 pages, 5 figure
Quantum simulation of a spin polarization device in an electron microscope
A proposal for an electron-beam device that can act as an efficient
spin-polarization filter has been recently put forward [E. Karimi et al., Phys.
Rev. Lett. 108, 044801 (2012)]. It is based on combining the recently developed
diffraction technology for imposing orbital angular momentum to the beam with a
multipolar Wien filter inducing a sort of artificial non-relativistic
spin-orbit coupling. Here we reconsider the proposed device with a fully
quantum-mechanical simulation of the electron beam propagation, based on the
well established multi-slice method, supplemented with a Pauli term for taking
into account the spin degree of freedom. Using this upgraded numerical tool, we
study the feasibility and practical limitations of the proposed method for
spin-polarizing a free electron bea
Optical anisotropy induced by torsion stresses in LiNbO3 crystals: appearance of an optical vortex
We report the results of studies of torsion effect on the optical
birefringence in LiNbO3 crystals. We have found that twisting of those crystals
causes a birefringence distribution revealing non-trivial peculiarities. In
particular, it has a special point at the center of cross section perpendicular
to the torsion axis where zero birefringence value occurs. It has also been
ascertained that the surface of the spatial birefringence distribution has a
conical shape, with the cone axis coinciding with the torsion axis. We have
revealed that an optical vortex, with the topological charge equal to unity,
appears under the torsion of LiNbO3 crystals. It has been shown that, contrary
to the q-plate, both the efficiency of spin-orbital coupling and the orbital
momentum of the emergent light can be operated by the torque moment.Comment: 28 pages, 8 figure
Generation and dynamics of optical beams with polarization singularities
We present a convenient method to generate vector beams of light having
polarization singularities on their axis, via partial spin-to-orbital angular
momentum conversion in a suitably patterned liquid crystal cell. The resulting
polarization patterns exhibit a C-point on the beam axis and an L-line loop
around it, and may have different geometrical structures such as \qo{lemon},
\qo{star}, and \qo{spiral}. Our generation method allows us to control the
radius of L-line loop around the central C-point. Moreover, we investigate the
free-air propagation of these fields across a Rayleigh range.Comment: 6 pages, 4 figures, appears on Optics Express
Generation of a spin-polarized electron beam by multipoles magnetic fields
The propagation of an electron beam in the presence of transverse magnetic
fields possessing integer topological charges is presented. The spin--magnetic
interaction introduces a nonuniform spin precession of the electrons that gains
a space-variant geometrical phase in the transverse plane proportional to the
field's topological charge, whose handedness depends on the input electron's
spin state. A combination of our proposed device with an electron orbital
angular momentum sorter can be utilized as a spin-filter of electron beams in a
mid-energy range. We examine these two different configurations of a partial
spin-filter generator numerically. The results of these analysis could prove
useful in the design of improved electron microscope.Comment: 7 pages, 7 figure
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