99 research outputs found
Optical vortices with starlight: Implications for ground-based stellar coronagraphy
Using an l = 1 blazed fork-hologram at the focal plane of the Asiago 122 cm
telescope, we obtained optical vortices from the stellar system Rasalgethi
(alpha Herculis) and from the single star Arcturus (alpha Bootis). We have
analyzed the structure of the optical vortices obtained from non-monochromatic
starlight under very poor seeing conditions using a fast CCD camera to obtain
speckle patterns and carry out the lucky imaging technique, alternative to
adaptive optics. With the insertion of a red filter and of a Lyot stop we
performed l = 1 optical vortex coronography the double star HD74010. The
results are in agreement with theory and numerical simulations. Our results
open the way to applications of optical vortices to ground based astronomical
observations, in particular for coronagraphy with l > 1 masks. No intrinsic
orbital angular momentum was detected in the starlight.Comment: 4 pages, 5 figures. Revised data analysi
Lyot-plane phase masks for improved high-contrast imaging with a vortex coronagraph
Context. The vortex coronagraph is an optical instrument that precisely removes on-axis starlight allowing for high contrast imaging at small angular separation from the star, a crucial capability for direct detection and characterization of exoplanets and circumstellar disks. Telescopes with aperture obstructions, such as secondary mirrors and spider support structures, require advanced coronagraph designs to provide adequate starlight suppression.
Aims. We introduce a phase-only Lyot-plane optic to the vortex coronagraph, which offers improved contrast performance on telescopes with complicated apertures. Potential solutions for the European Extremely Large Telescope (E-ELT) are described.
Methods. Adding a Lyot-plane phase mask relocates residual starlight away from a region of the image plane, thereby reducing stellar noise and improving sensitivity to off-axis companions. The phase mask is calculated using an iterative phase retrieval algorithm.
Results. Numerically, we achieve a contrast on the order of 10-6 for a companion with angular displacement as small as 4λ/D with an E-ELT type aperture. Even in the presence of aberrations, improved performance is expected compared to either a conventional vortex coronagraph or an optimized pupil plane phase element alone
Overcoming the Rayleigh Criterion Limit with Optical Vortices
We experimentally and numerically tested the separability of two independent
equally-luminous monochromatic and white light sources at the diffraction
limit, using Optical Vortices (OV), related to the Orbital Angular Momentum
(OAM) of light. The diffraction pattern of one of the two sources crosses a
phase modifying device (fork-hologram) on its center generating the
Laguerre-Gaussian (L-G) transform of an Airy disk. The second source, crossing
the fork-hologram in positions different from the optical center, acquires
different OAM values and generates non-symmetric L-G patterns. We formulated a
criterion, based on the asymmetric intensity distribution of the superposed L-G
patterns so created, to resolve the two sources at angular distances much below
the Rayleigh criterion. Analogous experiments carried out in white light allow
angular resolutions which are still one order of magnitude below the Rayleigh
criterion. The use OVs might offer new applications for stellar separation in
future space experiments.Comment: 4 pages, 5 figure
Vortex stabilization by means of spatial solitons in nonlocal media
We investigate how optical vortices, which tend to be azimuthally unstable in local nonlinear materials, can be stabilized by a copropagating coaxial spatial solitary wave in nonlocal, nonlinear media. We focus on the formation of nonlinear vortex-soliton vector beams in reorientational soft matter, namely nematic liquid crystals, and report on experimental results, as well as numerical simulations
Stability of vortex solitons in a photorefractive optical lattice
Stability of off-site vortex solitons in a photorefractive optical lattice is
analyzed. It is shown that such solitons are linearly unstable in both the high
and low intensity limits. In the high-intensity limit, the vortex looks like a
familiar ring vortex, and it suffers oscillatory instabilities. In the
low-intensity limit, the vortex suffers both oscillatory and Vakhitov-Kolokolov
instabilities. However, in the moderate-intensity regime, the vortex becomes
stable if the lattice intensity or the applied voltage is above a certain
threshold value. Stability regions of vortices are also determined at typical
experimental parameters.Comment: 3 pages, 5 figure
Detection of vorticity in Bose-Einstein condensed gases by matter-wave interference
A phase-slip in the fringes of an interference pattern is an unmistakable
characteristic of vorticity. We show dramatic two-dimensional simulations of
interference between expanding condensate clouds with and without vorticity. In
this way, vortices may be detected even when the core itself cannot be
resolved.Comment: 3 pages, RevTeX, plus 6 PostScript figure
Utilization of photon orbital angular momentum in the low-frequency radio domain
We show numerically that vector antenna arrays can generate radio beams which
exhibit spin and orbital angular momentum characteristics similar to those of
helical Laguerre-Gauss laser beams in paraxial optics. For low frequencies (< 1
GHz), digital techniques can be used to coherently measure the instantaneous,
local field vectors and to manipulate them in software. This opens up for new
types of experiments that go beyond those currently possible to perform in
optics, for information-rich radio physics applications such as radio
astronomy, and for novel wireless communication concepts.Comment: 4 pages, 5 figures. Changed title, identical to the paper published
in PR
Vortex precession in Bose-Einstein condensates: observations with filled and empty cores
We have observed and characterized the dynamics of singly quantized vortices
in dilute-gas Bose-Einstein condensates. Our condensates are produced in a
superposition of two internal states of 87Rb, with one state supporting a
vortex and the other filling the vortex core. Subsequently, the state filling
the core can be partially or completely removed, reducing the radius of the
core by as much as a factor of 13, all the way down to its bare value. The
corresponding superfluid rotation rates, evaluated at the core radius, vary by
a factor of 150, but the precession frequency of the vortex core about the
condensate axis changes by only a factor of two.Comment: 4 pages, 3 figure
Stable spinning optical solitons in three dimensions
We introduce spatiotemporal spinning solitons (vortex tori) of the
three-dimensional nonlinear Schrodinger equation with focusing cubic and
defocusing quintic nonlinearities. The first ever found completely stable
spatiotemporal vortex solitons are demonstrated. A general conclusion is that
stable spinning solitons are possible as a result of competition between
focusing and defocusing nonlinearities.Comment: 4 pages, 6 figures, accepted to Phys. Rev. Let
- …