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2007
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Optical Ferris wheel
(May 2007)
The overlap of two laser beams creates both constructive and destructive interference. For Laguerre-Gaussian modes, the resulting patterns appear as a circular array of petals. We have now shown that it the beams are chosen correctly, the destructive interference forms a circular array of dark holes, ideal for trapping atoms at very low temperatures. Shifting the frequency between the beam causes the pattern to rotate, creating what we call an "Optical Ferris wheel".
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movie
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S. Franke-Arnold, J. Leach, M. J. Padgett, V. E. Lembessis, D. Ellinas, A. J. Wright, J. M. Girkin, P. Öhberg and A. S. Arnold, Optical ferris wheel for ultracold atoms, Opt. Express 15, 8619-8625 (2007)
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2006
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Optical-vortex web
(April 2006)
Optical vortices are lines of darkness that thread volumes of light. These lines can form lines, loops, links and even knots. Even four plane waves interfering together can create vortex lines that are parallel, criss-crossed or looped, depends on a simple inequality involving the real amplitudes of the waves.
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Quantum entanglement measured using spatial light modulators (SLMs)
(Dec 2006)
Photons have long been the playground for experiments in quantum entanglement. We have just used spatial light modulators acting as holograms that measure the orbital angular momentum of down-converted photons, confirming their entanglement. The spatial light modulators simultaneously set the quantum state, form part of a scanning imaging system, and correct for optical aberrations.
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Eric Yao, Sonja Franke-Arnold, Johannes Courtial, Miles Padgett and Stephen Barnett, Observation of quantum entanglement using spatial light modulators, Opt. Express 14, 13089-13094 (2006)
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2004
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Knotted optical vortices
(November 2004)
Light is full of vortex lines - dark threads around which the light's momentum swirls. A careful examination of the light scattered from a wall, for example, would reveal many vortex lines. The scattered light would contain many vortex lines that form loops. But what other shapes can they have? It was only recently that Mark Dennis and Michael Berry from Bristol University succeeded in constructing mathematically light fields that contain knotted and linked vortex lines, and we have now succeeded in observing them experimentally!
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Jonathan Leach, Mark Dennis, Johannes Courtial and Miles Padgett, Laser beams - Knotted threads of darkness, Nature 432, 165 (2004)
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Optical communication using light's orbital angular momentum
(November 2004)
We have pioneered the use of orbital angular momentum to encode information in free-space optical communication. Just like an intensity that is switched on or off can represent a value of zero or one -- a bit! --, so different phase-front twists can represent different values. In principle, there are infinitely many different twists, and as a single photon can have these twists, single photons can carry arbitrary amounts of information.
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Graham Gibson, Johannes Courtial, Miles J. Padgett, Mikhail Vasnetsov, Valeriy Pas'ko, Stephen M. Barnett and Sonja Franke-Arnold, Free-space information transfer using light beams carrying orbital angular momentum, Opt. Express 12, 5448 (2004)
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Discovery Channel Canada's story about the group's twisted-light communicator
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(12 June 2004) Our group's work makes the cover of New Scientist!
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(May 2004) The Royal Society of Edinburgh's Science Scotland magazine publishes an article about our group's work (issue 2, spring 2004).
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Measurement of the angular uncertainty principle
(August 2004)
One of the best known physical laws, Heisenberg's
Uncertainty Principle, states that the momentum and the position of a
particle cannot be measured simultaneously with arbitrary precision. This
sets a lower bound for the product of the uncertainties of these
quantities. The uncertainty relation also applies to other pairs of
physical quantities, known as conjugate variables. In particular, the
angular uncertainty principle relates angular momentum and angular position. The
first experimental verification of this was performed in our laboratory
using light beams carrying orbital angular momentum and precise angular
position filters.
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Sonja Franke-Arnold, Stephen M. Barnett, Eric Yao, Jonathan Leach, Johannes Courtial and Miles Padgett, Uncertainty principle for angular position and angular momentum, New J. Phys. 6, 103 (2004)
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2003
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Observation of colours in white-light vortices
(November 2003)
We were the first to observe the colours near perturbed white-light vortices. Our observation of these fundamental patterns of nature has sparked great interest in popular scientific media.
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2002
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Measuring the orbital angular momentum of single photons
(June 2002)
Unlike spin angular momentum, which has only two quantum-mechanical eigenstates, orbital angular momentum has an infinite number. We showed that this could be measured for single photons. In addition opening opportunities in ground-breaking experiments on Quantum Mechanics this may lead to new ultra-high density communication optical communications.
[more...]
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Mechanical effects of spin and orbital angular momentum: spinning and orbiting
(January 2002)
Spin and orbital angular momentum, when transferred to objects, cause them to rotate. Using optical tweezers to trap micron-sized objects away from the beam centre we were able to demonstrate that this rotation is about the object's axis when spin is transferred, and about the beam's axis in the case of orbital angular momentum.
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A. T. O'Neil, I. MacVicar, L. Allen, and M. J. Padgett, Intrinsic and extrinsic nature of the orbital angular momentum of a light beam, Phys. Rev. Lett. 88, 053601 (2002)
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1998
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Rotational Doppler effect
(November 1998)
The Doppler effect is familiar to anyone who has heard a fire engine speed past — because the (sound) source is translated relative to the observer, its frequency is shifted. If a (sound or light) source is rotated, an analogous frequency shift occurs.
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1997
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Nonlinear optics of light with orbital angular momentum
(November 1997)
Both linear and angular momentum must be conserved in any interaction. Light beams can have their frequency doubled by passing through a non-linear crystal. While the conservation of linear momentum places stringent conditions on the temperature and orientation of the crystal, the conservation of angular momentum changes the shape of the light beam.
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J. Courtial, K. Dholakia, L. Allen, and M. J. Padgett, Second harmonic generation and the conservation of orbital angular momentum with high-order Laguerre-Gaussian modes, Phys. Rev. A 56, 4193-4196 (1997)
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Optical spanners
(January 1997)
Optical tweezers use a tightly focused beam of light to hold and translate micron-sized objects in three dimensions. When the light beam has (spin or orbital) angular momentum, slightly absorbing objects will absorb not just some of the energy in the light beam, but also some of its angular momentum, causing the objects to rotate. We cristened the resulting tool, which can not only translate microscopic objects but also rotate them, optical spanners [spanner (brit.) = wrench (am.)].
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N. B. Simpson, K. Dholakia, L. Allen, and M. J. Padgett, The mechanical equivalence of the spin and orbital angular momentum of light: an optical spanner, Opt. Lett. 22, 52-54 (1997)
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