>From water whirlpools to meteorological tornadoes to superfluid
Bose-Einstein Condensates (BEC), vortices abound in nature.
But it is difficult, if not impossible, to reverse the direction
of rotation in a vortex without first destroying it.
Gabriel Molina-Terriza, Technical University of Catalonia, Spain) has
observed in detail for the first time a reversal in the spin of an
optical vortex, a specially prepared light beam with a central dark core.
Studying the reversal of spin in this relatively simple type of vortex may
provide powerful insights into other vortices and whether they too can
reverse direction.
Around the dark eye of an optical tornado, the energy carried by the light
beam flows like a spiral staircase, in a clockwise or counterclockwise
direction. Researchers in the last decade have built devices to reverse the
spin of an optical vortex, but they have not observed what happens during
the reversal.
Now, the researchers employ a trick both to reverse and observe the optical
vortex. They pass it through a cylindrical lens. As the vortex travels
beyond the lens, its once-spherical core elongates like putty until it
is a vanishingly thin line. As the vortex moves farther beyond the
lens, the core eventually compresses itself into an ellipse but the
energy around it spins in an opposite direction.
These optical maelstroms can potentially carry several channels of quantum
data for such applications as quantum entanglement and teleportation, and
they can serve as optical tweezers for holding and rotating microscopic
objects.
They can also shed light on vortex behavior in Bose-Einstein
Condensates, since both optical and BEC vortices are described by
similar equations. The researchers' observations with light suggest
that BECs with weakly interacting atoms may have vortices whose
spins constantly reverse direction.
Molina-Terriza et al., Phys. Rev. Lett., 9 July 2001
http://xxx.lanl.gov/abs/cond-mat/0106210
http://www.optics.arizona.edu/wright/
http://www.aip.org/physnews/select
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