From water whirlpools to meteorological tornadoes to superfluid Bose-Einstein condensates, vortices abound in nature. But it is difficult, if not impossible, to reverse the direction of rotation in a vortex without first destroying it. Now, a Barcelona-Arizona collaboration 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. Their work is detailed in yesterday's (July 9) Physical Review Letters.
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 (see figures at this URL).
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.
Authors of the Physical Review Letters paper are Gabriel Molina-Terriza, Jaume Recolons, Juan P. Torres and Luis Torner of the Laboratory of Photonics, Department of Signal Theory and Communications, Universitat Politecnica de Catalunya, Barcelona, Spain and Ewan M. Wright of the Optical Sciences Center, University of Arizona, Tucson.
(Reference: Molina-Terriza et al., Phys. Rev. Lett., 9 July 2001; text at this URL.)
(Editor's Note: This story is adapted from PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 546, July 5, 2001, by Phillip F. Schewe, Ben Stein, and James Riordon.)
[Contact: Gabriel Molina-Terriza]
10-Jul-2001
