Watching Optical Tornadoes Reverse Their Spin

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 (Gabriel Molina-Terriza, Technical University of Catalonia, Spain, Molina@tsc.upc.es) 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 (see figures at Physics News Graphics).

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; text at Physics News Select.)

Physics News Graphics