Four snapshots of an optical vortex, a specially prepared light beam
with a dark central core (blue). A Barcelona-Arizona collaboration has
observed in detail for the first time what happens when the energy spiraling
around the vortex core changes the direction in which it spins. Such
a reversal in spin seems to be rare, if not impossible, in other types
of vortices (such as water whirlpools and meteorological tornadoes).
Therefore, studying the change in the direction of rotation in a relatively
simple optical vortex can provide deep insights into other types of
more complex vortices, including whether or not they too can reverse
their spin. From the first snapshot (upper left) to the last (bottom
right), the core of the optical vortex changes from a spherical shape
to an elongated ellipse (top right and bottom left) until it reverses
its spin (bottom right). The small arrows show the change in direction.
The colors show intensity of the light beam, with red the brightest
and blue the darkest.
Around the dark core of an optical vortex, the energy flow carried
by the light beam is like a spiral staircase, flowing in a clockwise
or counterclockwise direction.
The researchers prepare an optical vortex by sending a laser beam into
a computer-generated holographic mask (above), a film which is simply
a hologram of the kind of photographic mask used in lithography. Containing
an interference pattern of light and dark bands, the mask essentially
splits the light into several light waves and causes them to yield a
beam with a vortex core.
How an optical vortex reverses its spin: The laser beam propagates
in the horizontal (z) direction and the transverse (x-y) plane is the
cross-section of the beam at a given z-plane. The sketch shows a schematic
top view of what-happens. Omitted are all the arrangements needed to
prepare the vortex that illuminates the lens. After the lens, the beam
begins to compress up to the focus of the lens where the beam starts
expanding again. 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 flow around it spins in an opposite
direction.
Reported by: Gabriel
Molina-Terriza, Jaume Recolons, Juan P. Torres, Luis Torner, and Ewan
M. Wright in the July 9 issue of Physical Review Letters.
Associated
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