"Optical vortices" might extract abundant information from matter,
providing a new and potentially wide-ranging optical tool, a Spain-US
team has proposed theoretically. An ordinary light beam, when viewed
head-on, looks like a bright circle. But a special light beam called
an "optical vortex," when viewed head-on, looks like a bright ring surrounding
a dark central core (see www.aip.org/png/2001/133.htm).
Optical vortices are the simplest kind of beam carrying a property called
"orbital angular momentum" (see Update
Extensively studied since the early 1990s, such light beams, when viewed
from the side, trace out a three-dimensional corkscrew pattern (see
figure at www.aip.org/png/2005/229.htm);
the pattern represents regions of constant phase (for example, regions
of maximum electric field). This spiraling of light represents an extra
"degree of freedom" that researchers can use as a new handle to optically
encode information and subsequently to retrieve information from objects
the beam strikes. In conventional laser beams, the energy flows parallel
to the beam axis, like water in a jet.
However, for light with orbital angular momentum (OAM), the energy spirals
around the beam axis. Ordinary beams carry only "spin angular momentum,"
encoded in the polarization of light. All possible spin states can be
constructed with just two polarization states (vertical and horizontal,
or clockwise and counterclockwise). For light with nonzero OAM, however,
many states are possible, with higher states denoting tighter corkscrews
(and consequently, a faster spiraling of energy; see figure at www.aip.org/png/2005/229.htm).
For this reason, one can encode a huge amount of information in an OAM
beam by creating light made of a superposition of many OAM states.
call the different OAM components "spiral spectra." In the "digital
spiral imaging" concept now put forward by Lluis Torner at the new Institute
for Photonic Sciences (ICFO) in Barcelona and his colleagues, a light
beam of a convenient shape illuminates a sample to be probed. The sample
scatters the beam and alters its spiral components. Breaking down the
altered beam into its individual orbital-angular momentum components
(and thereby analyzing the “spiral spectrum” of the scattered beam)
can yield a wealth of information from the object.
The spiral spectra would, for example, be sensitive to nonuniformities
in geometrical and structural properties of objects, and could be potentially
useful for detecting biological and chemical agents, for probing biological
specimens sensitive to OAM light, and might even aide recent proposals
to increase the amount of data that can be imprinted on a compact disk
using OAM. (Torner,
Torres, Carrasco, Optics Express, Feb. 7, 2005; contact Lluis Torner,
http://www.icfo.es ; for more background
on OAM light, see Physics Today,
May 2004, and New Scientist,
12 June 2004).