PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 721 February 24, 2005
by Phillip F. Schewe, Ben Stein
        
THE BIGGEST SPLASH OF LIGHT FROM OUTSIDE THE SOLAR SYSTEM to be recorded
here at Earth occurred on December 27, 2004.  The light came from an object
called SGR 1806-20, about 50,000 light years away in our own galaxy.  SGR
stands for "soft gamma repeater," a class of neutron star
possessing a gigantic magnetic field.  Such "magnetars" can erupt
violently, sending out immense bolts of energy in the form of light at
gamma rays and other wavelength regions of the electromagnetic spectrum. 
The eruption was first seen with orbiting telescopes at the upper end of
the spectrum over a period of minutes and then by more and more telescopes;
at radio wavelengths emissions were monitored for months.  For an instant
the flare was brighter than the full moon. (NASA press conference, 18
February; www.nrao.edu/pr/2005/sgrburst/;
www.ras.org.uk/html/press/pn0505ras.html; many telescopes participated in
the observations and results will appear in a forthcoming issue of Nature.)

FRACTAL JAMMING OF NANOTUBES.  Carbon nanotubes, those tiny hollow carbon
whiskers nanometers wide but microns or longer in length, have intriguing
optical, electrical, thermal, and mechanical properties.  Perhaps the
earliest big practical use for nanotubes will be as an additive in many
composite materials, both liquid and
solid.   NIST physicist Erik Hobbie gauges nanotube flow properties
by suspending them in a liquid polymer solvent between two parallel plates
and then subjecting the fluid to shear force by moving one of the plates. 
In general getting the long nanotubes lined up is like herding cats; they
get tangled very easily.  But at low concentration and high enough shear,
the tubes do line up, as if the mixture were a "nematic" liquid
crystal, a liquid in which rod-shaped polymer molecules are aligned with
each other.  Lower the amount of shear or raise the nanotube concentration
and the tangles begin.  Increase the concentration further and the tangling
gets more elaborate; the nanotubes form bands (visible to the human eye)
parallel to the plates and perpendicular to the flow direction.  At even
higher concentrations (around 3%) the aggregation becomes so great that
fluid flow comes to a halt.  In this tangled state the web of
interconnections between nanotubes takes on a fractal-like geometry. 
Knowing this geometry well will be of use in numerous upcoming industrial
processes
involving carbon nanotubes.  Hobbie reported his results at last week's
meeting of the Society of Rheology in Lubbock, Texas.  (Paper MF9,
www.rheology.org/sor/annual_meeting/2005Feb/default.htm )
                                                                                
"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 639). 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. The researchers 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; text at
www.aip.org/physnews/select ; contact Lluis Torner, http://www.icfo.es ;
for more background on OAM light, see Physics Today, May 2004, and New
Scientist, 12 June 2004).

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