Number 721, February 24, 2005
by Phil Schewe and Ben Stein
The Biggest Splash of Light From Outside the Solar System
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
"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; 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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