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S.
Hill/Army
Research
Lab
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Hot
drop.
A
fine
droplet
of
water
will
focus
an
ultrafast
pulse
of
light
near
its
edge,
creating
a
confined
ball
of
plasma
that
emits
a beam
of
light.
The
hot
spot
appears
red in
an
image
from
the
theory
that
accompanied
the
experiments.
(Click
image
for
larger
version.)
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The
secret to probing clouds from afar may be
igniting tiny fires in their water
droplets. Researchers would like to detect
compounds in the air by watching the way
the atmosphere reflects brief pulses of
laser light, but so far this technique has
trouble producing a strong signal or
detailed data on particle sizes and
compositions. Now a team has found that
extremely brief light pulses reflect
strongly in one direction from single,
miniscule water droplets, by generating an
intense plasma within the drop. The
finding, reported in the 15
July print issue of PRL,
could potentially offer a more powerful
way of tracking everything from airborn
dust and minerals to pollen and biological
agents.
In
light detection and ranging (LiDAR),
detectors collect the light reflecting
back from a laser pulse sent into the
atmosphere. This technique gives the
concentration of a suspension of fine
drops of liquid, called an aerosol, in
space, but no other information. Shorter
pulses might improve the system, but
researchers expected nanosecond bursts to
vaporize small water droplets and scatter
their light in all directions.
So
Jean-Pierre Wolf of Claude Bernard
University in Lyon, France, and his
colleagues were surprised when they hit
single 30-micron-sized water droplets
falling from a nozzle with pulses lasting
just a few femtoseconds. The tiny globes
emitted light 35 times more intensely back
toward the laser source than in any other
direction. Equally curious, the emitted
light was white, whereas samples of water
in a centimeter-sized cuvette emit in the
infrared when given the same
treatment.
The
team's calculations point to plasma as the
explanation for both results. The curved
surface of the droplet strongly focuses
the incoming light pulse to a point near
the drop's edge. Water molecules at the
focal point rapidly ionize and heat up to
several thousand degrees Kelvin, forming a
tightly localized ball of plasma. Thanks
to the droplet's focusing abilities, this
plasma is hotter than that generated in a
cuvette of water hit with a similar laser
pulse, so the droplet's plasma glows in
visible light, rather than infrared.
Essentially, the droplet becomes white
hot. The white light is refocused back
toward the laser source, again by the
curved edge of the drop.
Wolf
and colleagues expect that the strong
directional emission will enhance LiDAR
signals and make it easier to discriminate
between particles of varying sizes,
because smaller particles focus light
differently from larger ones. They also
expect that the spectrum of the returning
light will suggest the composition of the
LiDAR target, something the standard
single-wavelength methods cannot do. The
team plans to test the technique in a
real-world experiment within the next
month. The Teramobile system--a mobile
terawatt-strength laser jointly operated
by academic institutions in France and
Germany--should give them this
chance.
The
paper reports a surprising effect that did
not have an immediately obvious
explanation, says Arnold Migus of the
Ecole Polytechnique in Paris. "It's a kind
of miracle." The results of the Teramobile
experiment will be instructive, he adds,
because the current experiment only looked
at one drop at a time. He isn't sure what
will happen when the pulse bounces around
inside a real aerosol, which "is not the
equivalent of a sum of drops."
--JR
Minkel
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White-Light
Nanosource with
Directional
Emission
Catherine
Favre, Véronique
Boutou, Steven C. Hill,
Wiebke Zimmer, Marcel
Krenz, Hendrik
Lambrecht, Jin Yu,
Richard K. Chang, Ludger
Woeste, and Jean-Pierre
Wolf
Phys.
Rev. Lett.
89,
035002
(print
issue of 15 July
2002)
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