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Holographic images may catch speed of light
Caltech scientists seek to catch movements at the speed
of light
By Becky Oskin
PASADENA -- The fastest digital cameras click at a doddering
1/1,000,000,000 of a second. While that's speedy enough for watching the
dancing molecules of chemical reactions and cracks tearing through steel,
scientists at Caltech are pushing the limits of technology by developing a
holographic imagining system quick enough to capture changes in a laser
pulse traveling at the speed of light.
Because it's creating
holograms instead of pictures, the tool may open up new research into
ultra-fast events such as the physics of filaments (self-focusing lasers),
an effect produced when intensely focused lasers tear apart the air.
"It's fascinating
because something so simple -- a single pulse propagating through water or
air -- can create such a huge complexity," said Demetri Psaltis, 50, a
Caltech electrical engineering professor and holography expert. "We're
capturing the fastest thing possible, looking at the speed of light."
Holography, the science
of making holograms, is an ideal way to record processes that happen too
fast for the human eye or the best digital equipment. Instead of a flat
picture, holograms are three-dimensional images, which offer more clues to
scientists trying to puzzle out the details of something they can only see
with the help of lasers and computers. For example, a hologram holds the
size, shape, brightness and contrast of an object, or in the case of a
laser pulse, the beam's phase and amplitude, as the physicists say.
"Holography is very
powerful, because it allows you to catch very fast events and also retain
all this information," Psaltis said.
To the casual observer,
a hologram is an indecipherable pattern created by a laser beam. Shining
light on the hologram re-creates the original image.
Here's where the
explanation can be a bit confusing, because Psaltis and his team are using
lasers to make holograms of other lasers.
A pulsed laser beam
aimed at what they want to record, in this case the laser burst producing a
filament, is split in two. One pulse is a reference, moving unchanged from
the laser source to the camera, while the other goes from the source to the
object of the interest (the laser pulse), then to the camera.
The slight difference
between the two pulses makes an interference pattern -- the hologram.
"It's basically
like strobe lighting. We illuminate it with short pulses," said Martin
Centurion, 26, a Caltech graduate student working on the holography system.
The Caltech researchers
are looking at ephemeral things, so their setup sends laser pulses so fast
a single camera picture records four holograms. Each pulse is aimed at a
different part of the camera lens, upper right, lower right and so on.
"They're so close
in time no electronics can detect (the different pulses)," Centurion
said.
By assembling a series
of holograms, Centurion produces a movie of a filament forming.
Centurion is a physicist
by training, and the challenge of explaining filaments piqued his interest
enough to dedicate a few years to engineering the holography laser system.
"Now that we have a
tool that works, I can amuse the physicist in me and try to find out how
(filaments) work," he said. Intensely focusing a laser pulse creates
so much energy the leading edge of the pulse ionizes atoms in the air,
making a plasma. Because the plasma has different properties than normal
air, the end of the laser pulse doesn't see the same thing the beginning
does.
Looking from the side,
the pulse appears to stretch into a fiber or filament, hence the name.
Filaments were discovered only eight years ago, Psaltis said. No one's
developed a good model to predict why they happen, he added.
Both filaments and
ultra-fast holography have practical applications. A French team with a
powerful laser has sent filaments into the sky -- the technique could one
day be used to study the atmosphere, for communication or to control where
lightning strikes by guiding electrical discharge from clouds. The
ultra-fast movies help scientists understand new phenomena such as
filaments and further delve into the properties of matter.
But for Psaltis, the joy
is in the engineering and discovery.
"When we started
this, I had friend here (at Caltech) using a camera to study cracks
propagating at the nanosecond time scale. I was looking at his setup and
thinking, 'Gosh, maybe we could do this holographically,' and indeed we
can.
"Then I thought,
'What if we push it to the femtosecond?' Then the question is: what are we
going to see?"
-- Becky Oskin can be
reached at (626) 578-6300, Ext. 4451, or by e-mail at becky.oskin@sgvn.com.
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