pulse penetrates glass
Technology Research News
As anyone familiar with laser pointers
knows, a laser beam can travel a fairly long distance through air before
the light spreads out enough to dissipate the red dot.
If you try pointing through a pane of glass, however, the red dot will
dissipate completely before it gets to the other side.
A team of researchers from France has shown that it is possible, however,
to push a very short pulse of laser light about a centimeter into a transparent
solid using the same principles that allow laser beams to stay focused
while they are traveling through air.
The researchers precisely focused femtosecond pulses of laser light very
close to the front surface of a block of glass in order to "propagate
a hairline laser beam... without the need of specially fabricated waveguides,"
said Stelios Tzortzakis, a research associate at Ecole Polytechnique in
France. "The laser beam is self-guided." A femtosecond is one million
billionth of a second.
Sending coordinated lightwaves a significant distance through solids before
the light starts to break up is potentially useful in several ways, Tzortzakis
The technique could be used to further shorten laser pulses, which could
speed optical communications. The experiments showed that a self-guided
pulse can break into two or more shorter pulses as it goes through the
glass. Optical communications use light pulses and the absence of pulses
to represent the ones and zeros of digital information; shorter pulses
mean more information can be transmitted in a given amount of time.
The ability to control tiny pulses of light as they pass through a solid
could also eventually lead to circuits that use light to compute, he said.
"Our work could probably give new design solutions," for all-optical circuits,
Tzortzakis said. Current research efforts rely on waveguides, which are
lines precisely etched in transparent material.
In both gases and solids, high-intensity laser pulses can travel long
distances when two competing phenomena are perfectly balanced. First,
the pulse changes the index of refraction of the medium it is traveling
through. This can cause the medium to act like a lens that focuses, or
converges the pulse further. At the same time, however, the high-intensity
of the pulse ionizes some of the molecules around it, lowering the index
of refraction, which causes the pulse to dissipate.
It was more difficult to find the right balance in a solid because light
does not shine through solids as easily as it shines through gases, said
Tzortzakis. The key is to focus the beam exactly, he said. "It is necessary
to focus the femtosecond laser beam near the front surface of the sample
with an adequate convergence. Too much or too little convergence is not
effective," he said.
The results are interesting from a materials physics point of view, said
Warren S. Warren, a chemistry professor at Princeton University. "Self-guiding
pulses have been observed in gases before, but not this cleanly in solids,"
The technique could be applied to shorten laser pulses within two years,
Tzortzakis said. Better understanding how laser pulses propagate through
solids may help overcome some of the barriers to developing intense femtosecond
lasers due to the damage the pulses cause to transparent optical elements,
Applications in optical computing could take five or even 10 years, he
Tzortzakises research colleagues were Lionel Sudrie, Michel Franco, Bernard
Prade, André Mysyrowicz, Arnaud Couairon and Luc Bergé of Ecole Polytechnique.
They published the research in the November 19, 2001 issue of Physical
Review Letters. The research was funded by the National School of Advanced
Techniques (ENSTA) in France, the French National Center for Scientific
Research (CNRS) the French Atomic Energy Authority (CEA).
Timeline: 5-10 years
Funding: University, Government
TRN Categories: Optical Computing; Optoelectronics and
Story Type: News
Related Elements: Technical paper, "Self-guided Propagation
of Ultrashort IR Laser Pulses in Fused Silica," Physical Review Letters,
November 19, 2001.
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