FEATURE ARTICLE
Filaments of Light
Pulsed terawatt lasers create some surprising effects when shone through the air—including the channeling of light
Jérôme Kasparian
Bug Zapper Extraordinaire
The fact that the Kerr effect can transform a high-power infrared
laser into a remote source of white light opens the door to a number
of exciting applications. For example, the tendency for some of the
light to be reflected backward suggests that we could create an
artificial "guide star" for use in adjusting astronomical
telescopes equipped with adaptive optics. But there are other
nonlinear optical effects of the Teramobile laser that can be
exploited as well. One is something called multiphoton fluorescence.
In normal fluorescence, a substance, say the phosphor powder that
coats the inside of a fluorescent lamp, absorbs high-energy photons
(typically in the ultraviolet) and releases lower-energy photons
(having, usually, visible-light wavelengths). In multiphoton
fluorescence, two or more low-energy photons are absorbed
simultaneously, raising an electron's energy level enough to allow a
single high-energy photon to be given off when the electron returns
to its original state. But because the chance of an atom absorbing
two photons at once is quite low, light of very high intensity (that
is, containing a very large number of photons) is needed. The pulsed
Teramobile laser provides just such light, which proves a great boon
for remotely sensing certain compounds using the phenomenon of
multiphoton fluorescence.
In a 2002 experiment, my colleagues and I showed that the Teramobile
beam and detection apparatus could sense biological aerosols at a
distance. The motivation was to be able to map a cloud, say, of
bacteria (perhaps given off during some industrial mishap or even a
biological attack) and to identify potentially pathogenic agents
among the various background atmospheric aerosols, among which may
be more mundane organic particles such as soot or pollen.
Our test used water droplets sized to mimic bacteria and laced with
the compound riboflavin, which fluoresces at visible wavelengths
when it absorbs two infrared photons, producing a characteristic
spectrum in the backscattered light. The experiment, carried out on
a cloud located about 45 meters from the Teramobile laser, showed
that it was easy to distinguish such a plume from a cloud of pure
water droplets. With refinement, this technique could, potentially,
be quite sensitive. We calculated that a laser tuned to excite
two-photon fluorescence in the amino acid tryptophan would boost
sensitivity by a factor of 10, allowing concentrations of as little
as 10 bacteria per cubic centimeter to be detected 4 kilometers
away. Although lidar systems based on normal fluorescence could also
be used to probe for biological agents, the laser employed would
have to operate at a shorter wavelength and thus be more prone to
attenuation, limiting the distance over which it could function effectively.
The ability of laser filaments to deliver high-intensity light at
substantial distances also opens the door to other very interesting
applications. For example, it becomes possible to conduct elemental
analyses of the surfaces of metals, plastics, minerals or liquids
from an appreciable distance, using a variation of a technique
called laser-induced breakdown spectroscopy. For that, a
powerful laser is focused on the material of interest, causing some
of it to be transformed into plasma. The emission spectrum of the
glowing plasma can then be analyzed, revealing the nature of the
substrate, with a detection limit that can be as little as a few
parts per million for some elements. This method is currently used
for such applications as the identification of highly radioactive
nuclear waste and for monitoring the composition of molten alloys,
because the tests can be performed without having to touch the
sample. Imagine being able to do such probing from a large distance
away! Normally, diffraction limits the intensity of light that can
be focused on a remote target. But laser filaments can deliver
intensities that are higher than the ablation threshold of many
types of materials, at distances of hundreds of meters or even kilometers.
Another application under investigation may prove more spectacular
yet—the control of lightning strikes. Lightning has always
fascinated people, in part because of its unpredictable nature and
destructive power—qualities that make these electrical
discharges very difficult to study. Investigators from
Electricité de France and CEA partially overcame those
obstacles in the 1970s, when they developed a technique to trigger
lightning on command using small rockets trailing thin wires. If
shot upward at the right moment, the rockets and the wires they
unspooled behind them served to initiate and channel the flow of
electric current.
One outgrowth of this work was the idea of using a high-intensity
laser to ionize air along the beam, thus forming a conducting
channel of plasma that could replace the rocket-hoisted wires. The
first attempts, mounted in the 1970s and '80s, used lasers that
produced nanosecond-long pulses. Those experiments were
unsuccessful, however, because the plasma created by such lasers is
largely opaque, which keeps the beam from extending a conductive
path very far. But recently this field of research has seen renewed
interest, because lasers can now provide higher intensities in
shorter pulses, thereby avoiding the severe absorption that would
otherwise occur. In particular, the team of Henri Pépin
(Institut national de la recherche scientifique) and Hubert P.
Mercure (Hydro-Québec) in Montreal have obtained quite
promising results, using pulsed lasers to trigger and guide
high-voltage discharges over several meters in the laboratory.
Spectacular experiments with the Teramobile system, installed in a
high-voltage facility at the Technische Universität Berlin,
showed that laser filaments can trigger and guide electric
discharges over distances exceeding 4 meters. Moreover, the
breakdown voltage is typically reduced by 30 percent. My colleagues
and I have also shown that rain (or rather simulated rain) does not
prevent the laser filaments from triggering these huge sparks.
Research now focuses on the possibility of extending the lifetime of
the plasma and increasing the length over which it is able to guide
a discharge. Although the control of real lightning remains science
fiction for the moment, recent progress in laser technology has
brought this three-decade-old dream much closer to reality.
Over the past few years, the capabilities of terawatt-class lasers
have improved markedly, while size and cost have come down. At the
same time, physicists have made great strides in understanding the
non-linear propagation of these high-power laser pulses in air. The
rapidity of this progress suggests that Teramobile-type lasers, or
systems like it, might soon be used widely, not just by scientists
in the course of their research but for any number of military,
commercial or public-safety applications.
Bibliography
- Kasparian, J, M. Rodriguez, G. Méjean, J. Yu, E.
Salmon, H. Wille, R. Bourayou, S. Frey, Y.-B. André, A.
Mysyrowicz, R. Sauerbrey, J.-P. Wolf and L. Wöste. 2003.
White-light filaments for atmospheric analysis. Science
301:61-64.
- Wille, H., M. Rodriguez, J.
Kasparian, D. Mondelain, J. Yu, A. Mysyrowicz, R. Sauerbrey,
J.-P. Wolf and L. Wöste. 2002. Teramobile: A mobile
femtosecond-terawatt laser and detection system. European
Physical Journal—Applied Physics
20:183-190.
- Kasparian, J, S. Frey, G.
Méjean, E. Salmon, J. Yu, J.-P. Wolf, R. Bourayou, J.-C.
Luderer, M. Rodriguez, H. Wille and L. Wöste. 2005.
Femtosecond white-light lidar. In Laser Remote Sensing.
Ed. T. Fujii and T. Fukuchi. New York: Marcel
Dekker.
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