WHAT are round and flat and travel at the speed of light? The answer is light
strings, super-powerful disc-shaped pulses of infrared radiation only a few
wavelengths long. Generated by powerful lasers, light strings are tiny pancakes
of energy鈥攔oughly a centimetre across鈥攖ravelling face-on at the
speed of light.
Although they sound odd, light strings are more than mere scientific
curiosities. Over the past few years, researchers have discovered that they have
an extraordinary property: they focus themselves as they pass through the air.
In other words, light strings can maintain their intensity over distances at
which conventional laser beams become hopelessly spread out.
This opens up a myriad of potential uses. Light strings promise to help
identify pollutants in the atmosphere, trigger lightning strikes and help
astronomers observe distant stars. They may also help in weather forecasting and
in rangefinding. But first physicists have a challenge on their hands, as the
underlying science of light strings is turning out to be more complex than
anyone imagined.
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One way to think about the passage of light through the atmosphere is to
imagine a photon being absorbed by a molecule in the air. The molecule undergoes
a momentary transition to an excited state and then re-emits the photon. The
photon is slowed down in this process which occurs many times as the photon
moves through the air. This process is called refraction and it is the reason
that light rays are bent when they pass between two materials of different
refractive indices, from air to water, for instance.
Since light is an electromagnetic wave, it has an electric field associated
with it. 鈥淭he trick with light strings is to create light fields that are so
intense that they modify the optical properties of the air,鈥� says Ewan Wright, a
physicist at the University of Arizona in Tucson who studies light strings. The
aim is to make the electric field so strong that it distorts the cloud of
electrons in the molecules in the air. This has the effect of increasing the
refractive index of the air, so that the path of any photon that interacts with
these distorted molecules is bent even more than usual.
With a pancake-shaped disc of laser light travelling through the air, light
at the edges of the pancake is refracted towards the middle. This increases the
strength of the associated electric field, which in turn increases the focusing
effect and creates even stronger fields. This positive feedback leads to a
hugely violent state of affairs. At the centre of the pancake, the fields become
so intense that they begin to rip electrons from the molecules in the air,
creating a plasma.
The plasma is a relatively long-lived phenomenon: it survives in the air for
a fraction of a second after the electromagnetic pancake has passed by.
Consequently, the pancake leaves a narrow cylinder of plasma trailing several
metres behind it. These trails are what give light strings their name.
The optical properties of the plasma are entirely different to the air
nearby. The plasma has a strong defocusing effect which works with the focusing
effect to create ideal conditions for the energy pancake to propagate, a kind of
optical fibre in the sky.
So how do you create a light string? Ionising air is relatively
straightforward. Many laboratories possess lasers that can do the trick. The
beam must have an intensity of around a ten thousand billion watts per square
metre to rip electrons from molecules in the air, and this can easily be
achieved by focusing beams to a point. But as more molecules get ripped apart,
this triggers a chain reaction that creates an explosive breakdown of the air.
Once under way, this breakdown is almost impossible to control. Yet, somehow the
processes at work inside light strings prevent it occurring.
The key turns out to be the generation of a laser pulse of just the right
length. A pulse lasting a matter of femtoseconds produces an electromagnetic
pancake just a few micrometres thick. Because it is moving at the speed of
light, there isn鈥檛 time for it to pull too many electrons from the surrounding
molecules. Instead it creates a low-energy channel through which the pulse of
light can pass unimpeded. If the pulses are too long, however, the air molecules
are ripped apart, soaking up energy and stopping the pulse in its tracks.
In 1995, when researchers first began to model light strings, they believed
that the focusing and defocusing effects balanced each other, creating a steady
state that allowed the light to propagate. But they soon realised that the
forces at work were so powerful and dynamic that this is simply not possible.
鈥淵ou get this very violent collapse from self-focusing, then all of a sudden,
kaboom, the defocusing effect kicks in,鈥� explains Wright.
Pancake power
To find out more, Wright and his colleague Jerry Moloney, an applied
mathematician at the University of Arizona, modelled the behaviour of light
strings on a powerful computer. What they found is astounding. It turns out that
the front of each electromagnetic pancake is highly unstable and tends to break
up into random filaments. 鈥淚f you have a pulse with enough power in it and
that鈥檚 wide enough, it will break up into very intense filaments that run side
by side,鈥� says Moloney. In their simulations, Moloney and Wright have seen up to
50 filaments in a single light string. They believe that real light strings may
split into thousands of filaments which can join together again or break apart
at any time. During the focusing, each filament is compressed violently until it
reaches a high enough intensity to break down air. At this point, the defocusing
effect kicks in and the cycle of focusing and defocusing begins again. 鈥淵ou get
this kind of breathing process, like an accordion,鈥� says Wright.
For their simulation, Moloney and Wright create a three-dimensional grid and
continually calculate at each point various properties of the pulse as it moves.
One of their biggest problems is concentrating the computing power in areas of
the grid where the important things are happening. For example, when a filament
divides, the computer must concentrate its power on simulating this rather than
the background noise. 鈥淵ou have to design an algorithm that redistributes the
mesh points on the grid,鈥� says Moloney. This is no easy task since the break-up
of the filaments is a random process.
Although the pulse鈥檚 鈥渂reathing鈥� is extraordinarily violent and turbulent,
very little energy is lost. This small energy loss means that the strings cannot
propagate indefinitely, however. Roland Sauerbrey of Jena University in Germany
has been measuring just how far they can go. He used infrared light strings no
wider than a human hair and with a peak power of 2 terawatts. The partially
ionised air in the light string鈥檚 wake radiates low levels of light in all
directions, and by observing this light Sauerbrey is able to see how far the
light strings have travelled. He found that they travelled more than 12
kilometres through the atmosphere.
This light trail has other uses too. Its frequencies provide a spectroscopic
fingerprint of the ions that created it, so Sauerbrey was also able to determine
the distribution of oxygen and water vapour in the atmosphere. Measuring
concentrations of chemicals in the atmosphere could turn out to be one of the
most important applications of light strings. Knowing the concentration of water
vapour in the atmosphere is a key factor in weather forecasting, for
example.
Another obvious application lies in environmental monitoring. Wright says it
would be possible to aim a light string at a smoke plume and retrieve a
spectroscopic fingerprint of the chemicals within it in seconds. And the fact
that this can be done from many kilometres away makes it attractive for the
detection of chemical and biological weapons. Laser beams can do a similar
job鈥攂ut not as well. Their range is limited and the beam has to be focused
onto the cloud of pollutants, a difficult task that can require expensive
optics.
Random filaments
The ability to illuminate the atmosphere high above the ground would also be
useful for astronomers. Atmospheric turbulence distorts the light from stars,
making the wavefront that reaches telescopes on the ground wrinkled rather than
perfectly flat. This makes it difficult to observe faint stars. However, a
growing number of telescopes are equipped with mirrors that rapidly flex to
compensate for the distortion of the incoming wavefronts. But the technique
depends crucially on knowing what turbulence exists in the atmosphere above the
telescope. Light strings projected from the ground could generate a reliable
light source high up in the atmosphere, making the turbulence much easier to
measure.
Another application is for rangefinding. Researchers have been toying
with a technique called light detection and rangefinding (lidar) since the
1960s. It works in a similar way to radar, but uses lasers instead of radio
waves. The idea is that a laser beam reflected off a target can be used to
determine the distance to the target and even to identify it. The short range of
conventional laser beams has proved a stumbling block, but this will be less of
a problem with light strings. Since a light string can deliver much more energy
to a given target, the reflection from that target will be greater. James
Murray, cofounder of Lite Cycles, a company specialising in lidar equipment in
Tucson, Arizona, believes that light strings could make lidar systems far more
sensitive, and also able to penetrate cloud and fog with less light
scattering.
Wright and Moloney are currently collaborating with Jean-Claude Diels, an
optical physicist at the University of New Mexico in Albuquerque, to generate
ultraviolet light strings. UV light ionises the air more readily than infrared
light, leaving more intense plasma trails. These trails could help to trigger
lightning strikes in specific places, allowing researchers to study them.
However, this extra ionisation and the resulting energy loss means that UV light
strings do not propagate as far as infrared strings. So far Diels has only
managed to send UV light strings a few tens of metres, but he expects to
increase this dramatically in future.
But if light strings are to be used widely, one other problem needs to be
overcome. Today, creating such high power but extremely short pulses requires
huge lasers and delicate optics. However, Wright is optimistic that the
technology will improve. 鈥淲hen I was a graduate student in the 70s, powerful
lasers were only available in a few of the very best labs. Nobody could have
foreseen the advances that we鈥檝e made since then and today you can buy these
lasers off the shelf,鈥� he says. 鈥淲ho knows what kind of lasers will be widely
available in 10 or 15 years鈥� time?鈥�