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LASERS have made their mark almost everywhere you look. Their beams can make
precise surgical incisions, or act as neat little particle accelerators. They
can recreate the white heat of the Sun鈥檚 surface in the laboratory. But the last
thing you鈥檇 expect a laser to do is drain materials of their heat until they鈥檙e
nearly as cold as the frozen planet Pluto.

Yet strangely enough, the latest in refrigeration techniques does just that.
Researchers in the US have developed prototype laser coolers, and hope to put
them to use on satellites. Eventually, laser beams might chill anything from
superconductors to optical computers.

On the atomic scale, lasers have been cooling matter for decades in a
technique known as Doppler cooling. This uses photons to slow atoms in much the
same way that firing ping-pong balls at an oncoming football would gradually
slow it down. The transfer of energy from the atom to the photons can cool atoms
to less than a millionth of a degree above absolute zero. But this only works on
the tiniest scales.

The idea of cooling large objects using light was first suggested by the
German physicist Peter Pringsheim in 1929. His idea was that materials could
cool when they fluoresce. When a molecule absorbs light, its electrons become
excited. This new state is unstable, and the molecule has to lose the extra
energy. It can do this by making a permanent chemical change to the molecule,
such as breaking a bond, or it can be changed to heat, warming up the molecule
and its surroundings. Alternatively, the excess energy can leave the molecule as
photons of light. This is fluorescence.

By ensuring that the total energy leaving as fluorescence is more than the
energy absorbed, it is possible to achieve a net cooling. This can be done by
choosing the energy of the photons in a laser beam so that they are only
absorbed by those molecules in a material which already have some vibrational
energy鈥攖hey are already 鈥渉ot鈥�. Statistically, a small proportion of
molecules in a material will always be hotter than the others. When they absorb
the photons, they are excited into a higher energy state.

Cool states

In certain materials, fluorescence will then bring the molecules to
vibrational states that are lower in energy, or cooler, than those from which
they originally came. The light leaving the molecules then contains more energy
than the light absorbed. This is known as anti-Stokes fluorescence. If all the
absorbed energy emerges as fluorescence, and none heats the material, it should
be possible to expel heat energy in the form of light.

Pringsheim鈥檚 idea was fine in theory, but fraught with difficulties in
practice. The main stumbling blocks were finding a suitable fluorescent material
and mounting it in a clear solid that lets in all the light to be absorbed, and
lets out all the fluorescence. But now a team of researchers from the Los Alamos
National Laboratory in New Mexico has managed to cool a solid in this way for
the first time. Richard Epstein, Tim Gosnell and their colleagues focused a
high-power infrared laser onto a glass host 鈥渄oped鈥� with ytterbium (Yb³⁺) ions.
They chose ytterbium specially for its highly efficient fluorescence and its
simple electronic structure, which reduces the chances that the absorbed energy
will be lost internally as heat.

In experiments on a matchstick-sized glass block during 1995, Epstein鈥檚 team
achieved a heat loss rate that was 2 per cent of the power of the laser
light鈥攁n efficiency 10 000 times better than that achieved in gases with
Doppler cooling. According to Gosnell, one of the secrets of their success was
the high purity of the glass host, which ensured that it did not scatter or
absorb the laser light. 鈥淟uckily, we鈥檙e now very good at making clear glasses
for optical fibres,鈥� he says. The temperature of the glass fell by only 0.3
掳C, but when the researchers used an optical fibre instead of a glass block
and increased the amount of laser light absorbed, they were able to cool the
sample to 16 掳C below room temperature.

Epstein and his colleagues have since improved and scaled up their technique
using a pair of state-of-the-art mirrors to form a cavity. The mirrors enclose a
ytterbium-doped glass block, about 3 centimetres across. They are transparent to
the fluorescence of the ytterbium so that the energy is free to leave. They
reflect infrared light, however, so that heat does not get in from the outside.
They also reflect the laser beam, so it bounces around inside this cavity,
making the cooling even more efficient. In the prototype cavity, the doped glass
loses heat at a rate of 0.5 watts. The researchers calculate that it should cool
to temperatures as low as 60 kelvin (around 鈭�210 掳C) when they have
fine-tuned their cavity. Epstein is upbeat: 鈥淲e should have a real cooler within
a year.鈥�

For its first application, Epstein is looking to the heavens. 鈥淥ur first
niche may be in space鈥攃ooling sensitive detectors and electronics on
satellites,鈥� he says. Anything that is warm emits infrared radiation. This is a
problem for infrared detectors in astronomy, for instance, because the 鈥渘oise鈥�
coming from warm instruments can swamp the signals from astronomical objects. So
efficient cooling of infrared detectors is vital.

Until now, the systems used to cool detectors in orbit have relied mainly on
tanks of liquefied gas, which boils away in a few years. Satellites on longer
missions can use mechanical heat pumps, but vibration and electrical
interference from their motors can affect the infrared sensor, which must be
carefully protected. With no moving parts, the laser cooler could be the best
option. 鈥淭hese devices should soon have comparable cooling power to the
cryocoolers used today,鈥� predicts Allan Mord of Ball Aerospace and Technologies
in Boulder, Colorado, the company that aims to put Epstein鈥檚 laser refrigerator
into space.

Despite Epstein and Mord鈥檚 optimistic forecasts, Gosnell is more cautious. He
foresees problems from cosmic rays鈥攈igh-energy particles and radiation in
space, which constantly bombard satellites. He says that they might damage the
ytterbium or the glass, upsetting the delicate photochemical balance and
increasing the proportion of absorbed light converted back to heat. 鈥淧ractical
space applications are a long way off,鈥� he warns, 鈥渕aybe 10 years away.鈥� But in
the meantime, he says, there is plenty of interesting physics to be done.

Yellow glow

At around the time of Epstein鈥檚 first experiments, a group at Imperial
College in London had also caught onto laser cooling鈥攂y accident. Chemists
Garry Rumbles and Joanne Clark noticed that they could generate yellow
fluorescence in a polymer film doped with rhodamine dye by shining red laser
light onto it.

鈥淲hen we cooled the polymer film by a few degrees, the yellow fluorescence
disappeared,鈥� says Rumbles. 鈥淭his suggested that absorption by `hot鈥� molecules
was responsible.鈥� Cooling the 鈥渉ot鈥� molecules meant that they could no longer
absorb the light, and the fluorescence vanished. When the researchers used a
liquid sample that contained even more dye, the sample cooled by 4 掳C.

Rumbles and Clark have found a way of using the effect鈥攖his time in
reverse. In 1995, they designed and patented a sensitive thermometer. Built from
an optical fibre doped with fluorescent dye, a laser and a simple light
detector, it can measure changes as small as 0.02 掳C in the fibre鈥檚
temperature by sensing how much fluorescence it produces. With no metal parts,
it may be ideal for use in environments where metals corrode.

Rumbles foresees other applications for laser cooling鈥攊n components for
optical computing, for instance. Some materials become warm and break down under
intense light. Choose the right material and shine a laser on it, and it would
cool down instead. As the devices cool, there will be fewer 鈥渉ot鈥� molecules to
absorb the laser energy and the rate of cooling will slow. In other words, the
materials will have a built-in thermostat.

鈥淲e could end up with stable, self-regulating materials for optical
computing,鈥� says Rumbles. Or perhaps these devices could be used to cool
superconductors. With all this activity, the future for laser coolers is
bright鈥攅ven glowing.