In cooler light
THE race of the cooling business may be
(the marketing mandarins of refrigerator companies can sl 'it up each other)
hotting up, but how about a laser-
operated fridge? Neighbours' envy,
owners' pride and all that... but optical
refrigeration technology is no longer
sci-fi. It has just been patented in the
us, and, is a device which can tuip -
under the right 'conditions - a hot
laser beam into a bright light which
can cool (and not heat) objects.
Scientists at the Los Alamos National
Laboratory in New Mexico, usA,
have dem onstrated. that optical refrigeration is now at last possible. While it
had been accepted as a theoretical
possibility since the '209, that bright
light could be used to chill solids,
advances in materials and techniques
were required before it could evolve
from theory to practice.
Most of us think of a laser beam as
a weapon of light with a burning
point so hot, that it can slice ilirough
most solid objects in a split second. But
since that tool and,power can also be
exploited for more than just one purpose, the future image of lasers may
soon include the idea of laser beams
To, create laser cooling, the Los
Alamos scientists shone A beam' of,
infrared light nearly as intense as
the light at the sun's surface, on to a,
1/4 inch slice of ultra-pure glass that
had been doped, or impregnated, with
ions of the element ytterbium (a rare
Excited 'by the laser beam's pulse
of energy, ytterbium vibrates at
much higher frequencies than the
ultra-pure glass. Consequently, the
ytterbium ions radiate more energy
than is - actually absorbed by the
glass, and the glass-ytterbium composite as a whole becomes cooler.
What are the potential applications
'Of this inverted laser process? In the
harsh environment of space,
this new solid-state, vibrationless
technology would be robust enough
to survive and function for years
while being used in satellites to cool
infrared cameras and detectors or
superconducting relays for cellular,
In homes, it could be used to
enable superconducting circuits to
operate tens or hundreds of times
faster than the conventional electronics of today. One can equate its use
to that of a supercomputer in one's
What is the theory behind this
new technology? Normally, light
hitting a solid object deposits energy
as heat. But what has been learnt at
Los Alamos is that under. some
well-defined and controlled circumstances, a 'tuned' laser light can
absorb energy from microscopic
thermal vibrations in a solid, and
radiate the energy out and away
from the solid, producing a drop in the
if an object excited by radiation
at one frequency, can be made to
emit radiation (heat) at higher frequencies carryirig more energy, it suffers
a net loss of energy. As a result,
the object cools down. . Scientists
have thought that by
pumping a fluorescent
cooling element with a
laser, it should be possible
to construct a compact,
all solid-state optical crycooler which would
enable the widespread
deployment of cryogenic
electronics and detectors
in space and elsewhere.
One can think of the
principle here as some
thing like cooling an object by'washing'
it with 'cool' light. Light pours on tothe
object, soaks up some of the vibrational
or heat energy of the object, then carries
away the excess energy.
it is not a very efficient process.
The cooling power is only a small
percentage of the absorbed laser power.
Thus, while at the moment, it is far too
inefficient to refrigerate food or cool
a house, it will do very nicely for cooling
high-tech devices to extremely. low
Scientists now feel that this will lead
eventually to their ability to cool objects
down, to about 200'c below zero.
Absolute zero, at which all motion
ceases is -273'c.
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