IMAGINE what you could do with a paint that thinks it鈥檚 a laser. You could
brighten up your neighbourhood by decorating walls or street signs, or create a
new form of illumination for bridges or other landmarks.
The idea may sound far-fetched but it could just become a reality. In 1999,
two groups of scientists who were developing new dyes noticed bizarre peaks in
the light emission spectra of their mixtures. Unlike fluorescent dyes or
light-emitting polymers, the new dyes seemed to be amplifying particular
wavelengths of light into coherent laser beams. But no one knew at the time how
this could be possible.
鈥淚t was very surprising,鈥� says Hui Cao of Northwestern University in
Evanston, Illinois, who discovered the phenomenon. Now Costas Soukoulis at the
US Department of Energy鈥檚 Ames Lab in Iowa has worked out a model which explains
Cao鈥檚 results. It confirms that the dye itself behaves like a laser.
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A conventional laser is a sandwich of two mirrors filled with gas, liquid or
a crystal whose atoms are in an 鈥渆xcited鈥� state. When an electron in such an
atom drops to a lower energy level, it emits a photon of light, which is trapped
in the cavity between the mirrors. The photon bounces back and forth and
eventually collides with another atom that is about to 鈥渄e-excite鈥�. That atom
will tend to emit its own photon in phase with the original one. After many of
these collisions, the bouncing beam has many photons in phase鈥攁nd is
called coherent or laser light.
Normally engineers take care to clear the cavity of any particles that might
scatter photons before they鈥檝e been amplified. Removing this contamination is
partly what makes conventional lasers so expensive.
This time though the scattering particles are vital. Soukoulis worked out
that if there are enough particles in a dye, they can act like mirrors, bouncing
photons around inside the dye until eventually they break out as a beam of
coherent laser light. Dubbed random lasers, because of the random distribution
of particles in the dye, they are far simpler and cheaper than conventional
lasers. Just like some conventional lasers, random lasers use an intense light
source to excite the electrons before lasing can begin. They also produce a beam
of comparable power though it鈥檚 less intense because it scatters in all
directions
Soukoulis鈥檚 model, which he hopes to publish in the journal Physical
Review Letters, enables him to predict the size of the scattering particles
needed to make different coloured dyes into lasers. The bulk of the dye is made
of the same material as conventional lasers, gallium nitride, for example, mixed
with zinc or titanium oxide paints which are good at scattering particles.
The researchers say laser paint could be dabbed onto deep-sea divers so
they鈥檒l be highly visible in an emergency. And it could give flat-screen
displays brighter pictures. Cao also expects the paint to be used in optical
networks, if she can confine the scattered beam so it can pass down an optical
fibre without wasting energy.