There's nothing like a coat of fresh paint to brighten up a room. But
you wouldn't expect an intense, room-filling glow to emanate from the
paint.
Now, researchers have discovered that certain dyes, when dissolved in a
liquid also containing tiny particles of titanium dioxide (a key
ingredient of white paint), generate light similar to that produced by a
laser. In essence, the randomly distributed titanium dioxide particles act
together to amplify light emitted by dye molecules that are excited by a
laser or some other external energy source.
"It was quite startling to see this," says' physicist Nabil M. Lawandy
of Brown University in Providence, R.I. Lawandy and his coworkers report
their discovery in the March 31 NATURE.
The researchers already have a number of applications in mind for their
"paint-on laser" ranging from display screens to the removal of
discolored skin resulting from tattoos or birthmarks.
Normally, lasers
require a source of energy, a material - such as a ruby rod or a liquid
dye -- that can be induced to emit light, and a resonator. in its
simplest form, a resonator may consist of nothing more than a pair of
mirrors at either end of the lasing medium. Light bounces back and forth
between the mirrors to stimulate the emission of additional radiation,
building up the emitted light into a strong beam.
Lawandy and his colleagues dispense with the mirrors. They use green
laser light having a wavelength of 532 nanometers to excite molecules of
a rhodamine dye dissolved in methanol. The dye in turn emits orange
light with a wavelength of 617 nanometers. Adding titanium dioxide
particles, averaging 250 nanometers in diameter, to the dye solution
greatly amplifies the emitted light.
The surprise is that a medium containing particles that reflect light
in all directions can somehow amplify the emitted radiation. Generally,
fabricators of lasers go to a lot of trouble to make the lasing medium
as uniform as possible, eliminating any impurities or inhomogeneities
that might scatter light and degrade the laser's performance.
"As lasing and disorder appear to be incompatible, it would seem to
be folly to attempt to promote lasing by deliberately introducing
scatterers into a medium," Azriel Z. Genack of Queens College of the
City University of New York in Flushing and J.M. Drake of Exxon Research
and Engineering Co. in Annandale, N.J., comment in the same NATURE. But
that's precisely what Lawandy and his colleagues accomplished.
It isn't clear yet why a suspension of titanium dioxide particles in
a liquid works effectively as a resonator and amplifier. Lawandy and his
collaborators are now conducting several experiments that may lead them
to a theory of how this effect occurs.
Although the light that emerges from dye-laced white paint appears as
a general glow rather than a definite beam, it still retains several
characteristics of laser light, including intense emission over a narrow
range of wavelengths. This laser like behavior could lead to advances in
such areas as laser medicine and display technology, Lawandy says.
For example, dermatologists use an array of lasers operating at
different wavelengths to treat and remove various types of skin
discolorations. Lawandy envisions the development of a cream or gel
containing an appropriate dye that could be applied to the affected area
and then excited to generate an intense burst of light of just the right
wavelength to erase the mark.
It may also be possible to get similar laser like behavior out of a
porous solid lasing medium, Lawandy says. Applied as tiny dots on the
inside surface of a television tube, such materials, when excited, could
generate intense light of precisely defined colors to create bright,
vivid displays.
