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Boosted light: laser action in white paintpaint-on laser discovered

Science News
April 9, 1994
by Ivars Peterson

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.

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