Temperature changes laser color

By Kimberly Patch, Technology Research News

What do you get when you cross tiny particles of opaque glass with the liquid crystals that make up the pixels of digital clocks and flat computer screens?

A pair of European researchers have found that the materials can form lasers that change colors as they change temperature.

The lasers, which could be made as small as a few dozen microns in diameter, could eventually be used as temperature sensors or in computer screens.

Lasers usually generate light using a cavity formed by two opposing mirrors positioned around an active element -- like a crystal -- that amplifies the light. In this type of system the lightwaves that eventually emerge are parallel, creating the narrow beams usually associated with lasers.

The researchers' lasers are essentially glass disks, and they are random lasers, which emit light in several directions.

The disks do not use mirrors, but instead trap light because the glass particles they're made out of are opaque and white, and so diffuse, or scatter, light. Lightwaves bounce around randomly inside the disk before they eventually pass through. "The light bounces randomly up and down -- up to thousands of times -- before it can escape," said Diederik Wiersma, a researcher at the European Laboratory for Non Linear Spectroscopy and the French National Institute for the Physics of Matter.

The researchers added a dye dissolved in liquid crystal to the disks. The dye, which readily emits light when energized, providing the amplification needed to create a laser.

The liquid crystal makes the disks respond to temperature changes, said Wiersma. At lower temperatures liquid crystal molecules line up like the molecules of a solid, while at higher temperatures they are less ordered, like a liquid. "The liquid crystal is what makes it special. It allows us to tune how much the system is... scattering -- so how much is white -- by changing the temperature," he said.

At temperatures below 42.5 degrees Celsius, the liquid crystal molecules line up and enough light scatters to trigger lasing. "If the system is scattering strongly, the light stays long enough inside for the laser to switch on," said Wiersma.

The disks also emit different light wavelengths, or colors, depending on temperature. This is because each liquid crystal phase has a different refractive index, meaning it scatters light differently, according to Wiersma.

Depending on the temperature, the wavelength of light can vary from 700 to 560 nanometers, which spans the visible light spectrum from red through yellow to green. "We have made a new type of light source that has emission characteristics that depend on temperature," said Wiersma.

At higher temperatures the light is dim and spans a wider range of colors. Below the threshold temperature for lasing, the light becomes intense and spans a narrower range of colors. "Its color spectrum becomes narrow at low temperatures, and broader at high temperatures," said Wiersma.

The disks are 1 centimeter in diameter. They could be made smaller than one tenth of a millimeter, however, according to the researchers.

The researchers can also change the glass/liquid crystal formula to design lasers that show different spectral behaviors. The researchers can vary the temperature that causes the laser to turn on and the abruptness or smoothness of the color changes, according to Wiersma.

The lasers could eventually be used as temperature sensors to investigate, for instance, biological processes, said Wiersma. They could also be used as pixels in temperature-sensitive computer screens, he said. "Imagine every random laser grain as a dot in a display."

The researchers are currently trying to understand the fundamental properties of how lightwaves behave in the tiny lasers, including the timing of the color switches, said Wiersma. The technology could be used in practical devices in two or three years, he said.

This type of laser has not been made before, but it's not obviously useful, said Eli Yablonovitch, an electrical engineering professor at the University of California at Los Angeles. "There are optical temperature sensors that don't lase, but they still measure the temperature," he added.

Wiersma's research colleague was Stefano Cavalieri of the European Laboratory for Non Linear Spectroscopy and the University of Florence in Italy. They published the research in the December 13, 2001 issue of the journal Nature. The research was funded by the European Laboratory for Non Linear Spectroscopy (LENS), the French National Institute for the Physics of Matter (INFM) and the University of Florence.

Timeline:   2-3 years
Funding:   Government
TRN Categories:  Optical Computing, Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "A Temperature-Tunable Random Laser," Nature, December 13, 2001.


December 19/26, 2001

Page One

LED fires one photon at a time

Chips turn more heat to power

Data protected on unlocked Web sites

Surgeons gain ultrasonic vision

Temperature changes laser color


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