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Scientists are creating unusual type of laser from a strange material

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David L. Shenkenberg,

Just what the heck is a random laser? Because lasers are supposed to produce coherent, linear beams of light, the notion of randomness in a laser beam seems at first to be an oxymoron.

As in an ordinary laser, light in a random laser is amplified by passing through a special material – the “gain medium,” but a random laser lacks one feature that most lasers have – a “microcavity,” or a narrow channel that confines the light into a linear beam. Instead of traveling as a linear beam, the light from a random laser radiates outward.

Not only does the laser lack a microcavity, but its gain medium also has been altered such that, besides amplifying light, the laser scatters it in multiple directions. This multiple scattering is the hallmark of the random laser. For example, laser dye can be encapsulated in particles that scatter the light amplified by the dye. In another example, a laser crystal is ground into a powder that amplifies and scatters the light.

A random laser shoots light outward in multiple directions. Courtesy of William Guerin.

Random lasers are interesting from a basic physics standpoint because the light moves randomly and not just in a straight line. Random movement notably has been characterized by P.W. Anderson who, in 1977, won one-third of the Nobel Prize in physics, and his “Anderson localization” can be used to describe the physics of random lasers.

Random lasers also could have various practical applications. Because the light shoots out in all directions in a unique array of colors, random lasers could be used for displays and also for street and home lighting. The unique array of colors also can be used for identifying objects spectroscopically.

Recently, a group of researchers decided to develop a random laser out of cold atoms. “The fact that the medium is very different from other random lasers – atoms versus condensed matter – makes the realization of a random laser with cold atoms both very interesting and very challenging,” said William Guerin, a researcher affiliated with the project.

Guerin and Robin Kaiser worked on this project at the University of Nice Sophia-Antipolis in France, and they also are both members of the National Center for Scientific Research. They collaborated with Luis Froufe-Pérez at the Materials Science Institute of Madrid in Spain and with Rémi Carminati at the Optical Physics Laboratory in Paris.

They used the simplest mathematical model of gain, Mollow gain, to calculate the diameter of the cloud in units of the typical distance a photon travels before scattering. This is the “optical thickness,” a measure of the opacity of the atom cloud.

As reported in the May 1, 2009, issue of Physical Review Letters, they found that they need an optical thickness ranging from approximately 200 to 250 to create the random laser out of the atom cloud. “We found a quite large optical thickness, but we think this is within reach of present cold atoms experiments,” Guerin said.

The researchers plan to use magnets and lasers to confine the atoms, slowing their motion and cooling them down to within a few micro-Kelvin. Then they will use tricks such as compressing the cloud to achieve the necessary opacity value. Finally, they will focus a pump laser on the cloud and, theoretically, will at last have their random laser developed out of cold atoms.

Photonics Spectra
Aug 2009
Basic ScienceConsumerlinear beammicrocavityrandom laserResearch & TechnologyTech Pulselasers

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