Stretching and twisting a bright idea

A typical laser produces one color of light with superb precision. Only uncommon, expensive lasers can be tuned to emit several colors. Nonetheless, both researchers and technology developers are finding more and more uses for tunable lasers.

Now, along comes a laser that changes color as easily as a rubber band stretches.

A team of German, U.S., Mexican, and Korean researchers has developed the heart of this laser: rubbery, translucent membranes made from liquid crystal molecules interspersed with dye particles. When stimulated by a conventional laser, such membranes produce laser light themselves. Moreover, when stretched, they shift the color of the light to shorter wavelengths. For instance, a relaxed membrane might make red light, but a taut one, green light. An account of the research is slated for a forthcoming issue of Advanced Materials.

Within the membranes, liquid-crystal molecules are oriented like the steps of a spiral staircase, says Antonio Munoz of the Autonomous Metropolitan University of Mexico City. These corkscrew structures trap light of certain colors in their twists. When energized by another laser, the imprisoned light then can itself intensify and some can escape as a laser beam.

Stretching the membrane squashes the spiral staircases. In turn, the wavelengths of both the trapped light and outgoing laser beam shorten, explains team member Peter Palffy-Muhoray of the Liquid Crystal Institute at Kent (Ohio) State University.

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