Laser taffy. Pulling a rubbery liquid crystal changes the wavelengths of laser light it emits.

Lasers Get a Stretch

A new artificial rubber that plays sophisticated tricks with light has found a niche in an unusual optical device: a stretchable laser that changes its wavelength when pulled like taffy. Although still very much a prototype, the contraption might lead to cheaper lasers that can be easily tuned to different wavelengths.

The laser is made of rubber that is also a liquid crystal, a material whose structure lies partway between the neat ordering of a true crystal and the haphazard disorder of a liquid. Unlike most liquid crystals, the new material contains rodlike molecules that twist like the steps of a spiral staircase. Such crystals can capture and reflect light, depending on the light's orientation to the molecule's twists. If light of just the right wavelength enters the material, the twisted molecules can trap it just as mirrors do in a regular laser.

To get such fine-tuned light, physical chemist Heino Finkelmann of the University of Freiburg, Germany, impregnated the specially designed liquid crystal with a fluorescent dye that, when excited by an external laser, gives off light of just the right wavelength to be trapped. If the exciting laser is powerful enough, the light emitted by the dye escapes as laser light at wavelengths both just above and just below the trapped wavelength band, Finkelmann and colleagues will report in an upcoming issue of the journal Advanced Materials. Shine a laser beam on one side, and laser light of a different frequency comes out the other. Stretch the rubber in two directions at once (see figure), and the molecular spirals shorten, causing the emitted light to shift toward the blue end of the spectrum. The color of the light leaving the rubber changes before your eyes, Finkelmann says.

The new work is a "significant step" in the progress of liquid crystal lasing, says physicist Cliff Jones of ZBD Displays in Malvern, United Kingdom. Applications may be in the offing. "I can imagine that this may lead to a low-cost, tunable laser," he says.

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