Laser lights up the spectrum

17 Jun 2002

Researchers have developed a new laser that could revolutionize the fiber-optics industry.

Heino Finkelmann from Albert-Ludwigs-Universität, Germany and US colleagues have developed the first mechanically tunable laser based on a liquid-crystal elastomer. The liquid crystal consists of a helical array of molecules that reflect laser light and remove the need for mirrors.

Previous work had shown that so-called helical cholesteric liquid crystals allow lasing without external mirrors. These structures consist of chiral molecules that are aligned to form a series of parallel helices. When light is directed along the liquid crystal, a combination of the periodic helix structure and the liquid crystal's birefringence prevents total propagation, leaving some of the light to reflect. Scientists can exploit this selective reflection, or photonic-bandgap effect by manipulating the reflected light to produce mirrorless lasing.

Finkelmann and his colleagues investigated this effect using a class of materials called elastomers. They synthesized cholesteric liquid single-crystal elastomers and by tweaking the concentration of the chiral components were able to produce the appropriate helical periodicity. They also added a fluorescent dye to enhance the selective reflective properties of the liquid crystal.

The team decided to deform the liquid crystal to see if this caused changes in the helix structure and if so, whether this would affect lasing. On stretching the elastomer, the team discovered that the molecules reoriented in such a way that the position of the reflection band moved. The team realized that if they could harness the lasing effect in their elastomers, they could be used as mechanically tunable lasing sources.

By pumping a beam of visible light at the standard and deformed liquid-single-crystal elastomers, Finkelmann's team produced lasing at different wavelengths. As the samples were stretched the laser light shifted from 544 to 640 nm, that is from green to red.

The researchers' mechanically tunable laser could find use in the fiber-optics communications industry, but first they intend to perform more detailed experiments.