Tiny crystals adjust laser colors

Researchers from the French National Office of Aerospace Study and Research, Pierre and Marie Curie University in France, the French National Center for Scientific Research, and Polytechnic University in France have shown it is possible to use a relatively inexpensive material to split and combine lightwaves to change the color of a light signal.

Today's communications equipment includes relatively expensive crystals that are engineered to capture lightwaves of certain frequencies, or colors, and split or combine them into new frequencies. The higher-energy lightwaves of blue light, for instance, can be split into a pair of lower-energy red lightwaves, and two waves of red light can be combined into a blue one.

Converters are generally used to produce frequencies that are difficult for lasers to produce directly. These single-crystals are fairly expensive to produce because they are precisely tuned to specific frequencies.

The new converter material could eventually be used in communications equipment and sensors and could be combined with the active material in lasers to make integrated light sources and converters, according to the researchers.

The researchers' innovation was to use a polycrystalline material, which contains clumps of crystal, or lattice-like arrangements of atoms. The micro-crystals that make up the material are naturally oriented in all possible directions. The researchers' prototype is made from zinc selenide, an inexpensive, transparent material.

In single-crystal materials different wavelengths of light spread out at different rates, which leads to the waves interfering with each other if the crystal and lightwaves are not carefully aligned. The researchers' polycrystalline material randomizes the phases of the lightwaves, which keeps them from interfering without the need for any alignment.

The work appeared in the November 18, 2004 issue of Nature.