Novel dyes alter the frequency of light.

In their efforts to use laser light for communications and computing, scientists are constantly conjuring up new materials capable of modulating and controlling the properties of those laser signals. Among the most intriguing--and perhaps most useful--materials of this sort are those exhibiting nonlinear optical behavior that increases the frequency of light passing through them.

This effect gives optical signals a boost; they pack a more powerful punch going out than they do coming in.

In the June 1 Nature, Geoffrey J. Ashwell, a materials scientist at Cranfield University in England, and his colleagues describe a "most unusual" result. They have found a new kind of dye--a specialized, nonlinear material used in lasers and optical communication--that appears to violate the traditional rules of optical physics.

The new dye doubles the frequency of light penetrating it, an established phenomenon known as second-harmonic generation. However, it does so in a completely unfamiliar way.

Ordinarily, researchers working with these special dyes emphasize the "noncentrosymmetry" of the molecules involved. In other words, dye molecules that consist of two mirror-image halves shouldn't produce the desired frequency-doubling effect.

Yet Ashwell's results show that this well-accepted rule does not always hold. His team synthesized a set of dyes, containing a central "squaraine" core, that have entirely centrosymmetric structures. According to current theoretical understanding, these molecules should not double a laser light's frequency. Yet they do.

"Heresy," says J.L. Bredas, a materials scientist at the University of Mons-Hainaut in Belgium, referring lightheartedly to the scientists' departure from conventional wisdom in the field of nonlinear optics.

"Whether or not the results of Ashwell and colleagues bring any direct benefits to nonlinear optical applications," Bredas adds, "it is clear that they are most exciting from a conceptual standpoint."

The unexpected properties of the new dyes not only make them potentially useful for optical signaling, but also stand accepted thinking in the field of optical communication on its head, says Bredas.

The finding points to the possibility of other mechanisms at work in the frequency-doubling process, suggesting that there may be alternative approaches to the control of laser light for signaling purposes.

Ashwell says that he finds these results more interesting than useful at the moment, though he agrees that the new dyes may have future applications in optical switching or laser modulation.

"Nonlinear optical phenomena are at the heart of modern communications systems in which optical signals need to be transmitted, processed, and stored," Bredas says.

Almost any research review or lecture dealing with nonlinear optical effects, he adds, "starts by emphasizing the overall noncentrosymmetry required for such processes to take place in any significant way."

In view of such molecular symmetry requirements, says Bredas, the approach of Ashwell and his colleagues "does indeed seem heretical."

Moreover, Ashwell's team tested the nonlinear optical effect with a very thin film of the dye, called a Langmuir-Blodgett monolayer, yet nevertheless obtained one of the strongest frequency-doubling signals ever reported.

Consequently, "the new results," Bredas observes, "open up an entirely new line of thinking in this technologically important field."