How the Solitary Wave Got Its Humps

Physicists have for the first time created a complicated wave that can cover large distances without losing its shape. Such waves, called solitons, usually travel with just one excited "mode" or intensity peak, but in the 25 May issue of Physical Review Letters a team reports that a crystal can focus laser light into a soliton with multiple modes, which could carry information.

Waves tend to spread out and lose their shape when they travel through anything other than a pure vacuum. That's because they contain a mixture of many other waves, which all travel at different speeds through a medium. But in 1844, engineer John Russell noticed an exception: A peculiar water wave that sped through a canal without any sign of weakening. This type of wave was dubbed a soliton. After the laser was invented, physicists found that optical solitons could be created when light passed through a "nonlinear" medium, which exactly compensated for the normal spread of the wave's single peak of brightness.

The team--Mordechai Segev and Matthew Mitchell of Princeton University and Demetrios Christodoulides of Lehigh University--knew that it should be possible to create solitons with multiple intensity peaks, or modes--as long as the modes did not interfere with each other. To make sure, they sent two laser beams, each having a different mode, through a nonlinear crystal of strontium barium niobate that creates solitons. One followed about 10 meters after the other so that their relative phases would change too quickly to interfere. When the researchers looked at the intensity profile of the beam that emerged from the crystal, they found that it was a soliton that had more than one hump of brightness.

It's too early to say just where these multimode solitons might be put to work, but computer science is one possibility. "The soliton work of Segev and his co-workers may have exciting implications for new ways to compute, such as quantum computing and DNA computing," says Kenneth Steiglitz, a computer scientist at Princeton. Solitons, Segev says, can cross each other without interference--a possible way to pack more information into a smaller space--and multiple peaks would boost their information-carrying ability.