23 March 2005--Light may move fast, but it generally takes its time to change a material's electrical properties. But now a Franco-Japanese team has turned an insulator into a conductor in just 2 picoseconds, a 50-fold speedup that should catapult optical switching from the gigahertz to the terahertz range.
"This is very meaningful for device applications," says Ying Fu, a researcher in optoelectronics at the Chalmers Institute of Technology in Gothenburg, Sweden. "In this experiment, the switching is very clear--the signal-to-noise ratio is very large."
The new material's speed could make optical mass data storage more attractive. Because it won't stay switched longer than a millisecond before decaying back to an insulator, any system using it would need to refresh at a rate of 1000 Hz. That's no problem for today's lasers. However, Fu says, the material's crystals would be hard to integrate into the nanometer-sized structures in memory chips.
"But for optical communications, they are fine," Fu says, " because you do not have to integrate an [optical] communications switch into the system." Terahertz-speed optoelectronic switches would be ideal for applications in high-speed routers and hubs in optical networks, explains team leader Shin-ya Koshihara of the Tokyo Institute of Technology in Japan.
The researchers, from Rennes University, in France, and from eight research centers in Japan, reported their work in the 7 January issue of Science .
They worked with an organic salt known as (EDO-TTF) 2 PF 6 , a member of a family known to have promising optoelectronic properties. They knew that the salt would change from insulator to metal when hit by light, but they were surprised to find just how fast it worked. " I doubted these results, and bet bottles of beer with them," says Koshihara. "And I was pleased to lose the bet."
The researchers hit their target with 1-kilohertz pulses from an 800-nanometer infrared laser while bouncing light from another source off the surface. That way, they could measure changes in the crystal's reflective powers. After 2 picoseconds, the material became shinier, indicating a transition to a metallic state. To prove the point, the team measured the material's electric conductivity and found that it, too, had increased.
But how this change takes place is not fully understood. Laurent Guerin, from Rennes University, notes that the salt's crystal lattice interacts very strongly with its electrons making it relatively easy to coax the electrons into the collective behavior characteristic of a metal. In any case, it happens fast and with relatively small doses of laser light compared to earlier experiments on other materials.
Because nobody imagined that such a material would exist, the discussion of applications has just started, says Koshihara.
To learn more, the team will conduct experiments with X-ray radiation from the European Synchrotron Radiation Facility in Grenoble and at KEK in Tsukuba, Japan. The extremely short pulses of very short-wave radiation will provide a series of snapshots of the atoms as they rearrange themselves into a metal.