Physicists build world's first antilaser

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Less than a year after it was first suggested, the world's first antilaser is here. A team of physicists have built a contraption that, instead of flashing bright beams, utterly extinguishes specific wavelengths of light.

Conventional lasers create intense beams of light by stimulating atoms to spit out a coherent beam of light in which all the light waves march in lockstep. The crests of one wave match the crests of all the others, and troughs match up with troughs.

The antilaser does the reverse: Two perfect beams of laser light go in, and are completely absorbed.

"There will be nothing coming out again," said experimental physicist Hui Cao of Yale University, whose research group built the new device.

The device could find uses in fields from computing to medical imaging, the researchers report in the Feb. 18 Science.

Yale physicist A. Douglas Stone, a coauthor of the paper, first suggested the antilaser in a theoretical paper last July. Stone and colleagues had noticed that several other researchers had hinted at the idea of a laser that runs backward, and some problems in engineering called for a way to completely snuff out light. But no one had ever put the two ideas together.

"Others discovered independently that there's an optimal condition where they can have the best absorption," Cao said. "But they didn't realize this was a time-reversed laser. They didn't know they can get in principle perfect absorption."

To build the antilaser, which Cao and colleagues call a "coherent perfect absorber," the researchers split a beam from a titanium-sapphire laser in two. The laser emitted light in the infrared part of the electromagnetic spectrum, with longer wavelengths than the human eye can see.

Some of the light continued forward through the beam splitter, and the rest was forced into a sharp right turn. The physicists guided the light beams into a cavity containing a silicon wafer one micrometer thick. One beam entered from the left and one from the right. The distance each beam travels determined the way the crests and troughs of the light waves aligned when they met in the wafer.

When the alignment was right, the light waves canceled each other out. The silicon absorbed the light and converted it to another form of energy, like heat or electrical current.

"It is a simple experiment," Cao said. "But it shows a very powerful way to control absorption."

The device can only absorb one wavelength of light at a time, but that wavelength can be adjusted by changing the thickness of the wafer.

Surprisingly, the antilaser switched from absorbent to reflective when the researchers changed the way the waves met in the wafer. Under certain conditions, the silicon crystal actually helped light escape.

"That is a little surprising," Cao said. "We can turn it on and off."

Theoretically, 99.999 percent of the light can be extinguished. Because of the physical limitations of the laser and the silicon wafer, the antilaser only absorbed 99.4 percent of the light.

That may be good enough, Cao says.

"For many applications, if you already have less than 1 percent coming out, you're already okay," she said. "I'm sure people in the community who have better lasers than us, I'm sure they will achieve much more impressive results. This is only the first demonstration of the principle."

The device may find uses in optical switches for future super-fast computer boards that use light instead of electrons. It may also have medical applications, such as imaging a tumor through normally opaque human tissue.

The most exciting applications will no doubt be the ones no one has thought of yet. The laser itself was called "a solution without a problem" when it first showed up.

"It is quite novel and indeed surprising that in such a mature field one can come up with something fundamentally new," said physicist Marin Soljačić of MIT, who was not involved in the new work. "I think it opens a few exciting venues."

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