Behold, the antilaser

Physicists conceive a ‘perfect absorber’

Fifty years after physicists invented the laser, ushering in everything from supermarket scanners to music CDs (SN: 5/8/10, p. 18), scientists have conceived its opposite — the “antilaser.”

Unlike its more popular cousin, the antilaser is unlikely to take over the world. Still, it could be useful one day, for instance in new types of optical switches for computers.

No one has yet reported building an antilaser, but a theoretical description of one appears in a paper published July 26 in Physical Review Letters.

“It’s kind of surprising that we’ve been using lasers for 50 years or so, and only now somebody noticed something pretty fundamental,” says Marin SoljaÄiÄ, a physicist at MIT who was not involved in the work.

Instead of amplifying light into coherent pulses, as a laser does, an antilaser absorbs light beams zapped into it. It can be “tuned” to work at specific wavelengths of light, allowing researchers to turn a dial and cause the device to start and then stop absorbing light.

“By just tinkering with the phases of the beams, magically it turns ‘black’ in this narrow wavelength range,” says team member A. Douglas Stone, a physicist at Yale University. “It’s an amazing trick.”

Stone and his colleagues thought up the antilaser while wondering what might happen if they replaced the material inside a laser that reflects photons — the “gain medium” — with a material that absorbs light. In the right configuration, the absorbing material sucks up most of the photons sent into it, while the remaining light waves cancel out by interfering with one another.

Stefano Longhi, a physicist at the Polytechnic Institute of Milan in Italy, calls the concept “very clever and simple.”

The Yale team refers to the device as a “coherent perfect absorber.” Another name is a “time-reversed laser,” since it is like running a laser in reverse using an absorbing medium rather than an amplifying one, says Yale postdoctoral fellow Yidong Chong.

Even though the antilaser absorbs perfectly, it does so only at specific wavelengths of light, making it unsuitable for applications like solar panels that take in a broad range of wavelengths.  (Other, specially engineered materials called metamaterials can perform those kinds of absorptions.) But because the antilaser can switch from absorbing to nonabsorbing just by changing the wavelength of the incoming light, it could prove useful in optical switches — for instance in futuristic computer boards that will use light instead of electrons.

Other Yale researchers, led by experimentalist Hui Cao, are now trying to build an antilaser. Stone says progress so far looks “very promising.”

One day the antilaser could even meet up directly with its relative, the laser. In a paper submitted for publication, Longhi argues it might be possible to make a device that combines an ordinary laser with one of these new absorbers — in essence, a laser and antilaser in one.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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