Friday, February 18, 2011

Anti-Laser - The "Coherent Perfect Absorber" Is Born

In the anti-laser, incoming light waves are trapped in a cavity where they bounce back and forth until they are eventually absorbed. Their energy is dissipated as heat. Image Credit: Yidong Chong/Yale University

Anti-Laser - The "Coherent Perfect Absorber" Is Born

Everyone is familiar with laser light emitting devices such as pointers used in presentations and lectures, lightshows performed at events, openings, and concerts, even with the red-light that hits a barcode on the front of one's morning newspaper and pastry purchase at the corner 7-11 ... but this was not the case 51 years ago.

Now there is a new tool that has been developed through the the use of focused wavelength of light but unlike with the laser, where the focused wavelength is passed through a material that amplifies the light, the anti-laser utilities the opposite concept of passing a focused wavelength of light through material that absorbs the light. The process has been given the name "Coherent Perfect Absorber" giving a new, future meaning to the an-acronym "CPA".

In an anti-laser, or coherent perfect absorber, the outgoing laser beams are replaced by incoming ones, and light flows into a light-absorbing material instead of out of a light-amplifying one. Image Credit: Science/AAAS

When the laser was first conceptualized and developed into a working device, no one knew that it would eventually lead to replacing records and needles when one listens to music or film projectors when one watches a home movie transferred from a computer to a laser/DVD disc. The same could be said at the dawn of the anti-laser CPA process, No one knows what this new tool will bring to the tool-box, and what new applications can be developed, to solve the many problems we encounter that make our lives easier and more efficient.

Coherent light is incident on an absorbing material in a resonator formed by two parallel reflective surfaces or mirrors. The interplay of absorption and interference leads to perfect absorption of the incoming radiation and its conversion into other forms of energy1. The schematic of a laser would be entirely analogous, with only the arrows for light and energy reversed: energy pumped in would result in coherent light out. Image Credit: Nature Volume: 467, Pages: 37–39 Date published: (02 September 2010)

Dr. Wenjie Wan, a Phd from Princeton University, is a post-doctoral associate in applied physics at Yale. In photo, Wan works with the optical set up for an anti-laser experiment in the applied physics lab at Yale which involves prisms, mirrors and silicon. An anti-laser (or, in technical terms, "coherent perfect absorber") works in the reverse of a conventional laser. Instead of emitting a beam of light, it absorbs it. Two laser beams with the exact same frequencies are emitted into a silicon wafer. The silicon aligns the light waves so that they become interlocked and oscillate until they are absorbed and transformed into heat. The concept is in it's infancy and may be adapted to new computer technology down the road. Image Credit: STEPHEN DUNN, Hartford Courant (2011)

This excerpted and edited from the Hartford Courant -

The Anti-Laser Is Here
Yale researchers butild device that absorbs light
By William Weir - Hartford Courant - Feb. 17, 2011

A. Douglas Stone, a physicist, and his team describe the anti-laser in Friday's issue of Science.
The possibility of an anti-laser had been suggested by other scientists, but only in passing, Stone said. And other physicists have stumbled upon the basic premise while working on other projects, he said, but they did not follow through.

"Nobody took it serious, until us," Stone said. "It was literally a footnote."
Any dark material can absorb light — a car's black interior on a summer day, for instance — but to absorb near 100 percent of the light of a laser beam requires a bit more precision. The difference in the anti-laser is that instead of using an amplifying material, it uses one that absorbs it — or a "loss medium." After his research team did the math, Stone said, they decided that silicon was the best choice.

The anti-laser is set up to split a single laser beam into two and direct the two beams to head toward each other, meeting at the paper-thin silicon wafer. The light's waves are precisely tuned to interlock with each other and become trapped. They then dissipate into heat.

Perhaps the most novel part of the device is that it allows the operator to tune the light's wavelengths and determine how much of the laser light is absorbed. That allows the device to work as an on-off switch for light.

Stone first proposed the idea last year, in a paper published in the journal Physical Review Letters. But it's one thing to write about it and do the math, and it's another to actually create it. That's where Stone's collaborators came in, a team of applied physicists headed by Hui Cao and Wenjie Wan. The divide between theoretical physics and applied physics is a stark one. As of Wednesday, Stone hadn't yet seen the finished device, built in another building on campus
Wan said it took about a year to build the device. Pointing at the mirrors, prism, beam splitter and the silicon wafer that make up the device's basic components, he said the design is fairly simple. But achieving the necessary level of precision was a challenge. Even now, they're fine-tuning it.
Now that the anti-laser has been built, what exactly do you do with it? Its best potential use, so far, appears to be in optical switches, used in the next generation of computers, which operate on light as well as electrons. Cao also has suggested that it could be useful in radiology, capturing images of human tissue normally too deep to see.

But as with much of science, the practical applications will be for others to figure out.
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