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Yale boffins have built a laser light cancelling device roughly analogous to noise-cancelling headphones.

Laser (Light Amplification by Stimulated Emission of Radiation) beams are made by pumping electricity or light into a device composed of two opposing mirrors and a so-called gain medium, such as gallium arsenide, between them. One of the mirrors is partially transparent. The gain medium, excited by the applied electricity or light, emits photons which bounce back and forth between the mirrors and the amount of light involved is amplified as more photons are emitted.

The device is a resonant optical cavity and the emitted photons oscillate between the two mirrors, with some escaping as an intense beam of coherent light through the partially transparent mirror, as the number of photons in the device continually increases.

"Coherence" means the laser beam is made of light waves with a single frequency and amplitude.

So we have light waves oscillating back and forth within the device. According to the El Reg science 101 manual, if a light wave travelling one way meets a light wave travelling the other way that has the same wavelength but is an inverse of itself, the two waves should cancel each other out. Put another way, if the incoming light enters what is effectively a loss medium instead of a gain medium it should disappear.

Laser killer chase

The laser killer chase was started last year when a team, led by Yale University physicist A Douglas Stone, published a paper theorising that a laser-killing device could be built using common or garden silicon. He then worked with another team of boffins led by Hui Cao at Yale, and they built and demonstrated a 1cm device, a Coherent Perfect Absorber (CPA) that worked almost perfectly, absorbing 99.4 per cent of the near-infrared laser light shone into it.

Anti-laser graphic (Yidong Chong, Yale University)

The concept is that laser beams of the same wavelength from two laser sources are shone directly at each other, meeting inside a cavity containing a silicon wafer, the loss medium, where the wavelengths bounce back and forth inside the wafer, cancelling each other out.

The Yale release says: "The wafer aligned the light waves in such a way that they became perfectly trapped, bouncing back and forth indefinitely until they were eventually absorbed and transformed into heat."

The boffins think they can build a device absorbing 99.999 per cent of incoming light, which measures only six microns across. They also expect to be able to absorb light at wavelengths visible to the human eye by altering the cavity dimensions and the loss medium.

The scientists say possible uses of the technology include an optical computer and radiological imaging or treatment of the human body for diseases such as cancer.

The device isn't exactly analogous to noise-cancelling headphones, as it doesn't generate the sound needed to cancel out incoming sound at certain wavelengths. However this is only a first step. It also can't function well as a defensive shield against laser light beams as, a) you need to generate an exactly identical opposing beam, and b) the absorbed light becomes heat, which would fry the shield.

Read more in the 18 February issue of Science (subscription). ®

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