In the anti-laser, incoming light waves are trapped in a cavity where they bounce back and forth until they are eventually absorbed.
Credit: Yidong Chong/Yale University
SYDNEY: More than 50 years after the invention of the laser, the world’s first functional ‘anti-laser’ – a device which causes two incoming beams of light to perfectly cancel each other out – has been developed by scientists in the U.S..
The invention could have future applications in everything from building certain components in the next generation of computers to radiology, according to A. Douglas Stone from Yale University in Connecticut, who came up with the idea while explaining to a colleague how a conventional laser works.
“I was trying to make an analogy to imagine instead of having an amplifying medium, you’d have an absorbing one – it was a backwards analogy to make it more intuitive. So I thought, “I wonder if that would really work?” It was sort of my Eureka moment,” said Stone.
Absorbing, not producing, light beams
Referred to as a coherent perfect absorber (CPA), the anti-laser works in the exact opposite way to a conventional laser. Where the laser uses what’s known as a ‘gain medium,’ - usually a semiconductor like gallium arsenide to produce a focused beam of coherent light - this is switched to a ‘loss medium’ in an anti-laser, which instead absorbs light.
The key to the anti-laser’s success is the use of a 100-micron-long, (a tenth of a millimetre) silicon wafer as the ‘loss medium’. Two laser beams with a specific frequency are aligned and perfectly trapped, causing them to bounce back and forth until they are eventually absorbed. Their energy is dissipated as heat.
"In principle the trapping of light for absorption can be achieved similarly to a laser, either via mirrored surfaces, or simply via the usual partial reflectivity of semiconductor surfaces,” said Stone. “In the current experiment we used only surface reflectivity but similar devices using mirrors have been studied and shown to work well theoretically."
Applications in next-gen communications
The CPA prototype, built by co-author Hui Cao from Yale University, currently works in a very narrow frequency range, using near-infrared radiation, but according to Stone, who published the research in Science, future versions should be more flexible.
By adjusting the size of the silicon wafer, which Stone’s computer simulations can shave down to six microns in length (about one-twentieth the width of an average human hair), the CPA will be able to absorb visible light as well as the specific infrared frequencies used in fibre optic communications.
“Perfect absorbers can be used to switch light on and off with great sensitivity, using external laser light. This opens up a number of applications in integrated photonic circuits (devices containing many optical components) which may find applications in the next generation of communication systems,” said Professor Ken Baldwin, of the Research School of Physics and Engineering at the Australian National University in Canberra.
Hot topic?
I am not sure what is happening here.
Is the silicone slice acting as an energy absorber? If so why is it necessary to have two opposite and opposed laser beams?
Would one beam also cancel out by multiple reflections?
With a single beam cancelling, the transformation to thermal radiation should equal the lazer energy input.
If two precisely equal and opposite beams cancel why should there be any thermal radiation as a result and what mechanism would convert the cancelled beams to thermal energy?
Is the heating observed to match the energy input of the twin lazers?
If not we might have a possible breach of the principle of conservation of energy/ matter.
If vacuum energy is supposed to be a field of virtual particles which cancel out to zero energy, perhaps leakage from imperfect cancellation might be the source of energy and matter in the universe?