2 minute read
Anti-lasers
TECHNOLOGY
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Anti-laser in action
See what happens when a laser beam meets its nemesis… Optical cavity
The laser beams enter a block of silicon that traps the light photons. The trick to stopping any laser is to disrupt the flow of coherent photons
Laser light
The incoming infrared laser beam is split into two, and directed at the device from opposite directions.
Going inside an anti-laser
The tiny device that can stop a laser in its tracks
Faced with the puzzle of creating a device that could absorb laser energy, researchers at Yale looked at how laser light is produced and simply reversed the process. What they created in 2011 was the first anti-laser, or coherent perfect absorber (CPA).
Conventional lasers work by stimulating atoms in what’s called a gain material. As these excited atoms drop back down to a less excited state, they emit photons with the same wavelength, creating light waves that are in step. Inside the laser amplification cavity, mirrors bounce these photons back and forth, causing excited atoms to emit photons of exactly the same wavelength. The result is a huge number of photons with the same frequency and direction, creating a focused beam of intense light energy.
The anti-laser demonstrated by Yale took this basic setup and switched it around. First a laser beam is split into two, with one of the two resulting beams being modified so it is out of step with its counterpart. The two incoming laser beams are directed at a small slab of silicon. The surface of the silicon acts as a one-way trapdoor, allowing the light to enter but not escape. As the two beams bounce around inside the silicon, they gradually lose energy as they cancel each other out through interference. Although the existing prototype can absorb 99.4 per cent of light, in theory it could be optimised to absorb 99.99 per cent.
More generally, the idea of reversing the process of laser light production by different materials could be used to investigate how those materials absorb light.
Trapped
As the photons bounce around they interfere with one another, neutralising the laser.
Heat
Laser energy is gradually dissipated as heat, although the device could be modified to make use of this lost energy.
Developing anti-lasers
We asked A Douglas Stone, professor of Applied Physics at Yale University, for his take on the anti-laser technology he developed
What’s so special about an anti-laser?
ADS: We realised that the strength of the anti-laser was that if you hit an opaque medium with exactly the right pattern of light then it would penetrate the medium and be absorbed. This is a totally new principle.
What applications does this have?
ADS: Suppose I want to know what’s happening inside a solar cell. Depending on where in the cell the light is absorbed, the cell’s efficiency at collecting energy varies. We can focus light deep within the cell to see how that changes things. If you’re trying to perfect solar cells it’s very exciting, although it might not resonate with the public just yet!
So there aren’t any direct defence applications for anti-lasers?
ADS: When our research was published a lot of people thought it was somehow a defence against laser [weapons]. A mirror is a better defence – it’s easier to get a laser to bounce off something than to try and absorb it. What we’ve developed is a selective absorption technique.
