What is inside an Anti-Laser .?

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 zon 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.