We're all familiar with the concept of a laser. They appear through our culture, from laser pointers to light shows, from metal cutting to CD players. The laser, or light amplification by stimulated emission of radiation device, works by exciting atoms. This causes electrons to jump up to an "excited state," and then drop down to normal, emitting a photon in the process. The diagram below shows how this works:
The electrons orbit the nucleus of protons and neutrons at certain, discrete levels. It can be at 1, 2, or 3, for instance, but not at 1.5 or 2.33. There is a specific amount of energy between each layer, and a wavelength of light (a photon) corresponds to that energy level. A laser uses this property to emit a focused light beam, the laser we're all familiar with.
As cool as lasers are, though, there's always a question in physics about "what would the opposite of this be?" In the case of a laser, it would be some sort of focused beam that cancelled out light, instead of amplifying it. It's theoretically possible; the incoming light photons would have to be captured and be absorbed, converting into heat. But, until recently, no one had ever tried to create this anti-laser. Yale physicist Douglas Stone, along with his research time, have now done a proof-of-concept, called the "coherent perfect absorber," or CPA. It uses a silicon wafer to capture and cancel two laser beams, absorbing light at specific frequencies in the exact opposite of a laser.
As with all innovations, there are lots of potential applications for this new technology. But the cool thing, to me, is not those uses. Instead, I admire the question itself. A scientist saw a hole in our knowledge and our technology, did some thought experiments to find potential ways it could work, then built the mechanism to show how it works. This creative innovation is one of the most fascinating and fun parts of doing science.
Sources: Science Daily- World's First Anti-Laser Built
How Stuff Works- "How Lasers Work"