14 DAY TRIAL // Lasers are some of the most useful scientific equipment ever devised. They have contributed to the development of many fields of research and have led to countless innovations, from medicine to particle physics. They work by amplifying certain wavelengths of light, by bouncing photons back and forth between two surfaces inside a cavity at the tip of the laser device. But a team of physicists was recently able to demonstrate that the effect can also work backwards, in the sense that light can be absorbed inside so-called “time-reversed” lasers, without it being able to escape.
Only certain materials can become lasers. They need to fulfill a very important condition, and namely be able to produce a photon when struck by an incoming photon. This leads to a chain reaction of sorts, in which more photons prompt the release of even more photons, until the growth curve becomes exponential. But bouncing the elementary particles back and forth – a process that has for a long time been thought to be absolutely essential to these instruments' operations – appears to not be all that necessary. Naturally-lasing materials have been discovered, which emit photons randomly.
A group of physicists from the Yale University, led by expert Yi Dong Chong, began to wonder some time ago whether the lasing process couldn't, by any chance, work in reverse to, not only in one direction. Much to their amazement, they learned that the process was fully-reversible. There are some materials that are capable of absorbing light flawlessly, though only photons of very specific wavelengths. The team has dubbed the new class of materials “coherent perfect absorbers” and published additional details about them in a paper appearing online, in the journal arXiv.
In the materials, coherent beams get separated into reflected and transmitted parts, which then interfere perfectly, as this happens, light of a particular wavelength is trapped perfectly, and the excess energy is vented off as either heat, or electron-hole pairs. But the effect evens out when many wavelengths are shone on the materials at the same time. Visible light, for example, spans from 400 to 750 nanometers, and contains many wavelengths. If only a specific one is shone on a suitable CPA, then it will be absorbed. Otherwise, the effect is negligible, Technology Review reports.