Scientists from Israel and Austria have developed a special and very effective trap for light. With the device, light can be optimally absorbed, ie recorded, by a thin layer. The researchers see possible applications primarily in science. In this way, faint light from very distant stars could be optimally captured by telescopes, despite the air movement in the atmosphere.
In the apparatus, the light is directed by mirrors and lenses in such a way that the light waves are extinguished by the incident light beam – the light is caught in a trap. Theoretically, 100 percent of the incident light can be absorbed with this arrangement. The group led by Ori Katz from the Hebrew University of Jerusalem and Stefan Rotter from the Technical University of Vienna presented their invention in the journal Science.
“A thick black wool sweater can easily absorb light, but in many technical applications you only have a thin layer of material and you want the light to be absorbed in exactly that layer,” explained Rotter. He and his team designed the device based on an idea from Katz. The experiments with the device were carried out in Jerusalem, the theoretical calculations in Vienna.
If someone throws two stones into a calm lake at the same time, the resulting waves overlap: When two wave crests meet, the wave crest is twice as high as the other wave crests, wave troughs are then twice as deep. But between mountains and valleys there is an area where the water is as high as the rest of the lake’s water level – at this point the superimposed waves cancel each other out. This is the same with light waves. The researchers directed the incident light in a tiny cavity in such a way that part of the light hits the incident beam again at the end of the path through the cavity.
“What is crucial is that the length of this path and the position of the optical elements are set in such a way that the returning light beam (and its multiple reflections between the mirrors) exactly cancels the light beam reflected directly from the first mirror,” explain the Jerusalem master’s students Yevgeny Slobodkin and Gil vineyard. The length of the light trap must therefore be set exactly to the wavelength of the light. Then the light can also have different intensities, come from different directions or be disturbed by air turbulence, but it is still effectively enclosed in the light trap.
The device consists of a semi-transparent mirror through which the light beam enters the cavity, two lenses that direct the light beam, and a highly reflective mirror at the back end. The absorbing material is placed between the semi-transparent mirror and the first lens. The light that passes through this material after it has been reflected is then extinguished, to a certain extent blocked, by the incident light beam at the semi-transparent mirror.
In the experiment, the scientists used a colored pane of glass as the absorbing material, which absorbs only 14.8 percent of the light that hits it. With the light trap, Katz, Rotter and colleagues made the glass absorb more than 94 percent of the light. But theoretically, with this device, 100 percent of the incident light of one wavelength can be absorbed in a thin layer, writes Jacopo Bertolotti from the University of Exeter (UK) in a comment, also in “Science”. He sees the possibility that light sensors in cameras and solar cells could be improved with the help of the device.
