A light trap perfectly absorbs light as the beam prevents itself from escaping the trap.
Absorption of light, in photosynthesis as well as in photovoltaic systems is a very important part and so is its use. The use of light will be efficient when the light is absorbed completely. Researchers have built a “light trap” around the thin layer using mirrors and lenses, in which the light beam is steered in a circle and then superimposed on itself in such a way that the beam of light blocks itself and can no longer leave the system.
“Absorbing light is easy when it hits a solid object,” says Prof. Stefan Rotter from the Institute of Theoretical Physics at TU Wien. “A thick black wool jumper can easily absorb light. But in many technical applications, you only have a thin layer of material available and you want the light to be absorbed exactly in this layer.”
There have been several attempts to improve the absorption of materials. But none actually enables the system to absorb light completely. However using the wave properties of light in a sophisticated way complete absorption could be achieved. “In our approach, we are able to cancel all back-reflections by wave interference,” says Prof. Ori Katz from The Hebrew University of Jerusalem. Helmut Hörner, from TU Wien, explains: “In our method, too, the light first falls on a partially transparent mirror. If you simply send a laser beam onto this mirror, it is split into two parts: The larger part is reflected, a smaller part penetrates the mirror.”
The light beam that penetrates the mirror passes through the transparent material and then returns to the partially transparent mirror with lenses and another mirror. “The crucial thing is that the length of this path and the position of the optical elements are adjusted in such a way that the returning light beam (and its multiple reflections between the mirrors) exactly cancels out the light beam reflected directly at the first mirror,” says Yevgeny Slobodkin and Gil Weinberg, the graduate students who built the system in Jerusalem. The two partial beams overlap in such a way that the light blocks itself.
“The system has to be tuned exactly to the wavelength you want to absorb,” says Stefan Rotter. “But apart from that, there are no limiting requirements. The laser beam doesn’t have to have a specific shape, it can be more intense in some places than in others – almost perfect absorption is always achieved.”
Reference: “Massively degenerate coherent perfect absorber for arbitrary wavefronts” 25 August 2022, Science.