VIENNA, Sept. 22, 2022 — Maximizing light absorption is essential for photovoltaics and many other photonic applications. It is also difficult, especially when the light is being absorbed by a thin layer of material that would normally allow a large portion of the light to pass through.

A way to achieve near-perfect light absorption, even with thin layers of material, has been developed through a collaboration between Vienna University of Technology (TU Wien) and The Hebrew University of Jerusalem.

“Absorbing light is easy when it hits a solid object,” TU Wien professor Stefan Rotter said. “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.”

Using lenses and mirrors, the researchers created a near-perfect “light trap” around a thin layer of material. The light beam is steered in a circle and then superimposed on itself so that the beam blocks itself and can no longer leave the system.

One way to improve light absorption has been to place the absorbing material between two mirrors. As light is reflected back and forth between the mirrors, it passes through the material, creating an opportunity for the material to absorb more of the light with each pass. However, one mirror must be partially transparent; otherwise, the light cannot penetrate the space between the mirrors. Whenever the light hits the partially transparent mirror, some of it is lost.

“In our approach, we are able to cancel all back-reflections by wave interference,” said Ori Katz, a professor of applied physics at Hebrew University.

In the new light-trapping technique, the light first hits the partially transparent mirror. “If you simply send a laser beam onto this mirror, it is split into two parts,” said Helmut Hörner, a researcher at TU Wien. “The larger part is reflected. A smaller part penetrates the mirror.”

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