A perfect ‘light trap’ to improve efficiency of photovoltaic systems

Using mirrors and lenses, a light beam is steered in a circle and then superimposed on itself.

The “light trap” set-up: a partially transparent mirror; a thin, weak absorber; two converging lenses; and a totally reflecting mirror. Normally, most of the incident light beam would be reflected. However, owing to precisely calculated interference effects, the incident light beam interferes with the light beam reflected back between the mirrors so that the reflected light beam is ultimately completely extinguished. The absorber completely sucks up the energy of the light.  | Photo Credit: Vienna University of Technology

If light is to be used efficiently, say, in a photovoltaic system, it has to be absorbed as completely as possible. However, this is difficult if the absorption is to take place in a thin layer of material that normally lets a large part of the light pass through.

Researchers of Vienna Technology University (TU Wien) and The Hebrew University of Jerusalem (THUJ) have together developed a scheme that allows a beam of light to be completely absorbed even in the thinnest of layers. Using mirrors and lenses around the thin layer, they have built a “light trap” 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. This absorption-amplification method was published in a recent issue of Science.

There have been earlier attempts to improve the absorption of materials. For example, the material can be placed between two mirrors. The light is reflected back and forth between the two mirrors, passing through the material each time and thus having a greater chance of being absorbed. However, for this purpose, the mirrors must not be perfect: one of them must be partially transparent, otherwise the light cannot be beamed into the inter-mirror space at all. But this also means that whenever light hits this partially transparent mirror, some of it is lost.

In order to prevent this, the TU Wien-THUJ group used the wave properties of light cleverly. “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,” explain the graduate students Yevgeny Slobodkin and Gil Weinberg, who built the system.

The two partial beams overlap in such a way that the light blocks itself, so to speak: although the partially transparent mirror alone would actually reflect a large part of the light, this reflection is rendered impossible by the other part of the beam travelling through the system before returning to the partially transparent mirror.

Therefore, the mirror that used to be partially transparent becomes completely transparent for the incident laser beam. This creates a one-way street for the light: the light beam can enter the system, but then it can no longer escape because of the superposition of the reflected portion and the portion guided through the system.