Shine a flashlight at a cat at night, and its eyes will appear to glow. That’s because cats—along with owls and many other nocturnal animals—have a reflective tissue layer behind their retinas. The adaptation increases their sensitivity to low levels of illumination by giving the retina a second chance to absorb photons.
A similar strategy can boost the amount of light absorbed by any material. For a material placed in an optical cavity, light passes through it many times. And under the right conditions, nearly all the light is eventually absorbed, even by a weakly absorbing medium. Such a system is an example of what’s known as a coherent perfect absorber (CPA), which achieves its performance with the help of interference effects.
Absorption is essential for the efficiency of solar panels and of light detectors, for example, particularly when the targeted signals are weak. Maximizing that efficiency is tricky, however, because absorption and reflection generally go hand in hand: A highly absorbent material is usually also highly reflective and sends away much of the light. CPAs don’t have that issue, and many of their designs could be incorporated into a detector without major alterations.
But until recently, CPAs worked only for a specific spatial mode and direction of propagation, both of which severely limit the eligible signals. Now Ori Katz of the Hebrew University of Jerusalem, Stefan Rotter of Technical University of Vienna, and their colleagues have demonstrated a CPA that overcomes those limitations.1 Taking inspiration from an established laser design, their simple setup, shown in figure 1, widens the range of acceptable wavefronts for perfect absorption.
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