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 light by giving the retina a second chance to absorb light.
A similar strategy can boost the amount of light absorbed by any material. In an optical cavity, light passes through the material many times. And under the right conditions, nearly all the light is eventually absorbed, even by a weakly absorbing material. Such a system is an example of what’s known as a coherent perfect absorber, which achieves its performance with the help of interference effects.
The conditions for perfect absorption are the same as those for lasing but run in reverse—the laser’s gain medium becomes an absorbing medium, and stimulated emission becomes absorption. The laser or absorber cavity generally does its job, however, only for a specific spatial mode and direction of propagation. Now Ori Katz of the Hebrew University of Jerusalem and his colleagues have demonstrated a simple design for a coherent perfect absorber (shown above) that overcomes those limitations.
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