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.
Katz and his team realized that to absorb various modes of light simultaneously, they could borrow ideas from a laser that emits multiple modes simultaneously: the degenerate-cavity laser. In a conventional coherent perfect absorber, light traveling normal to the cavity’s two mirrors bounces back and forth along the same path. Light at any other angle instead ricochets and eventually leaves the cavity.
The degenerate cavity (shown below) includes the addition of two lenses placed such that light traveling along any path retraces its steps after each round trip. Because light heading in any direction travels in a closed loop, different incoming angles and spatial modes are simultaneously trapped in the cavity. And because light ends up where it entered the cavity, it can also destructively interfere with any light that would otherwise be reflected at the cavity’s entrance mirror, so the light ends up in the cavity and bounces back and forth until it’s absorbed.
The researchers tested how well a thin slab of weakly absorbing colored glass absorbs light in a degenerate cavity. They used two forms of illumination: a laser sent through a spatial light modulator to produce a speckle pattern of over 1000 modes or the output from a multimode optical fiber that is shaken by airflows of various strengths and passes through turbulent air to produce a dynamic speckle pattern. The researchers found that absorption went from 15% for the glass alone to nearly 95% in the cavity, with negligible differences between modes or airflow strengths.
“We believe that our results open up new ways to detect weak signals of light,” says Katz, “even when they get perturbed by passing through the Earth’s turbulent atmosphere,” as is the case for faint starlight in astronomy, for example. But the current design is still limited to a narrow range of wavelengths for any given geometry. Overcoming that limitation is Katz and his colleagues’ next venture. (Y. Slobodkin et al., Science 377, 995, 2022.)