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.
A. Douglas Stone of Yale University and his colleagues introduced the theory behind CPAs2 in 2010. Consider a laser hitting a slab of material. Some of the light is reflected, some transmitted, and some absorbed. If two antiparallel beams enter opposite sides of the material, they can interfere such that the transmitted and reflected light in each direction cancel. In that case, all of the energy is absorbed. In the first experimental demonstration in 2011, a silicon slab absorbed over 99% of the light from two counter-propagating lasers.3 Total destructive interference requires matching frequencies, amplitudes, and phases and having the right material reflection and transmission coefficients.
The conditions for coherent perfect absorption can be reframed as those for lasing just run in reverse. With each round trip through a laser, the light amplification in the gain medium must balance the energy losses in the laser cavity. In a CPA, the light absorbed in each round trip must be equal to the light that enters the cavity during that time. If that condition is met—and if the cavity length is resonant with the wavelength—light that would be reflected at the cavity’s entrance mirror is canceled out by interference, so all the light ends up in the cavity and bounces back and forth until it’s absorbed.
Katz, who has long worked on novel imaging techniques, heard about the latest ideas for realizing CPAs at a February 2020 workshop on waves in complex media. Rotter’s then graduate student Matthias Kühmayer presented the group’s recent work showing coherent perfect absorption in a disordered medium, a feat that required devising a time-reversed version of what’s known as a random laser.4 As with other CPAs, the setup was limited to a single mode. Katz wondered if a time-reversed version of a laser that emits multiple modes simultaneously might overcome that single-mode limit.
Katz chatted with Rotter about the idea over one of the workshop’s coffee breaks, and the two decided to collaborate. “The innovation the project introduces is primarily conceptual: bringing a well-known cavity design from laser physics to a different field,” says Katz. That design came from a degenerate-cavity laser, a system he had worked on a few years earlier.
In a conventional CPA, light traveling perpendicular to the cavity’s two mirrors bounces back and forth along the same path. Light at any other angle instead ricochets before eventually leaving the cavity. The degenerate cavity, shown in figure 2a, includes the addition of two lenses placed in a telescopic arrangement such that the light is imaged back onto itself after a round trip. Because light heading in any direction travels a closed loop, light at various incoming angles and with multiple spatial modes is trapped in the cavity. And because light ends up where it entered the cavity, it can destructively interfere with any would-be reflected light.
À la mode
Katz and two of his grad students—Yevgeny Slobodkin and Gil Weinberg—came up with a few possible designs for a CPA based on a degenerate-cavity laser. Meanwhile, Rotter’s grad student Helmut Hörner did numerical calculations to verify that the idea was viable, particularly with any expected experimental imperfections. The predicted tolerance was enough that commercial optics were an option.
Bolstered by those promising calculations, Katz’s group moved forward with an experimental setup: a thin slab of weakly absorbing colored glass with a partially reflecting entrance mirror on one side of it and two lenses and a nearly fully reflecting back mirror on the other. To get as close as possible to perfect absorption, the group members ferreted out misalignment and other sources of imperfections. In the end, spurious reflections from the lenses remained the main performance limiter; in the future, they could be reduced with better antireflection coatings or alternate cavity designs that swap the lenses for curved mirrors.
The researchers measured the spatial distribution of the reflected intensity when light impinged on the cavity. (The intensity transmitted through the cavity was consistently near zero.) They used two forms of illumination: a laser sent through a spatial light modulator to produce a speckle pattern of over 1000 modes, shown in figure 1, 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 increased from 15% for the glass alone to nearly 95% in the cavity, with negligible differences between modes or airflow strengths.
The phenomenon could also be intentionally turned off—unlike more conventional forms of absorption—a feature that could be used for filtering or modulating. Changes of less than a micron in the cavity length altered the absorption dramatically. Dips in the reflected power occurred when the cavity length was resonant with the wavelength, as shown in figure 2b. When the cavity was detuned by fractions of a wavelength, the system’s absorption was even lower than the colored glass alone. Modes from different spots in the speckle pattern all reached a minimum reflected intensity at the same cavity length, whose value matched numerical calculations.
Of course, the number of modes and incoming angles that experience perfect absorption is limited. Light incident at too great an angle or too far from the mirror’s center hits the aberrant edges of the lenses and doesn’t achieve the requisite self-imaging. The precise limits depend on the setup’s specific optics and geometry. The researchers demonstrated consistent performance for more than 1000 modes covering a 3 mm × 3 mm area and with up to a 0.5° input angle.
The degenerate-cavity concept should work for all electromagnetic waves, for acoustic waves, and for other waves. “We believe that our results open up new ways to detect weak signals,” 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 a given cavity length. Overcoming that limitation is Katz and his colleagues’ current venture.