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How black is the blackest?

How black is the blackest black in the world? You may have heard of a painting called Vantablack, which claims to be the blackest black in the world. Objects painted with this paint are so black that only outlines remain. In recent years, though, someone has broken their record, producing colors that are darker than the blackest.

When all light goes to zero, we get black. Theoretically pure black would absorb 100% of incoming light, converting it all into heat. Just as the whitest white can cool a room by reflecting light, the perfect black paint has many uses. Probably the most famous is the Hubble Space Telescope. In order to avoid stray light hitting the mirrors and interfering with observations, NASA painted the lens barrel with the darkest paint they could find.

Vanta Black

Now that black paint works, there's no reason scientists shouldn't look for darker paints. The most famous of them may be Vantablack (transliterated as Vantablack).

In 2014, Ben Jensen disclosed his invention of ultra-black paint, which absorbs 99.965% of light at a wavelength of 663 nanometers. Ben Jensen then commercialized the material and named it Vantablack, named after its special structure, Vertically Aligned Nanotube Arrays (Vanta).

Traditional black materials can only absorb at most 90% of light, and this structure allows incoming light to bounce back and forth between microscopic structures and eventually be absorbed completely - just like a single piece of metal can reflect light (think ancient bronze mirrors) ), while the metal is black after being pulverized. Extreme blackness is essentially achieved with the microstructure of the material.

To achieve Vantablack's microstructure, Ben Jensen fabricated the material using chemical vapor deposition (CVD). It's not clear if that's why, after commercialization, the paint is still very expensive -- so expensive that there are no plans to sell it to private individuals.

When the paint is black enough to absorb 99.965% of incoming light, it can bring a lot of unexpected visual effects. When people apply Vantablack to the sculpture, any structural structure, or undulations on the sculpture will disappear, leaving only an outline. British sculptor Anish Kapoor even bought out the artistic use rights of Vantablack directly.

But kingship doesn't last forever, and Vantablack won't always occupy the "blackest" throne. In 2019, scientists at the Massachusetts Institute of Technology announced that they had made a material that was 10 times darker than Vantablack meaning the material could absorb 99.995 percent of incoming light.

The Artwork "Vanity Redemption" was made by the new materials team using the new blackest paint, on the right is the visual effect of the gemstone painted with the latest black material. Image credit: Diemut Strebe

Trap the light

For the whitest whites, near 100% reflectivity is its primary characteristic. And carbon nanotube coatings like Vantablack just achieve the blackest color, and after absorbing this light, it just converts the light energy into heat. There are many devices today that need to absorb as much light as possible to perform optimally, such as light sensors in cameras and solar panels. In order for devices to work at their highest efficiency, they are often designed to be thin, but the light absorption rate of the material itself is not very high. If we can make solar panels and light sensors that are not so dark and darker and absorb more light, we can improve the efficiency of these devices.

Obviously, brushing a layer of black paint on the surface of the solar panel not only fails to achieve this purpose, but because the black paint blocks the light, the solar panel cannot generate electricity at all (the light sensor has no signal). More than ten years ago, a group of scientists at Yale University in the United States struggled with this problem. They were thinking about how to make materials with low light absorption rates absorb more light. And in the process, the laser's production process gave them inspiration.

The three main structures in a laser are the pump source, the gain medium, and the resonator. The pump source will excite the electrons in the gain medium to a high energy state, and the electrons in the gain medium will spontaneously emit photons after reaching the high energy state. Once these photons collide with other high-energy electrons, they will induce it to transition to a low-energy state and release a photon with the same frequency, phase, and direction, which is called stimulated radiation, and it can excite more Exactly the same photon.

The resonator is a mirror with two parallel sides, one side is totally reflective and the other side is semi-reflective and semi-transmissive. These photons are reflected back and forth between the two mirrors of the resonator, and each time they pass through the gain medium, the laser is intensified by one point, and finally, a beam of photons with extremely high brightness is emitted from the semi-reflective and semi-transmissive mirrors.

In the helium-neon laser, the flashing part in the middle is the gain medium, in which the laser is continuously reflected to obtain gain, and finally hits the paper. Image credit: David Monniaux

If this process is reversed, the gain medium is replaced with an absorbing medium, and the optical path is reversed so that the beam is shot into the laser, can the light be completely absorbed between continuous reflections? In 2010, scientists at Yale University actually realized this structure and called it the perfect light absorber. A related paper was published in Physical Review Letters.

However, this structure is not perfect. Because the light is injected into the resonator through a semi-reflective and semi-transmissive mirror, when the light first enters the resonator, a part of the light is reflected. After that, every time the light in the resonator hits the mirror, a portion of the light leaks out.

The original light absorber is imperfect, and some light leaks out with every reflection. Image credit: Science 377, 995–998 (2022)

Last month, scientists from the Technical University of Vienna in Austria and the Hebrew University of Jerusalem in Israel fixed the bug and published a paper in the journal Science. The structure they designed is basically the same as the original light absorber, but a group of lens structures is added to it. This set of lens structures allows the incoming light to return along a specific path. When it reaches the mirror where the light entered, it will coincide with the reflected light at the beginning, and produce destructive interference, canceling all the reflected light.

The new light absorber can cancel out the reflected light by interference, thereby absorbing more light. Image credit: Science 377, 995–9 (2022)

In this way, no light can escape, and all light is absorbed by the absorbing medium in the back-and-forth reflections. Even though the absorbing medium would otherwise absorb only 15 percent of incoming light, the structure eventually allowed it to absorb at least 94 percent of the light, and in some directions, even 98 percent, the researchers said.

Although the absorption rate of 99.995% is not comparable to that of black paint, the use value of this light-absorbing structure is much higher. Both optical sensors and optical computing need to improve the light absorption rate of specific structures and convert weak optical signals into electrical signals as much as possible.

In this darker race, astronomers are the undefeated winners. Regardless of the purpose of the two technical routes, they can enjoy the benefits of technological progress. They can use black paint to eliminate unnecessary scattered light and avoid stray light interference; they can also use light-absorbing structures to make the light sensor absorb as much as possible. More light to achieve a better observation effect.

After all, there's probably no one on Earth who crave night more than astronomers.

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