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Apply Vantablack aluminum foil and note that the aluminum foil that is blackened has the same degree of wrinkling as the aluminum foil that is not blackened.

Photo Source: Surrey NanoSystems

How black is the blackest black in the world? You may have heard of a paint called Vantablack, which claims to be the blackest black in the world. An object coated with this paint is so black that there is only an outline. In recent years, however, some people have broken their records and created darker colors than the blackest.

When all the light goes to 00:00, we get black. In theory, pure black absorbs 100% of the incident light, converting it all into heat. Just as the whitest white cools the room by reflecting light, the perfect black paint has many uses. Perhaps the most famous is the Hubble Space Telescope, where NASA painted the tube with the blackest paint they could find to prevent stray light from hitting the lens.

Since black paint is useful, there is no reason why scientists should not look for darker paint. Perhaps the most famous of these is Vantablack.

In 2014, Ben Jensen unveiled his ultra-black paint, which absorbs 99.965% of light at a wavelength of 663nm. Ben Jensen then commercialized the material and named it Vantablack, named after its special structure, "vertically aligned carbon nanotube array" (Vertically Aligned NanoTube Arrays,Vanta).

Traditional black materials can only absorb up to 90% of the light, and this structure allows incident light to reflect back and forth between microstructure and eventually be fully absorbed-just as a whole piece of metal can reflect light (think of ancient bronze mirrors). The metal is black after being ground into powder. The extreme black is essentially realized by the microstructure of the material.

In order to realize the microstructure of Vantablack, Ben Jensen uses chemical vapor deposition (CVD) to fabricate the material. It is not clear whether it is for this reason that after commercialization, the paint is still very expensive-so expensive that there is no plan to sell it to the private sector at all.

When the paint is so dark that it can absorb 99.965% of the incident light, it can bring a lot of unexpected visual effects. When people apply Vantablack to the sculpture, any structure and ups and downs on the sculpture will disappear, leaving only an outline. Anish Kapoor, a British sculptor, even bought out the right to use Vantablack directly.

But the monarchy does not last forever, and Vantablack will not occupy the "darkest" throne forever. In 2019, scientists at the Massachusetts Institute of Technology announced that they had made a material 10 times darker than Vantablack-meaning it could absorb 99.995% of incoming light.

The New Materials team uses the art "Redemption of Vanity" made of the new blackest paint, and on the right is the visual effect of the gemstone painted with the latest black material. Photo Source: Diemut Strebe

Trapping light for the whitest white, close to 100% reflectivity is its most important feature. Carbon nanotube coatings such as Vantablack are only the darkest in color, and after absorbing this light, it just converts light energy into heat. Today there are many devices that need to absorb as much light as possible to achieve the best results, such as camera light sensors and solar panels. In order to achieve the highest efficiency, the equipment is often designed to be very thin, but the light absorptivity of the material itself is not very high. If we can make the not-so-black solar panels and light sensors darker and absorb more light, we can improve the efficiency of these devices.

Obviously, painting the surface of the solar panel with a layer of black paint not only does not achieve this goal, but because the black paint blocks the light, the solar panel cannot generate electricity at all (there is no signal instead of a light sensor). More than a decade ago, a group of scientists at Yale University in the United States were wrestling with this problem, thinking about how to make materials with low light absorption rate absorb more light. In this process, the laser generation process inspired them.

The three main structures of the laser are pump source, gain medium and resonant cavity. The pump source will excite the electrons in the gain medium to the high energy state, and the electrons in the gain medium will spontaneously emit photons when they reach 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 exactly the same frequency, phase and direction, which is called stimulated radiation, and it can excite more exactly the same photons. The resonant cavity is two mirrors parallel to each other, one with total reflection and the other with half reflection and half transmission. These photons are reflected back and forth between the two mirrors of the resonant cavity, and each time they pass through the gain medium, the laser will be enhanced by one point, and eventually the photon beam will be emitted from the other end of the semi-reflection and semi-transmission mirror.

The part of the he-ne laser that flashes in the middle is the gain medium, in which the laser constantly reflects and gets the gain, and finally hits the paper. Photo Source: David Monniaux

If we reverse this process, replace the gain medium with an absorption medium, and then reverse the light path and let the beam into the laser, won't it be able to completely absorb the light between continuous reflection? In 2010, scientists at Yale University actually implemented this structure and called it the perfect light absorber. The related paper was published in the physical Review KuaiBao.

However, this structure is not perfect. Because the light is emitted into the cavity through a semi-reflective and semi-transparent mirror, part of the light is reflected off as soon as it begins to enter the cavity. After that, every time the light in the resonant cavity touches the mirror, some of the light will leak out.

The original light absorber is not perfect, and some light leaks out every time it is reflected. Photo: 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 loophole and published the paper in the journal Science. The structure they designed is basically the same as the original light absorber, except that a set of lens structure is added to it. This lens structure allows the incident light to return along a specific path, and when it reaches the mirror in which the light enters, it coincides with the initial reflected light and cancels out all the reflected light.

The new light absorber can counteract the reflected light through interference, thus absorbing more light. Photo: Science 377,995-9 (2022)

In this way, no light can be emitted, and all light will be absorbed by the absorbing medium in the back and forth reflection. The researchers say that even if the absorbing medium absorbs only 15% of the incident light, the structure will eventually allow it to absorb at least 94% of the light and, in some directions, 98% of the light.

Although it can not be compared with the absorption rate of 99.995% of black paint, the use value of this light absorption structure is much higher. Optical sensors and optical calculations all need to improve the light absorptivity of specific structures and convert weak optical signals into electrical signals and other information as far as possible.

Astronomers are the undefeated winners in this competition that is darker than anyone else. No matter what the purpose of the two technological routes is, 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 absorption structure to enable light sensors to absorb as much light as possible to achieve better observation results.

After all, there may be no one on this planet who craves night more than astronomers.

Reference link:

Https://www.science.org/doi/10.1126/science.abq8103

Https://physics.aps.org/articles/v3/61

Https://physics.aps.org/articles/v15/131

Https://news.mit.edu/2019/blackest-black-material-cnt-0913

Https://en.wikipedia.org/wiki/Vantablack

Https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.105.053901

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by Wang Yu, revised by Chestnut

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