Most of us have been controlling the light without even thinking about it, usually in a mundane way: we put on a pair of sunglasses and apply sunscreen, and then close or open our blinds.

But the control of light can also take the form of high technology. The screen of the computer, tablet, or phone on which you are reading this article is an example. The other is telecommunications, which controls light to generate signals that transmit data along fiber optic cables.

Scientists are also using high-tech methods to control light in the laboratory, and now, thanks to a new breakthrough using special materials that are only three atoms thick, they can control light more precisely than ever before.

This work was carried out in the laboratory of Harry Atwater, the Otis Booth Leadership Chair of the Engineering and Applied Sciences Division, the Howard Hughes Professor of Applied Physics and Materials Science, and the Liquid Sun Alliance ( LiSA). It appeared in a paper in the journal Science published on October 22.

To understand this work, we must first remember that light exists in the form of waves and it has a characteristic called polarization, which describes the direction of wave vibration. Imagine a boat floating on the sea: the waves have vertical polarization, which means that when the waves pass through the bottom of the boat, they will wave up and down. Light waves behave in roughly the same way, except that these waves can be polarized at any angle. If a ship can steer light waves, it may swing from side to side, or on a diagonal, or even in a spiral manner.

Polarization is useful because it allows light to be controlled in a specific way. For example, the lenses in sunglasses can block glare (light rays usually become polarized when they reflect off a surface, such as a car window). The screen of a desktop calculator creates clear numbers by polarizing light and blocking it in the area. The area where the polarized light is blocked looks dark, and the area where the light is not blocked looks bright.

In this paper, Atwater and his co-authors describe how they used three layers of phosphorous atoms to create a tunable, precise, and extremely thin polarized light material.

This material is composed of so-called black phosphorous, which in many respects is similar to graphite or graphene, that is, in the form of carbon consisting of a thick layer of a single atom. However, although the graphene layer is completely flat, the black phosphorous layer is ribbed, like a corduroy pants or corrugated cardboard texture. (Phosphorus also comes in red, white and purple forms, depending on the arrangement of its internal atoms.)

Atwater said that this crystal structure gives black phosphorus significant anisotropic optical properties. "Anisotropy means it has to do with the angle," he explained. "In materials like graphene, light is equally absorbed and reflected regardless of the polarization angle. In a sense, black phosphor is very different. If the polarization of light is aligned along the ripples, its response Align with if it is perpendicular to the ripple."

When polarized light passes through the ripples of black phosphorous, it interacts with the material differently than when it is oriented along the ripples—it's a bit like rubbing your hands along the ribs of corduroy to pass through them more easily than rubbing them with your hands.

However, many materials can polarize light, and this ability alone is not particularly useful. Atwater said that the special thing about black phosphorus is that it is also a kind of semiconductor. This material has better conductivity than insulators (such as glass), but not as good as metals such as copper. Silicon in microchips is an example of semiconductors. Just as tiny structures built with silicon can control the current in a microchip, structures built with black phosphorous can control the polarization of light when electrical signals are applied to them.

"These tiny structures are doing this kind of polarization conversion," Atwater said, "So now I can make very thin and tunable nano-scale materials. I can make a series of these small components, each of which can Switch the polarization into a different reflected polarization state."

Liquid crystal display (LCD) technology in mobile phone screens and TVs already has some of these capabilities, but black phosphorous technology may be far ahead of it. The "pixels" of the black phosphor array may be 20 times smaller than the "pixels" in the LCD, but the response speed to input is one million times faster.

Atwater said such speeds are not necessary for watching movies or reading articles online, but they can revolutionize telecommunications. The fiber optic cables that transmit optical signals in telecommunications equipment can only transmit so many signals, and then they will start to interfere with each other and overwhelm each other, making them messy (the picture tries to listen to what friends say in a crowded and noisy bar). But Telecommunications equipment based on a thin layer of black phosphorous can adjust the polarization of each signal so that it does not interfere with each other. This will enable fiber optic cables to carry more data than they do now.

Atwater said the technology could also open the door to light-based Wi-Fi alternatives, which researchers in the field call Li-Fi.

"We will pay more and more attention to lightwave communication in free space," he said. "Lighting like this cool-looking lamp above my desk does not carry any communication signals. It just provides light. But there is no reason why you can’t sit in the future Starbucks and let your laptop receive light for wireless devices. Signal communication rather than radio signals. It has not yet been fully implemented, but when it gets here, it will be at least a hundred times faster than Wi-Fi."

The title of the paper describing this work is "Broadband Electro-Optical Polarization Conversion with Atomic-Level Thin Black Phosphorus." The first author is Souvik Biswas, a graduate student in applied physics. Other co-authors are Meir Y. Grajower, a postdoctoral researcher in the field of applied physics and materials science, and Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan.

"This is an exciting time for the discovery of new materials that can shape the future of photonic devices, and we barely touch the surface," Biswas said. "If one day you can buy a commercial product made of atomically thin materials, it will be comforting, and that day may not be far away."

Research funding is provided by the U.S. Department of Energy; Japan’s Ministry of Education, Culture, Sports, Science and Technology; Japan Society for the Promotion of Science; and Japan Science and Technology Promotion Agency.