Control light more precisely than ever before – using material only three atoms thick
Most of us control the light all the time without even thinking about it, usually in a trivial way: we put on a pair of sunglasses and put on sunscreen, and close or open our blinds.
But light control can also take high-tech forms. The screen of the computer, tablet, or phone you’re reading this on is one example. Another is telecommunications, which controls light to create signals that carry data along fiber optic cables.
Scientists are also using high-tech methods to control light in the lab, and now, thanks to a new breakthrough that uses a specialized material just three atoms thick, they can control light more precisely than ever before.
The work was carried out in the lab of Harry Atwater, Otis Booth Leadership Chair of the Division of Engineering and Applied Science, Howard Hughes Professor of Applied Physics and Materials Science, and Director of the Liquid Sunlight Alliance (LiSA). He appears in an article published in the Oct. 22 issue of Science.
To understand the work, it helps to first remember that light exists as a wave and has a property called polarization, which describes the direction in which waves vibrate. Imagine being in a boat that floats on the ocean: Ocean waves have vertical polarization, which means that when the waves pass under the boat, they rise and fall. Light waves behave in much the same way, except that these waves can be polarized at any angle. If a boat could ride waves of light, it could sway side to side, or diagonally, or even spiral.
Polarization can be useful because it allows light to be controlled in specific ways. For example, the lenses in your sunglasses block glare (light often becomes polarized when it reflects off a surface, such as a car window). The screen of a desktop calculator creates readable numbers by polarizing light and blocking it in certain areas. Areas where polarized light is blocked appear dark, while areas where light is not blocked appear bright.
In the article, Atwater and his co-authors describe how they used three layers of phosphorus atoms to create a material to polarize light that is tunable, precise, and extremely fine.
The material is constructed from black phosphorus, which is similar in many ways to graphite, or graphene, forms of carbon that consist of a singleatom– thick layers. But while the graphene layers are perfectly flat, the black phosphorus layers are ribbed, like the texture of corduroy pants or corrugated cardboard. (Phosphorus also comes in red, white, and purple forms, which are distinct due to the arrangement of the atoms within.)
This crystal structure, according to Atwater, gives black phosphorus significantly anisotropic optical properties. âAnisotropy means it depends on the angle,â he explains. âIn a material like graphene, light is absorbed and reflected equally, regardless of the angle at which it is polarized. Black phosphor is very different in that if the polarization of light is aligned along the ripples, it has a very different response than if it is aligned perpendicular to the ripples.
When polarized light is directed through the ripples in black phosphor, it interacts with the material differently than when it is directed along the ripples, much like how it is easier to rub your hand along the ribs. corduroy than rubbing your hand through them.
However, many materials can polarize light and this ability alone is not particularly useful. What makes black phosphorus special, says Atwater, is that it is also a semiconductor, a material that conducts electricity better than an insulator, like glass, but not as well as a metal like the copper. Silicon in electronic chips is an example of a semiconductor. And just as tiny structures made from silicon can control the flow of electricity in a microchip, structures made from black phosphorus can control the polarization of light when an electrical signal is applied to them.
âThese tiny structures do this polarization conversion,â Atwater explains, âso now I can make something very fine and tunable, and at the nanoscale. I could make an array of these small elements, each of which can convert to polarization to a different reflected polarization state.
Liquid crystal display (LCD) technology found in phone screens and televisions already has some of these capabilities, but black phosphor technology has the potential to get a head start. The “pixels” of a black phosphor matrix could be 20 times smaller than those of LCD screens, while responding to inputs a million times faster.
Such speeds aren’t necessary to watch a movie or read an article online, but they could revolutionize telecommunications, says Atwater. The fiber optic cable through which light signals are sent into telecommunication devices can only transmit a limited number of signals before they start interfering and overwhelming, jamming them (image trying to hear this a friend says in a crowded and noisy bar). But a telecommunications device based on thin layers of black phosphorus could adjust the polarization of each signal so that none interfere with each other. This would allow a fiber optic cable to carry much more data than it currently does.
Atwater says the technology could also open the door to a light-based replacement for Wi-Fi, which researchers in the field are calling Li-Fi.
More and more, we will be looking at light wave communications in free space, âhe says. âLighting like this cool looking lamp above my desk does not transmit any communication signal. It just provides light. But there’s no reason you can’t sit in a future Starbucks and have your laptop take a light signal for its wireless communication rather than a radio signal. It’s not quite there yet, but when it gets here, it’ll be at least a hundred times faster than Wi-Fi.
The article describing the work is titled “Wideband Electro-Optical Polarization Conversion with Atomically Thin Black Phosphor”. The main author is Souvik Biswas, a graduate student in applied physics. The other co-authors are Meir Y. Grajower, Postdoctoral Fellow in Applied Physics and Materials Science, and Kenji Watanabe and Takashi Taniguchi from the National Institute of Materials Science in Japan.
âThese are exciting times for the discovery of new materials that can shape the future of photonic devices, and we’ve barely scratched the surface,â Biswas said. “It would be gratifying if one day you could buy a commercial product constructed from such atomically thin material, and that day might not be very far away.”
Reference: âBroadband electro-optic polarization conversion with atomically thin black phosphorusâ by Souvik Biswas, Meir Y. Grajower, Kenji Watanabe, Takashi Taniguchi and Harry A. Atwater, October 22, 2021, Science.
DOI: 10.1126 / science.abj7053
Research funding was provided by the US Department of Energy; the Japanese Ministry of Education, Culture, Sports, Science and Technology; the Japanese Society for the Promotion of Science; and the Japan Science and Technology Agency.