From California Institute of Technology (US) : “Controlling Light with a Material Three Atoms Thick” 

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From California Institute of Technology (US)

October 22, 2021
Emily Velasco
(626) 372‑0067
evelasco@caltech.edu

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Credit: California Institute of Technology (US).

Most of us control light all the time without even thinking about it usually in mundane ways: we don a pair of sunglasses and put on sunscreen, and close—or open—our window blinds.

But the control of light can also come in high-tech forms. The screen of the computer, tablet, or phone on which you are reading this is one example. Another is telecommunications, which controls light to create signals that carry data along fiber-optic cables.

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

The work was conducted in the lab of Harry Atwater, the 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). It appears in a paper published in the October 22 issue of Science.

To understand the work, it is helpful first to remember that light exists as a wave and that it has a property known as polarization, which describes the direction in which the waves vibrate. Imagine being in a boat bobbing on the ocean: Ocean waves have a vertical polarization, which means that as the waves pass under the boat, it goes up and down. Light waves behave in much the same way, except these waves can be polarized at any angle. If a boat could ride waves of light, it might bob from side to side, or on a diagonal, or even in a spiraling fashion.

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Polarization refers to the orientation in which a wave (including light) vibrates. The angle of polarization can be changed. Credit: Smouss/Wikimedia Commons.

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, like the window of a car). The screen of a desk calculator creates legible numbers by polarizing light and blocking it in areas. Those areas where the polarized light is blocked appear dark, while areas where the light is not blocked appear light.

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The display of a calculator that uses the properties of polarized light to create light and dark areas that are readable as numbers and other figures. Credit: David R. Tribble/Wikimedia Commons.

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

The material is constructed from so-called black phosphorous which is similar in many ways to graphite or graphene, forms of carbon that consist of single-atom-thick layers. But whereas the layers of graphene are perfectly flat, black phosphorous’s layers are ribbed, like the texture of a pair of corduroy pants or corrugated cardboard. (Phosphorus also comes in red, white, and violet forms, distinct because of the arrangement of the atoms within it.)

That crystal structure, Atwater says, makes the black phosphorus have significantly anisotropic optical properties. “Anisotropy means is that it’s angle dependent,” he explains. “In a material like graphene, light is absorbed and reflected equally no matter the angle at which it’s polarized. Black phosphorus is very different in the sense that if the polarization of light is aligned along the corrugations, it has a very different response than if it’s aligned perpendicular to the corrugations.”

When polarized light is oriented across the corrugations in black phosphorous, it interacts with the material differently than when it is oriented along the corrugations—kind of like how it is easier to rub your hand along the ribs in corduroy than it is to rub your hand across them.

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Sheets of black phosphorus, much like this corduroy fabric, are ribbed. Credit: Ariel Glenn/Wikimedia Commons.

Many materials can polarize light, though, and that ability alone is not especially useful. What makes black phosphorous special, Atwater says, 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 copper. The silicon in microchips is an example of a semiconductor. And just as how tiny structures built from silicon can control the flow of electricity in a microchip, structures built from black phosphorous can control the polarization of light as an electric signal is applied to them.

“These tiny structures are doing this polarization conversion,” Atwater says, “so now I can make something that’s very thin and tunable, and at the nanometer scale. I could make an array of these little elements, each of which can convert the polarization into a different reflected polarization state.”

The liquid crystal display (LCD) technology found in phone screens and TVs already has some of those abilities, but black phosphorous tech has the potential to leap far ahead of it. The “pixels” of a black phosphorous array could be 20 times smaller than those in LCDs, yet respond to inputs a million times faster.

Such speeds are not necessary for watching a movie or reading an article online, but they could revolutionize telecommunications, Atwater says. The fiber-optic cable through which light signals are sent in telecommunications devices can transmit only so many signals before they begin to interfere with and overwhelm each other, garbling them (picture trying to hear what a friend is saying in a crowded and loud bar). But a telecommunications device based on thin layers of black phosphorous could tune 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 does now.

Atwater says the technology could also open the door to a light-based replacement for Wi-Fi, something researchers in the field refer to as Li-Fi.

“Increasingly, we’re going to be looking at light-wave communications in free space,” he says. “Lighting like this very cool-looking lamp above my desk doesn’t carry any communication signal. It just provides light. But there’s no reason that you couldn’t sit in a future Starbucks and have your laptop taking a light signal for its wireless communication rather than a radio signal. It’s not quite here yet, but when it gets here, it will be at least a hundred times faster than Wi-Fi.”

The lead author is Souvik Biswas, graduate student in applied physics. Other co-authors are Meir Y. Grajower, postdoctoral scholar research associate in applied physics and materials science, and Kenji Watanabe and Takashi Taniguchi of the NIMS-National Institute for Materials Science [物質・材料研究機構] (JP).

“These are exciting times for new materials discovery that can shape the future of photonic devices, and we have barely scratched the surface,” Biswas says. “It would be gratifying if some day you could buy a commercial product constructed out of such atomically thin materials, and that day might not be very far.”

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

See the full article here .


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The California Institute of Technology (US) is a private research university in Pasadena, California. The university is known for its strength in science and engineering, and is one among a small group of institutes of technology in the United States which is primarily devoted to the instruction of pure and applied sciences.

Caltech was founded as a preparatory and vocational school by Amos G. Throop in 1891 and began attracting influential scientists such as George Ellery Hale, Arthur Amos Noyes, and Robert Andrews Millikan in the early 20th century. The vocational and preparatory schools were disbanded and spun off in 1910 and the college assumed its present name in 1920. In 1934, Caltech was elected to the Association of American Universities, and the antecedents of National Aeronautics and Space Administration (US)’s Jet Propulsion Laboratory, which Caltech continues to manage and operate, were established between 1936 and 1943 under Theodore von Kármán.

Caltech has six academic divisions with strong emphasis on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. First-year students are required to live on campus, and 95% of undergraduates remain in the on-campus House System at Caltech. Although Caltech has a strong tradition of practical jokes and pranks, student life is governed by an honor code which allows faculty to assign take-home examinations. The Caltech Beavers compete in 13 intercollegiate sports in the NCAA Division III’s Southern California Intercollegiate Athletic Conference (SCIAC).

As of October 2020, there are 76 Nobel laureates who have been affiliated with Caltech, including 40 alumni and faculty members (41 prizes, with chemist Linus Pauling being the only individual in history to win two unshared prizes). In addition, 4 Fields Medalists and 6 Turing Award winners have been affiliated with Caltech. There are 8 Crafoord Laureates and 56 non-emeritus faculty members (as well as many emeritus faculty members) who have been elected to one of the United States National Academies. Four Chief Scientists of the U.S. Air Force and 71 have won the United States National Medal of Science or Technology. Numerous faculty members are associated with the Howard Hughes Medical Institute(US) as well as National Aeronautics and Space Administration(US). According to a 2015 Pomona College(US) study, Caltech ranked number one in the U.S. for the percentage of its graduates who go on to earn a PhD.

Research

Caltech is classified among “R1: Doctoral Universities – Very High Research Activity”. Caltech was elected to the Association of American Universities in 1934 and remains a research university with “very high” research activity, primarily in STEM fields. The largest federal agencies contributing to research are National Aeronautics and Space Administration(US); National Science Foundation(US); Department of Health and Human Services(US); Department of Defense(US), and Department of Energy(US).

In 2005, Caltech had 739,000 square feet (68,700 m^2) dedicated to research: 330,000 square feet (30,700 m^2) to physical sciences, 163,000 square feet (15,100 m^2) to engineering, and 160,000 square feet (14,900 m^2) to biological sciences.

In addition to managing JPL, Caltech also operates the Caltech Palomar Observatory(US); the Owens Valley Radio Observatory(US);the Caltech Submillimeter Observatory(US); the W. M. Keck Observatory at the Mauna Kea Observatory(US); the Laser Interferometer Gravitational-Wave Observatory at Livingston, Louisiana and Richland, Washington; and Kerckhoff Marine Laboratory(US) in Corona del Mar, California. The Institute launched the Kavli Nanoscience Institute at Caltech in 2006; the Keck Institute for Space Studies in 2008; and is also the current home for the Einstein Papers Project. The Spitzer Science Center(US), part of the Infrared Processing and Analysis Center(US) located on the Caltech campus, is the data analysis and community support center for NASA’s Spitzer Infrared Space Telescope [no longer in service].

Caltech partnered with University of California at Los Angeles(US) to establish a Joint Center for Translational Medicine (UCLA-Caltech JCTM), which conducts experimental research into clinical applications, including the diagnosis and treatment of diseases such as cancer.

Caltech operates several Total Carbon Column Observing Network(US) stations as part of an international collaborative effort of measuring greenhouse gases globally. One station is on campus.