Californian scientists created a specialized meta­material to control light more precisely than ever before. The work was conducted in the lab of Harry Atwater at California Institute of Technology in Pasadena. Atwater and his colleagues 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 black phosphorous, which is similar in many ways to graphene. But whereas the layers of graphene are perfectly flat, black phos­phorous's layers are ribbed.

That crystal structure makes the black phosphorus have signi­ficantly anisotropic optical properties. “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”, Atwater says. 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.

Many materials can polarize light, though, and that ability alone is not especially useful. What makes black phosphorous special is that it is also a semiconductor. 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 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. 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.”

“These are exciting times for new materials discovery that can shape the future of photonic devices, and we have barely scratched the surface,” Souvik Biswas, graduate student in applied physics, 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.” (Source: Caltech)

Reference: S. Biswas et al.: Broadband electro-optic polarization conversion with atomically thin black phosphorus, Science 374, 448 (2021); DOI: 10.1126/science.abj7053

Link: Atwater Research Group, Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, USA