A team of Caltech scientists announced in the journal Science the development of a new advance that uses a specialized material only three atoms thick and opens the doors to Li-Fi technology.
To understand work, it helps to first 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 on a boat floating in the ocean – ocean waves are vertically polarized, which means that when the waves pass under the boat, they rise and fall. Light waves behave in the same way, except that these waves can be polarized at any angle. If a ship could ride light waves, it could rock back and forth, or diagonally, or even in a spiral fashion.
Polarization can be useful because it allows you to control light in specific ways. For example, the lenses of your sunglasses block glare (light often polarizes 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 areas. Those areas where polarized light is blocked appear dark, while areas where light is not blocked appear light.
The properties of black phosphorus
In the study, Harry Atwater, professor of Applied Physics and Materials Science, and his co-authors describe how they used three layers of phosphor atoms to create a material for polarizing light that is tunable, precise, and extremely thin.
The material is built from so-called black phosphorus, which is similar in many ways to graphite or graphene, forms of carbon that consist of single-atom-thick layers. But while the graphene layers are perfectly flat, the black phosphor layers have ribs, like the texture of a pair of corduroy pants or corrugated cardboard. (Phosphorus also comes in red, white, and purple forms, distinct due to the arrangement of the atoms within it.)
That crystalline structure, Atwater says, makes black phosphorus significantly anisotropic optical properties. “Anisotropy means that it depends on the angle,” he explains in a statement. “In a material like graphene, light is absorbed and reflected equally regardless of the angle at which it is polarized. The black phosphor is very different in the sense that if the polarization of the light is aligned along the corrugations, has a very different response than is aligned perpendicular to the corrugations. “
When polarized light is oriented through the black phosphor corrugations, it interacts with the material differently than when it is oriented along the corrugations, kind of like the way it is easier to rub the hand along the ribs in corduroy to rub it through them.
However, many materials can polarize light, and that ability alone is not particularly useful. What makes black phosphorus 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 tiny structures made of silicon can control the flow of electricity in a microchip, structures made of black phosphor can control the polarization of light when an electrical signal is applied to them.
“These little structures are doing this polarization conversion,” Atwater says, “so now I can make something that is very thin and tunable, and on 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. “
Improved LCD screens
The liquid crystal display (LCD) technology found in phone and television screens already has some of those capabilities, but black phosphor technology has the potential to be far ahead. The “pixels” of a black phosphor matrix could be 20 times smaller than those of LCD screens, but respond to inputs a million times faster.
These speeds aren’t necessary to watch a movie or read an article online, but they could revolutionize telecommunications, Atwater says. The fiber optic cable through which the light signals are sent in telecommunication devices can only transmit a limited number of signals before they start to interfere and overwhelm each other, confusing them (image trying to hear what a friend is saying in a crowded and noisy bar). But a telecommunications device based on thin layers of black phosphor 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.
¿Bye bye wifi?
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 will analyze light wave communications in free space,” he says. “Lighting like this very cool looking lamp on my desk doesn’t transmit any communication signals. It just provides light. But there’s no reason why you can’t sit in a future Starbucks and have your laptop receive a light signal. for your wireless connection instead of a radio signal. It’s not here yet, but when it arrives, it will be at least a hundred times faster than Wi-Fi. “
