Controlling light with a material three atoms thick

Artist's rendering of light reflecting off a black phosphorus surface, which alters its Polarization. Credit: Caltech
We all control the light we see every day without thinking about it. Most people don't even need to think about it.

However, light control can be achieved in highly-technical ways. This is just one example of how light can be controlled on a computer screen, tablet or phone. Telecommunications is another example. It controls light to create signals which carry data along fiber optic cables.

Scientists use high-tech methods to control the light in the laboratory. Now, scientists can do this more precisely thanks to a breakthrough that uses three atoms thick specialized materials.

This work was done in the laboratory of Harry Atwater (Otis Booth Leadership Chair of Division of Engineering and Applied Science), Howard Hughes Professor of Applied Physics and Materials Science and director of Liquid Sunlight Alliance). The paper was published in Science on October 22nd, 2012.

It is important to understand that light exists in waves and that polarization describes how the waves vibrate. Imagine yourself in a boat bobbling on the ocean. Ocean waves have vertical polarization. This means that the waves move under the boat and up and down. The same behavior applies to light waves, but these waves can also be polarized at any angle. A boat can ride waves of light and it may bob in any direction, including on a diagonal or spiraling.

Polarization is the direction in which waves (including light) vibrate. You can change the angle of polarization. Credit: Smouss/Wikimedia commons

Because it allows light to controlled in certain ways, polarization can be very useful. The lenses of your sunglasses block glare. Light often becomes polarized as it reflects off surfaces like windows in cars. A desk calculator's screen creates legible numbers by blocking out certain areas and polarizing light. The dark areas that have the polarized light blocked are those areas which appear dark while the areas with the light not blocked are light.

Atwater and his coauthors explain how three layers of phosphorous molecules were used to create a material that polarizes light. It is extremely thin, precise, and tunable.

This material is made from black phosphorous. It is similar to graphite or graphene forms of carbon, which consist of single-atom-thick layers. Black phosphorous layers are not flat like graphene's layers. They are more like a pair corduroy pants or corrugated card. (Phosphorus can also be found in white, red, and violet forms. This is due to the way the atoms are arranged within it.

Atwater claims that the crystal structure of black phosphorus makes it have significant anisotropic optical characteristics. He explains that anisotropy is the fact that it is angle dependent. "Graphene is able to absorb and reflect light at any angle, no matter how it's polarized. Black phosphorus has a different response to light if it is aligned along corrugations.

Polarized light interacts differently with material when it is directed across corrugations in blackphosphorous. It is similar to how it is easier for you to rub your hands along corduroy ribs than to rub them across.

A calculator display that uses the properties polarized light to create dark and light areas that can be read as numbers. Credit: David R. Tribble/Wikimedia commons

Although many materials are capable of polarizing light, it is not an essential property. Atwater said that black phosphorous is a semiconductor. This means that it conducts electricity more efficiently than an insulator like glass but not as well a metal such as copper. A semiconductor is the silicon found in microchips. Just as tiny structures made from silicon can control electricity flow in microchips, structures made from black phosphorous can also control the polarization light when an electric signal is applied.

Atwater states that "these tiny structures are doing the polarization conversion." He says, "so now, I can make something very thin and tunable at the nanometer level." These little elements can each convert the polarization to a different reflected state. I could make a whole array.

Although liquid crystal display (LCD), technology used in TVs and phones already has some of these capabilities, black phosphorous tech could be far more advanced. Black phosphorous arrays could have "pixels" 20 times smaller than LCDs but respond to inputs one million times faster.

Atwater states that such speeds are not required for viewing movies or reading articles online. However, they could revolutionize telecommunications. Fiber-optic cables that transmit light signals in telecommunications devices only can transmit so many signals before they become interconnected and overwhelmed, garbling them. (Imagine trying to hear a friend in a noisy bar. A telecommunications device that is based on thin layers black phosphorous could adjust the polarization so that no signal interferes with another. This would enable a fiber-optic cable carry more data than it currently does.

Atwater believes that the technology could also be used to replace Wi-Fi with a light-based alternative, which researchers in the field call Li-Fi.

Ribbed sheets of black phosphorus are similar to this corduroy fabric. Credit: Ariel Glenn/Wikimedia Commons

He says, "Increasingly we're going be looking at lightwave communications in free space." "Lighting such as this cool-looking lamp that hangs above my desk does not carry any communication signal. It only provides light. It doesn't mean that your laptop can't receive a light signal to its wireless communication from a future Starbucks. Although it's not yet here, it will arrive at least 100 times faster than Wi Fi.

The paper that describes the work is called "Broadband Electro-Optic Polarization Conversion with Atomically Thin Black Phosphorus." Souvik Biswas is a graduate student in applied Physics. Meir Yo. Grajower is a postdoctoral scholar researcher in applied physics. Kenji Watanabe, Takashi Taniguchi, and Takashi Taniguchi are also co-authors.

Biswas states that these are exciting times in new materials discovery, which can help shape the future photonic devices. Biswas believes they have only scratched the surface. It would be wonderful if you could one day buy a product made from such atomically-thin materials. That day may not be far away.

Continue reading Investigating optical activity in an external magnetic field

More information: Science Souvik Biswas and colleagues, Broadband electrooptic polarization transformation with atomically-thin black phosphorus, Science (2021). Information from Science Souvik Biswas and colleagues, Broadband electrooptic polarization transformation with atomically-thin black phosphorus. Journal (2021). DOI: 10.1126/science.abj7053