Directing light from one place to another is the backbone of our modern information world. Fiber optic cables cross deep oceans and vast continents, carrying light that contains everything from YouTube video clips to bank transfers in hair-thin fibers. However, Professor Jiwoong Park of the University of Chicago wanted to know what would happen if the fiber was made thinner and flatter - so thin that it was actually two-dimensional instead of three-dimensional. What happens to light?
University of Chicago scientists have discovered that a glass crystal just a few atoms thick can capture and carry light -- and could be used in a variety of applications. The thin thread in the center of the plastic held by study co-author Hanyu Hong is this material. Image source: Jean Lachat
Through a series of innovative experiments, he and his team discovered that thin slices of glass crystals just a few atoms thick can capture and carry light. Not only that, it's surprisingly efficient and can travel a relatively long distance - one centimeter, which is very far in the world of light-based computing.
Professor Jiwoong Park (left) and scientist Hanyu Hong (right) at the Laser Laboratory, where they confirmed that the material can carry light - even though it is smaller than the light itself. Image source: Jean Lachat
The research, recently published in the journal Science, demonstrates what is essentially a two-dimensional photonic circuit and could open the way to new technologies.
"We were completely surprised by how powerful this ultrathin crystal is; not only can it hold energy, but it can also deliver it a thousand times farther than anyone has seen in similar systems. The trapped light also behaves as if it is traveling in two-dimensional space," said Jiwoong Park, lead author of the study and professor and chair of the Department of Chemistry at the James Franck Institute and Pritzker School of Molecular Engineering.
guide light
The newly invented system is a method of guiding light, known as a waveguide, that is two-dimensional in nature. In tests, the researchers found that they could direct light along the chip's path using extremely tiny prisms, lenses and switches - all elements of circuitry and computing.
Photonic circuits already exist, but they are much larger and three-dimensional. Crucially, in existing waveguides, light particles - so-called photons - always propagate enclosed within the waveguide.
The scientists explain that in this system, the glass crystal is actually thinner than the photon itself, so part of the photon actually sticks out of the crystal as it travels.
Professor Jiwoong Park (left) and scientist Hanyu Hong (right) examine the material in Park's laboratory at the University of Chicago. In tests, they can use tiny prisms, lenses and switches to direct light along the chip's path - all elements of circuitry and computing. Photo credit: Jean-Rachat
It's a bit like building a tube to carry suitcases in an airport versus putting suitcases on a conveyor belt. On the conveyor belt, the suitcases are open-air and you can easily see and adjust them on the way. This approach makes it easier to create complex devices from glass crystals, since light can move easily through lenses or prisms.
Photons can also experience information about conditions along the way. Think about it, checking your suitcase coming in from outside to see if it's snowing outside. Likewise, scientists could imagine using these waveguides to create microscopic-level sensors.
"Say you have a sample of liquid and you want to sense the presence of a specific molecule," Park explains. "You can design it so that the waveguide passes through the sample, and the presence of that molecule will change the behavior of the light."
Scientists are also interested in building very thin photonic circuits that can be stacked on top of each other to integrate more tiny devices on the same chip area. The glass crystal they used in these experiments was molybdenum disulfide, but the principle applies to other materials as well.
While theoretical scientists have predicted that this behavior should exist, actually achieving it in the lab has been a years-long process, the scientists said.
"This was an extremely challenging but satisfying problem because we entered a completely new field. Therefore, everything we needed had to be designed ourselves—from growing the material to measuring how light moves," said Hanyu Hong, a graduate student and co-first author of the paper.