Vortex lasers could help photons carry more data, a new study finds.
Modern optical telecommunications encode data in multiple aspects of light, such as its brightness and color. In order to store even more data in light, scientists are exploring other properties of light that have proven more difficult to control.
One promising feature of light under investigation has to do with momentum. Light has momentum, just like a physical item moving through space, even though it does not have mass. As such, when light shines on an object, it exerts a force. Whereas the linear momentum of light exerts a push in the direction that light is moving, angular momentum of light exerts torque.
A beam of light can possess two kinds of angular momentum. The spin angular momentum of a ray of light can make objects it shines on rotate in place, whereas its orbital angular momentum can make objects rotate around the center of the ray. A beam of light that carries orbital angular momentum resembles a vortex, moving through space with a spiraling pattern like a corkscrew. Whereas a conventional light beam is brightest at its center, vortex beams have ringlike shapes that are dark in the center, due to how some of the waves making up vortex beams can interfere with one another.
A potentially extraordinarily useful property of vortex beams is that they do not interfere with each other if they all possess different twisting patterns. This means a theoretically infinite number of vortex beams can get overlaid on top of each other to carry an unlimited number of data streams at the same time.
However, until now, all microchip-scale vortex lasers firing at telecommunications wavelengths were each limited to transmitting a single orbital angular momentum pattern. At the same time, existing detectors for vortex beams relied on complex filtering techniques using bulky components, which prevented them from being integrated on chips and made them incompatible with most practical optical telecommunications approaches.
Now scientists at the University of Pennsylvania and their colleagues have made breakthroughs with both vortex lasers and vortex beam detectors. They detailed their findings in two studies in the 15 May issue of the journal Science.
The researchers began with a microring laser consisting of a ring of indium gallium arsenide phosphide only 7 microns in diameter in which light could flow in a loop, via a channel 650 nanometers wide. By varying the light pumped into this circle from microscopic arms on either side of this ring, the researchers could alter the orbital angular momentum of the beam emitted from the laser. Instead of emitting a single orbital angular momentum mode, they showed it could emit five distinct modes.
The scientists also developed a light detector based on tungsten ditelluride, which can act like a so-called Weyl semimetal, a material with properties lying between a conductive metal and a pure semiconductor. Their experiments found that different orbital angular momentum modes of light each generated unique patterns of electrical current within the photodetector, and they suggest this electronic method of detecting the orbital angular momentum of light could be scaled to work on microchips.
"By generating five different orbital angular momentum modes using our laser and sorting them with our detector, the data capacity of orbital angular momentum channels can be boosted by up to five times," says Liang Feng, an optical engineer at the University of Pennsylvania and lead author of the study describing the laser.
Ritesh Agarwal and Liang Feng, pictured in 2018. Photo: Scott Spitzer
"We now have both essential integrated elements—that is, both source and detector—for implementing high-capacity optical communication via orbital angular momentum modes," says Ritesh Agarwal, a materials scientist at the University of Pennsylvania and lead author of the study describing the detector.
In the future, instead of using lasers to tune the orbital angular momentum of the vortex beams, Feng says they could do it electrically, which could help better integrate these devices onto microchips. He also suggests they could increase the number of orbital angular momentum modes to which they could set the vortex beam to whatever they might like.
The scientists also plan on increasing the sensitivity of the detector to single photons so that it could serve in quantum communications and other quantum applications, Agarwal says. "However, it will be challenging to achieve high sensitivity and signal purity, so we will keep on searching for better material platforms and refining fabrication techniques."