An optical antenna developed by researchers at the University of California, Berkeley, can, in principle, provide limitless bandwidth. The technology takes advantage of orbital angular momentum (OAM), a characteristic of light that enables multiplexing, or simultaneous transmission, exponentially greater than current technology. According to Boubacar Kanté, the study’s principal investigator and the Chenming Hu Associate Professor at UC Berkeley’s Department of Engineering and Computer Sciences, current methods of transmitting signals through electromagnetic waves are approaching their limit. Frequencies are becoming clogged, and while the process and effect of polarization is able to double the amount information that can be multiplexed, there is need to scale up data communication capabilities beyond the capacities of current technologies. Artist depiction of the topological antenna containing three quantum wells in concentric circles, designed to create the quantum Hall effect, depicted above the antenna’s surface. Courtesy of Boubacar Kanté. “We've been experiencing an explosion of data in our world, and the communication channels we have now will soon be insufficient for what we need,” Kanté said. “The technology we are reporting overcomes current data capacity limits through a characteristic of light called the orbital angular momentum.” OAM, Kanté said, can be understood conceptually by comparing it to the vortex of a tornado. “The vortex in light, with its infinite degrees of freedom, can, in principle, support an unbounded quantity of data,” he said. “The challenge has been finding a way to reliably produce the infinite number of OAM beams. No one has ever produced OAM beams of such high charges in such a compact device before.” The researchers created a topological antenna by etching a grid pattern into indium gallium arsenide phosphide, a semiconductor material, which they then bonded onto a surface of yttrium iron garnet. The grid was designed to form quantum wells in a pattern of three concentric circles — with the largest being about 50 µm in diameter — to trap photons. The design created the necessary conditions for the photonic quantum Hall effect, which describes the movement of photons when a magnetic field is applied, forcing the light to travel in only one direction in the rings. “People thought the quantum Hall effect with a magnetic field could be used in electronics but not in optics because of the weak magnetism of existing materials at optical frequencies,” Kanté said. The new work showed that the quantum Hall effect does work for light. By applying a magnetic field perpendicular to their two-dimensional microstructure, the researchers generated three OAM laser beams. These beams traveled in circular orbits above the surface. Experimentation showed the beams had quantum numbers as high as 276, referring to the number of times light twists around its axis in one wavelength. The study demonstrated the capability at telecommunication wavelengths, though Kanté said it can, in concept, be adapted for other frequency bands; while the study created three beams, the potential number is limitless, he said. The lab will focus next on making quantum Hall rings that use electricity as a power source. The work was published in Nature Physics (www.doi.org/10.1038/s41567-021-01165-8).