Data limits could disappear with new optical antennas

Researchers at the University of California at Berkeley have found a new way to harness the properties of light waves that can dramatically increase the amount of data they carry. They demonstrated the emission of discrete torsion laser beams from antennas made up of concentric rings roughly equal to the diameter of a human hair, small enough to be placed on computer chips.

The new work, reported in an article published Thursday, February 25 in the journal Physics of nature, opens wide the amount of information that can be multiplexed, or transmitted simultaneously, by a coherent light source. A common example of multiplexing is the transmission of multiple telephone calls over a single wire, but there were fundamental limits to the number of coherent twisted light waves that could be directly multiplexed.

“This is the first time that lasers producing twisted light have been directly multiplexed,” said Boubacar Kanté, principal investigator of the study, associate professor Chenming Hu in the Department of Electrical Engineering and Computer Science at UC Berkeley. “We have experienced an explosion of data in our world and the communication channels we have now will soon be insufficient for what we need. The technology we are reporting is breaking current limits on data capacity with a feature of light called orbital angular momentum. . This is a game-changer with applications in biological imaging, quantum cryptography, high-capacity communications and sensors. “

Kanté, who is also a professor in the Materials Sciences Division at the Lawrence Berkeley National Laboratory (Berkeley Lab), continued this work at UC Berkeley after starting research at UC San Diego. The first author of the study is Babak Bahari, a former Ph.D. student in Kanté’s lab.

Kanté said that current methods of transmitting signals using electromagnetic waves are reaching their limits. The frequency, for example, has become saturated, which is why there is only a limited number of stations that can be tuned to on the radio. Polarization, where light waves are split into two values ​​- horizontal or vertical – can double the amount of information transmitted. Filmmakers take advantage of this when creating 3D movies, allowing viewers equipped with specialized glasses to receive two sets of signals – one for each eye – to create a stereoscopic effect and the illusion of depth.

Harnessing the potential of a vortex

But beyond frequency and polarization, there is orbital angular momentum, or OAM, a property of light that has caught the attention of scientists because it offers exponentially greater capacity for data transmission. One way to think of OAM is to compare it to a tornado vortex.

“The vortex of light, with its infinite degrees of freedom, can, in principle, withstand an unlimited amount of data,” Kanté said. “The challenge has been to find a way to reliably produce the infinite number of OAM beams. No one has ever produced OAM beams of such high loads in such a compact device before.”

The researchers started with an antenna, one of the most important components of electromagnetism and, they noted, at the heart of current 5G and 6G technologies. The antennas in this study are topological, meaning that their essential properties are maintained even when the device is twisted or bent.

Create rings of light

To fabricate the topological antenna, the researchers used electron beam lithography to etch a grid pattern on indium gallium arsenide phosphide, a semiconductor material, and then bonded the structure to a yttrium and iron garnet surface. The researchers designed the grid to form quantum wells in a pattern of three concentric circles – the largest about 50 microns in diameter – to trap photons. The design created the conditions to withstand a phenomenon known as the Photonic Quantum Hall Effect, which describes the movement of photons when a magnetic field is applied, forcing light to travel in a single direction in the rings.

“People thought that the quantum Hall effect with a magnetic field could be used in electronics but not in optics due to the weak magnetism of existing materials at optical frequencies,” Kanté said. “We are the first to show that the quantum Hall effect works for light.”

By applying a magnetic field perpendicular to their two-dimensional microstructure, the researchers succeeded in generating three OAM laser beams moving in circular orbits above the surface. The study further showed that laser beams have quantum numbers as large as 276, referring to the number of twists of light around its axis in a wavelength.

“Having a bigger quantum number is like having more letters to use in the alphabet,” Kanté said. “We allow the light to expand its vocabulary. In our study, we demonstrated this capability at telecommunication wavelengths, but in principle it can be adapted to other frequency bands. Even though we’ve created three lasers, multiplying the data rate by three, there is no limit to the possible number of beams and the data capacity. “

Kanté said the next step in his lab is to make quantum Hall rings using electricity as a source of energy.