Data limits may disappear with new optical antennas

Researchers at the University of California, Berkeley, have found a new way to take advantage of features of light waves that can drastically increase the amount of data they carry. They showed that the emission of discrete, rotating laser beams from antennas consists of concentric rings that are approximately equal to the diameter of a human hair, small enough to be placed on computer chips.

The new work, reported in a paper published in the magazine on Thursday 25 February Natural Physics, wide open the amount of information that can be multiplexed or transmitted simultaneously by a coherent light source. A common example of multiplexing is the sending of multiple telephone calls over a single wire, but there were fundamental limits to the number of contiguous twisted light waves that could be directly multiplexed.

“This is the first time that lasers that produce twisted light are directly multiplexed,” said the study’s chief investigator Boubacar Kanté, associate professor Chenming Hu at the Department of Electrical Engineering and Computer Science. “We’ve experienced an explosion of data in our world, and the communication channels we have now will soon be inadequate for what we need. The technology we report overcomes current data capacity limits by a feature of light that’s the angular momentum. “It is a game changer with applications in biological imaging, quantum cryptography, high capacity communication and sensors.”

Kanté, who is also a faculty scientist in the Department of Materials Science at Lawrence Berkeley National Laboratory (Berkeley Lab), continued this work at UC Berkeley after beginning his research at UC San Diego. The first author of the study is Babak Bahari, a former Ph.D. student in Kanté’s laboratory.

Kanté said that current methods of transmitting signals by electromagnetic waves reach their limit. For example, the frequency has become saturated, which is why there are only so many stations on which you can switch on the radio. Polarization, where light waves are separated into two values ​​- horizontal or vertical – can double the amount of information transmitted. Filmmakers use it when making 3D movies, allowing viewers with specialized glasses to receive two sets of signals – one for each eye – to create a stereoscopic effect and the illusion of depth.

Exploiting the potential in a vortex

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

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

The researchers started with an antenna, one of the most important components in electromagnetism, and, according to them, central to continuous 5G and upcoming 6G technologies. The antennas in this study are topological, meaning that their essential properties are retained even when the device is twisted or bent.

Create rings of light

To make the topological antenna, the researchers used electron beam lithography to etch a grid pattern on indium gallium arsenide phosphide, a semiconductor material, and then bond the structure to a surface of yttrium iron garnet. The researchers designed the grid to form quantum pits in a pattern of three concentric circles – the largest about 50 microns in diameter – to capture photons. The design created conditions to support a phenomenon known as the photonic quantum Hall effect, which describes the motion of photons when a magnetic field is applied, forcing the light to move in only one direction in the rings.

“People thought that 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. “We are the first to show 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 moving in circular orbits above the surface. The study further showed that the laser beams have quantum numbers as large as 276, referring to the number of times light rotates around its axis in one wavelength.

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

Kanté said the next step in his laboratory is to make quantum Hall rings that use electricity as power sources.