Data limits can disappear with new optical antennas

Researchers at the University of California, 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 rotating 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 a paper published in the journal Thursday, Feb. 25 Natural physics, throws wide open the amount of information that can be multiplexed or simultaneously transmitted by a coherent light source. A well-known example of multiplexing is the transmission of multiple telephone calls over a single wire, but there were fundamental limitations to the number of coherent twisted light waves that could be multiplexed directly.

“It is the first time that lasers that produce rotated light have been multiplexed directly,” said lead researcher Boubacar Kanté, the Chenming Hu associate professor in UC Berkeley’s Department of Electrical Engineering and Computer Sciences. “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 report is overcoming current data capacity limits through a characteristic of light that called the orbital angular momentum. It’s a game-changer with applications in biological imaging, quantum cryptography, high-capacity communications and sensors. “

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

Kanté said the current methods of transmitting signals via electromagnetic waves are reaching their limits. For example, the frequency has become saturated and so there are only a limited number of stations that you can tune into on the radio. Polarization, where light waves are separated into two values ​​- horizontal or vertical – can double the amount of information sent. Filmmakers take advantage of this when creating 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.

Harnessing the potential in a whirlpool

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

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

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

Creating rings of light

To create 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 onto a yttrium iron garnet surface. The researchers designed the grid to form quantum wells in a pattern of three concentric circles – the largest with a diameter of about 50 microns – to collect photons. The design created conditions to support 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 the rings in only one direction.

“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,” said Kanté. “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 successfully generated three OAM laser beams that travel in circular orbits above the surface. The study further showed that the laser beams had 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 to use in the alphabet,” said Kanté. “We allow light to expand its vocabulary. In our study we demonstrated this capability on telecommunication wavelengths, but in principle it can be adapted to other frequency bands. Even though we made three lasers, multiplying the data rate by three. , there is no limit to the possible number of bundles and data capacity. “

Kanté said the next step in his lab is to create quantum Hall rings that use electricity as an energy source.