New Optical Antennas for Vortices of Light

Illustration of the emission of discrete twisting laser beams from antennas made up of concentric rings. (Source: B. Kanté)

Researchers at the University of California, Berkeley, have found a new way to harness properties of light waves that can radi­cally increase the amount of data they carry. They demonstrated the emission of discrete twisting laser beams from antennas made up of concentric rings, small enough to be placed on computer chips. The new work throws wide open the amount of information that can be multiplexed, or simul­taneously transmitted, by a coherent light source. A common example of multi­plexing is the transmission of multiple telephone calls over a single wire, but there had been funda­mental limits to the number of coherent twisted lightwaves that could be directly multiplexed.

“It’s the first time that lasers producing twisted light have been directly multiplexed,” said study principal inves­tigator Boubacar Kanté, the Chenming Hu Associate Professor at UC Berkeley’s Department of Electrical Engineering and Computer Sciences. “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. The technology we are reporting overcomes current data capacity limits through a charac­teristic of light, the orbital angular momentum. It is a game-changer with appli­cations in biological imaging, quantum crypto­graphy, high-capacity communications and sensors.”

Kanté said that current methods of transmitting signals through electro­magnetic waves are reaching their limit. Frequency, for example, has become saturated, which is why there are only so many stations one can tune into on the radio. Polarization, where lightwaves are separated into two values – horizontal or vertical – can double the amount of information trans­mitted. 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.

But beyond frequency and polari­zation is orbital angular momentum, or OAM, a property of light that has garnered attention from scientists 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 light, with its infinite degrees of freedom, can, in principle, support an unbounded quantity of data,” said Kanté. “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 started with an antenna, one of the most important components in electro­magnetism and, they noted, central to ongoing 5G and upcoming 6G technologies. The antennas in this study are topo­logical, which means that their essential properties are retained even when the device is twisted or bent. To make the topological antenna, the researchers used electron-beam litho­graphy to etch a grid pattern onto indium gallium arsenide phosphide, a semi­conductor material, and then bonded the structure onto a surface made of yttrium iron garnet. The researchers designed the grid to form quantum wells in a pattern of three concentric circles to trap 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 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,” said Kanté. “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 success­fully generated three OAM laser beams traveling 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 twists around its axis in one wavelength.

“Having a larger quantum number is like having more letters to use in the alphabet,” said Kanté. “We’re allowing 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 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 lab is to make quantum Hall rings that use elec­tricity as power sources. (Source: UCB)

Reference: B. Bahari et al.: Photonic quantum Hall effect and multiplexed light sources of large orbital angular momenta, Nat. Phys., online 25 February 2021; DOI: 10.1038/s41567-021-01165-8

Link: Dept. of Electrical and Computer Engineering, University of California San Diego, La Jolla, USA

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