Knowledge boundaries might disappear with new optical antennas and “rings of sunshine”
New research is opening up the amount of information that can be transmitted simultaneously from a single light source.
Researcher at the University of California, Berkeleyfound a new way to harness the properties of light waves that can radically increase the amount of data they carry. They demonstrated the emission of discrete twisted laser beams from antennas made up of concentric rings roughly the diameter of a human hair and small enough to be placed on computer chips.
The new work, published in an article published February 25, 2021 in the journal Nature Physics, opens up the amount of information that can be multiplexed or transmitted simultaneously from a coherent light source. A common example of multiplexing is the transmission of multiple phone calls over a single line. However, the number of coherent twisted light waves that could be directly multiplexed was fundamentally limited.
“It is the first time that lasers that produce twisted light are directly multiplexed,” said study director Boubacar Kanté, associate professor of Chenming Hu at the Department of Electrical and Computer Science at UC Berkeley. “We have seen an explosion of data in our world, and the communication channels we have now will soon no longer be sufficient to achieve what we need. The technology we report on overcomes current data capacity limits through a property of light known as angular momentum. It is a milestone for applications in biological imaging, quantum cryptography, high capacity communications, and sensors. “
Kanté, who is also a faculty scientist in the materials science department at Lawrence Berkeley National Laboratory (Berkeley Lab), continued that work at UC Berkeley after starting research at UC San Diego. The study’s first author is Babak Bahari, a former Ph.D. Student in Kanté’s laboratory.
Kanté said that current methods of transmitting signals using electromagnetic waves are reaching their limits. For example, the frequency is saturated, which is why there are only so many stations that you can tune into on the radio. Polarization, in which light waves are divided into two values - horizontal or vertical – can double the amount of information transmitted. Filmmakers take advantage of this when creating 3D movies so that viewers wearing special glasses can receive two sets of signals – one for each eye – to create a stereoscopic effect and the illusion of depth.
Using the potential in a vortex
Beyond frequency and polarization, however, lies the orbital angular momentum (OAM), a property of light that scientists have noticed because it offers an exponentially greater capacity for data transmission. One way to think about OAM is to compare it to the vortex of a tornado.
“The vortex of light with its infinite degrees of freedom can in principle support an unlimited amount of data,” said Kanté. “The challenge was to find a way to reliably generate the infinite number of OAM beams. Never before has anyone produced OAM beams with such high charges in such a compact device. “
Researchers started with an antenna, one of the most important components of electromagnetism, and they found it to be central to the ongoing 5G and upcoming 6G technologies. The antennas in this study are topological, which means that their essential properties are retained even if the device is twisted or bent.
Create rings of light
To create the topological antenna, the researchers used electron beam lithography to etch a grid pattern onto indium gallium arsenide phosphide, a semiconductor material, and then tied the structure onto a surface made from yttrium iron garnet. The researchers designed the lattice to form quantum wells in a pattern of three concentric circles – the largest around 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 movement of photons when a magnetic field is applied, forcing the light to move in only one direction in the rings.
“People thought the quantum Hall effect with a magnetic field could be used in electronics but not optics because existing materials have weak magnetism 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 its two-dimensional microstructure, the researchers were able to successfully generate three OAM laser beams that move in circular paths over the surface. The study also showed that the laser beams had quantum numbers of up to 276, based on the frequency with which light rotates around its axis in a wavelength.
“Having a larger quantum number is like having more letters in the alphabet,” said Kanté. “We allow light to expand its vocabulary. In our study we demonstrated this capability at telecommunications wavelengths, but in principle it can be adapted to other frequency bands. Although we created three lasers with the data rate multiplied by three, there is no limit to the number of beams that can be used or the data capacity. “
Kanté said the next step in his lab would be to make quantum Hall rings that use electricity as a power source.
Reference: February 25, Natural Physics.
DOI: 10.1038 / s41567-021-01165-8
This research was supported primarily by the Office of Naval Research, the National Science Foundation, and the Berkeley Lab’s Laboratory Directed Research and Development Program.