Researchers have discovered a 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 roughly equal to the diameter of a human hair — small enough to be placed on computer chips.
The work will significantly increase 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 light waves that could be directly multiplexed.
The technology overcomes current data capacity limits through a characteristic of light called the orbital angular momentum. It has applications in biological imaging, high-capacity communications, and sensors.
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 light waves 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.
Beyond frequency and polarization is orbital angular momentum (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. The challenge has been finding a way to reliably produce the infinite number of OAM beams.
The researchers started with an antenna, one of the most important components in electromagnetism and central to ongoing 5G and upcoming 6G technologies. The antennas in this study are topological — 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. They 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 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.
By applying a magnetic field perpendicular to the 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.