Photon diodes -- devices that allow light to flow in only one direction -- are ubiquitous in modern electronics and can be found in LEDs, solar cells and integrated circuits used in computing and communications. But most diodes are not small enough for consumer electronics.
To that end, researchers Mark Lawrence and Jennifer Dionne made a compact, efficient diode that can set the stage for next-generation computing, communication and energy conversion technologies.
"One grand vision is to have an all-optical computer where electricity is replaced completely by light and photons drive all information processing," said Lawrence, a postdoctoral scholar of materials science and engineering. "The increased speed and bandwidth of light would enable faster solutions to some of the hardest scientific, mathematical and economic problems."
The researchers detailed their findings in the journal Nature Communications.
There are two main challenges in creating photon diodes. First, as stated by the laws of thermodynamics, light should move forward through an object with no moving parts in the exact same way it would move backward. Making it flow in one direction requires new materials that break this law, which is known as time-reversal symmetry. Second, light is much more difficult to manipulate than electricity since it doesn't have a charge.
Other researchers previously addressed these challenges by creating a so-called Faraday isolator. This three-part device works by running light through a polarizer, which makes light waves oscillate in a uniform direction, creating polarized light.
The light is then beamed through a crystalline material within a magnetic field, which causes light waves to rotate several degrees to angle their orientation. Another polarizer is then used to usher the light out with near-perfect transmission. If light is run through the device from in a backward direction, no light gets out.
Lawrence likened this one-way action of a Faraday isolator to a moving sidewalk between two doors. If someone tried to go back through the last door, the one-way sidewalk would prevent the person from reaching the first door.
But to sufficiently rotate the polarization of light, diodes that use a Faraday isolator must be relatively large – much too large to fit into smartphones and consumer computers. To that end, Lawrence and Dionne used another light beam instead of a magnetic field to rotate polarized light and at the same time reduce the size of the rotator.
This beam is polarized so that its electric field takes on a spiral motion. This, in turn, generates rotating acoustic vibrations in the crystalline material, giving it magnetic-like spinning abilities and allowing more light to get transmitted.
The researchers then used tiny nano-antennas and nanostructured materials called metasurfaces to create arrays of ultra-thin silicon disks. These disks work in pairs to trap light and enhance its spiraling motion until it finds its way out, causing more light to travel in a forward direction. When light is run through the device in a backward direction, the acoustic vibrations spin back, canceling out any light trying to exit.
In theory, there was no limit to how small this system could be, but based on their simulations, the researchers imagined structure as thin as 250 nanometers. For reference, a sheet of paper is about 100,000 nanometers thick. (Related: "Ultra-thin" gold is a MILLION times thinner than a fingernail, but has applications in medicine and water purification.)
In all, the researchers said that their photon diode could lay the groundwork for several cutting-edge technologies, including all-optical computers, all-optical chips and neuromorphic computers that perform computations by mimicking how the brain works.
Computing.news has more on photon diodes and other computing technologies.