Tiny gears that twist light enable huge bandwidth when printed on microchips

Tiny gears that twist light will enable optical computing with a huge bandwidth when 250,000 of them are printed on one square millimetre of a microchip.

Lasers on a microchip are needed to realise optical computing and its high bandwidth data transfer rates for faster computation. Silicon’s light emission efficiency is not good enough for it to emit laser light. But Germanium can be engineered to emit light efficiently and is compatible with silicon and the lithographic process used to make microchips. Twisting a wavelength of Germanium emitted light like a corkscrew can make it represent a value or symbol. While a laser can send data in the form of a varied number of photons being emitted or by switching between light’s two polarisation states, each twist of that wavelength now represents one bit; and the number of twists could be many.

“Now, I can use blue with one twist or blue with two or three twists and this is another dimension of multiplexing,” says Abdelrahman Al-Attili, a researcher at the University of Southampton’s school of electronics and computer science. “The same optical fibre for three signals can now send nine, or twelve or multiples of the twists I have.”

The twist or change in the orbital angular momentum of the light is achieved by making germanium gears, tiny structures with radii of less than a micron, that have teeth. Al-Attili and his fellow researchers have shone a green laser light onto the micro-gears, and they emit a light beam with the twist each individual gear’s dimensions has been designed for.

Caption: This imagery represents the different twists that could be given to each micro-gear. Credit: University of Southampton

The gear designs were improved with simulations, using computers to understand how the light propagates in the gears in less than a nanosecond. The researchers would then compare the prototype gears’ light emission with the results of the simulation to determine if the correct twist had been achieved. Essentially, the gear generates its own photons that then circulate around the gear’s edges, forming twisted light after less than a nanosecond, and that is then reflected vertically away from the gear by its own teeth.

To make germanium emit a light beam, it is put under strain by stretching it. To help germanium produce this coherent light beam it also needs to be protected from heat. Doping, the adding of a chemical to improve conductivity is also another option the researchers could investigate.

The next step for Al-Attili and his researchers is to get the germanium to emit laser light when it receives electrons from an electrical supply like a normal electronic circuit. This would enable optical computing to be introduced into mobile phone and other computing device applications, with the 250,000 gears or more on each microchip. Al-Attili added that for optical communications the micro-gear technology is already ready for commercialisation because they would be activated by a laser light for this purpose; just like the laboratory experiments.

Caption: Doctoral student Abdelrahman Al-Attili works on germanium lasers that could enable optical computing on microchips. Credit: University of Southampton

This research was supported by the United Kingdom’s Engineering and Physical Sciences Research Council, and was carried out in collaboration with Hitachi, the University of Tokyo and the Toyohashi University of Technology.