Twisting Light for Data Transmission

For the first time, researchers from the Uni­versity of Southampton in the UK and Uni­versity of Tokyo, Toyohashi Uni­versity of Tech­nology and Hitachi Ltd., all in Japan, have used tiny gears made of germanium to generate a vortex of twisted light that turns around its axis of travel much like a corkscrew. Because germanium is compatible with the silicon used to make computer chips, the new light source could be used to boost the amount of data that can be trans­mitted with chip-based optical computing and communi­cation. With a radius of one micron or less, 250,000 of the gears could be packed into just one square millimeter of a computer chip.

Researchers created tiny gears that generate a vortex of twisted light. One square millimeter of a computer chip could hold 250,000 of these gears, which could be used to boost the amount of data transmitted with chip-based optical computing and communication. (Source: A. Al-Attili, Univ. Southampton)

There is a great deal of interest in generating light that is twisted, or has orbital angular momentum, because of its advantages for communi­cations and computing. Today, light is used to carry information by varying the number of photons emitted or switching between light’s two polari­zation states. With twisted light, each twist can represent a different value or letter, allowing the encoding of a great deal more infor­mation using less light. “Our new micro­gears hold the potential for a laser that can be inte­grated on a silicon substrate – the last component needed to create an inte­grated optical circuit on a computer,” said Abdel­rahman Al-Attili from the Uni­versity of Southampton. “These tiny optical-based circuits use twisted light to transmit large amounts of data.”

It has been impossible to make a usable minia­turized light source on silicon, the material commonly used to make computer chips and associated components, because the material’s pro­perties led to poor light-generating effi­ciency. Although germanium has similar limi­tations, applying strain by stretching it can improve its light emission effi­ciency. “Previously, the strain that could be applied to germanium was not large enough to effi­ciently create light without degrading the material,” said Al-Attili. “Our new micro­gear design helps overcome this challenge.”

The new design features micro­gears that are free­standing at the edges so that they can be stretched by an oxide film deposited over the structures. This allows tensile strain to be applied without breaking the germanium’s crystal structure. The gears stand on a silicon pedestal that connects it to the top of the silicon substrate and allows heat to dissi­pate during operation.

To demon­strate their new design, the researchers used electron beam litho­graphy to fabricate the very fine physical features that form the gears’ teeth. They then illu­minated the gears with a standard green laser that did not emit twisted light. After the micro­gear absorbed the green light it generates its own photons that are circu­lated around the edges forming twisted light that is reflected verti­cally out of the gear by the periodic teeth.

The researchers tested and tweaked their design using computer simu­lations that model the way light propa­gates in the gears over nano­seconds or even shorter time periods. By comparing the prototype’s light emission with computer simu­lation results, they were able to confirm that the gears generated twisted light. “We can precisely design our device to control the number of rota­tions per propa­gation wave­length and the wave­length of the emitted light,” said Al-Attili.

The researchers are now working to further improve the effi­ciency of light emission from the germanium micro­gears. If successful, this tech­nology would make it possible to inte­grate thousands of lasers onto a silicon chip for trans­mitting infor­mation. “Silicon fabri­cation tech­nologies that were developed to make elec­tronic devices can now be applied to make various optical devices,” said Al-Attili. “Our microgears are just one example of how these capa­bilities can be used to make nano- and micro­scale devices.”(Source: OSA)

Reference: A. Z. Al-Attili et al.: Germanium vertically light-emitting micro-gears generating orbital angular momentum, Opt. Exp. 26, 34675 (2018); DOI: 10.1364/OE.26.034675

Link: Sustainable Electronic Technologies, Electronics & Computer Science, University of Southampton, Southampton, UK

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