Microgears Made from Germanium Generate Twisted Light
SOUTHAMPTON, England, Dec. 31, 2018 — Miniature gears made from germanium have demonstrated the ability to generate light with orbital angular momentum (OAM). This new light source could be used to boost the amount of data that can be transmitted through optical computing. Light carries information by varying the number of photons emitted or switching between two polarization states. With OAM, or twisted light, each twist can represent a different value or letter, allowing the encoding of more information using less light.
Researchers from the University of Southampton, the University of Tokyo, Toyohashi University of Technology, and Hitachi Ltd. built the tiny gears on silicon pillars using germanium-on-silicon-on-insulator (Ge-on-SOI) wafers with a high-crystalline-quality germanium layer. The microgears are freestanding at their edges and can be stretched by an oxide film deposited over the structures. This feature allows tensile strain to be applied without breaking the germanium’s crystal structure. The gears stand on silicon pillars that connect them to the top of the silicon substrate and allow heat to dissipate during operation. With a radius of 1 μm or less, 250,000 microgears can be packed into just 1 sq mm 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. Courtesy of Abdelrahman Al-Attili, University of Southampton.
So far, generating a miniaturized light source on silicon has been hampered by silicon’s inefficiency as a light-generating material. Although germanium has similar limitations, applying strain by stretching it can improve its light emission efficiency, the researchers said.
“Previously, the strain that could be applied to germanium was not large enough to efficiently create light without degrading the material,” said researcher Abdelrahman Al-Attili. “Our new microgear design helps overcome this challenge.”
To demonstrate their new design, the researchers used electron beam lithography to fabricate the 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 generated its own photons, which circulated around the edges of the gear, forming twisted light that was reflected vertically out by the gear’s teeth.
The team tested and tweaked its design using computer simulations to model the way light propagates in the gears over nanoseconds and even shorter time periods. By comparing the prototype’s light emission with computer simulation results, the researchers were able to confirm that the gears generated twisted light.
Abdelrahman Al-Attili, from the University of Southampton, designed the new microgears and developed methods for fabricating the gears from germanium. Courtesy of University of Southampton.
“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 working to further improve the efficiency of light emission from the germanium microgears. If successful, this technology could make it possible to integrate thousands of lasers onto a silicon chip for transmitting information.
“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,” Al-Attili said.
The researchers believe that their microgears represent the first demonstration of OAM generation within a germanium light source. They further believe that germanium microgears show potential as CMOS-compatible OAM light sources for emerging applications.
“Silicon fabrication technologies that were developed to make electronic devices can now be applied to make various optical devices,” Al-Attili said. “Our microgears are just one example of how these capabilities can be used to make nano- and microscale devices.”
The research was published in Optics Express, a publication of OSA, The Optical Society (https://doi.org/10.1364/OE.26.034675).
Researchers made tiny gears that can generate a vortex of light. The new light source could be used to boost the amount of data that can be transmitted with chip-based optical computing and communication. Courtesy of Abdelrahman Al-Attili, University of Southampton.
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