Researchers from University of Southampton used micro-gears of germanium that vertically emit light to generate orbital angular momentum
A team of researchers from the University of Southampton in the U.K., and University of Tokyo, Toyohashi University of Technology and Hitachi Ltd., developed a new light-emitting gears with a radius of one micron or less. According to the researchers, 250,000 of the gears can be packed into just one square millimeter of a computer chip. A light with orbital angular momentum offers several advantages in communications and computing. Light is a medium to shuttle information by varying the number of photons that are emitted. Moreover, switching between light’s two polarization states also offers transfer of information. Each twist in the light can represent a different value or letter to allow encoding of large packs of data using less light.
A usable miniaturized light source on silicon cannot be achieved as the material’s properties lead to poor light-generating efficiency. The team found that although germanium has similar limitations, its light emission efficiency can be improved by applying strain. The new design includes micro-gears that are freestanding at the edges. This enables to stretch the micro-gears by an oxide film deposited over the structures. This in turn allows tensile strain to be applied without breaking the germanium’s crystal structure. The gears are located on a silicon pedestal that connects it to the top of the silicon substrate. This allows heat to dissipate during operation.
The researchers used electron beam lithography to develop the very fine physical features that form the gears’ teeth. The gears were illuminated with a standard green laser, which did not emit twisted light. The micro-gear absorbed the green light to produce its own photons. These photons are circulated around the edges to form twisted light, which is reflected vertically out of the gear via the periodic teeth. The team tested and modified the design with the help of computer simulations that model the pattern light propagates in the gears over a few nanoseconds. The team compared the prototype’s light emission with computer simulation results and confirmed that the gears generated twisted light. The research was published in the journal Optics Express on December 21, 2018.