The researchers built the components from germanium (Ge). “Germanium has several properties that suit it to transmit and guide mid-infrared light,” explained Jian Kang, a Ph.D. candidate in the Department of Electrical Engineering and Information Systems, University of Tokyo.
Germanium has high optical transparency in the mid-infrared range so mid-infrared light passes through it. Compared to silicon, germanium has a number of other optically interesting properties, including a higher refractive index, which means light passes more slowly through it. Germanium also has larger third-order nonlinearity, an optical effect that can be exploited to amplify or self-focus beams of light. It has a stronger free-carrier effect, which means charge carrying electrons and holes in the material can help modulate light. Germanium also has a stronger thermo-optic effect than silicon, which means the refractive index can be more easily controlled with temperature.
“These properties could make Ge-based devices show higher performance or even realise new functionalities in the mid-infrared,” said Kang. Furthermore, recent progress on lasers made from strained-Ge and GeSn-based materials make germanium a promising material for integrating both the light producing and light steering components on the same photonic chip, according to Kang.
The researchers designed and tested several fundamental photonic waveguide components made from germanium, including grating couplers, MMI couplers, and micro-ring resonators. The biggest challenge the team faced was controlling the device fabrication process, including the polishing and etching of the germanium wafer.
“Currently, the Ge device performance may be not as good as state-of-the-art Si-based ones, because the study of Ge-based photonic components for mid-infrared is quite new and there remain many issues in the optimisation of the fabrication process,” Kang admitted. “Nevertheless, we believe that Ge-based devices have intrinsic advantages.”
Germanium’s attractive optical properties in the mid-infrared mean that an optimised Ge waveguide could be more compact than a similar silicon device, meaning more chips could fit into the same space.
Molecules, such as carbon dioxide, absorb and emit light in the mid-infrared when they change vibrational states, so mid-infrared photonics could serve as the basis for new sensors. Kang said that monitoring and detecting carbon emissions, hidden explosives, and health conditions like liver disease and cancer could all be possible with Ge-based sensors.
Ge-based photonic chips also have the potential to increase the bandwidth of optical fibre communications, increasing Internet speeds.
The researchers are now working on improving their fabrication techniques. Afterwards they plan to build more devices, such as optical switches, and to integrate a GeSn laser and Ge waveguide devices onto the same chip.
