Researchers from University of Tokyo have successfully built fundamental components from germanium for integrated photonic circuits that work at mid-infrared wavelengths. The research could enable improved Internet speeds and novel sensors.
The researchers built the new components from the material germanium (Ge) which has similar electrical properties like silicon commonly used in conventional near-infrared photonics. Germanium has high optical transparency in the mid-infrared range (which has many important applications in remote sensing and communication technologies), so mid-infrared light can easily pass through it, said Jian Kang from University of Tokyo, Japan.
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 a larger third-order nonlinearity, an optical effect that can be exploited to, for example, amplify or self-focus beams of light.
Additionally, Germanium 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 realize new functionalities in the mid-infrared. 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, Kang said.
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.
Furthermore, many important 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. Monitoring and detecting carbon emissions, hidden explosives, and health conditions like liver disease and cancer are all possible with Ge-based sensors. Ge-based photonic chips also have the potential to increase the bandwidth of optical fiber communications. This can make the internet much faster.
Several fundamental photonic waveguide components made from germanium were tested, including grating couplers, MMI couplers, and micro-ring resonators. Grating couplers are used to couple light efficiently from free space into a waveguide, and vice versa, MMI couplers are used as routers or couplers for light signal processing in the waveguide, and micro-ring resonators are used to filter certain wavelengths of light passing through.
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 optimization of the fabrication process.
Nevertheless, Ge-based devices have intrinsic advantages. The researchers are working on improving their fabrication techniques and also plan to build more devices, such as optical switches, and to integrate a GeSn laser and Ge waveguide devices onto the same chip.