Germanium Mid-Infrared Integrated Photonic Circuits

This project involves the integration of III-V light sources with photonic integrated circuits for the development of a comprehensive mid-infrared silicon photonic platform which can be potentially used as a gas sensor.

Group IV photonics has been an emerging topic of research over the last decade which focuses on the development of photonic devices which operate in the Near-Infrared (NIR) Mid-Infrared (MIR) wavelength range. However, this work focuses only in the development of devices for MIR.These devices are suitable for numerous applications including chemical, biological and environmental sensing, security, communications, as well as astronomy. The main point which the overall project aims to achieve is the development of integrated photonic devices that can potentially be integrated on a single silicon chip.
This report presents the work that has been carried out towards this aim, over the last 8 months. More specifically, this includes the development of low loss waveguides for the MIR wavelength range. These waveguides were demonstrated using the platform of suspended Germanium-on-Silicon-on-Insulator (Ge-on-SOI). The waveguides were initially designed using Lumerical Mode Solutions. The simulations were carried out for 400 nm and 1 _m thick Germanium (Ge) rib waveguides grown on an Silicon on Insulator (SOI) substrate formed of a 220 nm thick Silicon (Si) layer and a 3 _m thick Silicon Dioxide (SiO2) layer (also known as the Buried Oxide (BOX)). The measured propagation loss was 16.7 dB/cm at 3.8 _m. Then the waveguides were suspended by removing the BOX using a wet etch step with Hydrofluoric Acid (HF). Access to the BOX was achieved by designing subwavelength holes alongside the waveguides. After removing the BOX, a propagation loss of (2.82 dB/cm) at 3.8 _m was observed. The measured bend loss was 0.13 dB/bend. Samples of 1 _m Ge-on-SOI have also been fabricated and are due to be measured. Integration of Quantum Cascade Lasers (QCLs) on Si has also been investigated. A QCL-waveguide overlap analysis has been carried out showing that a lateral misalignment of 1 _m corresponds to an insertion loss of approximately 0.34 dB which is increased to 10 dB for 5.5 _m of misalignment. The respective values of the vertical misalignment are 2.71 dB for 1 _m and 10 dB for 1.9 _m.