Germanium mid-infrared photonic integrated circuits

Abstract

Group-IV photonics has been an emerging topic of research over the last decade, which focuses on the development of photonic devices that operate in the Near-Infrared (NIR) and the Mid-Infrared (MIR) wavelength ranges. The work presented in this thesis focuses on the development of devices for the MIR. These devices are suitable for numerous applications including chemical, biological and environmental sensing, security, communications, as well as astronomy. Silicon (Si) does not emit light efficiently, therefore the integration of other light-emitting materials is highly demanded for Silicon Photonic Integrated Circuits (SPICs). The main subject, which this project addresses is the integration of light sources, in particular Quantum Cascade Lasers (QCLs) emitting light in the 3-5 µm wavelength region, that are waveguide-coupled on-chip using flip-chip boning. This thesis presents the progress made towards this aim. This includes the development of various components for the MIR wavelength range. Firstly, waveguides based on the suspended Germanium (Ge) platform were designed, fabricated and characterised. The main aim of investigating this platform is the fact that Ge is transparent in the whole MIR wavelength range, unlike all the other group-IV platforms demonstrated to date. Suspended Ge waveguides have been simulated, fabricated and characterised using Germanium-on-Silicon-on-Insulator (Ge-on-SOI) as the initial platform for wavelengths of 3.8 and 7.67 µm. The lowest measured propagation loss values were 2.82 and 2.65 dB/cm respectively. Simulations have also been carried for the wavelength of 9.5 µm. Lastly, the integration of QCLs with waveguides based on the the Germanium-on-Silicon (GOS) platform using flip-chip bonding was demonstrated. In this approach QCLs operating at a wavelength of 5.5 µm were flip-chip bonded on a 3 µm processed GOS wafer. Light was then butt-coupled to rib waveguides and subsequently to an optical fibre using grating couplers. The light sources were in a form of bars, each one containing 24 lasers and were provided by the University of Sheffield. Simulations have been carried out to evaluate the effect of lateral and vertical misalignment on the coupling efficiency and the performance of the grating couplers. Laser-bars where characterised prior to bonding. The voltage of the laser-bar was measured at 15 ◦C, whereas the optical power was measured at temperatures raising from 8 ◦C to room temperature ∼20 ◦C. The laser turn-on current was between 200 and 225 mA. The emitted optical power was ranging from a minimum of ∼25 to a maximum of ∼35 mW. Laser-bars of the same material were characterised once they were flip-chip bonded. The measured coupling loss was 25 dB at the point where the maximum coupled optical power was obtained, which corresponded to a laser current of ∼300 mA. This coupling loss is attributed to various mechanisms discussed in this thesis

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ORCID for Ahmed Osman: orcid.org/0000-0001-6575-3861

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