Photonic Damascene process for integrated high Q microresonator based nonlinear photonic devices

Chip-based optical waveguides formed by a silicon nitride (Si3N4) core and a silicon dioxide (SiO2) cladding are an important platform for low loss waveguides and nonlinear optics. High Q microresonators based on this material system have become a standard platformfor Kerr frequency comb generation. After their discovery in 2007, Kerr frequency combs have attracted significant interest due to their novel properties, featuring repetition rates from the microwave to the terahertz range, and the potential to realize mass-producible, compact frequency comb systems. The excitation of dissipative Kerr solitons (DKS) has quickly become the preferred operational state, allowing generation of fully coherent frequency combs with designable properties. However, the design and fabrication of microresonators that allow for DKS excitation has remained challenging. In this thesis, a novel fabrication process, the photonic Damascene process, is introduced that solves problems of previous fabrication processes. Inverting the process order and depositing the Si3N4 thin film onto a pre-patterned substrate allows for wafer-scale, crack-free fabrication of low loss photonic waveguides. Through process optimization, high dimensional accuracy similar to existing process schemes and good process stability is achieved. This enables a significant advance in the sample design and understanding of the important design elements required for microresonators with excellent linear and nonlinear performance. Especially, the design of the coupling between the microresonator and the bus waveguide is found to be critical. Together, the advances in design and fabrication allow for the demonstration of octave spanning DKS frequency combs in microresonators with 1-THz free spectral range. Finally, a systematic study of the loss processes in the photonic waveguides is presented and allows for the first time a quantification of the scattering and absorption loss rates. The ultra-smooth sidewalls of waveguides fabricated with a novel reflow process lead to dominant absorption losses. Material analysis techniques reveal for the first time the critical role of transition metal impurities, found to be present in significant amounts, for integrated photonic waveguides. The findings of this thesis provide the technological basis for design and fabrication of chipscale microresonator based Kerr frequency comb generators. In the future it will enable custom-designed and application-specific nonlinear waveguide devices that drive the further proliferation of Kerr comb technology.