Ion Implantation of Germanium Into Silicon for Critical Coupling Control of Racetrack Resonators

Critical coupling control is an important concept used in integrated photonics to obtain functionalities such as single and coupled resonator optical filters and wavelength multiplexers. Realization of critical coupling depends strongly on device fabrication, and reproducibility is therefore an ongoing challenge. Post-fabrication trimming offers a solution for achieving optimal performance for individual devices. Ion implantation into silicon causes crystalline lattice damage which results in an increase of the material's refractive index and therefore creates a platform for realization of various optical devices. In recent years, we have presented results on the development of erasable gratings, optical filters and Mach-Zehnder interferometers using ion implantation of germanium into silicon. Here, we report the design, fabrication and testing of silicon-on-insulator racetrack resonators, trimmed by localised annealing of germanium ion implanted silicon using continuous and pulsed wave laser sources. The results demonstrate the ability to permanently tune the critical coupling condition of racetrack resonators. Compared to the pulsed lasers used for annealing, continuous wave lasers revealed much higher extinction ratio due to improved material quality after silicon recrystallization.