Seedlayer-Free Rare-Earth Garnet for TE Mode Waveguides

Integrating rare-earth iron garnets with silicon structures for non-reciprocal photonic devices is rife with challenges specific to material processing. In this work, we present the integration of a seed-layer free garnet for TE mode waveguide along with enhancement of high-temperature annealing processes. Current research in integration has been limited to Si-garnet wafer bonding or vacuum deposition (sputtering or pulsed laser deposition) of garnet top claddings followed by annealing. Some of these techniques are difficult to scale and all pose limitations to TE mode waveguides which require significant sidewall coating. Here, we introduce the use of cerium-doped terbium iron garnet, which can be sputtered on integrated photonic structures, have acceptable sidewall coating, and do not require seed layers for garnet formation. These terbium-based garnets have Faraday rotations of -2600°/cm at 1550nm. This value is similar or higher than the effective Faraday rotation of Ce-doped yttrium iron garnet grown by many of the vacuum deposition techniques. It is also important to minimize the thermal processing budget, an important industry metric, without sacrificing the properties of garnet. This work focusses on the application of a two-step anneal and optimizing the quenching rate, resulting in better crystallization and by extension superior material properties. Our results show that the two-step anneal can reduce the peak temperature for crystallization and increase the Faraday rotation by almost 10%. This co-optimization of growth techniques and thermal processing has resulted in better performance for garnets on TE-mode structures.