Development and characterization of low-loss waveguides and top claddings for germanium-on-silicon mid-wave infrared integrated photonics

We present a mid-wave infrared photonic integrated circuit platform based on germanium-on-silicon waveguides - the first to include dielectric top cladding, which improves robustness and allows for the complicated metal routing needed for practical, large-scale photonic integrated circuits. We conduct the first comprehensive review of top cladding materials for germanium-on-silicon waveguides and find that Nb₂O₅ (loss: 3.5-4.6 dB/cm at 4.6 µm; ring resonator loaded Q >37,000 and unloaded Q >58,000) and epitaxial Si (loss: 4.1-6.3 dB/cm at 4.6 µm; ring resonator loaded Q >23,000 and unloaded Q >40,000) achieve the best results. For sensing applications, which can require waveguides exposed to the environment, we demonstrate state-of-the-art air-clad waveguides and ring resonators that achieve losses of 0.6-2.5 dB/cm for both TE and TM light and high-Q factors (loaded Q of >106,000 and unloaded Q >190,000 for TE light).