Low-loss amorphous silicon (a-Si) cladded germanium-on-silicon waveguides for on-chip mid-infrared spectroscopy

Germanium-on-Silicon (GoS) waveguides are of growing interest for environmental monitoring and chemical sensing due to their wide transparency window (2–15 μm) and potential for low optical loss. These waveguides are particularly suited for on-chip mid-IR spectroscopy. Air-cladded GoS waveguides have achieved losses as low as 0.6 dB/cm but would only be practical for many real-world applications if a top cladding is used to isolate the optical mode from the chip environment. Amorphous silicon (a-Si) is CMOS-compatible and therefore an attractive cladding material. In this work, we present an a-Si-cladded GoS waveguide with a propagation loss of 4 dB/cm at a wavelength of 6 μm.