Graded SiGe waveguides with broadband low-loss propagation in the mid infrared

Mid-infrared (mid-IR) silicon photonics is expected to lead key advances in different areas including spectroscopy, remote sensing, nonlinear optics or free-space communications, among others. Still, the inherent limitations of the silicon-on-insulator (SOI) technology, namely the early mid-IR absorption of silicon oxide and silicon at λ~3.6 µm and at λ ~8.5 µm respectively, remain the main stumbling blocks that prevent this platform to fully exploit the mid-IR spectrum (λ ~2-20 µm). Here, we propose using a compact Ge-rich graded-index Si_1-xGe_x platform to overcome this constraint. A flat propagation loss characteristic as low as 2-3 dB/cm over a wavelength span from λ = 5.5 µm to 8.5 µm is demonstrated in Ge-rich Si_1-xGe_x waveguides of only 6 µm thick. The comparison of three different waveguides design with different vertical index profiles demonstrates the benefit of reducing the fraction of the guided mode that overlaps with the Si substrate to obtain such flat low loss behavior. Such Ge-rich Si_1-xGe_x platforms may open the route towards the implementation of mid-IR photonic integrated circuits with low-loss beyond the Si multi-phonon absorption band onset, hence truly exploiting the full Ge transparency window up to λ ~15 µm.