Ge-on-Si Mid-Infrared Waveguide Platform for Molecular Fingerprint Sensing

The mid-infrared spectral region is key to several large market applications such as security, healthcare and environmental monitoring. This is due to how chemical compounds can be detected from unique vibrational modes that absorb in the molecular ‘fingerprint’ region ( $6.7-20\ \mu \mathrm{m}$ wavelength). There is significant interest in realizing cheaper and smaller spectrometers based on waveguide integrated photonics. Here we demonstrate Ge-on-Si waveguides operating up to $11\ \mu \mathrm{m}$ wavelength with low propagation loss (∼ 1 dB/cm). The performance of the waveguides is then demonstrated by measuring thin films that are used to calibrate the modal overlap for molecular sensing. Lastly, an ultra-broadband Ge-on-Si waveguide polarization rotator is presented. The fabricated polarization rotator devices demonstrate a polarization extinction ratio of ≥ 15 dB over a $2\ \mu \mathrm{m}$ bandwidth ( $9-11\ \mu \mathrm{m}$ wavelength) with an average insertion loss < 1 dB.