Mid-infrared high-Q germanium microring resonator.

Germanium is a promising material for mid-infrared (MIR) integrated photonics due to its CMOS compatibility and wide transparency window covering the fingerprint spectral region (2-15 μm). However, due to the limited quality and structural configurations of conventional germanium-based integration platforms, the realization of high-Q on-chip germanium resonators in the MIR spectral range remains challenging to date. Here we experimentally demonstrate an air-cladding MIR germanium microring resonator with, to the best of our knowledge, the highest loaded Q-factor of ∼57,000 across all germanium-based integration platforms to date. A propagation loss of 5.4 dB/cm and a high extinction ratio of 22 dB approaching the critical coupling condition are experimentally realized. These are enabled by our smart-cut methods for developing high-quality germanium-on-insulator wafers and by implementing our suspended-membrane structure. Our high-Q germanium microring resonator is a promising step towards a number of on-chip applications in the MIR spectral range.