High-Q microresonators based on native germanium for precision sensing

We will present our recent work on achieving a high-quality factor (Q) in microresonators operating in the longwave infrared (LWIR) range of 8 to 14 microns.1 Advances in this area have the potential to drive new developments in integrated non-linear optics and chip-based sensing, due to the availability of powerful integrated light sources such as solid-state quantum cascade lasers and strong demand for sensing applications in the LWIR atmospheric transparency window. However, until recently limitations in low-loss materials and fabrication processes have resulted in Q factors that are only several thousand. We will discuss the use of germanium as a high-quality material and heterogeneous fabrication process that produces ultra-smooth surfaces. By coupling the output of a QCL into a partially suspended Ge-on-glass waveguide, we were able to achieve an intrinsic Q of 2.5 ×105 . Our results demonstrate the importance and potential of using high-quality native materials for integrated photonics in the LWIR range and portends new sensor topologies.