High-Q whispering gallery mode germanium microresonators at 8 um

Achieving a high-quality (Q) factor for microresonators at a longer wavelength, e.g. longwave infrared (LWIR) with a wavelength from 8 to 14 um, will trigger new development in integrated non-linear optics and sensing on a chip. Although there are both powerful integrated light sources like solid-state quantum cascade lasers (QCLs) and strong driving force from chip-based sensing applications in the LWIR atmospheric transparent window,(1, 2) the Q factors of the microresonators are only several thousand, due to limited choices of low-loss materials and complicated fabrication procedures.(3) Here, we report on the realization of a germanium (Ge) whispering gallery mode microresonator from a facile non-epitaxy fabrication process of high-quality Ge material with an ultra-smooth surface. By coupling the output of a QCL at 7.8 um into a partially suspended Ge on glass waveguide, an intrinsic Q of 2.5x10(5) are reported.(4) Compared with the previous study, our work shows the importance and great promise of maintaining high-quality material for integrated photonics at LWIR.