Mid-infrared high-Q germanium resonators (Conference Presentation)

Mid-infrared (MIR) resonators with high quality (Q) factors play crucial roles in a variety of applications in nonlinear optics, lasing, biochemical sensing, and spectroscopy by virtue of their features of long photon lifetime as well as strong field confinement and enhancement. Previously, such devices have been mainly studied on silicon integration platforms while the development of high-Q germanium resonators is still in its infancy due to quality limitations of current germanium integration platforms. Compared with silicon, germanium possesses a number of advantages for MIR applications, such as a wider transparency window (2 - 15 µm), a higher refractive index (~4), and a higher third-order nonlinear susceptibility. Here we present our experimental demonstration of two types of MIR high-Q germanium resonators, namely, a microring resonator and a photonic crystal nanobeam cavity. A maximum Q factor of ~57,000 is experimentally realized, which is the highest to date on germanium platforms. Moreover, we demonstrate a monolithic integration of the high-Q germanium resonators with suspended-membrane waveguides and focusing subwavelength grating couplers. Our resonators pave a new avenue for the study of on-chip light-germanium interactions and development of on-chip MIR applications in sensing and spectroscopy.