High-Q nanobeam cavities on a silicon nitride platform enabled by slow light

Silicon nitride integrated photonic devices benefit from a wide working spectral range covering the visible and near-infrared spectra, which in turn enables important applications in bio-photonics, optical communications, and sensing. High-quality factor optical resonators are essential photonic devices for such applications. However, implementing such resonators on a silicon nitride platform is quite challenging due to the low refractive index contrast attainable with this material. Here, we demonstrate that silicon nitride photonic cavities comprising a slow-light waveguide bounded by mirrors can in principle exhibit quality factors in the order of several millions despite a relatively low refractive index contrast. We show that the energy stored in such a slow-light cavity exhibits a cubic dependence on the cavity length, which can enable extremely large quality factors with modest-length cavities. We present the design and experimental characterization of silicon nitride slow-light nanobeam-type cavities. Two sets of nanobeam cavities were fabricated to experimentally verify the cubic dependence of the Q factor on the cavity length. The highest measured Q factor in our devices is 4.42 × 105, which is limited by fabrication imperfections.