Lithium niobate microring with ultra-high Q factor above 108

Abstract

We demonstrate ultra-high quality (Q) factor microring resonators close to the intrinsic material absorption limit on lithium niobate on insulator. The microrings are fabricated on pristine LN thin-film wafer thinned from LN bulk via chemo-mechanical etching without ion slicing and ion etching. A record-high Q factor up to 10 8 at the wavelength of 1550 nm is achieved because of the ultra-smooth interface of the microrings and the absence of ion-induced lattice damage, indicating an ultra-low waveguide propagation loss of ∼0.28 dB/m. The ultra-high Q microrings will pave the way for integrated quantum light source, frequency comb generation, and nonlinear optical processes. 1. Introduction Lithium-niobate-on-insulator wafer (LNOI) is considered as an important candidate platform for photonic integrated circuits (PICs), owning to its outstanding material properties that include a broad transparency window (350 nm to 5 m), a linear electro-optic effect, and a large second-order nonlinearity susceptibility (χ (2) =30 pm/V) [1-5]. A wide of high-performance device applications, such as waveguide/resonator optical frequency convertors, high-speed Mach-Zehnder modulators, and multiplexers have been demonstrated due to the rapid developments in ion-slicing technique and LNOI nanofabrication technology [6-18]. Among the various devices, optical waveguides with ultra-low propagation loss and high refractive index contrast are building elements for the realization of large-scale PICs [19-23]. And the propagation losses in optical waveguides are susceptible to sidewall roughness induced by the fabrication imperfection. To reduce the sidewall roughness of the waveguide, diamond-blade dicing, precision cutting, focused ion beam milling, argon ion milling, and chemo-mechanical etching have been successively used to etch the LNOI into waveguides with smoother sidewalls, resulting in propagation losses ranging from 120 dB/m, below 10 dB/m, to 2.7 dB/m [19-30].