A lithium-niobate-on-insulator (LNOI) wafer is considered as an important candidate platform for photonic integrated circuits (PICs), owing to its outstanding material properties featuring a broad transparency window (350 nm to 5 µm), a linear electro-optic effect, and a large second-order nonlinearity susceptibility (χ(2)=30pm/V)^[1–5]. A wide range 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 the ion-slicing technique and LNOI nanofabrication technology^[6–23]. 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^[24–28]. 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 polishing (CMP) have been successively used to etch the lithium niobate (LN) thin film into waveguides with smooth sidewalls to improve propagation losses^[29–34]. For instance, an ultra-smooth sidewall roughness as small as 0.1 nm has been achieved by CMP etching, leading to an ultra-low propagation loss of ridge waveguides of 0.027 dB/cm^[25,35]. However, the minimum propagation loss of the waveguide determined by the intrinsic material absorption is only 0.001 dB/cm^[36]. There is still room to reduce the propagation loss by improving the quality of LN thin film.