Ultrahigh‐Quality 780 nm Bragg Grating Fabry‐Pérot Cavity on an Etchless Lithium Niobate Platform

Integrated Fabry‐Pérot (FP) cavities on the thin‐film lithium niobate on insulator (LNOI) platform harness their local field‐enhancement and standing‐wave properties to exploit the advantages of the LiNbO 3 material, enabling linear and nonlinear applications such as atomic trapping and quantum sources. To explore such potential, achieving ultrahigh‐quality (Q) FP cavities on LNOI in the visible band is essential but remains challenging. This challenge stems from the finer nanostructures required at visible wavelengths, which are difficult to realize by high‐fidelity etching in LiNbO 3 , together with higher Rayleigh scattering losses compared with near‐infrared operation. In this work, we demonstrate Bragg‐grating‐based FP cavities on the LNOI platform with an intrinsic Q up to 6.6 × 10 5 at 780 nm. The key is to avoid direct etching of LiNbO 3 by patterning a polymer waveguide atop the LNOI. This facilitates the formation of ultra‐low‐loss TE guided modes as well as fine nanostructures down to 100 nm by taking advantage of ultrahigh resolution of electron‐beam lithography (EBL) and the chemical bond‐cleavage mechanism. Our approach is also experimentally validated to be compatible with another material platform. Our results open a route toward visible ultrahigh‐Q integrated FP cavities for atomic spectroscopy, solid‐state quantum emitters and high‐sensitivity sensing.