Ultralow‐Loss Etchless Lithium Niobate Integrated Photonics at Near‐Visible Wavelengths

Abstract Lithium niobate is considered as the “silicon of photonics” because of its many excellent optical properties as well as a wide transparency window covering the visible to mid‐infrared wavelength range. Here, by using the principle of bound states in the continuum, lithium niobate integrated photonics at near‐visible wavelengths with an etchless fabrication process is developed, which achieves a minimal propagation loss of 1.8 dB cm −1 in straight waveguides at the wavelength of ≈771 nm and a maximal intrinsic Q factor of 1.1 × 10 6 in disk microcavities at ≈765 nm. Typical integrated photonic components including Mach–Zehnder interferometers and multimode directional couplers are also demonstrated. Based on the experimental results, a convenient scheme of second‐harmonic generation is proposed for converting light in the fundamental TE‐polarized mode at a telecom wavelength into the second‐order TM‐polarized mode at a near‐visible wavelength. The demonstrated etchless lithium niobate integrated photonic platform can support both passive and active optical devices and systems on a chip for various visible‐light applications including optical imaging, nonlinear optics, and quantum information processing.