Integrated Photonics Based on Rare‐Earth Ion‐Doped Thin‐Film Lithium Niobate

Abstract Rare‐earth (RE) ion doped crystalline materials have a wide spectrum of applications in lasers, amplifiers, sensors, as well as classical and quantum information processing. The incorporation of RE ions into integrated photonics holds great promise for enriching the designer's toolbox with a view to addressing key performance features not available in existing photonic integration platforms. RE‐ion‐doped thin‐film LiNbO 3 (also called RE‐ion‐doped lithium‐niobate‐on‐insulator, RE:LNOI), which inherits nearly all the material advantages as well as nanophotonic integration from the LNOI technology, meets the urgent demands for chip‐integrated laser sources, optical amplifiers, and quantum memories based on LNOI photonics. In this article, a timely review is provided on the development of RE:LNOI photonics in terms of ion‐doping techniques, chip‐integrated lasers, and amplifiers, as well as low‐temperature optical characterizations for quantum photonics. To conclude, some well‐noted topics that may shape the future directions in lithium–niobate integrated photonics are discussed.