  Lithium niobate (LN) is an excellent nonlinear optical and electro-optic
material that has found many applications in classical nonlinear optics,
optical fiber communications and quantum photonics. Here we review the recent
development of thin-film LN technology that has allowed the miniaturization of
LN photonic devices and microcavities with ultrahigh quality factors. We
discuss the design principle of LN devices that makes use of the largest
nonlinear coefficients, various device fabrication approaches and resulting
device performances, and the current and potential applications of LN
microcavities.
