  Lithium niobate (LN), an outstanding and versatile material, has influenced
our daily life for decades: from enabling high-speed optical communications
that form the backbone of the Internet to realizing radio-frequency filtering
used in our cell phones. This half-century-old material is currently embracing
a revolution in thin-film LN integrated photonics. The success of manufacturing
wafer-scale, high-quality, thin films of LN on insulator (LNOI), accompanied
with breakthroughs in nanofabrication techniques, have made high-performance
integrated nanophotonic components possible. With rapid development in the past
few years, some of these thin-film LN devices, such as optical modulators and
nonlinear wavelength converters, have already outperformed their legacy
counterparts realized in bulk LN crystals. Furthermore, the nanophotonic
integration enabled ultra-low-loss resonators in LN, which unlocked many novel
applications such as optical frequency combs and quantum transducers. In this
Review, we cover -- from basic principles to the state of the art -- the
diverse aspects of integrated thin-film LN photonics, including the materials,
basic passive components, and various active devices based on electro-optics,
all-optical nonlinearities, and acousto-optics. We also identify challenges
that this platform is currently facing and point out future opportunities. The
field of integrated LNOI photonics is advancing rapidly and poised to make
critical impacts on a broad range of applications in communication, signal
processing, and quantum information.
