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 successes of manufacturing wafer-scale, high-quality thin films of LN-on-insulator (LNOI) and 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 has enabled ultra-low-loss resonators in LN, which has 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.

We demonstrate an ultralow loss monolithic integrated lithium niobate photonic platform consisting of dry-etched subwavelength waveguides with extracted propagation losses as low as 2.7 dB/m and microring resonators with quality factors up to ${10}^7$.

Lithium niobate (LN), first synthesized 70 years ago, has been widely used in diverse applications ranging from communications to quantum optics. These high-volume commercial applications have provided the economic means to establish a mature manufacturing and processing industry for high-quality LN crystals and wafers. Breakthrough science demonstrations to commercial products have been achieved owing to the ability of LN to generate and manipulate electromagnetic waves across a broad spectrum, from microwave to ultraviolet frequencies. Here, we provide a high-level Review of the history of LN as an optical material, its different photonic platforms, engineering concepts, spectral coverage, and essential applications before providing an outlook for the future of LN.

Periodically poled lithium niobate (PPLN) waveguides are a powerful platform for efficient wavelength conversion. Conventional PPLN converters, however, typically require long device lengths and high pump powers due to the limited nonlinear interaction strength. Here we use a nanostructured PPLN waveguide to demonstrate an ultrahigh normalized efficiency of $2600\%/\mathrm{W}-{\mathrm{cm}}^2$ for second-harmonic generation of 1.5 μm radiation, more than 20 times higher than that in state-of-the-art diffused waveguides. This is achieved by a combination of sub-wavelength optical confinement and high-fidelity periodic poling at a first-order poling period of 4 μm. Our highly integrated PPLN waveguides are promising for future chip-scale integration of classical and quantum photonic systems.