Lithium niobate is one of the most widely explored materials for photonic applications due to its excellent optical properties such as strong electro-optic effect, high second-order optical nonlinearity, wide optical transparency window, and low material loss. Since its first synthesis ${\sim}{70}$ years ago, lithium niobate has served optics, photonics, and the related communication technologies in the forms of bulk crystals and weakly confining waveguides for decades. In recent years, the realization of tightly confining waveguides in lithium niobate has opened new avenues for integrated photonics owing to the emergence and development of lithium-niobate-on-insulator (LNOI) technology. Nowadays, LNOI wafers are commercially available, which consist of a single-crystalline lithium niobate thin film on silicon oxide on a lithium niobate or silicon substrate. By directly etching the lithium niobate device layer or patterning another hybrid integrated dielectric material, people have realized various types of photonic integrated circuits and devices on the LNOI platform for both classical and quantum applications. These applications include, for example, passive and active integrated photonic devices, electro-optic and acousto-optic modulators, nonlinear optical processes, and quantum photonic devices. The state of the art in this subject has been comprehensively covered in several recent review articles [1,2].

This feature issue focuses on the most recent advances in integrated lithium niobate photonics and is devoted to the promotion of the latest developments in both fundamental science and practical applications of novel devices and systems on the LNOI platform.

This feature issue includes two review articles. In [3], Han et al. review the recent progress in dielectrically loaded LNOI waveguides, with summary of the advantages and disadvantages of different loading materials, comparison of the performance of different platforms, and discussion of the future of these platforms for photonic integrated circuits. In [4], Wang et al. review the material properties of lithium niobate and fabrication approaches for heterogeneous integration, introduce various photonic devices involving different functionalities, and present the advances in photonic–electronic convergence based on heterogeneous platform of merging lithium niobate into silicon.

This feature issue also includes 11 research articles, which cover the areas of passive integrated photonic components, microring-based photonic devices, electro-optic modulators, and nonlinear optical processes.

Two research articles are in the area of passive integrated photonic components. In [5], Kang et al. designed and optimized chirped grating couplers on Z-cut lithium niobate on insulator using a backward-propagation neural network combined with the particle swarm optimization algorithm, achieving the maximum coupling efficiency of 63% at 1550 nm. In [6], Quan et al. report a broadband adiabatic mode division (de)multiplexer in thin-film lithium niobate waveguide, which can realize multiplexing or demultiplexing of the ${{\rm TE}_0}$, ${{\rm TE}_1}$, and ${{\rm TE}_2}$ modes, with the simulated conversion efficiencies being 96.3% and 94.6% for the ${{\rm TE}_2}$ and ${{\rm TE}_1}$ modes and the 1-dB operation bandwidth being ${\sim}{420}\;{\rm nm}$.

Two research articles are in the area of microring-based photonic devices. In [7], Lin et al. propose and demonstrate a tunable microring resonator based on the thermo-optic effect in a Z-cut LNOI platform with a trench structure, achieving a measured extinction ratio of 25 dB, $Q$ factor of 12,000, and thermo-optic tuning efficiency of 8.6 pm/mW. In [8], Ma et al. demonstrate an on-chip microring laser on ${{\rm Yb}^{3 +}}$-doped thin-film lithium niobate fabricated by photolithography-assisted chemo-mechanical etching technology, achieving multiwavelength laser emission around 1025 nm with the lasing threshold of 10 mW. Such an on-chip microring laser is a promising light source for multiple wavelength channels in optical communications and biosensors.

Three research articles are in the area of electro-optic modulators. In [9], Mere et al. report strategies for improved fabrication of scalable hybrid silicon nitride–thin-film lithium niobate electro-optic modulators. By incorporating appropriately designed outgassing channels and modifying certain key processing steps, they achieved a ${\gt}{99}\%$ reduction in void density during bonding, which contributed to a high extinction ratio, low half-wave-voltage–length product, and high modulation bandwidth in their fabricated devices. In [10], Ghoname et al. demonstrate a compact electro-optic modulator based on a spiral-shaped waveguide Bragg grating on Z-cut thin-film lithium niobate. The integrated design wrapped a 2.2-mm-long grating into a ${120} \times {120}\;{{\unicode{x00B5}{\rm m}}^2}$ area. The modulator bandgap, with an extinction ratio of ${\gt}{35}\;{\rm dB}$ at 1550 nm, could be efficiently tuned with a sensitivity of 8.36 pm/V and a 3-dB operating bandwidth of 25 GHz. In [11], Bente et al. propose an integrated photonic design for optical pattern generation on thin-film lithium niobate. By combining the electro-optic properties of LNOI and the nonvolatile, reconfigurable nature of the phase-change material ${{\rm Ge}_2}{{\rm Sb}_2}{{\rm Te}_5}$, they demonstrate with calculations and preliminary experimental results the possibility of generating arbitrary optical patterns on a chip.

Four research articles are in the area of nonlinear optical processes. In [12], Lv et al. designed and simulated a chiral metasurface using topological patterns in lithium niobate thin film to obtain the optimal circular dichroism and its corresponding nonlinear effect. This tunable chiral metasurface is expected to be applicable to nonlinear chiroptical responses and chiral light modulation. In [13], Zhu et al. demonstrate efficient second-harmonic generation (SHG) and sum-frequency generation (SFG) in an X-cut thin-film lithium niobate microdisk with a high intrinsic quality factor over ${{10}^6}$, with a measured normalized SHG and SFG conversion efficiency of 1.53%/mW and ${2.52} \times {{10}^{- 4}}/{\rm mW}$, respectively. In [14], Ye et al. propose a continuous-wave triggered method to enhance the mid-infrared dispersive wave generation at 3 µm in lithium niobate waveguides, which has potential for multispecies greenhouse gas detection through gas absorption spectroscopy. In [15], Rodrigues et al. conducted a comprehensive numerical investigation of stimulated Brillouin scattering in lithium niobate waveguides, demonstrating that surface acoustic waves have a strong interaction with the optical fields. These results could unleash new opportunities to study its second-order nonlinearities and piezoelectric properties.