Second-order nonlinear frequency conversion in lithium niobate waveguides: principles, fabrication, and recent advances

Lithium niobate waveguides have emerged as a leading platform for second-order nonlinear frequency conversion because they combine a large second-order nonlinear coefficient with mature phase-matching schemes and low-loss integrated photonic implementations. Recent advances in thin-film lithium niobate and related fabrication technologies have substantially improved device performance and enabled efficient nonlinear frequency conversion over a broad spectral range extending from the visible to the mid-infrared. In this review, we summarize recent progress in second-order nonlinear frequency conversion in lithium niobate waveguides, with an emphasis on waveguide design, phase-matching strategies, fabrication techniques, and representative nonlinear processes. We first outline the fundamental design principles, including modal confinement, dispersion engineering, and the implementation of quasi-phase matching and modal phase matching. We then discuss key fabrication processes, such as lithography, etching, electric-field poling, and post-processing, and analyze their influence on propagation loss, domain fidelity, and conversion efficiency. On this basis, we systematically review and compare recent advances in second-harmonic generation, sum-frequency generation, difference-frequency generation, optical parametric generation/amplification, and cascaded secondorder nonlinear processes. Finally, we discuss the advantages and limitations of different approaches, as well as the key challenges and future opportunities for developing efficient, broadband, tunable, and scalable on-chip nonlinear photonic devices.