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 second-order 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.