Approaching Ideal Second-Harmonic Generation in Polymer Etchless Periodically Poled Thin-Film Lithium Niobate Waveguides with Efficient Domain Characterization

The etchless thin-film lithium niobate (TFLN) platform, with its simple fabrication and low cost, has made the investigation of large-area photonic integrated circuits (PICs) more feasible. Frequency converters, as the critical components in PICs, exhibit low conversion efficiencies on the reported etchless TFLN platforms. The absence of an efficient domain characterization method also hinders the achievement of wafer-scale nonlinear integrated photonics. Herein, we experimentally demonstrate the first ultrahigh-efficiency and near-ideal second-harmonic generation (SHG) on a polymer etchless periodically poled (PP) TFLN platform. The waveguide bound states in the continuum mechanism are utilized to mitigate propagation loss. By optimizing the poling waveform and employing polarization-contrast microscopy for domain characterization, high-quality periodic poling is achieved. The measured normalized conversion efficiency (NCE) is 1728%W–1 cm–2, marking a two-orders-of-magnitude enhancement over previously reported polymer etchless TFLN waveguides that rely on modal phase matching, significantly advancing the practical applications of frequency converters based on etchless TFLN platforms. Notably, this NCE reaches 97% proximity to the theoretical value, surpassing all reported SHG performances based on PPTFLN waveguides. Our work, which benefits from easy fabrication, cost-effective domain characterization methods, and superior performance, paves a preferable path for wafer-scale multifunctional nonlinear PICs.