Efficient Cascaded Third‐Harmonic Generation in Sampled‐Grating Periodically‐Poled Lithium Niobate Waveguides.

Periodically poled thin‐film lithium niobate (PPTFLN) waveguides enable efficient quadratic nonlinear processes due to the strong light confinement and the utilization of the largest nonlinear coefficient d33 of lithium niobate. Here, an efficient third harmonic (TH) generation is reported with a normalized conversion efficiency of 6481% W−2 cm−4 in a sampled‐grating PPTFLN waveguide under the pump of a continuous laser, which is improved by 4 orders of magnitude compared with reported results. Such an efficient TH generation is realized by cascaded second‐harmonic (SH) generation and sum‐frequency (SF) generation supported by a sampled‐grating domain structure with two Fourier coefficients of 0.44 and 0.37, respectively. Phase matching conditions for the SH and cascaded SF processes are simultaneously achieved by finely tuning the geometric dispersion, domain structure, and operating temperature of the PPTFLN waveguides. [ABSTRACT FROM AUTHOR]