Shallow-etched thin-film lithium niobate waveguides for highly-efficient second-harmonic generation Zhao, Jie Rüsing, Michael Javid, Usman A. Ling, Jingwei Li, Mingxiao Lin, Qiang Mookherjea, Shayan Atomic and Molecular Physics and Optics High-fidelity periodic poling over long lengths is required for robust, quasi-phase-matched second-harmonic generation using the fundamental, quasi-TE polarized waveguide modes in a thin-film lithium niobate (TFLN) waveguide. Here, a shallow-etched ridge waveguide is fabricated in x-cut magnesium oxide doped TFLN and is poled accurately over 5 mm. The high fidelity of the poling is demonstrated over long lengths using a non-destructive technique of confocal scanning second-harmonic microscopy. We report a second-harmonic conversion efficiency of up to 939 %/W (length-normalized conversion efficiency 3757 %/Wcm²), measured at telecommunications wavelengths. The device demonstrates a narrow spectral linewidth (1 nm) and can be tuned precisely with a tuning characteristic of 0.1 nm/°C, over at least 40 °C without measurable loss of efficiency. Optica Publishing Group 2020 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_6501 https://ris.uni-paderborn.de/record/47958 Zhao J, Rüsing M, Javid UA, et al. Shallow-etched thin-film lithium niobate waveguides for highly-efficient second-harmonic generation. <i>Optics Express</i>. 2020;28(13). doi:<a href="https://doi.org/10.1364/oe.395545">10.1364/oe.395545</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1364/oe.395545 info:eu-repo/semantics/altIdentifier/issn/1094-4087 info:eu-repo/semantics/closedAccess