Chip-integrated whispering-gallery resonators (WGRs) enable compact and wavelength-agile nonlinear optical frequency synthesizers. So far, the most flexible phase-matching technique, i.e., quasi-phase matching, has not been applied in this configuration. The reason is the lack of suitable thin films with alternating crystal structure on a low-refractive-index substrate. Here, we demonstrate an innovative method of realizing thin film substrates suitable for quasi-phase matching by field-assisted domain engineering of lithium niobate, and subsequent direct bonding and polishing. We are able to fabricate high-$Q$ on-chip WGRs with these substrates by using standard semiconductor manufacturing techniques. The $Q$-factors of the resonators are up to one million, which allows us to demonstrate quasi-phase-matched second-harmonic generation in on-chip WGRs for the first time, to the best of our knowledge. The normalized conversion efficiency is $9\times {10}^{-4}{\mathrm{mW}}^{-1}$. This method can also be transferred to other material systems.

中文翻译：

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片上回音壁中准相位匹配的非线性光学频率转换

芯片集成的耳语画廊谐振器（WGR）使紧凑和波长敏捷的非线性光频率合成器成为可能。迄今为止，最灵活的相位匹配技术（即准相位匹配）尚未在此配置中应用。原因是在低折射率衬底上缺少合适的具有交替晶体结构的薄膜。在这里，我们演示了一种创新的方法，该方法可通过现场辅助铌酸锂磁畴工程以及随后的直接键合和抛光来实现适用于准相位匹配的薄膜基板。我们能够制造出高$问$通过使用标准的半导体制造技术在这些基板上进行片上WGR。这$问$谐振器的最大因子高达一百万，这使我们能够据我们所知，首次在片上WGR中演示准相位匹配的二次谐波产生。归一化转换效率为$9\times {10}^{-4}{\mathrm{兆瓦}}^{-\mathrm{1个}}$。该方法也可以转移到其他材料系统。