Lithium niobate (LN), dubbed by many as the silicon of photonics, has recently risen to the forefront of chip-scale nonlinear optics research since its demonstration as an ultralow-loss integrated photonics platform. Due to its significant quadratic nonlinearity (${\chi }^{(2)}$), LN inspires many important applications such as second-harmonic generation (SHG), spontaneous parametric downconversion, and optical parametric oscillation. Here, we demonstrate high-efficiency SHG in dual-resonant, periodically poled $z$-cut LN microrings, where quasi-phase matching is realized by field-assisted domain engineering. Meanwhile, dual-band operation is accessed by optimizing the coupling conditions in fundamental and second-harmonic bands via a single pulley waveguide. As a result, when pumping a periodically poled LN microring in the low power regime at around 1617 nm, an on-chip SHG efficiency of 250,000%/W is achieved, a state-of-the-art value reported among current integrated photonics platforms. An absolute conversion efficiency of 15% is recorded with a low pump power of 115 μW in the waveguide. Such periodically poled LN microrings also present a versatile platform for other cavity-enhanced quasi-phase-matched ${\chi }^{(2)}$ nonlinear optical processes.

中文翻译：

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具有二次谐波产生效率为250,000％/ W的周期性极化薄膜铌酸锂微环谐振器

铌酸锂（LN）被誉为光子学的硅，自从其作为超低损耗集成光子学平台的展示以来，最近已升至芯片级非线性光学研究的最前沿。由于其明显的二次非线性（${\chi }^{（\mathrm{2个}）}$），LN激发了许多重要的应用，例如二次谐波生成（SHG），自发的参数下变频和光学参数振荡。在这里，我们展示了双共振，周期性极化的高效SHG$ž$切割的LN微环，其中通过场辅助域工程实现准相位匹配。同时，通过单滑轮波导，通过优化基频和二次谐波频带中的耦合条件，可以访问双频带操作。结果，当在低功率状态下在约1617 nm处泵浦周期性极化的LN微环时，片上SHG效率达到250,000％/ W，是当前集成光子平台中报告的最先进的值。在波导中以115μW的低泵浦功率记录了15％的绝对转换效率。这种周期性极化的LN微环还为其他腔增强的准相位匹配提供了通用平台${\chi }^{（\mathrm{2个}）}$ 非线性光学过程。