AI 中文总结
本研究探究氮化硅中直接相位匹配光电流二次谐波产生，实现亚赫兹级频率比保真度，为片上计量、光谱学及光学时钟提供稳健方案。
AI 中文摘要
相干光电流（PG）效应在本征为χ(3)的氮化硅集成光子学中诱导出有效χ(2)非线性，通过二次谐波产生（SHG）为芯片级精密光谱学和光学时钟装置开辟了途径。利用空间变化内部电场的准相位匹配PG-SHG虽具备调谐灵活性，但常伴随依赖泵浦功率和失谐的频率偏移。本研究探讨利用产生空间均匀电场的模间方案实现直接相位匹配是否能支持计量兼容的SHG。通过对比氮化硅微谐振器中的基频与倍频，测试直接相位匹配PG-SHG中2:1频率比的保持情况，观测到频率偏移小于1赫兹，与此前准相位匹配结构的局限形成鲜明对比。此外，测得1秒时的残余分数频率不稳定度为2×10^-15，在1000秒时平均降至10^-16量级，多小时偏差保持在1赫兹以下。这些结果确立了直接相位匹配PG-SHG作为一种稳健、计量兼容的有效χ(2)功能实现途径，在成熟集成平台上结合亚赫兹频率比保真度与高相干性，可应用于光学时钟装置、自参考及精密光谱学。
英文摘要
The coherent photogalvanic (PG) effect induces an effective $χ^{(2)}$ nonlinearity in natively $χ^{(3)}$ silicon nitride integrated photonics, unlocking pathways toward chip-scale precision spectroscopy and optical clockworks via second harmonic generation (SHG). While quasi-phase-matched PG-SHG using spatially varying internal electric fields offers tuning flexibility, it is often accompanied by pump-power- and detuning-dependent frequency offsets. Here, we investigate whether direct phase-matching---utilizing an intermodal scheme that generates a spatially uniform electric field---can support metrologically compatible SHG. By comparing the fundamental and doubled optical frequencies in a silicon nitride microresonator, we test the preservation of the (2:1) frequency ratio in directly phase-matched PG-SHG. We observe a frequency offset of $< 1\mathrm{~Hz}$, contrasting with previous limitations in quasi-phase-matched configurations. Furthermore, we measure a residual fractional frequency instability of $2\times 10^{-15}$ at $1\mathrm{~s}$, averaging down to the $10^{-16}$ level at $1000\mathrm{~s}$, with multi-hour deviations remaining below $1\mathrm{~Hz}$. These results establish directly phase-matched PG-SHG as a robust, metrologically compatible route to effective $χ^{(2)}$ functionality, combining sub-Hz frequency-ratio fidelity and high coherence on a mature integrated platform for optical clockworks, self-referencing, and precision spectroscopy.
