Silicon nitride has emerged as a prominent platform for building photonics integrated circuits. While its nonlinear properties based on third-order effects have been successfully exploited, an efficient second harmonic generation in standard stoichiometric silicon nitride (Si3N4) waveguides can also be achieved after all-optical poling, as was recently shown. The root of such a phenomenon has been attributed to the inscription of a self-organized periodic space-charge grating along the waveguide, allowing an effective χ(2) and automatic quasi-phase-matching of pump and second harmonic. However, the different parameters and their role in increasing the efficiency of the process are still not fully comprehended. In this work, we use optical means to identify the general conditions of mode matching occurring during all-optical poling. The overlap integral between pump and second harmonic optical modes is shown to be the governing parameter in determining the features of the χ(2) gratings. Two-photon microscopy measurements of the χ(2) gratings reveal the presence of a secondary periodicity in some of the waveguides used in the study. According to overlap integral simulations, such an effect can occur due to mode mixing in the waveguide bends. From a study of poling dynamics, we observe that poling efficiency and rate increase as a function of optical pump power and waveguide length. However, in order to initiate poling, a critical pump intensity, which is lower for longer waveguides, must be coupled into a waveguide. Temporal and thermal stability tests reveal the nature of charge traps responsible for grating inscription. After applying thermally activated hopping as a conductivity mechanism in our samples, we show that only shallow traps seem to be activated during the all-optical poling process.

中文翻译：

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氮化硅波导中光诱导χ（2）光栅的形成规则和动力学。

氮化硅已经成为构建光子集成电路的重要平台。尽管已成功开发出基于三阶效应的非线性特性，但在全光极化后，也可以在标准化学计量的氮化硅（Si3N4）波导中实现高效的二次谐波生成，如最近所示。这种现象的根源归因于沿着波导的自组织周期性空间电荷光栅的刻写，从而实现了有效的χ（2）以及泵浦和二次谐波的自动准相位匹配。但是，仍然不能完全理解不同的参数及其在提高过程效率中的作用。在这项工作中，我们使用光学手段来确定全光极化过程中发生模式匹配的一般条件。泵浦和二次谐波光学模式之间的重叠积分显示为确定χ（2）光栅特征的控制参数。χ（2）光栅的双光子显微镜测量揭示了该研究中使用的某些波导中存在次级周期性。根据重叠积分模拟，由于波导弯曲中的模式混合，可能会发生这种影响。通过对极化动力学的研究，我们观察到极化效率和速率随光泵浦功率和波导长度的变化而增加。但是，为了启动极化，必须将临界泵浦强度耦合到波导中，该临界泵浦强度对于较长的波导而言较低。时间和热稳定性测试揭示了负责刻写光栅的电荷陷阱的性质。