Nitiss Edgars, Liu Tianyi, Grassani Davide, Pfeiffer Martin, Kippenberg Tobias J, Brès Camille-Sophie
Ecole Polytechnique Fédérale de Lausanne, Photonic Systems Laboratory (PHOSL), STI-IEL, Station 11, CH-1015 Lausanne, Switzerland.
Ecole Polytechnique Fédérale de Lausanne, Laboratory of Photonics and Quantum Measurements (LPQM), SB-IPHYS, Station 3, CH-1015 Lausanne, Switzerland.
ACS Photonics. 2020 Jan 15;7(1):147-153. doi: 10.1021/acsphotonics.9b01301. Epub 2019 Dec 9.
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 (SiN) 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 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 gratings. Two-photon microscopy measurements of the χ 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.
氮化硅已成为构建光子集成电路的一个重要平台。虽然基于三阶效应的非线性特性已得到成功利用，但最近研究表明，在经过全光极化处理后，标准化学计量比的氮化硅（SiN）波导中也能实现高效的二次谐波产生。这种现象的根源被归因于沿波导写入的自组织周期性空间电荷光栅，它能实现有效的χ并使泵浦光和二次谐波自动准相位匹配。然而，不同参数及其在提高该过程效率中的作用仍未被完全理解。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。泵浦光和二次谐波光学模式之间的重叠积分被证明是决定χ光栅特性的关键参数。对χ光栅的双光子显微镜测量揭示了研究中使用的一些波导中存在二次周期性。根据重叠积分模拟，这种效应可能是由于波导弯曲处的模式混合引起的。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增加而提高。然而，为了启动极化，必须将一个临界泵浦强度耦合到波导中，该强度对于较长的波导更低。时间和热稳定性测试揭示了负责光栅写入的电荷陷阱的性质。在将热激活跳跃作为我们样品中的导电机制应用后，我们表明在全光极化过程中似乎只有浅陷阱被激活。
