[论文解读] Probing the loss origins of ultra-smooth $\mathrm{Si_3N_4}$ integrated photonic waveguides

Martin H. P. Pfeiffer, Junqiu Liu+4 more|arXiv (Cornell University)|Feb 12, 2018

Advanced Fiber Laser Technologies参考文献 55被引用 72

一句话总结

该论文提出了一种光子Damascene回流工艺，用于制造超光滑的Si3N4波导，达到创记录的低传输损耗，并解析散射与吸收损耗，识别金属杂质为关键吸收源。

ABSTRACT

On-chip optical waveguides with low propagation losses and precisely engineered group velocity dispersion (GVD) are important to nonlinear photonic devices such as soliton microcombs. Yet, despite intensive research efforts, nonlinear integrated photonic platforms still feature propagation losses orders of magnitude higher than in standard optical fiber. The tight confinement and high index contrast of integrated waveguides make them highly susceptible to fabrication induced surface roughness. Therefore, microresonators with ultra-high Q factors are, to date, only attainable in polished bulk crystalline, or chemically etched silica based devices, that pose however challenges for full photonic integration. Here, we demonstrate the fabrication of silicon nitride ($\mathrm{Si_3N_4}$) waveguides with unprecedentedly smooth sidewalls and tight confinement with record low propagation losses. This is achieved by combining the photonic Damascene process with a novel reflow process, which reduces etching roughness, while sufficiently preserving dimensional accuracy. This leads to previously unattainable \emph{mean} microresonator Q factors larger than $5 imes10^6$ for tightly confining waveguides with anomalous dispersion. Via systematic process step variation and two independent characterization techniques we differentiate the scattering and absorption loss contributions, and reveal metal impurity related absorption to be an important loss origin. Although such impurities are known to limit optical fibers, this is the first time they are identified, and play a tangible role, in absorption of integrated microresonators. Taken together, our work provides new insights in the origins of propagation losses in $\mathrm{Si_3N_4}$ waveguides and provides the technological basis for integrated nonlinear photonics in the ultra-high Q regime.

研究动机与目标

• 展示一种制备方法（光子Damascene并回流）以实现超光滑、强限制的Si3N4波导，具备高Q值。
• 量化并区分波导和微腔中的散射损耗与吸收损耗机制。
• 识别与杂质相关的吸收作为集成光子学中的重要损耗源。
• 提供见解以引导在非线性光子学中实现超高Q的制备策略。

提出的方法

• 采用光子Damascene工艺，通过在预制件中布置凹槽并充填Si3N4来反转图案化，从而缓解薄膜应力。
• 引入预制件回流步骤，在略高于玻璃转变温度的温度下加热，以抚平侧壁并降低粗糙度。
• 通过分析微腔谐振来表征损耗，以提取在多谐振下的固有损耗率kappa0/2pi。
• 使用具有复耦合系数的共振双峰分析以分离相干与耗散散射贡献。
• 进行热双稳态（加热引起的谐振位移）测量来限定吸收损耗率kappa_abs并与固有损耗相关联。
• 使用SIMS和GDMS来识别过渡金属杂质（特别是Cu）以及与氢相关的物种，作为潜在的吸收源。

实验结果

研究问题

• RQ1 在不同晶圆和加工参数下，高约束Si3N4波导的固有损耗率分布是什么？
• RQ2 在超光滑的Si3N4波导中，散射与吸收对传播损耗的相对贡献是多少？
• RQ3 光子Damascene回流步骤是否在强约束的波导中降低了散射损耗？
• RQ4 是否存在与杂质相关的（特别是金属杂质）吸收机制，导致集成Si3N4波导的损耗？
• RQ5 包层材料和加工步骤如何影响总体损耗预算和Q因子？

主要发现

• 回流步骤在1.0–1.5 μm宽度波导中显著降低散射损耗，为强约束几何形状带来创纪录的平均Q因子>5×10^6。
• 固有损耗率可估计1.5 μm宽度的散射损耗约为45 MHz，在最佳样品中吸收约占总损耗的一半。
• 吸收损耗率κ_abs/2π在通信波段约为9–20 MHz，与宽带吸收种类相符，而不仅仅是氢相关的泛音吸收。
• 热双稳态测量给出κ_abs的上限，与剩余非散射损耗一致，表明宽带吸收源在最佳器件中具有重要作用。
• SIMS/GDMS分析显示Si3N4芯材中存在过渡金属（Cu ~10 ppm wt）以及高含量的氢/氯，表明可能存在超出典型Si–H/N–H泛音吸收的杂质相关吸收机制。
• 未包层的包层氧化物（LTO）影响被确认是限制当前器件的主要因素，未包层的器件表现最好。