[論文レビュー] 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被引用数 72

ひとこと要約

この論文は、超滑らかな Si3N4 波長導波路を実現する光子 Damascene リフロープロセスを提示し、散乱損失と吸収損失を分解して金属不純物を主要な吸収源として特定する。

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.

研究の動機と目的

• Demonstrate a fabrication method (photonic Damascene with reflow) to achieve ultra-smooth, tightly confined Si3N4 waveguides with high Q.
• Quantify and distinguish scattering and absorption loss mechanisms in the waveguides and microresonators.
• Identify impurity-related absorption as a significant loss contributor in integrated photonics.
• Provide insights to guide fabrication toward ultra-high Q regimes in nonlinear photonics.

提案手法

• Adopt photonic Damascene process to invert patterning by structuring a preform with recesses and filling with Si3N4, alleviating film stress.
• Introduce a preform reflow step by heating slightly above the glass transition temperature to smooth sidewalls and reduce roughness.
• Characterize losses by analyzing microresonator resonances to extract intrinsic loss rate kappa0/2pi across many resonances.
• Use resonance doublet analysis with a complex coupling coefficient to separate coherent and dissipative scattering contributions.
• Perform thermal bistability (heating-induced resonance shifts) measurements to bound absorption loss rate kappa_abs and correlate with intrinsic losses.
• Employ SIMS and GDMS to identify transition metal impurities (notably Cu) and hydrogen-related species as potential absorption sources.

実験結果

リサーチクエスチョン

• RQ1 What is the distribution of intrinsic loss rates in high-confinement Si3N4 waveguides across different wafers and processing parameters?
• RQ2 What are the relative contributions of scattering versus absorption to propagation losses in ultra-smooth Si3N4 waveguides?
• RQ3 Does the photonic Damascene reflow step reduce scattering losses in tightly confining waveguides?
• RQ4 Are there impurity-related (especially metal impurity) absorption mechanisms contributing to loss in integrated Si3N4 waveguides?
• RQ5 How do cladding materials and processing steps influence overall loss budgets and Q factors?

主な発見

• The reflow step substantially reduces scattering losses in 1.0–1.5 μm wide waveguides, contributing to record mean Q factors >5×10^6 for tightly confined Geometries.
• Intrinsic loss rates allow estimating scattering losses around 45 MHz for 1.5 μm wide, with absorption making up about half of total losses in the best samples.
• Absorption loss rates κ_abs/2π are bounded around 9–20 MHz in the telecom range, compatible with broadband absorbing species rather than hydrogen-related overtone absorption alone.
• Thermal bistability measurements place upper limits on κ_abs that align with residual non-scattering losses, suggesting broadband absorbers are significant in the best devices.
• SIMS/GDMS analyses reveal transition metals (Cu ~10 ppm wt) in the Si3N4 core and high hydrogen/chlorine contents, indicating possible impurity-related absorption mechanisms beyond typical Si–H/N–H overtones.
• Cladding oxide influence (LTO) is identified as a major factor limiting current devices, with uncladded devices showing the best performance.