Silicon-integrated nonlinear III-V photonics
Fig. 1. (a) Schematic of the AlGaAs-Si integration architecture. (b) Vertical heterogeneous integration process using wafer bonding technique.
Fig. 2. (a) Measured Q factor of AlGaAs microring on SOI. (b) Kerr frequency comb generated in AlGaAs microring with pump/output accessed via Si waveguides.
Driven by cost-effective and massive mature CMOS technology, silicon photonics (SiPh) based on Si-on-insulator (SOI) substrates has made a big success to provide various photonics integrated circuits (PICs) building blocks from individual devices to systems in high-volume manufacturing, including nanowaveguide-based passive components, Si PN junction-based modulators, SiGe detectors, and even laser sources based on III-V/Si heterogeneous integration technique.
Undoubtedly, SiPh plays a central role in today's PICs community and has found applications in a large number of areas such as optical interconnects, telecommunications, computing, and so on. Meanwhile, integrated nonlinear photonics ⎯a counterpart of conventional linear integrated photonics ⎯ has emerged as a new direction for scientific exploration over the last decade, primarily due to its unqie ability to generate on-chip new classes of coherent, ultra-broadband light sources (e.g., microcombs), and consequently opened up new perspectives for a wide variety of applications such as coherent datacom, precision measurement, metrology, quantum optics, etc.
In particular, recent breakthroughs in ultralow-loss nanophotonics have enabled high-efficiency nonlinear operation at dramatically reduced power levels of milliwatts or even lower and really allowed the possibility of fully chip-scale nonlinear implementation that is capable of both nonlinear signal generation and processing.
In such context, naturally, it is desirable to incorporate the intriguing nonlinear functionalities into conventional mature SiPh platform towards fully-integrated nonlinear engines for innovative applications. Unfortunately, Si has inherent bottlenecks in nonlinearity (e.g., two-photon absorption and the absence of the 2nd nonlinearity) at the most important telecom bands, hampering its application in efficient nonlinear PICs.
Therefore, it has to be necessary to search for a suitable material candidate to combine its potential excellent nonlinear properties with existing SiPh for fully functional nonlinear chips. More importantly, this combination should fulfill the requirement of compatibility in both power budget and fabrication technology for future scalable and advanced integration.
To address the above issue, in this article, the researchers from the Prof. John E. Bowers' group at the University of California, Santa Barbara experimentally realized high-Q AlGaAs microresonators heterogeneously integrated on Si waveguides on the SOI substrate and demonstrated low-power-threshold microcomb generation at telecom bands on this AlGaAs-SOI platform. Related results were published in Photonics Research Vol. 10, Issue 2, 2022 (Weiqiang Xie, Chao Xiang, Lin Chang, Warren Jin, Jonathan Peters, John E. Bowers. Silicon-integrated nonlinear III-V photonics[J]. Photonics Research, 2022, 10(2): 02000535).
They report a vertical heterogeneous integration approach to bridge high-index, highly-nonlinear AlGaAs, which has been termed as "the silicon of nonlinear optical materials", and SiPh on SOI platform for the development of power-efficiency, scalable Si nonlinear photonics in future, as shown in Fig. 1.
In this work, the authors propose a general architecture in which the independent Si and AlGaAs PICs layers can be coupled efficiently with each other. Meanwhile, this architecture possesses compatibility in both function and fabrication. They develop a reliable fabrication technology using the wafer bonding method (Fig. 1b), which can integrate high-quality (Q over 106) AlGaAs microresonators with SiPh on a heterogeneous SOI platform.
Such heterogeneous integration can be realized in a back-end-of-line integration approach at relatively low processing temperature (≤200°C) and hence fully compatible with existing passive/active Si PICs (waveguides, Si PN junction-based modulators, SiGe detectors, III-V/Si lasers, etc.) for developing large-scale and advanced applications. The fabricated device shows submilliwatt-threshold (~ 0.25 mW) frequency comb generation at the telecom bands (~1550 nm) on the SOI platform for the first time, as shown in Fig. 2.
This work bridges the superior nonlinear AlGaAs and the mature SiPh in seamless integration and demonstrates the compatibility in both fabrication technology and operation power budget between the two classes of photonic functionalities. This would enable further scalable and complex nonlinear PICs and miniaturized systems hosted in the SOI platform, by leveraging mature SiPh technology. The result provides a promising route towards fully-integrated nonlinear chips and propels the related fundamental research and applications into a new phase.
"This work successfully adds the missing building blocks ⎯ nonlinear functionalities ⎯ to the current Si photonics platform, by integrating AlGaAs, which is well-known for its both strong second- and third-order nonlinearities, on SOI substrate", Dr. Weiqiang Xie comments, "The technique itself is straightforward but very effective and could be extended to construct a complex structure that consists of more functional SiPh and AlGaAs nonlinear functions without limitations in processing. Thus, we can see that this work will open many new possibilities for future nonlinear PICs in many exciting application areas."
Currently, Prof. John E. Bowers' group is making an effort on the further improvement of Q factor of AlGaAs microresonators on SOI platform thus to lower the power threshold for nonlinear processes and to investigate the approaches for generating soliton microcombs. Meanwhile, more building blocks from Si and AlGaAs photonics will be integrated together for fully chip-scale nonlinear signal generation and processing. The relevant applications based on integrated microcombs are also under development.
硅集成的三五族非线性光子学
图1、（a） AlGaAs-Si 集成架构；（b）采用晶圆键合技术的垂直异质集成过程
图2、（a）实验测试的SOI上集成的AlGaAs微环的Q值；（b）在AlGaAs微环中的克尔光频梳的产生，其中泵浦和输出通过Si波导
凭借CMOS技术的推动，硅光子学无疑在今天的集成光子学领域位居核心，并在光互联、光通信、光计算等领域得到了广泛应用。与此同时，发端于过去十多年的集成非线性光子学成为新的科学探索方向并为诸如相干光通信、精密测量、光量子集成芯片等领域打开了全新的视野。特别是近年来在超低损耗纳米光子学方面的突破，根本上导致了阈值功率达毫瓦甚至更低的高效非线性过程，真正意义上使全集成片上非线性应用成为可能。
在此背景下，人们便自然地渴望将非线性光子学和主流硅光子学结合，来开发全集成的非线性引擎及其应用。然而，在集成光子最重要的通信波段，硅具有内在的瓶颈限制，比如双光子吸收以及二阶非线性的缺失。
因此，寻找一种合适的材料并将其潜在优异的非线性特性和硅光子结合就显得非常必要。更重要的是，这种结合要满足两方面的兼容性：系统的光功率预算和制备技术，以面向未来规模化的、更先进的集成。
为了解决以上问题，近日，加州大学圣塔芭芭拉分校John E. Bowers 教授研究团队在实验中实现了集成在硅波导上面的高质量的AlGaAs微腔，同时在这种AlGaAs-SOI平台上首次验证了在通信波段的低功率阈值光频梳的产生。相关结果发表于 Photonics Research 2022年第2期 (Weiqiang Xie, Chao Xiang, Lin Chang, Warren Jin, Jonathan Peters, John E. Bowers. Silicon-integrated nonlinear III-V photonics[J]. Photonics Research, 2022, 10(2): 02000535)。
该团队报道了一种垂直结构异质集成的方案（图1a），目标是将同样高折射率、高非线性系数的砷化镓铝(AlGaAs)集成到SOI平台，为未来在硅光平台上开发高效的、规模化的非线性集成光子；而AlGaAs材料本来就被称为是非线性材料里面的“硅”，因此可以说这是一种完美的结合。
研究团队首先设计了一种普遍的架构，其中相互独立的Si和AlGaAs光子集成层可以高效地耦合起来。同时，该架构具有功能和制备方面的兼容性。进而，他们利用晶圆键合的方法（图1b）开发了一种可靠的制备技术，该技术能够将高质量的AlGaAs微腔（Q值高达106以上） 和硅光在SOI平台上实现异质集成。
这种异质集成能在温度相对较低的（≤200°C）后端硅光工艺中实现，因此完全兼容现有的无源和有源硅光器件（包括波导、Si PN结调制器、SiGe探测器，III-V/Si异质集成激光器等）来开发大规模、更高级的应用。研究制作的器件展示了在1550 nm通信波段、亚毫瓦阈值功率（约0.25 mW）的微腔光频梳产生（图2），该结果也是首次在SOI平台上实现这一功能。
这一工作将优异非线性AlGaAs材料和相对成熟的硅光子学无缝衔接，并证明了这两种不同类型的光子功能在制备技术和功率预算方面的兼容性。通过充分利用成熟的硅光技术，该成果将进一步赋能规模化、复杂化的、基于SOI平台的片上微型非线性光子集成系统。这一结果为全集成的非线性芯片提供了一种有希望的途径，并将相关的基础研究和应用推到一个新的阶段。
“这项工作通过在SOI衬底上集成AlGaAs ⎯以其强的二阶、三阶非线性著称的材料成功地将缺失的非线性功能组件加入到了目前的硅光平台”，谢卫强博士表示，“该技术本身直接而又非常有效，未来可以扩展到构建包含了更多硅光和AlGaAs非线性功能的复杂结构而没有加工工艺的限制。因此，我们能够看到这项工作将为未来非线性集成光子在诸多令人振奋的应用领域带来了新的可能性。”
目前，John E. Bowers 教授课题组正在努力探索如何进一步提高AlGaAs微腔在SOI平台上的Q值来进一步降低非线性阈值，并研究如何产生孤子光频梳。同时，基于该平台，他们还在探索将更多功能的硅光子和AlGaAs非线性相结合以期望实现全芯片级的非线性光信号产生和处理。基于集成光频梳的相关应用也在开发中。