Highly integrated photonic crystal bandedge lasers monolithically grown on Si substrates

Advanced microscale silicon photonics technology has emerged as a promising candidate for the next-generation chip-scale data communication network due to its unique advantages of low cost, high integration density, high speed and energy efficient. However, a highly efficient microscale Si-based light source is still considered as the obstacle for realizing a practical Si-based photonic integrated circuit (PIC), due to the indirect bandgap nature of bulk Si and Ge materials.

Various approaches towards integration of microcavity lasers on Silicon substrates have been demonstrated extensively, including co-packing, hybrid integration, wafer bonding, etc. But these methods more or less include cumbersome fabrication process. Monolithic integration method, however, is another promising route towards low cost and scalable Si-based PICs. Because monolithic integration takes advantages of advanced fabrication process of contemporary integrated circuits, in other words, it is unnecessary to consider heterogeneous way to make precise alignment between light source and other optical components. Nevertheless, the fundamental challenge of epitaxial III-V on Si is the degraded material quality caused by large material dissimilarities. The planar defect, anti-phase boundary, due to the polar on non-polar growth, can be avoided by employing pretreated double-atomic-step Si substrate. Furthermore, strained-layer superlattice (SLS) has been used as defect filter layers to significantly reduce the threading density defect caused by the lattice mismatched heteroepitaxial growth of III-V materials on Si. In addition, semiconductor lasers with quantum dots (QDs) as gain material have been extensively investigated due to its robust tolerance to defects, which is significant to PIC.

The research group led by Professor Zhaoyu Zhang from The Chinese University of Hong Kong, Shenzhen demonstrated InAs/GaAs QD square-lattice photonic crystal bandedge lasers monolithically grown on on-axis Si (001) substrates. The lasers operated under continuous-wave optical pumping at room-temperature. Ultra-low lasing threshold was observed in a multi-mode lasing operation（Fig. 1）. The research results are published in Chinese Optics Letters, Volume 20, No. 4, 2022 （Yaoran Huang, et al. Highly integrated photonic crystal bandedge lasers monolithically grown on Si substrates.）

In this work, they demonstrated InAs/GaAs QD square lattice photonic crystal bandedge lasers monolithically grown on on-axis Si (001) substrate with ultra-low lasing threshold under 632.8 nm He-Ne gas laser source at room temperature. The fabricated photonic crystal slab in this work utilizes the band-edge effect of photonic crystals to achieve in-plane light resonance. By tuning the structural parameters of the band-edge photonic crystals, photonic crystal surface-emitting lasers (PCSELs) with a much smaller footprint can achieve precise wavelength control and mode field confinement, and finally achieve a higher quality factor than conventional vertical-cavity surface-emitting lasers (VCSELs). The far-field pattern with Gaussian-like distribution from PCSEL is easier to be coupled into single-mode fiber, which is beneficial to realize silicon photonics, gas sensing and other related applications. In order to further enhance the selection and confinement of optical modes, the optical band gap (PBG) effect of photonic crystals may be utilized to design point defects or line defects within the photonic crystals, enabling selection of defect modes with different symmetries by destroying its periodic structure.

The corresponding mode will be localized near the defect center, thereby achieving controllable emission direction, low divergence angle and higher output power. The narrow linewidth of PCSEL makes it possible to greatly reduce modal dispersion and transmission loss during transmission after subsequently coupling to a single-mode waveguide. Therefore, this square-lattice photonic crystal band-edge laser is expected to become a coherent light source for large-scale monolithic integrated silicon photonics systems in the future.

可单片集成硅基带边型光子晶体激光器

硅光技术具有高集成度、高速、能耗低等优点，被视为下一代芯片级通讯传输网络的候选人。但是硅或锗等四族材料间接带隙的特征，使得其辐射复合效率低，因此迫切需要高效率的微米级硅基光源来突破硅基光子集成电路的这一障碍。

近年来，共封装、混合集成和晶圆键合等将激光器集成在硅衬底上的方案被相继提出，但其制造工艺较为复杂。单片集成技术利用了集成电路发达的制造工艺，使光源在其他光学无源器件集成成为可能，并极大程度提高了容错率，降低了制造成本。同时，三五族材料外延在（001）晶相的硅衬底的方案利用应变层超晶格层、缺陷过滤层等外延层可以有效减少螺纹缺陷对材料质量的影响。而且，利用 InAs 量子点作为增益材料能够进一步增强三五组外延材料对晶格缺陷的抵抗能力，这为实现光源在硅基上单片集成奠定了坚实的基础。

与此同时，光子晶体带边激光器阵列可实现大面积相干震荡，因此衍生出大量具有吸引力的应用，如基于激光的加工和光探测。在光子晶体能带图中，高度对称点的群速度较低，当在材料增益区间内的模式的群速度接近于零时，就会出现带边效应。带边点的光程长度大幅增加，提高了有效增益。目前，已有多种高性能光子晶体表面发射激光器（PCSEL）被证实具有高输出功率和单模激射。

香港中文大学（深圳）的张昭宇教授团队推出了一种可高度集成的硅基带边型二维光子晶体InAs量子点激光器，该激光器在室温连续光泵浦的条件下可实现1.3 μm通讯波段的超低阈值激射，有望作为 CMOS 工艺兼容的光电子集成电路的光源。相关研究成果发表在Chinese Optics Letters 2022年第20卷第4期上（Yaoran Huang, et al. Highly integrated photonic crystal bandedge lasers monolithically grown on Si substrates），并被选为当期封面。

该工作提出了在同轴（001）硅衬底上单片生长的InAs量子阱光子晶体带边型激光器，见图1。借由632.8 nm的He-Ne激光器作为光泵浦源，该激光器在室温条件下实现了超低阈值激射输出。其中，光子晶体平板利用光子晶体的带边效应实现面内光的谐振。通过调控带边光子晶体的结构参数，PCSEL可以在较小的尺寸上实现精确的波长调控和模式场限制，最终实现较传统的垂直腔表面发射激光器（VCSEL）更高的品质因子。

此外，PCSEL具有更接近高斯分布的远场光斑，更易耦合至单模光纤中，这有利于实现其在硅光、气体传感等相关领域的应用。为进一步增强光学模式的选择和调控，可利用光子晶体的光学带隙（PBG）作用，在光子晶体内部设计点缺陷或线缺陷，通过破坏其周期性结构的方式选取不同对称性的缺陷模式，将对应的模式局域化在缺陷中心附近，进而实现可控的发射方向、低发散角和更高的输出功率。

PCSEL窄线宽的特性，使得其在后续耦合到单模波导传输后，模式色散和传输损耗大大降低。因而这种正方晶格光子晶体带边激光器有望成为未来大规模单片集成硅光系统的相干光源。