RESEARCH ARTICLE

Performance optimization of planar photonic crystal bound states in the continuum cavities: mitigating finite-size effects  

Ran Hao, Bilin Ye, Jinhong Xu, Yonggang Zou

2025, 18(1): 3.https://doi.org/10.1007/s12200-025-00147-5

Abstract：Bound states in the continuum (BICs) offer a promising solution to achieving high-quality factor (Q factor) cavities. However, finite-size effects severely deteriorate the BIC mode in practical applications. This paper reports the experimental demonstration of an electrically pumped 940 nm laser based on optimized BIC cavity, achieving a high Q factor of up to 1.18 × 10⁴ even with finite photonic crystal footprint, which is two orders of magnitude larger than un-optimized BIC design. Two strategies have been systematically investigated to mitigate finite-size effects: reflective photonic crystal cavity design and graded photonic crystal cavity design. Both methods significantly improve the Q factor, demonstrating the effectiveness of preserving BIC characteristics in finite-sized photonic crystal cavities. In addition, the reflective boundary photonic crystal design is fabricated and experimentally characterized to demonstrate its lasing characteristics. The fabricated laser exhibits single-mode operation with a signal-to-noise ratio of 38.6 dB. These results pave the way for future designs of BICs with finite size in real applications, promoting the performance of BIC-based integrated lasers.

研究背景

光子晶体腔因其高度可调控的腔行为而备受关注，但传统光子晶体激光器难以同时实现高Q值、窄线宽和高输出功率。近年来，基于束缚态在连续体（BIC）的光子晶体激光器因其理论上无限大的Q值而受到关注，然而实际应用中有限尺寸的光子晶体会严重破坏BIC模式，导致Q值大幅下降。

主要内容

本文研究了如何在有限尺寸的光子晶体中优化BIC腔的性能，以实现高Q值激光器。研究提出了两种策略：反射光子晶体腔设计和渐变光子晶体腔设计，并通过实验验证了这些设计的有效性。实验中展示了基于BIC的940 nm电泵浦激光器，实现了高达1.18×10⁴的Q值，比未优化时高出两个数量级。

方法

1. 垂直腔激光器设计：采用分布式布拉格反射镜（DBR）实现激光发射，P型和N型DBR镜分别位于顶部和底部，通过设计P-DBR镜的反射率低于N-DBR镜，实现激光从P-DBR表面单向发射。

2. 工艺方案：采用三步对准电子束光刻工艺，首先定义并蚀刻台面图案，随后精确对准并蚀刻光子晶体图案，最后进行电极图案化；通过优化反应离子蚀刻（RIE）参数，确保精确控制空气孔的尺寸和侧壁轮廓，这对于维持高Q值性能至关重要。

创新点

1. 首次实验展示：首次实现了基于BIC的940 nm电泵浦激光器，填补了该波段电泵浦BIC激光器的空白。

2. 反射光子晶体设计：通过在BIC光子晶体周围添加反射光子晶体，利用其带隙特性反射BIC模式，减少边界截断效应，Q值提升至13,974。

3. 渐变光子晶体设计：通过逐渐改变空气孔半径，创建平滑的模式过渡区，减少模式截断和辐射损耗，Q值最高可达92,091。

4. 高性能验证：实验验证的激光器展现出单模运行，信噪比高达38.6 dB，证明了设计的实用性。

总结

本研究通过创新的光子晶体设计，成功克服了BIC在有限尺寸光子晶体中的限制，为高性能BIC激光器的设计提供了新思路，推动了BIC技术在光子集成领域的应用前景。