Combining Light Enhancement and Incident Angle Robustness by Merging Bound States in the Continuum

ABSTRACT

Despite decent progress on BIC‐based devices, they are essentially sensitive to structural deformations and excitation conditions, making them vulnerable to fabrication imperfections and incident angle. Furthermore, their resonance modes are highly localized and strongly confined inside the structure, leading to insufficient overlap between field distribution and active emitters such as quantum dots and dye molecules. Here, to overcome these drawbacks, we present a subtle strategy that combines high imperfection tolerance and adequate light‐matter interaction. In our design, a flat band is formed along high‐symmetry directions in the Brillouin zone by merging the symmetry‐protected BICs and accidental BICs. Different from conventionally established BICs, the quality factors of the proposed design are maintained > 10 ⁸ within a wide range even though the structure parameters and excitation angles largely deviate from the BIC condition. More importantly, a hole is purposely designed to effectively concentrate the electric field, which can significantly overlap with the active emitters and subsequently ensure strong light‐matter interactions. Finally, when the fluorescent dye molecules are aligned with the electromagnetic field hotspot region, a certain fluorescence enhancement effect occurs. Our results illustrate an effective way to take advantage of BICs and have potential applications in bioimaging, sensing, and fluorescence enhancement.