Robust single-mode laser via merging bound state in the continuum.

Bound states in the continuum (BICs) are optical states that remain perfectly confined despite existing within the radiation spectrum, enabling strong light confinement and light-matter interactions. These unique properties make BICs a promising platform for high-performance photonic crystal lasers. However, achieving robust, single-mode BIC lasers with compact footprints remains challenging due to mode competition and fabrication imperfections. Here, we demonstrate a robust single-mode laser by leveraging the concept of "merging BIC" in momentum space, which enables stable lasing behavior up to 80 times the threshold power, showcasing exceptional mode stability under high-power excitation. Furthermore, we realize an ultra-compact photonic crystal laser by combining the BIC mode with edge engineering, achieving a 5 × 5 periodic array in which the entire patterned photonic crystal region has an area smaller than 15 μm². These results provide a promising pathway toward high-performance, miniaturized lasers for photonic applications.