Near-unity spontaneous emission factor InP surface-emitting lasers based on quasi-bound states in the continuum

Surface-emitting lasers featuring optical bound states in the continuum (BICs) have recently emerged as a promising alternative to vertical cavity surface-emitting lasers. However, structural damage caused by top-down fabrication processes remains as a major obstacle that limits device performance. Here, we overcome this bottleneck by demonstrating surface-emitting quasi-BIC lasers fabricated with a bottom-up, etching-free process. We epitaxially grow arrays of crystal phase-engineered InP nanosheets with atomically smooth sidewalls as both the laser active medium and a symmetry-broken quasi-BIC cavity, strategically optimizing coupling between spontaneous emission and the lasing mode. Furthermore, we leverage the dipole selection rule associated with the wurtzite crystal structure of the nanosheets for side mode suppression, achieving single-mode lasing at room temperature with lasing threshold as low as 14 μJ cm^-2 pulse^-1. Optimal light-cavity coupling is evidenced by spontaneous emission factors as high as 0.8, showcasing the potential of bottom-up BIC lasers in realizing near-thresholdless lasing at room temperature.