Colloidal‐quantum‐dot nanolaser oscillating at a bound‐state‐in‐the‐continuum with planar surface topography for a high Q ‐factor

Solution‐based optical gain materials offer a cost‐effective path to coherent light sources. Further, bound states in the continuum (BICs) have garnered great interest owing to their diverging quality ( Q ) factors. Therefore, a hybrid of these – a solution‐based material for optical gain and a BIC structure for the lasing mode – should constitute an ideal form factor for low‐cost and low‐threshold nanolasers. However, the nonuniform surface topography induced during the thin‐film formation of a solution‐based material, especially on top of a prepatterned substrate, can easily disrupt the structural symmetry required for a high‐ Q BIC, resulting in a degradation of Q . Thus, in this study, a simple surface‐flattening technique utilizing a soft and flexible squeegee was applied, which realized the planar surface topography crucial for preserving the high Q promised by the BIC and achieving low‐threshold lasing. We fabricated BIC nanolasers by incorporating colloidal quantum dots (CQDs) for optical gain into a two‐dimensional photonic crystal backbone layer composed of Si 3 N 4 . By leveraging the unique properties of the BIC mode with a well‐ordered surface, our CQD‐based BIC laser exhibited a lasing threshold as low as 10.5 kW/cm 2 , which is significantly lower than those reported in previous studies.