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 Si3N4. 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/cm2, which is significantly lower than those reported in previous studies.