Room-Temperature Lasing at Flatband Bound States in the Continuum

High-quality factor optical modes with a low dispersion in the momentum space are highly desirable for applications such as low-threshold lasers, strong light-matter interactions, and optical trapping. Bound states in the continuum (BICs) have recently gained attention as a promising optical cavity concept due to their theoretically infinite quality factors. However, their quality factor decreases exponentially when deviating from the BIC singularity in the momentum space, which limits their practical use. Here, we present a design concept and experimental realization of flatband BICs in a rectangular array of titanium dioxide nanopillars. By precisely engineering the interaction between four counterpropagating guided modes in the array, a nondispersive BIC band can be obtained. The flatband BIC exhibits an enhanced quality factor near the Γ-point by 2 orders of magnitude compared to that of the symmetry-protected BIC mode in a square array, along with an exceptionally high optical density of states. As a result, we achieve room-temperature lasing at the flatband BIC with a quality factor of ∼9100 and a threshold 4 times lower than that of the symmetry-protected BIC. The flatband-BIC lasing properties, such as directionality and topological charge, are also studied in detail. The concept and outstanding lasing performance of the flatband BICs presented in our work mark an important step toward efficient optical cavities and microlasers and hold great potential for advanced photonic and optoelectronic devices.