BIC lasing from first principles

One of the main applications of the bound state in the continuum (BIC) is the low-threshold lasing. Ideally, the infinite quality factor of the BIC results in the zero-threshold and zero-linewidth lasing. However, various effects disturb the infinite quality factor. Instead, we have a dense distribution of eigenmodes around a target frequency of ideal systems, giving rise to a complex mode selection under finite pumping. We here study how the lasing modes are selected in finite photonic crystals composed of dielectric spheres with gain media. We employ a semi-classical approach, the so-called steady-state ab-initio laser theory combined with the multiple-scattering method. The theory predicts that the BIC certainly results in the low-threshold lasing in the finite systems. Moreover, the localfield enhancement of the BIC stabilizes the single-mode lasing far beyond the higher lasing threshold in the noninteracting limit.