Merging bound states in the continuum to enhance the Purcell effect in nanostructures

High-refractive-index dielectric subwavelength resonators enable the enhancement of spontaneous emission at the nanoscale. Merging bound states in the continuum (M-BICs) can suppress radiation losses and achieve an ultrahigh quality factor (Q-factor), offering an important platform for tailoring spontaneous emission. Herein, we investigate the Purcell effect in finite-size structures based on M-BICs. The proposed nanostructure consists of a metagrating composed of the phase-change material antimony selenide (Sb2Se3). By merging two accidental BICs, each with a topological charge of +1, M-BICs at the Γ point are constructed. This M-BICs exhibits high robustness and enable an enhanced Q-factor in finite-size nanostructures, thereby boosting the Purcell effect. A finite structure with four unit cells retains a high Q-factor of approximately 7.6 × 103 with a Purcell factor approaching 1.3 × 103. Moreover, the Purcell effect can be actively tuned by changing the crystallinity of Sb2Se3. This work provides a viable scheme for tunable light–matter interactions in finite-size structures, with potential applications in low-threshold micro/nanolasers, high-performance quantum light sources, and so on.