High-efficiency third-harmonic generation enabled by merging bound states in the continuum

Ultracompact third-harmonic generation (THG) devices are critical for integrated photonics, nonlinear sensing, ultrafast optical modulation and more. However, conventional systems based on silicon or lithium niobate suffer from severe radiative losses in resonant modes, fundamentally limiting THG efficiency to below ${10}^{-6}$ even under gigawatt-level pump intensities. Recently, the system based on isolated bound states in the continuum (BIC) have shown that THG efficiency can be enhanced to ${10}^{-3}$, but it remains at a relatively low level. Here, we propose a strategy to significantly enhance third-harmonic generation (THG) efficiency by merging BICs in gallium arsenide (GaAs) photonic crystal slabs. In contrast to previous studies utilizing lithium niobate or silicon-based platforms for second-harmonic generation (SHG) or THG, our GaAs-based design leverages its high third-order susceptibility. Numerical simulations demonstrate a THG efficiency of $3.2\times {10}^{-2}$ at an ultralow pump intensity ($2\text{kW}/{\text{cm}}^2$). Furthermore, we analyze the topological charge dynamics during the merging process between symmetry-protected and accidental BICs, revealing the physical mechanism behind the radiation suppression. Our work not only advances the application of merged BICs in third-order nonlinear processes, but also provides insights into tailoring BIC topology for efficient frequency conversion applications.