Efficient second-harmonic generation based on off-Γ merging bound states in the continuum

Ultracompact devices engineered for second-harmonic generation (SHG) hold a significant promise across a diverse spectrum of applications. Here, we propose a merging bound state in the continuum at an off-Γ point in a reciprocal space with the anisotropic materials lithium niobate. Such a merging BIC yields a profound reduction in radiative loss and scattering losses while concurrently exhibiting a substantial enhancement in the quality factor. As a result, we achieved a noteworthy SHG efficiency ( η = 3.7%) at the incident angle θ = 10° when the pump intensity I 0 = 2 kW/cm 2 , outperforming alternative nanostructures designed for SHG. This research contributes valuable insights into the feasibility of metadevices founded on the principles of nanoengineering applied to traditional nonlinear crystals. Such advancements hold a considerable promise for the development of compact, high-performance SHG devices across a range of applications.