Enhanced Second Harmonic Generation in Dual-Resonance 3R-MoS2 Metasurfaces, Roles of the Q Factor and Absorptance

Abstract

Resonant dielectric metasurfaces supporting quasi-bound states in the continuum (q-BICs) provide a powerful platform for enhancing light-matter interactions and nonlinear optical processes such as second-harmonic generation (SHG). In this thesis, SHG in dual-resonance 3R-MoS2 metasurfaces is investigated through a combined theoretical, numerical, and experimental study. Traditionally, SHG intensity has been assumed to scale with the square of the resonance quality factor (Q^2). However, experiments show that samples with similar Q values can exhibit very different SHG outputs, indicating that, for the sample-to-sample variations studied here, pump absorptance also plays an important role in determining the nonlinear response. Based on temporal coupled-mode theory (TCMT), a compact expression is used to describe the dependence of SHG intensity on both the resonance quality factor (Q) and the pump absorptance (A), highlighting the balance between field confinement and pump coupling in the present structure. To verify this, dual-resonance 3R-MoS2 metasurfaces were designed and fabricated, combining reflective and q-BIC resonances. Numerical simulations and optical measurements show overall consistency with the TCMT-based description. The best-performing sample exhibits a conversion efficiency of about 0.002% and an SHG enhancement of approximately 40 times compared with an unpatterned flake of the same thickness. These results clarify the role of pump absorptance together with the resonance Q factor in the dual-resonance metasurfaces studied in this thesis and provide guidance for the analysis and design of nonlinear metasurfaces based on similar resonant platforms. Show more