Polarization-Independent Enhancement of Third-Harmonic Generation Empowered by Doubly Degenerate Quasi-Bound States in the Continuum

Recent advancements in nonlinear nanophotonics are driven by the exploration of sharp resonances within high-index dielectric metasurfaces. In this work, we leverage doubly degenerate quasi-bound states in the continuum (quasi-BICs) to demonstrate the robust enhancement of third-harmonic generation (THG) in silicon metasurfaces. These quasi-BICs are governed by <i>C</i>_4<i>v</i> symmetry and therefore can be equally excited with the pump light regardless of polarization. By tailoring the geometric parameters, we effectively control <i>Q</i>-factors and field confinement of quasi-BICs and thus regulate their resonantly enhanced THG process. A maximum THG conversion efficiency up to 1.03 × 10^-5 is recorded under a pump intensity of 5.85 GW/cm². Polarization-independent THG profiles are further confirmed by mapping their signals across the polarization directions. This work establishes foundational strategies for the ultracompact design of robust and high-efficiency photon upconversion systems.