ultrathin wse₂ quantum metasurfaces enabling efficient shg for integrated photonic devices

Ultrathin quantum materials such as WSe₂ offer strong nonlinear optical response, but their nanometric thickness and centrosymmetric phases limit second-harmonic generation (SHG) without resonant enhancement. We design symmetry-engineered WSe₂ metasurfaces that use geometric asymmetry to activate bright quasi-bound states in the continuum (q-BICs). Using COMSOL Multiphysics with Floquet boundaries, anisotropic refractive index, and the χ² tensor of 3R-WSe₂, we model 20–25 nm films patterned into circular and triangular nanohole lattices. Circular apertures preserve near-C₄ symmetry and support symmetry-protected BICs, while triangular apertures introduce C₃v symmetry breaking, enabling strong radiative coupling and intense near-field localization. Simulations solve the pump field at ω, compute the nonlinear polarization P(2ω), and evaluate SHG using frequency-domain excitation with perfectly matched layers. Triangular lattices show over an order-of-magnitude SHG enhancement, Q-factors approaching 10³, and peak efficiencies near 10⁻³ at ~816 nm. These results demonstrate a computationally validated approach for ultrathin nonlinear devices, enabling compact frequency converters, modulators, and integrated quantum photonic interfaces.