Quasiphase matching (QPM) serves as an effective strategy to achieve second harmonic generation (SHG) with high conversion efficiency, which compensates for phase mismatch through periodic poling of nonlinear coefficients. Unlike traditional materials that rely on high-voltage electric fields or complex fabrication for periodic poling, two-dimensional (2D) layered materials enable QPM through nanoscale thickness control and tunable twist angles. In addition, the larger nonlinear susceptibility of 2D materials enables the realization of high-power coherent light sources using SHG technology. By combining the nonlinear transfer matrix method with experimental validation, we demonstrate that reverse stacking of ${\mathrm{NbOBr}}_2$ with optimized thickness achieved a four to five times SHG enhancement in both reflection and transmission configurations. Notably, the transmission SHG attained a conversion efficiency of $5.8\times {10}^{-6}\%$ under an ultralow power density of $10\mathrm{MW}/\mathrm{c}{\mathrm{m}}^2$. Our experiments confirm that ${\mathrm{NbOBr}}_2$ exhibits low dispersion and a large SHG response in the visible range, making it an ideal 2D nonlinear crystal for phase-matching enhancement through reverse stacking.

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• Layered crystals
• Nonlinear optical materials
• Optical second-harmonic generation