Quasi-Phase-Matched Frequency Conversion in van der Waals Semiconductors: Thicker May Not Be Better.

Emerging nonlinear van der Waals (vdW) crystals show strong nonlinearity for various applications in nonlinear optics. Here, we investigate the characteristics of second-harmonic generation in vdW semiconductors under transmission loss conditions using 3R-stacked MoS as a model system. Incorporating dissipation terms and Fabry-Pérot interference effects, we establish a nonlinear transmission model applicable to finite-thick lossy van der Waal materials. Our analysis reveals that strong second-harmonic absorption leads to rapid intensity saturation, rendering conventional phase-matching strategies ineffective. To overcome this limitation, we propose a quasi-phase-matching scheme based on multilayer stacking with controllable orientation angles, enabling effective enhancement of second-harmonic signals even under finite-loss conditions by adjusting the thickness and lattice orientation of each layer. This work uncovers the loss mechanisms and quasi-phase-matching behavior in nonlinear-optical processes of two-dimensional materials, important for high-efficiency ultrathin nonlinear devices, integrated photonic chips, and quantum photonic technologies based on vdW structures.