Nonlinear frequency conversion provides essential tools for generating new colors and quantum states of light. Transition metal dichalcogenides possess huge nonlinear susceptibilities; further, 3R-stacked transition metal dichalcogenide crystals possess aligned layers with broken inversion symmetry, representing ideal candidates to boost the nonlinear optical gain with minimal footprint. Here we report the second-order nonlinear processes of 3R-MoS₂ along the ordinary and extraordinary directions. Along the ordinary axis, by measuring the thickness-dependent second-harmonic generation, we present the first measurement of the second-harmonic-generation coherence length of 3R-MoS₂ and achieve record nonlinear optical enhancement from a van der Waals material, >10⁴ stronger than a monolayer. It is found that 3R-MoS₂ slabs exhibit similar conversion efficiencies of lithium niobate, but within 100-fold shorter propagation lengths. Furthermore, along the extraordinary axis, we achieve broadly tunable second-harmonic generation from 3R-MoS₂ in a waveguide geometry, revealing the coherence length in such a structure. We characterize the full refractive-index spectrum and quantify its birefringence with near-field nanoimaging. Our results highlight the potential of 3R-stacked transition metal dichalcogenides for integrated photonics, providing critical parameters for designing highly efficient on-chip nonlinear optical devices including periodically poled structures, optical parametric oscillators and amplifiers, and quantum circuits.

非线性频率转换为生成光的新颜色和量子态提供了必要的工具。过渡金属二硫化物具有巨大的非线性磁化率；此外，3R 堆叠的过渡金属二硫属化物晶体具有具有破坏反转对称性的对齐层，代表了以最小的占用空间提高非线性光学增益的理想候选者。_{在这里，我们报告了 3R-MoS 2}沿普通和非常方向的二阶非线性过程。沿着普通轴，通过测量与厚度相关的二次谐波产生，我们首次测量了 3R-MoS ₂的二次谐波产生相干长度，并从范德华材料实现了创纪录的非线性光学增强，>10 ⁴比单层强。发现 3R-MoS ₂平板表现出与铌酸锂相似的转换效率，但传播长度短了 100 倍。此外，沿着非凡轴，我们在波导几何结构中实现了从 3R-MoS ₂产生广泛可调的二次谐波，揭示了这种结构中的相干长度。我们对全折射率光谱进行了表征，并通过近场纳米成像对其双折射进行了量化。我们的研究结果突出了 3R 堆叠过渡金属二硫化物在集成光子学中的潜力，为设计高效的片上非线性光学器件提供了关键参数，包括周期性极化结构、光学参量振荡器和放大器以及量子电路。