Two-dimensional transition metal dichalcogenides possess unprecedentedly strong optical nonlinear properties associated with exciton complexes, which are highly promising for developing novel optoelectronics and nanophotonics. In this work, we investigate various exciton complexes that exist in WSe₂ and WS₂ triangular monolayers, through second-harmonic generation (SHG) and two-photon excited photoluminescence (2P-PL) images that are generated rapidly by multiphoton laser scanning microscopy. These large-scale images taken at different photon energies reveal the spatial distribution of the exciton complexes. The SHG images capture the exciton and trion resonances but not the biexciton resonance, despite a prominent biexciton signature in 2P-PL. The peculiar absence of biexciton signature is explained using time-dependent perturbation theory. SHG can also resonate with the band nesting states to produce images with high contrast, which is out of reach in conventional PL. By analyzing the crystal structure and growth dynamics of WS₂ monolayers using a high-angle annular dark-field scanning transmission electron microscope, we further establish the link between the oxidized triangular holes induced by S atom vacancies and the formation of biexcitons at the triangle edges. This newfangled approach using multiphoton microscopy shows that the spatial characteristics and excitation energy dependence of the exciton complexes are crucial for a deep understanding of the interactions between excitons and charged carriers and the two-dimensional materials in general.

中文翻译：

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使用多光子显微镜对WSe ₂和WS _2单分子膜中的激子复合物进行快速实空间成像

二维过渡金属二卤化物具有与激子配合物相关的空前强大的光学非线性特性，这对于开发新型光电子学和纳米光子学非常有前途。在这项工作中，我们研究了WSe ₂和WS ₂中存在的各种激子复合体通过二次谐波生成（SHG）和双光子激发光致发光（2P-PL）图像形成的三角形单层，这些图像是通过多光子激光扫描显微镜快速生成的。这些在不同光子能量下拍摄的大规模图像揭示了激子复合物的空间分布。尽管在2P-PL中有显着的双激子签名，但SHG图像捕获了激子和三重子共振，但没有捕获双激子共振。使用依赖时间的扰动理论解释了双激子信号特有的缺失。SHG还可以与波段嵌套状态产生共振，以产生具有高对比度的图像，这是常规PL无法达到的。通过分析WS ₂的晶体结构和生长动力学使用高角度环形暗场扫描透射电镜观察单层，我们进一步建立了由S原子空位引起的氧化三角孔与在三角边缘形成双激子之间的联系。这种使用多光子显微镜的新方法表明，激子配合物的空间特性和激发能依赖性对于深入理解激子和带电载流子与二维材料之间的相互作用至关重要。