Atomically thin 2D materials, currently being at the forefront of scientific and technological interest, can be categorized as metallic, semimetallic, semiconducting, insulating, or superconducting, depending on their chemical composition and structural configuration. They also exhibit, in some cases, a transition from an indirect to a direct bandgap alignment when bulk materials are scaled down to monolayers. An important class of 2D materials is layered transition metal dichalcogenides (TMDs) with a tunable bandgap, because photogenerated optical excitations and subsequent excitation dynamics, which produce energy migration and photogenerated charge carrier transport, make them promising candidates for a variety of optoelectronic devices, including solar cells, photodetectors, light-emitting diodes, and phototransistors. In this work, we probe the excitation dynamics following nonlinear optical absorption/scattering in two unexplored TMDs, metallic NbS₂ and semimetallic ZrTe₂, using a combination of the standard optical Z-scan and photoacoustic Z-scan techniques, and compare them with semiconducting MoS₂. The comparison of optical Z-scan (OZ-scan), which depends on the contributions of both nonlinear scattering and nonlinear absorption, with photoacoustic Z-scan (PAZ-scan), which depends only on nonlinear absorption due to local heating from nonradiative relaxation, allows us to separate these contributions from the total nonlinear response. In addition, these studies also allow us to look at the nature of nonlinear absorption as to whether it is due to saturable absorption (SA) of a one-photon transition, reverse saturable absorption (RSA) derived from two-photon excitation processes, or any combination thereof. In MoS₂, NbS₂, and ZrTe₂, we observed both SA and RSA. The relevant nonlinear absorption coefficient parameters were obtained. Density functional theory modeling provides an insight onto possible underlying physical processes.

中文翻译：

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光学和光声Z扫描方法探测二维分层过渡金属二硫属化物中的非线性光学相互作用和弛豫

原子薄的2D材料（目前处于科学技术前沿）可以根据其化学组成和结构配置分为金属，半金属，半导体，绝缘或超导。在某些情况下，当将大块材料缩小到单层时，它们还表现出从间接带隙对准到直接带隙对准的过渡。一类重要的2D材料是具有可调带隙的层状过渡金属二硫化碳（TMD），因为产生能量迁移和光生电荷载流子传输的光生光激发和随后的激发动力学使其成为各种光电器件的有希望的候选者，包括太阳能电池，光电探测器，发光二极管和光电晶体管。在这项工作中₂和半金属ZrTe ₂结合使用标准的光学Z扫描和光声Z扫描技术，并将它们与半导体MoS ₂进行比较。光学Z扫描（OZ扫描）与光声Z扫描（PAZ扫描）的比较取决于光学非线性Z扫描的非线性散射和非线性吸收的贡献，光声Z扫描（PAZ扫描）仅取决于非辐射弛豫引起的局部加热导致的非线性吸收，使我们可以将这些贡献与总非线性响应分开。此外，这些研究还使我们能够研究非线性吸收的性质，例如它是由于单光子跃迁的饱和吸收（SA），还是由于双光子激发过程产生的反向饱和吸收（RSA）还是由于它们的任何组合。在MoS ₂，NbS ₂和ZrTe _2中，我们观察到了SA和RSA。获得了相关的非线性吸收系数参数。密度泛函理论建模可洞悉可能的基础物理过程。