Ultrafast electron diffraction has been proven to be a powerful tool for the study of coherent acoustic phonons owing to its high sensitivity to crystal structures. However, this sensitivity leads to complicated behavior of the diffraction intensity, which complicates the analysis process of phonons, especially higher harmonics. Here, we theoretically analyze the effects of photoinduced coherent transverse and longitudinal acoustic phonons on electron diffraction to provide a guide for the exploitation and modulation of coherent phonons. The simulation of the electron diffraction was performed in 30-nm films with different optical penetration depths based on the atomic displacements obtained by solving the wave equation. The simulation results exhibit a complex relationship between the frequencies of the phonons and diffraction signals, which highly depends on the laser penetration depth, sample thickness, and temporal stress distribution. In addition, an intensity decomposition method is proposed to account for the in-phase oscillation and high harmonics caused by inhomogeneous excitation. These results can provide new perspectives and insights for a comprehensive and accurate understanding of the lattice response under coherent phonons.

中文翻译：

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相干声声子下超快电子衍射强度模拟

由于其对晶体结构的高敏感性，超快电子衍射已被证明是研究相干声声子的有力工具。然而，这种敏感性导致衍射强度的复杂行为，这使得声子，尤其是高次谐波的分析过程变得复杂。在这里，我们从理论上分析了光致相干横向和纵向声声子对电子衍射的影响，为相干声子的开发和调制提供指导。基于求解波动方程获得的原子位移，在不同光学穿透深度的30 nm薄膜中进行电子衍射模拟。模拟结果展示了声子频率和衍射信号之间的复杂关系，这在很大程度上取决于激光穿透深度、样品厚度和时间应力分布。此外，还提出了一种强度分解方法来解决由不均匀激励引起的同相振荡和高次谐波。这些结果可以为全面、准确地理解相干声子下的晶格响应提供新的视角和见解。