The reflection of waves from rough surfaces is a fundamental process that plays a role in diverse fields such as optics, acoustics, and seismology. While a quantitative understanding of the process has long been established for many types of waves, the precise manner in which thermal phonons of specific wavelengths reflect from atomically rough surfaces remains unclear owing to limited control over THz frequency phonon generation and detection. Knowledge of these processes is critical for many applications, however, and is particularly important for recent attempts to create novel materials by coherently interfering thermal phonons. Here, we report measurements of a key property for these efforts, the phonon wavelength-dependent specularity parameter, which describes the probability of specular reflection of thermal phonons at a surface. Our observations demonstrate that THz thermal phonons specularly reflect from surfaces with Angstrom-scale roughness, and further show that completely diffuse reflections occur with surface imperfections on the scale of only a few atomic planes. Our work provides key insights into the surface perfection required for coherent manipulation of heat and demonstrates a general approach to probe thermal phonon interactions with surfaces with considerable microscopic detail.

中文翻译：

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原子粗糙表面上的热声子的光谱分辨镜面反射

来自粗糙表面的波反射是一个基本过程，在光学，声学和地震学等各个领域都发挥着作用。尽管已经对许多类型的波建立了对该过程的定量理解，但是由于对THz频率声子的产生和检测的控制有限，特定波长的热声子从原子粗糙表面反射的精确方式仍不清楚。然而，对这些过程的了解对于许多应用而言至关重要，并且对于最近通过相干干涉热声子来创建新型材料的尝试尤为重要。在这里，我们报告了这些工作的关键特性的测量值，即与声子波长有关的镜面反射参数，该参数描述了表面上的热声子的镜面反射概率。我们的观察结果表明，太赫兹热声子从具有Angstrom尺度粗糙度的表面镜面反射，并且进一步表明，仅在几个原子平面的尺度上，具有表面缺陷的情况下会发生完全漫反射。我们的工作提供了对热的连贯处理所需的表面完善度的关键见解，并展示了一种探测热声子与表面的微观细节相互作用的通用方法。