We use micro-focussed Brillouin light scattering spectroscopy ($\mu$BLS) to investigate surface acoustic waves (SAWs) in a GaN layer on a Si substrate at GHz frequencies. Furthermore, we discuss the concept of $\mu$BLS for SAWs and show that the crucial parameters of SAW excitation and propagation can be measured. We investigate a broad range of excitation parameters and observe that Rayleigh and Sezawa waves are excited simultaneously at the same frequency. Spatially resolved measurements of these co-propagating waves show a periodic pattern, which proves their coherent interference. From the periodicity of the spatial phonon patterns, the wavevector difference between the two waves has been identified and compared to the dispersion relation. This concept of co-propagating phonons might be used to produce acoustic or magneto-elastic fields with a time-independent spatial variation similar to the situations realized using counter-propagating waves. However, co-propagating SAW have a well defined direction of the wave vector and thus, posses a finite phonon angular momentum which offers new opportunities, e.g. for angular momentum conversion experiments.

中文翻译：

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用微聚焦布里渊光散射光谱观察到共同传播的瑞利波和 Sezawa 波的干涉

我们使用微聚焦布里渊光散射光谱 ($\mu$BLS) 在 GHz 频率下研究 Si 衬底上 GaN 层中的表面声波 (SAW)。此外，我们讨论了 SAW 的 $\mu$BLS 概念，并表明可以测量 SAW 激励和传播的关键参数。我们研究了广泛的激发参数，并观察到瑞利波和 Sezawa 波以相同的频率同时激发。这些共同传播的波的空间分辨测量显示出周期性模式，这证明了它们的相干干涉。从空间声子图案的周期性，已经确定了两个波之间的波矢差异，并将其与色散关系进行了比较。这种共同传播声子的概念可用于产生具有与时间无关的空间变化的声学或磁弹性场，类似于使用反向传播波实现的情况。然而，共同传播的 SAW 具有明确定义的波矢量方向，因此具有有限的声子角动量，这提供了新的机会，例如角动量转换实验。