Picosecond ultrasonics, which studies laser-induced high-frequency strain waves, is a reliable and versatile method for nondestructive materials' characterization. Strain waves are generated through a light interaction with charges and their subsequent relaxation, and these waves conceal a wealth of information on the material. However, strain waves are detected through their convolution with a sensitivity function, which blurs much of this information. Here, we show that the reflection of strain waves at a free surface leads to the appearance of a Fano resonance in the reflectivity spectrum, accompanied by a drastic increase in the detection bandwidth. We take advantage of this feature to provide a method for the reconstruction of strain waves. We apply it to unambiguously highlight the exact origin of the generation of coherent acoustic phonons in Stranski–Krastanov grown quantum dots, revealing that both the wetting layer and quantum dots are responsible for the generation. Our results will offer the possibility to understand better the interaction of light with charges and their interactions with the lattice.

中文翻译：

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从相干声学声子反射重建超快应变波

皮秒超声波研究激光诱导的高频应变波，是一种可靠且通用的无损材料表征方法。应变波是通过与电荷的光相互作用及其随后的弛豫产生的，这些波隐藏了材料的大量信息。然而，应变波是通过它们与灵敏度函数的卷积来检测的，这会模糊大部分信息。在这里，我们表明应变波在自由表面的反射导致反射率谱中出现 Fano 共振，伴随着检测带宽的急剧增加。我们利用这一特点提供了一种重建应变波的方法。我们应用它来明确强调 Stranski-Krastanov 生长的量子点中相干声子产生的确切起源，揭示润湿层和量子点都对产生负责。我们的结果将为更好地理解光与电荷的相互作用及其与晶格的相互作用提供可能性。