Ultrafast structural probing has greatly enhanced our understanding of the coupling of atomic motion to electronic and phononic degrees-of-freedom in quasi-bulk materials. In bi- and multilayer model systems, additionally, spatially inhomogeneous relaxation channels are accessible, often governed by pronounced interfacial couplings and local excitations in confined geometries. Here, we systematically explore the key dependencies of the low-frequency acoustic phonon spectrum in an elastically mismatched metal/semiconductor bilayer system optically excited by femtosecond laser pulses. We track the spatiotemporal strain wave propagation in the heterostructure employing a discrete numerical linear chain simulation and access acoustic wave reflections and interfacial couplings with a phonon mode description based on a continuum mechanics model. Due to the interplay of elastic properties and mass densities of the two materials, acoustic resonance frequencies of the heterostructure significantly differ from breathing modes in monolayer films. For large acoustic mismatch, the spatial localization of phonon eigenmodes is derived from analytical approximations and can be interpreted as harmonic oscillations in decoupled mechanical resonators.

中文翻译：

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纳米级双层系统中的超快应变传播和声共振


超快结构探测极大地增强了我们对准块体材料中原子运动与电子和声子自由度耦合的理解。此外，在双层和多层模型系统中，可以访问空间不均匀的弛豫通道，通常由明显的界面耦合和受限几何形状中的局部激发控制。在这里，我们系统地探索了由飞秒激光脉冲光学激发的弹性失配金属/半导体双层系统中低频声声子谱的关键依赖性。我们采用离散数值线性链模拟来跟踪异质结构中的时空应变波传播，并使用基于连续介质力学模型的声子模式描述来访问声波反射和界面耦合。由于两种材料的弹性特性和质量密度的相互作用，异质结构的声共振频率与单层薄膜中的呼吸模式显着不同。对于大的声学失配，声子本征模的空间定位是从解析近似导出的，并且可以解释为解耦机械谐振器中的谐波振荡。