Interface Scattering of THz Coherent Acoustic Phonons
Date Issued
2010
Date
2010
Author(s)
Wen, Yu-Chieh
Abstract
THz coherent acoustic phonons have capability to interact with sub-surface nanostructures and quanta with comparable energies due to their nanoscaled wavelengths and high penetration depths. The development of THz acoustics thus stimulates the advances of condense-matter physics and leads to new applications. Based on ultrafast optical spectroscopies, this thesis describes a series of THz acoustic experiments which investigate interactions between coherent acoustic phonons and crystal boundaries. The goal is to facilitate our understanding of a fundamental issue: how does vibration energy transmit through an interface?
The thesis is composed of three parts: solid/air, solid/solid, and solid/liquid-water interfaces. Microscopic investigations on the destruction of phonon coherence during interface reflections clarify the origins of phonon scatterings at different interfaces, which are critical for unraveling the long-standing debate on Kapitza anomaly. Phonon scatterings at epitaxial-quality free surface are first examined. We show that specular phonon scatterings are the predominant scattering type for atomically flat surfaces, while the specular scattering probability decreases as the surface (frequency) becomes irregular (higher). The phonon-interface interaction is found to agree well with the macroscopic theory on wave scattering from rough surfaces. Our study thus quantitatively verifies the responsibility of corrugations in diffuse scatterings of the sub-THz phonons.
An extended study on a solid/solid interface confirms the dominant role of interface irregularity in the phonon scatterings. Based on these finding, we estimate the threshold frequency (and threshold temperature) for the transition of normal-to-anomalous Kapitza resistance, satisfactorily agreeing with previous cryogenic observations. From an application viewpoint, this study opens a way to nondestructively probe interface roughness at an atomic level; however, nowadays nanometrologies are either restricted to surface measurement or highly destructive.
The ability of THz acoustic waves to explore buried interfaces at room temperature enables us to investigate phonon scatterings at a solid/liquid-water interface. The measured acoustic reflectivity spectrum shows several remarkable minima in the sub-THz range and a frequency threshold, above that interface phonon scattering transits from specular to diffuse type. All possible mechanisms are examined in details. Violation of the continuum elastic theory indicates the influence of discrete water molecules and their assemblies on our experiments, revealing molecular-level resolutions of the adopted nanoultrasonic technique.
The observed scattering transition discloses local structural order of interfacial water and indicates a substantial increase of phonon transmission coefficient from 0.1 to 0.88 in the sub-THz range. This feature implies a close relation of the local hydrogen-bond network to the heat conduction at wetting interfaces. Moreover, the observed resonant transmission, resulting from stimulated vibrations of interfacial molecular layers, implicates the potential of the nanoultrasonics in determination of intermolecular force interactions within interfacial water. Our dynamic study indicates that the acoustic attenuation in the highly crystalline interfacial water is less than the liquid and polycrystalline iced water, and the interfacial water is composed of 4~5 monolayers.
Subjects
phonon
ultrafast laser
Kapitza
thermal boundary resistance
interfacial water
Type
thesis
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