同調兆赫聲學聲子之介面散射

Interface Scattering of THz Coherent Acoustic Phonons

溫昱傑(Yu-Chieh Wen)

指導教授 ： 孫啟光

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.01451

摘要

由於兆赫頻段的同調聲學聲子兼具奈米尺度波長與高穿透深度之特性，故其可和次表面奈米結構與相似能量之粒子相互作用。兆赫聲學的發展因此可促進物理學的進步，並引發新應用的可能。作者以超快光譜技術進行一系列的兆赫聲學實驗，藉以探討同調聲學聲子與晶體表面的交互作用，此研究的目的在於釐清振動能量如何傳遞並穿越界面。
本論文分三組探討界面，分別為固態/空氣、固態/固態，與固態/液態水界面。作者微觀地評估界面反射對聲子同調性所造成的破壞，藉以瞭解造成聲子散射的主要機制，俾解決長期在Kapitza anomaly議題上的爭論。首先，本文檢視在晶體表面所發生的聲子散射，得出在原子等級平坦的表面，聲子散射的主要型態為鏡向散射。然而隨著表面變粗糙或聲子頻率增高，鏡向散射機率會逐漸下降。本研究發現聲子與界面之交互作用，符合描述波在粗糙界面散射的巨觀理論。此結論定量地證明原子等級表面不平整對於次兆赫聲子散射的重要影響。
次於固態/固態界面所進行的延伸性研究，確認了界面不平整對聲子散射的支配地位。基於此發現，作者對一般固態表面，進行正常-異常邊界熱阻變遷的臨界頻率/溫度估算，其符合過去在低溫熱傳導實驗的觀測結果。在應用的層面上，此研究展示了一個具有原子解析度與非破壞優點的界面粗糙度評估方法，而該方法有助於奈米量測技術突破現今受表面量測與樣品破壞的限制。
由於兆赫音波得在室溫下評估埋藏之界面，因此可以進一步應用在固態/液態水界面的聲子散射行為研究。本文指出，在此界面所量得的聲學頻譜顯現出數個反射低點，與由鏡向散射變遷至漫射的過渡變化。因為連續體彈性理論無法解釋實驗結果，因此可知離散的水分子與其群聚結構對於實驗觀察有關鍵性的影響。鏡向散射變成漫射的過渡變化，揭露了界面鄰近水分子的區域結構規則性。從能量傳遞的觀點而言，散射型態的轉變指出在兆赫頻段，聲子穿透係數由0.1大幅增加為0.88，此特徵暗示了區域氫鍵網路結構與固態/水界面熱傳導的關連性。此外，實驗發現，界面分子膜受激振動能引起聲子共振穿透，此透露出奈米超音波技術具有評估界面水中跨分子作用力的潛力。本研究指出，音波在介面水中的衰減速度小於其在液態水與多晶相冰中的衰減。此外，介面水是由四到五層水分子膜所組成。

關鍵字

聲子 ； 超快雷射 ； 卡披薩 ； 邊界熱阻 ； 介面水

並列摘要

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.

並列關鍵字

phonon ； ultrafast laser ； Kapitza ； thermal boundary resistance ； interfacial water

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