奈米音波之介面反射特性

Abstract

本篇論文利用光壓電換能器產生具有奈米波長的聲波，並且將此奈米聲波應用在高解析度表面偵測。透過超快光學之激發探測技術，吾人在具有壓電效應之氮化鎵與氮化銦鎵多重量子井中產生並且偵測同調聲子震盪。由於氮化鎵材料具有很強的壓電效應，產生的同調聲子震盪可視為具有奈米波長的同調聲波。因此氮化鎵與氮化銦鎵多重量子井可視為光壓電換能器，將飛秒脈衝雷射之電磁波能量轉換為具有奈米波長之聲波能量。該光壓電換能器產生的奈米聲波具有兩個優異特性：它具有奈米聲波波長以及相位資訊，這些特性是目前任何其他聲波產生機制所無法達到的。所產生的奈米聲波首次成它a應用在高解析度表面不平整偵測，其解析度高達次奈米等級，與目前最精密的表面探測原子力顯微鏡相當。不僅如此，奈米聲波探測技術具有穿透特性，能夠偵測固體內之介面特性，這是任何原子力顯微鏡所無法達到的。此外論文中也應用奈米聲波之相位反射特性，首次設計並實現新型奈米壓電換能器；透過此新型壓電換能器結構，將可提升過去壓電換能器之輸出必v。

In this thesis, we have used optical piezoelectric transducers to generate acoustic waves with nanometer acoustic wavelength and demonstrated the high-resolution surface detection with initiated nano-acoustic waves. The generation and detection of coherent longitudinal-acoustic phonon oscillations were demonstrated in piezoelectric InGaN/GaN multiple quantum wells by pump-probe technique. Due to strong piezoelectric effect in GaN-based system, the coherent phonon oscillations can be treated as a coherent acoustic wave with nanometer acoustic wavelength. Therefore a piezoelectric InGaN/GaN multiple-quantum-wells structure can be regarded as an optical piezoelectric transducer converting electromagnetic energy of femtosecond laser pulses into acoustic energy. The nano-acoustic wave generated by the optical piezoelectric transducer has two promising features: its nanometer-scaled wavelength and its phase information, which is inaccessible to acoustic waves generated by any other mechanism. The initiated nano-acoustic waves were applied to high-resolution surface detection for the first time. The depth resolution reached sub-nanometer which is equal to the most accurate atomic force microscope, one of the most widely used surface detection equipments. Moreover taking advantage of the penetration characteristic of nano-acoustic waves, one can detect the interface pattern inside a solid, which is unreachable for any atomic force microscope. In addition, we demonstrated a novel designed structure of nano-piezoelectric transducer by means of the phase characteristic of reflected nano-acoustic waves. Compared with traditional piezoelectric transducers, this novel transducer structure enhances the acoustic output power. Our study not only provides the design guideline for future nano-piezoelectric-transducers, but also reveals the fact that strain of nano-acoustic wave experiences a 180-degree sign change after total internal reflection at air-solid interface.