Reflection Property of Nano-Acoustic-Waves at Interface

• Main Authors: Chia-Lung Hsieh, 謝佳龍
• Other Authors: Chi-Kuang Sun
• Format: Others
• Language: en_US
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/24100315642909671011

碩士 === 國立臺灣大學 === 光電工程學研究所 === 92 === 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.