Coherent Opto-Acoustic Behaviors of GaAs Nanorods
Date Issued
2010
Date
2010
Author(s)
Chen, Yi-Hsin
Abstract
Longitudinal Coherent Acoustic Phonons (CAPs) in 2D ordered cylindrical GaAs nanorods were excited and investigated by the ultrafast two-color transient reflection measurement. The generated CAPs are guided along the axis of the cylindrical rod in a small region. The boundary conditions at the interfaces can affect the behaviors of the CAPs. Such CAPs are called guided CAPs. The vibrational modes are formed by the superposition of two guided modes traveling along the cylindrical rod in opposite directions. The longitudinal vibrational modes of a cylindrical rod with a high aspect ratio (aspect ratio > 2.5) can be classified into two kinds of modes: the extensional modes and the breathing modes. Both the fundamental extensional mode and the fundamental breathing mode are observed experimentally in the GaAs nanorod. The characteristic frequencies of both kinds of modes are analytically calculated under the assumption that the rod is isotropic. The phases of the vibrational modes are also studied, which is related to the generation and detection mechanisms of the CAPs.
In order to study the guided modes in the GaAs nanorods, we performed experiments on another sample with a 15 nm-thickness gold film on top of rods. By absorbing the energy of the pump laser beam, the gold film would launch CAPs which propagate along the axis of the rod and couple to the propagating guided modes of the nanorod. The propagating guided modes can then be detected through the backward Brillouin scattering. Two additional oscillations whose frequencies at 6 ± 1 GHz and 12.1 ± 1 GHz are observed experimentally. To explain this phenomenon qualitatively, the dispersion relation of the guided modes is calculated, and a mechanism used to detect the propagating acoustic waves, the so-called backward Brillouin scattering, is also discussed. In this thesis, according to the mode patterns simulated by the finite element method (FEM), we infer that the 6 ± 1 GHz oscillation is due to the vibrational mode of the nanorod coupled by the laser-induced thermal stress in the disk-like gold films, and the 12.1 ± 1 GHz oscillation is due to the backward Brillouin scattering caused by the fundamental guided mode according to the calculated waveguide dispersion relation.
Subjects
Coherent Acoustic Phonons
Femtosecond Laser
Nanorod
Propagating Guided Modes
Vibrational Modes
Nano-Ultrasonic Waveguide
Type
thesis
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