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  4. Nano-Acoustic Guided Waves in GaAs Nanorods
 
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Nano-Acoustic Guided Waves in GaAs Nanorods

Date Issued
2012
Date
2012
Author(s)
Wu, Yueh-Chun
URI
http://ntur.lib.ntu.edu.tw//handle/246246/253781
Abstract
Picosecond ultrasonics for its mechanisms of generation, detection and applications has been widely studied. Most of previous studies treated the excited acoustic waves as longitudinal and transverse modes due to the neglect of boundary confinement. On the other hand, using femtosecond lasers to excite the confined acoustic modes of nanostructures were also been discussed greatly, but the acoustic guided modes confined to the nanostructures were rarely discussed. In this thesis, we used ultrafast optical pump-probe technique to generate and detect nano-acoustic waves (NAWs) confined to a GaAs nanorod. In our designed structure, a gold nanodisk that deposited on the top of the nanorod was designed to acts as an opto-acoustic transducer. NAWs are launched for the thermal-induced vibration of the disk, and then the waves evolve to guided modes that propagate inside the nanorod. In theory, continuum linear elastic model – resonant ultrasound spectroscopy (RUS) is adopted for the anisotropic elastic properties of GaAs. Under the assumptions of infinite rod length and stress free boundary conditions, the dispersion relation of guided modes is obtained. From experiment, we concluded the detection sensitivity of acoustic responses of the designed samples depend on the probe wavelength: (1) for near-infrared probe (880nm), the dominating oscillatory signal observed is induced from the radial breathing mode of GaAs nanorods. However, (2) for infrared probe (1120nm) condition, the dominant signal converts into the vibration of nanodisk. Under this scheme, echoes with relatively slow velocity are observed as well. We suggested the intervention of localized surface plasmon resonance (LSPR) should be the key to the change of the signal. With the aid of LSPR, the Au nanodisk is not only a transducer but also a highly sensitive acoustic detector to detect the returning echo. The roundtrip time of observed echo shows a good agreement with the simulation of the anisotropic waveguide theory. The results reflect the fact that propagation of NAWs confined to nanostructures are different to that in bulk crystal. Additionally, in this thesis we further demonstrated the reflection coefficient of the fundamental mode at the rod-substrate interface is roughly 0.5+-0.2 , while the other modes of higher frequency suffer lower reflection for the issue of mode matching between the nanorod and the substrate.
Subjects
Nano-acoustic waves (NAWs)
ltrafast optic technique
nanorod
guidedmodes
radial breathing mode
resonant ultrasound spectroscopy (RUS)
dispersion relation
localized surface plasmon resonance (LSPR)
Type
thesis
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