Investigation of Gold Nanodisk on GaN Nanorod Arrays for Hypersonic Detection
Date Issued
2015
Date
2015
Author(s)
Yang, Szu-Chi
Abstract
Acoustic imaging technique was demonstrated as an efficient method to obtain the structure below the opaque sample surface non-destructively. In traditional ultrasonic imaging, acoustic transducers such as single element transducers or phased array systems are widely utilized. In these setups, acoustic waves are generated by piezoelectric materials, such as ZnO or lead zirconate titanate (PZT). Theoretically the system resolution will be limited by the diffraction of the acoustic waves. In order to achieve high resolution acoustic image, imaging systems intend to utilize high frequency acoustic waves. For example, an acoustic microscope applied the ZnO single element transducer for the detection of 15.3 GHz hypersonic waves in pressurized superfluid helium. However the acoustic lens was always necessary in the system based on a single element transducer. On the contrary, phased array system allows dynamic image reconstruction at different depths below the sample surface, which provides better flexibility and capability in detection setups. However, the highest operation frequency of the phased array systems is in the sub-GHz region. Therefore it is highly desirable to extend the detection frequency of phased arrays to above 10 GHz for high resolution imaging. In this thesis, we demonstrate that gold nanodisks on GaN nanorod array have a great potential to be utilized as a hypersonic array. The lowest detection frequency is the fundamental confined acoustic vibrations of gold nanodisks, which is around 10GHz. In this structure, the hypersound detection sensitivity can be enhanced by optically exciting localized surface plasmons at the gold/GaN interface, which makes each gold nanodisk as an independent opto-acoustic detector through eliminating the plasmonic coupling between gold nanodisks. For array imaging application, we further apply this structure to passively detect the hypersonic waves and to study the effect of the array periodicity. Our results show that the hypersound signal detected by single gold nanodisk depends on the array periodicity. When the periodicity is smaller than the surface hypersonic wavelength, signal detection would be affected by the coupling of the extensional-vibration-like mode of neighboring nanorods as the detection frequency approached such vibrational mode frequency. This coupling effect could be avoided by increasing the nanorod length to shift the frequency of the extensional mode away from the detection frequency. On the contrary, when the periodicity is on the order of or longer than the wavelength of surface hypersonic waves, the detected signal is affected by the period-dependent resonance of surface hypersonic waves scattered from the nanorod/substrate interface. By studying the transport behavior of hypersonic-frequency acoustic phonons at the bulk-material/nano-structure interface, this work not only investigate the possibility of hypersonic array for high resolution acoustic imaging purpose, but also suggests that effects of the periodicity and nanorod length on the individual nanodisk responses need to be taken into consideration for future hypersonic imaging array design.
Subjects
Hypersonic waves
GaN nanorod
Gold nanodisk
Type
thesis
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