Investigation of Gold Nanodisk on GaN Nanorod Arrays for Hypersonic Detection

近年來，超音波成像已經被廣泛應用來取得非透明表面下之影像。在成像系統中，單一元件超音波換能器或超音波陣列可利用氧化鋅或壓電陶瓷來產生超音波。理論上，成像系統的解析度會被音波波長所決定，因此高頻的音波可以提供更高的影像解析度。舉例來說，過去曾有團隊利用單一元件超音波換能器來產生15.3吉赫的超音波來成像。但是在單一元件的系統中，必須要搭配音波的聚焦鏡來收集音波。相反地，超音波陣列只需要分析每個像素所量測的訊號，即可重建出影像。但目前的超音波陣列所能偵測的頻率仍在次吉赫波段，若是能將超音波陣列所偵測的波段提升到10吉赫以上，對未來的特高頻音波成像上可以產生極大的幫助。 在這篇論文中，我們證明了金奈米圓盤於氮化鎵奈米柱陣列上的結構可用來偵測10吉赫以上的音波。在此結構中，侷限性表面電漿子可大幅提高偵測靈敏度，並且消除每個奈米圓盤之間的電漿耦合。因此每個金奈米圓盤可以視為獨立的偵測結構。此外，我們也發現金奈米圓盤所偵測的訊號與陣列的週期，以及奈米柱的長度都有關係。當週期小於音波的波長時，所偵測的訊號會受到奈米柱間的震動膜態耦合所影響。這個效應可以藉由改變奈米柱長度，使得奈米柱的震動頻率遠離我們的操作頻率來消除。而當週期小於音波的波長時，表面音波會在奈米柱間產生共振，進而影響音波的穿透率。此研究不僅探討特高頻音波在奈米柱與材料基板間的傳遞，此外也闡明了在未來特高頻音波陣列的設計上所需注意的事項，對未來的高解析特高頻音波成像系統可望做出許多貢獻。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