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Title: | 砷化鎵奈米柱之同調聲學聲子行為 Coherent Opto-Acoustic Behaviors of GaAs Nanorods |
Authors: | Yi-Hsin Chen 陳奕欣 |
Advisor: | 孫啟光(Chi-Kuang Sun) |
Keyword: | 同調聲學聲子,飛秒雷射,奈米柱,波導模態,振動模態,奈米超音波波導, Coherent Acoustic Phonons,Femtosecond Laser,Nanorod,Propagating Guided Modes,Vibrational Modes,Nano-Ultrasonic Waveguide, |
Publication Year : | 2010 |
Degree: | 碩士 |
Abstract: | 我們利用超快雷射暫態反射量測技術在整齊排列的砷化鎵奈米圓柱二維陣列中激發縱向同調聲學聲子並研究其行為。由於奈米柱的邊界效應,產生的音波會被侷限在奈米柱裡,並且音波的傳播行為會受到改變,我們稱此為波導模態。當兩個沿著奈米柱往相反方向傳播的波導模態重疊時,即會產生駐波,我們稱此為振動模態。當奈米柱的長度遠大於其寬度時,其振動模態可以被分類為兩類:軸向延伸模態(extensional mode)和徑向延伸模態(breathing mode)。我們成功地在實驗上觀察到砷化鎵奈米柱的基諧軸向延伸模態以及基諧徑向延伸模態。根據一個簡化的等向性圓柱體模型,我們計算出此兩類振動振動模態之特徵頻率的解析解,並將實驗結果與有限元素分析法模擬結果做比較,我們發現理論計算與模擬結果大致上與實驗結果吻合。另一方面,我們也對實驗觀察到的振動模態的相位進行分析。
為了要觀察砷化鎵奈米柱的波導模態,我們對另一個在砷化鎵奈米柱頂端鍍了十五奈米厚金膜的樣品進行了量測。藉由吸收雷射光的能量,金膜會因為熱膨脹而對奈米柱施以應力,進而產生沿著奈米柱傳播的音波,此音波會將其能量耦合到波導模態,故我們將有機會藉由背向布里淵散射(backward Brillouin scattering)機制觀察到砷化鎵奈米柱中波導模態的傳播行為。根據此實驗,我們可在觀察到兩個頻率為6 ± 1 GHz 和 12.1 ± 1 GHz的額外振盪。為了在定量上解釋此現象,我們計算了波導膜態的色散關係,並且探討了一個用來偵測音波傳播行為的機制,背向布里淵散射。在此論文中,藉由與有限元素法所模擬出來的振動模態場型做比較,我們推測頻率為6 GHz的振盪是來自金膜內由雷射產生的熱應力所耦合到的奈米柱振動模態。而根據計算出來的波導色散關係,我們推測頻率為12.1 ± 1 GHz的振盪是來自於基頻波導模態所造成的背向布里淵散射,而其光學折射率,符合等效介質理論。 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. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/46451 |
Fulltext Rights: | 有償授權 |
Appears in Collections: | 光電工程學研究所 |
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