奈米音波於砷化鎵奈米柱之波導模態

Nano-Acoustic Guided Waves in GaAs Nanorods

吳岳駿(Yueh-Chun Wu)

指導教授 ： 孫啟光

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.01385

摘要

利用超快光學方式來產生並偵測具奈米尺度波長與次兆赫(sub-terahertz)頻率音波的技術(picosecond ultrasonics) 已廣泛地被用來評估與研究奈米尺度下的物理現象。先前研究中，因忽略邊界效應，激發之音波主要被視為於晶體塊材內傳播的縱波與橫波模態。而另一方面，奈米結構之共振特性(confined acoustic modes)雖亦被大量地探討與研究，然而音波因受奈米結構邊界之侷限而產生離散波導模態(guided modes)與特定色散關係的傳播特性卻鮮少被討論。故在此研究內，我們欲利用激發探測法(pump-probe technique)，以具奈米厚度之金膜吸收激發光(pump beam)而熱膨脹，進而產生奈米等級波長之音波在砷化鎵奈米柱內以波導模態傳播，再以探測光(probe beam)偵測其反應。在理論計算上，我們用發展於共振超音波頻譜分析(resonant ultrasound spectroscopy)之計算方法處理砷化鎵奈米柱的非等向性，在假設柱體為無限長，邊界沒有受應力的條件下，可得被侷限在奈米柱內傳播之波導模態的色散關係與特徵場型。而在實驗上，我們對一系列不同尺寸的奈米柱分別在不同的探測光(probe beam)波長下做量測，由實驗結果我們歸納：(1) 在探測光波長為880奈米，所觀測到的震盪訊號主要來自於柱體的徑向呼吸震動模態 (radial breathing mode)，然而(2)在波長為1120奈米的探測光的條件下，柱體直徑小於300奈米以下的樣品，所觀測到的震盪訊號主要來自於表面奈米金盤的振動。在此情況下，此訊號同時伴隨著回音的產生。我們推測此震盪訊號的轉變和奈米金盤的侷域表面電漿子共振(localized surface plasmon resonance)有關。藉由其效應，奈米金盤可視為一感測器去偵測由底部反射回來的回音。此回音代表由金盤振動所耦合的波導模態在柱體內的傳播以及在柱底不連續介面的反射行為。由實驗結果分析，音波在奈米柱內的確呈現出不同於塊材內的傳播行為。且其回音時間符合波導理論預期。同時實驗中也顯示，不同模態因介面不連續所造成的反射率不同，其中我們推估主要激發之低頻模態其反射率為0.5+-0.2。

關鍵字

奈米超音波 ； 超快光學 ； 奈米柱 ； 音波波導模態 ； 共振超音波頻譜分析 ； 色散關係 ； 徑向呼吸模態 ； 侷域表面電漿子共振

並列摘要

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.

並列關鍵字

Nano-acoustic waves (NAWs) ； ltrafast optic technique ； nanorod ； guidedmodes ； radial breathing mode ； resonant ultrasound spectroscopy (RUS) ； dispersion relation ； localized surface plasmon resonance (LSPR)

參考文獻

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