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標題: | 利用飛秒聲學量測非晶二氧化矽薄膜的超音波衰減性質 Measuring Hypersound Attenuation in Vitreous Silica Films by Femtosecond Acoustics |
作者: | Tsung-Chi Hung 洪從豈 |
指導教授: | 孫啟光(Chi-Kuang Sun) |
關鍵字: | 玻色子峰,飛秒聲學,非晶材料,熱傳導,不規則結構,超音波衰減, boson peak,femtosecond acoustics,amorphous material,thermal conductivity,disordered system,hypersound attenuation, |
出版年 : | 2018 |
學位: | 碩士 |
摘要: | 非晶(amorphous)材料是一種沒有晶格結構的材料,其結構為不規則、隨機無序的分子排列,此結構造成了著名的熱學上的異常現象,而這些異常現象出現的溫度 (約1~10K),對應於熱聲子的頻率約為 1THz,所以一般我們把這些異常歸因於玻色子峰(boson peak),指非晶材料的振動狀態密度(vibrational density of states)在約 1THz處超過傳統的德拜模型而呈現一個峰值。因此科學家積極地使用各種技術,試圖量測靠近1THz的超音波在非晶材料中傳遞的情形,來解釋這些熱傳異常現象。而從超音波傳遞的角度來看,當超音波頻率不斷提高,波長到了奈米尺度,在聲波傳遞的過程中即會受到不規則的結構的影響,改變其傳遞的性質。以標準非晶材料玻璃石英(vitreous silica)來說,當頻率達到600GHz時,波長約為10nm,因此量測玻璃石英中600GHz~1THz這段頻率的超音波實驗意義非凡,但於各種實驗方法均有其限制,在室溫玻色子峰出現的頻率(約1THz)仍缺少了關鍵的實驗量測;當波長和不規則結構相當時,超音波傳播問題也仍缺乏實驗數據。
在此論文中,我們將使用飛秒聲學的技術,補足以往實驗技術所沒有辦法量測的頻段。我們改善了激發-偵測(pump-probe)系統和樣品結構,使之能發出高於過去發表的研究中所發出的音波,藉由使用更薄(1.8nm, 3.2nm, 6.4nm, 9.2nm, 13.8nm)的玻璃石英薄膜,來增加可量測的聲波訊號頻寬。而過去的分析方法可能有一些關於樣品粗糙度所造成的散射的疑慮,因此我們採用比較不同厚度的分析方法,來消除介面散射的影響。在室溫的量測中我們發現了從 300GHz到700GHz的超聲波衰減係數和頻率成二次方的關係,而700GHz到 880GHz的趨勢則超過的頻率的二次方,藉由低溫150K的實驗我們確認了為頻率四次方的趨勢,此外接近波色子峰頻率的地方都出現了負的聲波色散關係。這些量測結果呼應了非彈性X光散射在1620K的實驗結果,也符合最早的在玻璃石英低溫實驗中所量測的熱傳導率和比熱,而這樣頻率四次方的損耗關係可以追溯到瑞利散射(Rayleigh scattering)。這是第一次有實驗可以在室溫下量測到石英玻璃中的聲波衰減係數從二次方轉四次方,以往非彈性X光散射實驗所做的高於1THz的量測,和皮秒聲學等技術所量測的低於400GHz的結果,順利被我們連接起來,填上了最後一塊拼圖。我們的研究能提供不論是玻色子峰的相關熱學問題,或是超音波在和波長相當的不規則材料中傳導等研究有利的實驗證據,期盼解決長久以的爭議。 Amorphous material is a non-crystalline structured material. The disorder of the structure results in the well-known thermal anomalies found in low-temperature experiments. The dominant phonon frequency of the temperature (1~10K) of the anomalies is around 1THz. Therefore, these anomalies are often related to the boson peak, which is the reduced vibrational density of states shows a peak around 1THz, disobeying the Debye model. Numerous studies try to measure the hypersound transportation around 1THz in amorphous materials by using different techniques. From the sound wave point of view, as the frequency becomes higher, the wavelength will shrink to the nanometer scale, which is comparable to the disordered structure in the amorphous material. The high-frequency sound wave starts to be influenced by the random structure. For instance, in vitreous silica, a typical amorphous material, the wavelength is 10nm when frequency approaches 600GHz. Hence, measuring the hypersound wave transportation in 600GHz~1THz is critical for understanding the thermal anomalies as well as discovering the phonon transportation of whose wavelength comparable to the disordered structure. However, since the shortages in different experimental techniques, the most critical evidence of this frequency range has not been measured yet. In this thesis, we measured the hypersound attenuation in vitreous silica by femtosecond acoustics in the frequency range where other techniques are not able to measure. An optimized pump-probe system and thinner InGaN single quantum well are used to generate a broader band acoustic signal than the previous study from our group. Thinner vitreous silica films (1.8nm, 3.2nm, 6.4nm, 9.2nm, 13.8nm) are measured to attain the high-frequency acoustic signal. The uncertainty of the surface scattering is eliminated by comparing two samples of different thickness silica film. The room temperature results show a quadratic dependency from 300GHz to 600GHz and turn into a higher frequency dependency above 600GHz. With the 150K experiment result, we confirm the fourth power frequency dependency of the room temperature results above 600GHz. In addition, a negative dispersion is measured near the boson peak frequency. Our results concur with the 1620K inelastic X-ray scattering data; the sound attenuation of Rayleigh-like forth power dependency is in a good agreement with the phonon mean free path calculated by the specific heat and thermal conductivity measured in 1~10K. This is for the first time the turning from an f^2-dependency into an f^4-dependency of sound attenuation in vitreous silica is measured at room temperature. The gap between the results above 1THz, measured by IXS, and below 400GHz, measured by picosecond ultrasonics, is successfully bridged by our measurement. Our study sheds light on the long-debated boson peak anomalies and provides critical evidence of how the acoustic wave interacts with the disorder structure. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/72166 |
DOI: | 10.6342/NTU201803301 |
全文授權: | 有償授權 |
顯示於系所單位: | 光電工程學研究所 |
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