利用光激發二維電漿產生超短音波脈衝

Abstract

飛秒聲學的應用主要分為結構成像以及熱聲子頻譜分析兩種。為了提升量測的空間解析度、得到更多高頻聲子的資訊以及有效的增強高頻聲子的訊雜比，如何產生超短音波脈衝至今仍然是非常重要的議題。根據彈性理論，影響音波脈衝寬度主要分為音波產生的範圍以及音波產生的機制。而我們認為介面是最薄的二維結構，因此在此論文中，我們想藉由累積在介面上的二維電子氣體來產生超短音波脈衝。 為了研究超快的光激發二維電子氣體產生音波脈衝的機制，我們建立了一個簡單的音波產生的模型。根據模擬的結果，音波脈衝寬度變化主要是被熱電子形成的膨脹壓力在空間中的分佈所影響。從模型模擬出來的音波脈衝寬度來看，二維電子氣體是有潛力去產生約100飛秒的超短音波脈衝的超音波產生源。為了證明理論模型的結果，實驗上我們成功的利用氮化鎵以及非晶氧化鎵的介面所生成的二維電子氣體去產生時間寬度為266飛秒、頻寬為2.1兆赫茲的超短音波脈衝。 因此本篇的研究不僅推導出新穎的光激發二維電子氣體產生音波的機制，實驗上我們更成功的利用二維電子氣體產生至今最短的超音波脈衝。

The applications of Femtosecond Acoustics are mainly divided into structure imaging and thermal phonon spectroscopy. For increasing spatial resolution, obtaining higher frequency information, and enhancing the signal to noise ratio (SNR) of higher frequencies, finding the approaches to generate an ultra-short acoustic pulse is a very critical issue. According to the continuum elastic theory, the acoustic pulsewidth is influenced by the thickness of acoustic generation region and acoustic generation mechanisms, respectively. In this thesis, we utilized 2-dimensional electron gas (2DEG) accumulated at the interface to generate the ultra-short photoacoustic pulse since we consider that the interface is the thinnest 2D structure. To study the ultrafast photoacoustic mechanism of 2DEG under impulsive photo-excitation, we develop a simple theoretical model in this thesis. According to our numerical studies, the acoustic pulsewidth is dominated by the contribution of hot electron pressure and 2DEG is a potential source to generate a ~100-fs super-short acoustic pulse. Experimentally, to prove our theoretical study finding, we created the 2DEG at the a-Ga2O3/GaN interface. Through photo-exciting 2DEG, we successfully measured an ultra-short acoustic pulse with a pulsewidth of 266 fs after deconvolution with a bandwidth up to 2.1 THz. Our study not only demonstrated a novel photoacoustic pulse generation mechanism in 2DEG, but also experimentally generated a record-short acoustic pulse which has never been achieved before.