利用超寬頻中紅外光激發探測光譜研究狄拉克費米子之超快動力學

Ultrafast dynamics of Dirac fermions studied by ultra-broadband mid-infrared pump-probe spectroscopy

葉恬恬(Tien-Tien Yeh)

指導教授 ： 羅志偉

國立交通大學/理學院/電子物理系所/博士(2020年)

摘要

我們已成功建立了800奈米光激發及超寬頻中紅外光探測之超快光譜系統。此具飛秒級時間解析度之光譜系統是藉由四波混頻技術將中紅外光脈衝載在800奈米啾頻脈衝上並轉成可見光來進行量測。產生的中紅外光探測脈衝波長可涵蓋整個中紅外光的範圍，從200到5000波數(cm-1)。此系統結合了單發測量的技術來記錄每發脈衝的信號以提高訊噪比。此激發探測系統被運用在測量未摻雜半導體鍺的晶片上，並利用測量瞬時反射率變化以得知其超快動力學的行為。藉由德汝德模型(Drude model)的擬合，具飛秒時間解析的瞬時光激發載子遷移率、電漿濃度、載子散射率及載子濃度可以被計算。更進一步的，在100皮秒內主宰的歐傑複合(Auger recombination)效應及作用時間較長的擴散(diffusion)現象，也在我們的實驗中被觀察到。此外，在瞬時反射光譜的2000 cm-1處，觀察到一奇異的吸收峰。此吸收峰在高激發能量且延遲時間在20皮秒內尤為明顯。這個類似羅倫茲震盪(Lorentz oscillation)的吸收峰透露出，在高濃度下的載子因為有較強的庫倫作用力進而造成光譜的扭曲。
除了對半導體鍺的研究以外，此光譜技術也成功地被應用在新興的材料拓樸絕緣體(Topological insulators)上。這類材料擁有塊材態能隙，但表面卻似金屬沒有能隙的特性，此無能隙的結構稱為狄拉克錐(Dirac cone)。這個新興材料的發現，為近十年來凝態物理研究帶來爆炸性的發展。由於中紅外光的光子能量小，不足以激發價電子，因此取得單純的狄拉克錐的訊號變成可能。由於其光譜特徵與自由載子吸收相似，使得光譜分析上難以區隔。然而，這兩者特徵在非外在環境擾動之超寬頻中紅外光時間解析光譜中呈現了截然不同的特徵。在此研究中，我們展示了狄拉克費米子躍遷與自由載子吸收的光譜特性，並利用格林久保定理(Green–Kubo formula)在線性能帶的光學特性理論進一步擬和，由擬和結果提出了表面態複合過程中的瞬時濃度、溫度、散射率及化學位能的變化。

關鍵字

非線性光學 ； 激發探測光譜儀 ； 超快動力學 ； 中紅外光 ； 半導體 ； 拓樸絕緣體

並列摘要

We developed an 800-nm pump and ultra-broadband midinfrared (MIR) probe spectroscopy by utilizing the chirped-pulse up-conversion technique with four-wave difference-frequency generation in a gas. The generated ultra-broadband MIR probe pulses were in the range 200–5000 cm−1, which were recorded by single-shot detection using the chirped-pulse up-conversion technique.
This pump-probe spectroscopy was used to measure the ultrafast carrier dynamics of a semiconductor and topological insulators (TI). By fitting the transient reflectivity changes (ΔR/R) with the Drude model, the time-resolved photoexcited carrier mobility, plasma frequency, scattering rate, and carrier concentration were obtained. In the intrinsic germanium crystal wafer, the Auger recombination was found to virtually dominate the fast relaxation of photoexcited carriers within 100 ps, followed by slow relaxation because of diffusion. Moreover, a novel oscillation feature near 2000 cm-1 was clearly observed, especially for high pump fluences, which is a Lorentz oscillation occurring at about 20 ps because of the Coulomb force generated immediately after excitation.

Furthermore, considering the small photon energy of the MIR region, this spectroscopy can potentially be employed to study the ultrafast dynamics of gapless materials. However, the Dirac fermion transitions in Dirac cones are veiled by free carrier absorption with similar spectral line shapes in static MIR spectra. Thus, distinguishing the Dirac fermion transition from the bulk free carrier absorption is difficult. In this study, we distinguish the free carrier absorption and the Dirac fermion transition in TIs using linearly polarized ΔR/R with no environmental perturbation. Although both the effects exhibit similar features in reflection, completely different variations for the non-equilibrium states are found in ΔR/R. Furthermore, this study provides femtosecond time evolutions of several important physical parameters, such as the chemical potential and the hot carrier temperature of TIs, by employing the ultra-broadband ΔR/R spectra.

並列關鍵字

nonlinear optics ； pump-probe spectroscopy ； ultrafast dynamics ； mid-infrared ； semiconductor ； topological insulator

參考文獻

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