利用光激發超寬頻中紅外光探測光譜研究拓樸絕緣體超快動力學

Abstract

本論文中，我們利用脈衝寬度為8.2 fs、波長範圍200-4000 cm-1的超寬頻中紅外光做為激發探測系統的探測光來量測類鉍系列(Bi2Te3-xSex, x = 0、1、2)與類銻系列(Sb2Te2-xSex, x = 0、1)拓樸絕緣體的瞬時反射率光譜，發現這兩類拓樸絕緣體展現截然不同的結果。對於類鉍系列樣品，藉由自由載子模型擬合並參考相關文獻分析，我們發現所測得的弛緩時間分別來自於低能量塊材態與表面態載子的弛緩行為。另外，在類銻系列樣品則是先利用庫柏公式(Kubo’s formula)將光導率轉換成反射率後，再進行擬合，此分析結果可闡釋費米面在激發後的弛緩行為。 這本研究中，我們不僅觀察到各式拓樸絕緣體的弛緩行為，也展示了超寬頻中紅外光譜作為研究拓樸絕緣體的表面態行為的潛力與價值。

We investigated the ultrafast carrier dynamics of topological insulators (TIs) including Bi2Te3-xSex and Sb2Te3-xSex (x=0,1) by the optical pump and ultra-broadband mid-IR probe spectroscopy, which was performed by using four-wave difference frequency generation. The mid-IR spectrum used in this study covers the region of 200-4000 cm-1 with pulse duration of 8.2 fs. Although both of Bi-series and Sb-series TIs have the surface states, they show totally different pattern in the transient reflectivity change (ΔR/R) spectrum. Moreover, by fitting with free carrier absorption model and optical conductivity of Kubo’s formula, the ultrafast dynamics of Bi2Te3-xSex are mainly dominated by the relaxation processes of free carrier at bulk and surface states. On the other hand, the ultrafast dynamics of Sb2Te3-xSex are caused by the shift of Fermi level.