以光激發中紅外光探測光譜研究拓樸絕緣體硒化鉍晶體之超快動力學

Abstract

在本論文中，我們利用超快動力學實驗室自行建立的光激發中紅外光探測光譜系統研究拓樸絕緣體硒化鉍晶體之載子超快動力學行為。由於所使用之中紅外光光子能量(約100 meV)小於拓樸絕緣體硒化鉍晶體之塊材能隙(約300 meV)，因此，從瞬時反射率變化光譜上可清楚觀察到表面態載子及塊材載子在超快動力學行為上之差異。此外，透過改變激發光脈衝能量密度，我們成功的抑制了塊材載子於瞬時反射率變化光譜的貢獻，以避開塊材載子對信號的干擾，進而可單一且清晰地觀察表面態載子動力學行為並建立其弛緩模型。最後，我們得到了表面態狄拉克錐中載子之能量散失速率約為 1 meV/ps，以及狄拉克點附近之電子聲子耦合強度約為0.088。此實驗結果與使用角解析光電子能譜得到之結果十分吻合。因此，本研究所建立之光激發中紅外光探測光譜技術可望成為檢驗拓樸絕緣體表面態之新利器。

In this thesis, we investigated the ultrafast dynamics of carriers in topological insulators (TI) Bi2Se3 by using the homemade optical pump mid-infrared probe (OPMP) spectroscopy system. Due to the photon energy of the mid-infrared beam is about 100 meV which is smaller than the bulk bandgap of Bi2Se3 (about 300 meV), the difference between the ultrafast dynamics of carriers in bulk and surface states can be unambiguously recognized in the transient reflectivity changes spectroscopy (ΔR/R). Additionally, the signal in ΔR/R from bulk states has been certainly reduced by decreasing the pumping fluences to independently and clearly observe the ultrafast dynamics of carriers in the Dirac-cone surface state. Therefore, the relaxation model for carriers in the Dirac-cone surface state can be established with the energy loss rate of 1 meV/ps. Furthermore, the electron-phonon coupling constant was estimated to be ~0.088 near the Dirac point. Finally, the optical pump mid-infrared probe spectroscopy would be a new technique to study the Dirac-cone surface states in topological insulators.