二硒化鎢之載子與能谷極化動力學

Abstract

本論文中我們利用激發探測與柯爾旋轉光譜探測層狀二硒化鎢樣品的載子與能谷極化動力學。第一部分研究中我們分析了大面積單層二硒化鎢中的情況。首先我們發現柯爾訊號的正負號可直接對應到自旋與能谷極化的方向，並且有能谷生命期長達數奈秒時間的情況。以上觀察到的衰減，都由三種不同的衰減時間所組成。由於較大的雷射光點會涵蓋許多晶界、邊緣、甚至不同層的問題，因此我們無法排除這些因素對於生命期的影響。因此在第二部分中，我們進一步將柯爾系統拓展到顯微尺度，以其針對單晶或特定區域進行量測。與大面積樣品相當不同，我們發現在單晶樣品中僅有一段長時間的衰減，證明了僅由一種機制主導電洞的能谷去極化現象。更進一步，我們發現晶界或樣品邊緣會使得能谷生命期下降。最後，我們也分析了雙層樣品的能谷生命期，我們發現雙層樣品中的生命期普遍都較單層樣品來的長。因此我們證明了這些結構的不完美以及層數問題將是未來探討層狀WSe2材料的一大重點。

Carrier and valley polarization dynamics in layered WSe2 were investigated by pump-probe and time-resolved Kerr rotation (TRKR) spectroscopy. The large-scale monolayer WSe2 was analyzed in the first part. We found the sign of Kerr rotation is directly related to the spin and valley polarization, in which the valley lifetime is as long as ~ns scale. However, due to the fact that there are many different crystals in a laser spot, we cannot exclude the effects of grain boundary, sample edge and layer number on the decay dynamics. In order to probe the dynamics in some specific regions, we further improve the spatial resolution of TRKR system in the second part. In contrast to the large-scale sample, we found there is only one decay in a single-crystal region, demonstrating only one depolarization mechanism of hole spin. Furthermore, we found the valley lifetime is shorter in the grain boundary and edge region, and the bilayer samples generally have longer lifetimes than monolayer samples. Our works here demonstrate the importance of these structural imperfections on the study of carrier and valley polarization dynamics.