單層二硒化鎢之時間解析克爾旋轉光譜

Abstract

二維材料，如石墨烯、六方氮化硼及過渡金屬二硫族化物等材料，近年都被發現具有特殊的載子傳輸特性。其中過渡金屬二硫族化物屬於擁有能隙的二維半導體材料，使得過渡金屬二硫族化物比石墨烯能在電子元件發展上有更好的應用。本論文利用超快雷射光譜量測單層二硒化鎢的時間解析克爾旋轉及橢圓率，研究其能谷極化動力學。首先，我們分析在室溫下的時間解析克爾旋轉光譜，發現在室溫下單層二硒化鎢的能谷生命期約為200 皮秒。在改變激發功率的實驗中，也發現激子-激子交互作用對克爾訊號沒有明顯的影響。在室溫改變偵測波長的實驗中，也分析了克爾訊號和介電函數之間的對應關係。在變溫的實驗中，隨著溫度變化，可藉由改變波長得到最強的克爾訊號。隨著溫度升高，能谷生命週期逐漸下降，可以根據簡單的聲子散射模型分析單層二硒化鎢能谷去極化的機制。

Two-dimensional (2D) materials, such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDCs) have been found to exhibit fascinating transport properties. In particular, TMDCs are 2D semiconducting materials with energy band gap, which makes TMDCs to have greater potential than graphene for electronic device applications. In this thesis, valley polarization dynamics in monolayer WSe2 were investigated by time-resolved Kerr rotation (TRKR) spectroscopy. Firstly, we performed TRKR spectroscopy at room temperature and found that the Kerr lifetime is about 200 ps. In pump beam power dependent experiments, the exciton-exciton interactions were found to play a minor role in the Kerr rotation signals. In probe-wavelength dependent measurements, we found that the Kerr rotation and Kerr ellipticity can be well described by the real and the imaginary parts of the complex dielectric function of monolayer WSe2. In temperature dependent measurements, the Kerr lifetime decreases with increasing temperature, which can be analyzed by a simple phonon scattering model to understand the valley depolarization mechanism in monolayer WSe2.