Ultrafast spectroscopy of two-dimensional crystals in external magnetic fields

在这个项目中，我们将用时间积分四波混频光谱来研究原子薄的过渡金属二卤化物单分子膜中激子和激子复合体的相干性质。实验将采用三光束技术，在光学分裂线圈磁体低温恒温器中施加不同的偏振配置。调查将集中在MoSe2单分子膜上。在这种材料中，相干激子复合体将通过不同的偏振构型的不同选择而被激发。其目的是研究络合物的量子力学耦合以及外加磁场中的四粒子和多粒子关联。我们将从在法拉第组态中应用磁场开始。在这种构型中，我们预期多粒子关联和非马尔可夫记忆效应的特征会被急剧放大。另一方面，这些实验也将在Voigt配置下进行，在那里可以实现不同自旋态的混合。我们将研究自旋混合对激子复合体相干性质的影响。在项目接近尾声时，还计划对包裹在六方氮化硼(HBN)中的MoSe2单层和MoSe2-WSe2异质结构进行实验。在hBN中的封装显著降低了TMDC单分子膜中带电激子的不均匀展宽和无序势，并潜在地影响了相干时间。在MoSe_2-WSe_2异质结中，MoSe_2中的电子密度可以通过额外的激光照射来调节，这是因为异质结中的第二类能带排列以及由此产生的光激电子-空穴对的电荷分离。我们将探索背景载流子浓度对激子复合体相干性质的影响。

In this project, the coherence properties of excitons and excitonic complexes - like charged excitons, neutral and charged biexcitons - in atomically thin transition-metal dichalcogenide monolayers in an external magnetic field shall be explored by time-integrated four-wave-mixing spectroscopy. The experiments will be performed in three-beam technique by applying different polarization configurations in an optical split-coil magnet cryostat. The investigations will be focused on MoSe2 monolayers. In this material, coherent excitonic complexes will be excited via distinct choices of different polarization configurations. The aim is to investigate of the quantum mechanical coupling of the complexes and the four- and many-particle correlations in external magnetic fields. We will start by applying the magnetic field in the Faraday configuration. In this configuration we expect a drastic amplification of many-particle correlations and signatures of non-Markovian memory effects. On the other hand, the experiments will also be performed in Voigt configuration, where a mixing of different spin states can be achieved. We will study the impact of the spin mixing on the coherence properties of excitonic complexes. Towards the end of the project also experiments on MoSe2 monolayers and MoSe2-WSe2 heterostructures, which are encapsulated in hexagonal Boron nitride (hBN), are planned. Encapsulation in hBN reduces significantly the inhomogeneous broadening and the disorder potential for charged excitons in TMDC monolayers and can potentially also influence the coherence times. In MoSe2-WSe2 heterostructures, the electron density in the MoSe2 can be tuned via an additional laser illumination, due to the type-II band alignment of the heterostructure and the resulting charge separation of photo-excited electron-hole pairs. We will explore the influence of the background charge-carrier concentration on the coherence properties of excitonic complexes.