Valley-polarized excitons in 2D semiconductors

Speaker: 
Dr. Ziliang Ye
Event Date and Time: 
Mon, 2017-03-27 11:00 - 12:30
Location: 
Hennings 318
Local Contact: 
Leanne Ebbs / Andrea Damascelli
The successful isolation and manipulation of atomically thin two-dimensional (2D) materials have ushered in a rich new era of fundamental scientific research and technological innovation. Among them, the transition metal dichalcogenide (TMDC) family of the form MX2 (M = Mo, W; X = S, Se, Te) has attracted significant attention as a new class of 2D semiconductors. While preserving the extraordinary tunability as found in graphene, the monolayer TMDC has a broken inversion symmetry, which gives rise to a series of emerging properties such as the direct band gap at the K point and the access to the valley degree of freedom associated with the propagation direction of the electron in the crystal. In this talk, I will discuss our effort to understand and control the electronic structure of TMDCs with ultrafast optical spectroscopy. I will firstly present our discovery of a strongly correlated electron-hole pair, known as exciton, in the WS2 monolayer.[1] This excitonic effect largely modifies the optical property of the material up to room temperature and exhibits a nonhydrogenic Rydberg series as a consequence of an unscreened Coulomb interaction in the 2D limit. In addition, I will report our recent success on coherently manipulating the valley polarization in the WSe2 exciton.[2] As an analogy to the spin degree of freedom, we are able to initialize, rotate, and read the valley pseudospin in an all-optical way. The ability to control the valley degree of freedom potentially can enable a new platform for quantum information applications. [1] Z. Ye et al. Nature 513, 214 (2014) [2] Z. Ye et al. Nature Physics 13, 26 (2017)
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