Monolayer transition-metal dichalcogenides such as MoS2 and WS2 are prime examples of atomically thin semiconducting crystals that exhibit remarkable electronic and optical properties. They have a pair of valleys that can serve as a new electronic degree of freedom, and these valleys obey optical selection rules with circularly polarized light. Here, we discuss how ultrafast laser pulses can be used to tune their energy levels in a controllable valley-selective manner. The energy tunability is extremely large, comparable to what would be obtained using a hundred Tesla of magnetic field. We will also show that such valley tunability can be performed while we effectively manipulate the valley selection rules. Finally, we will explore the prospect of using this technique through photoemission spectroscopy to create a new phase of matter called a valley Floquet topological insulator.
Edbert J. Sie and Nuh Gedik, "Optical tuning of electronic valleys (Conference Presentation)," Proc. SPIE 10102, Ultrafast Phenomena and Nanophotonics XXI, 101021Q (Presented at SPIE OPTO: February 02, 2017; Published: 19 April 2017); https://doi.org/10.1117/12.2254939.5391659190001.
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