Physics (Internal)

Valley-selective optical Stark effect in monolayer WS2

This event is part of the Condensed Matter Theory Seminar Series.

Semiconductors that are atomically thin can exhibit novel optical properties beyond those encountered in the bulk compounds. Monolayer transition metal dichalcogenides (TMDs) are leading examples of such semiconductors that possess remarkable optical properties. They obey unique selection rules where light with different circular polarization can be used for selective photoexcitation at two electronic valleys. These valleys constitute bandgaps that are normally locked in the same energy. Selectively varying their energies is of great interest for applications because it offers a means to carry information in next-generation valleytronics. Here, we demonstrate that the bandgaps at the two valleys can be shifted relative to each other by means of the optical Stark effect in a controllable valley-selective manner. We will discuss the physics of this effect, and describe the mechanism that leads to its valley-selectivity in monolayer TMD tungsten disulfide (WS2). Finally, by using this technique we propose to realize a conceptually unique topological phase transition that can be made valley-selective in this system. [Reference: E J Sie et al., Nature Materials 14, 290 (2015)]