Optical control of exciton valley polarization in MoS$_2$

Atomic monolayers of transition metal dichalcogenides have emerged as an interesting class of 2-dimensional (2D) crystals beyond graphene. In particular, the isoelectronic family of MoS$_{2}$, MoSe$_{2}$, WS$_{2}$ and WSe$_{2}$ monolayers are direct band gap semiconductors.\footnote{Mak, K. F., Lee, C., Hone, J., Shan, J. {\&} Heinz, T. F. \textit{Phys Rev Lett} \textbf{105}, 136805 (2010); Splendiani, A.\textit{ et al.} \textit{Nano Lett} \textbf{10}, 1271-1275 (2010).}$^,$\footnote{Xiao, D., Liu, G.-B., Feng, W., Xu, X. {\&} Yao, W. \textit{Phys Rev Lett} \textbf{108}, 196802 (2012); Zhu, Z. Y., Cheng, Y. C. {\&} Schwingenschlogl, U. \textit{Phys Rev B} \textbf{84}, 153402 (2011).} Unlike graphene, because of the lack of inversion symmetry and the presence of strong spin-orbit interactions, the fundamental energy gaps of these compounds are located at two inequivalent high-symmetry valleys in the Brillouin zone (K and K') with coupled valley and spin degrees of freedom.\footnote{Ibid.} This electronic property makes them unique from conventional semiconductors. In this talk, we will discuss the properties of MoS$_{2}$ atomic layers as a prototype. Through characterization of the optical properties of the material as a function of thickness, we show that quantum confinement effects lead to a crossover in MoS$_{2}$ from a bulk indirect gap semiconductor to a direct gap semiconductor at monolayer thickness.\footnote{Mak, \textit{PRL} 105, 2010} With this basic property established, we show that complete valley polarization of the excitons in monolayer MoS$_{2}$ can be achieved by optical pumping with circularly polarized light.\footnote{Mak, K. F., He, K., Shan, J. {\&} Heinz, T. F. \textit{Nat Nano} \textbf{7}, 494-498 (2012); Zeng, H., Dai, J., Yao, W., Xiao, D., {\&} Cui, X. \textit{Nat Nano} \textbf{7}, 490-493 (2012); Cao, T. \textit{et al.} \textit{Nat Commun} \textbf{3}, 887 (2012); Sallen, G. et al. \textit{Phys Rev B} \textbf{86}, 081301(R) (2012).} Furthermore, this polarization can be retained for longer than 1ns. Our results thus highlight the great potential of this material family for studies of valley and spin Hall physics.\footnote{Xiao, D., Yao, W. {\&} Niu, Q. \textit{Phys Rev Lett} \textbf{99}, 236809 (2007); Yao, W., Xiao, D. {\&} Niu, Q. \textit{Phys Rev B} \textbf{77}, 235406 (2008); Xiao, D., Chang, M.-C. {\&} Niu, Q. \textit{Rev Mod Phys} \textbf{82}, 1959-2007 (2010).}