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Control of valley polarization in monolayer MoS2 by optical helicity


Electronic and spintronic devices rely on the fact that free charge carriers in solids carry electric charge and spin. There are, however, other properties of charge carriers that might be exploited in new families of devices. In particular, if there are two or more minima in the conduction band (or maxima in the valence band) in momentum space, and if it is possible to confine charge carriers in one of these valleys, then it should be possible to make a valleytronic device1,2,3,4. Valley polarization, as the selective population of one valley is designated, has been demonstrated using strain5,6 and magnetic fields7,8,9,10, but neither of these approaches allows dynamic control. Here, we demonstrate that optical pumping with circularly polarized light can achieve complete dynamic valley polarization in monolayer MoS2 (refs 11, 12), a two-dimensional non-centrosymmetric crystal with direct energy gaps at two valleys13,14,15,16. Moreover, this polarization is retained for longer than 1 ns. Our results, and similar results by Zeng et al.17, demonstrate the viability of optical valley control and suggest the possibility of valley-based electronic and optoelectronic applications in MoS2 monolayers.

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Figure 1: Atomic structure and electronic structure at the K and K′ valleys of monolayer (a–c) and bilayer (d–f) MoS2.
Figure 2: Optical absorption and photoluminescence spectra of monolayer MoS2.
Figure 3: Optical control of valley-spin polarization in monolayer MoS2.
Figure 4: Substrate-independent photoluminescence helicity.


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This research was supported by the National Science Foundation (grant DMR-1106172 at Columbia University and grant DMR-0907477 at Case Western Reserve University). Additional support for the optical instrumentation at Columbia University was provided by the Center for Re-Defining Photovoltaic Efficiency Through Molecule Scale Control, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Basic Energy Sciences (grant DE-SC0001085). The authors thank G.H. Lee and J. Hone for help with sample preparation and I. Aleiner, W. Lambrecht and P. Kim for fruitful discussions.

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K.F.M. and J.S. developed the concept, designed the experiment and prepared the manuscript. K.F.M. performed the polarization resolved photoluminescence measurements. K.H. prepared the samples and contributed to the study of temperature dependence. All authors contributed to the interpretation of the results and writing the manuscript.

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Correspondence to Tony F. Heinz.

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Mak, K., He, K., Shan, J. et al. Control of valley polarization in monolayer MoS2 by optical helicity. Nature Nanotech 7, 494–498 (2012).

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