Source: http://aoot.osa.org/ome/abstract.cfm?uri=ome-9-3-1479
Timestamp: 2019-04-21 02:11:19+00:00

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The valley degree of freedom possessed by electronic excitations in transition metal dichalcogenides is providing new opportunities for information processing and optoelectronics. Valley contrasting polarization selection rules present unique opportunities for optical control in valleytronic devices. Critical to devices leveraging the valley degree of freedom is the ability to tailor optical valley polarizability and its degree of coherence. In this manuscript, we demonstrate the electric field control of both valley polarization and valley coherence in a monolayer of tungsten diselenide that has been incorporated into a van der Waals heterostructure. We find the competition between electron-hole exchange, radiative decay and pure dephasing determines the ability to control valley polarization and coherence of both excitons and trions.
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de Andrada e Silva, E. A.
van der Zant, H. S. J.
Zant, H. S. J. v. d.
Fig. 1 (a) Schematic of the van der Waals heterostructure used to form a vertical field effect diode. Inset: Band diagram of the monolayer under a vertically applied electric field. (b) Voltage controlled PL spectra. The neutral exciton X 0 and negative charged exciton, the trion, X − 1 are indicated.
Fig. 2 (a) Energy and (b) PL intensity of the neutral exciton (X 0 ) and trion (X − ) as a function of voltage extracted by peak fitting from Fig. 1(b).
Fig. 3 (a) Circular polarization resolved PL spectra at 0V. Black (Gray) curve is co-(cross-) polarized PL signal. Excitation is σ + polarized. (b) Exciton and trion degree of circular polarization (DoCP) as a function of voltage. (c) Illustration depicting valley depolarization due to the electron-hole exchange interaction.
Fig. 4 (a)-(b) Linear polarization resolved PL spectra at -8V and 8V respectively. Black (Grey) curve is co-(cross-) polarized signal with respect to the vertically polarized excitation laser. (c) DoLP as a function of voltage for the neutral exciton peak for H (solid blue triangle) and V (empty triangle) polarized excitation. (d) DoLP as a function of voltage for the trion peak for H (solid red triangle) and V (empty triangle) polarized excitation. (e) Illustration of the coherent superposition of thetwo valleys with no change in relative phase for the neutral exciton upon excitation with linearly polarized light (f) Illustration of an intervalley and intravalley trion with electron-hole (eh) and electron-electron (ee) exchange interactions. Linear excitation will also generate the time reversal partners.

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