Abstract
A robust valley polarization is a key prerequisite for exploiting valley pseudospin to carry information in next-generation electronics and optoelectronics. Although monolayer transition metal dichalcogenides with inherent spin–valley coupling offer a unique platform to develop such valleytronic devices, the anticipated long-lived valley pseudospin has not been observed yet. Here we demonstrate that robust valley-polarized holes in monolayer WSe2 can be initialized by optical pumping. Using time-resolved Kerr rotation spectroscopy, we observe a long-lived valley polarization for positive trion with a lifetime approaching 1 ns at low temperatures, which is much longer than the trion recombination lifetime (∼10–20 ps). The long-lived valley polarization arises from the transfer of valley pseudospin from photocarriers to resident holes in a specific valley. The optically initialized valley pseudospin of holes remains robust even at room temperature, which opens up the possibility to realize room-temperature valleytronics based on transition metal dichalcogenides.
Highlights
A robust valley polarization is a key prerequisite for exploiting valley pseudospin to carry information in next-generation electronics and optoelectronics
We used time-resolved Kerr rotation (TRKR) spectroscopy (Fig. 1a), which gives a direct measurement of the population imbalance between valleys induced by helical light
Using TRKR spectroscopy, which directly measures the population imbalance between valleys induced by helical light, we observe a long-lived valley polarization for positive trion with a lifetime approaching B1 ns, which is much longer than the trion lifetime (B10–20 ps)
Summary
A robust valley polarization is a key prerequisite for exploiting valley pseudospin to carry information in next-generation electronics and optoelectronics. Monolayer TMDs further feature their coupled spin and valley physics stemming from the inherent inversion symmetry breaking in the honeycomb lattice structure and the strong spin–orbit coupling in the d-orbits of the transition metal, leading to strong valley-contrasting optical selection rules at ±K valleys[3,4,5] (Fig. 1a). This unique property provides an unprecedented opportunity to generate, control and detect valley polarization (that is, the population imbalance between ±K valleys) by optical means. The sample was studied at T 1⁄4 10 K, unless otherwise specified in temperature-dependent measurements
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