Long-Lived Valley Polarization of Intra-Valley Trions in Monolayer WSe2*

We report distinct valley dynamics associated with intervalley and intravalley trions in monolayer WSe2. Using circular two-color pump-probe spectroscopy, we observe no decay of a near-unity valley polarization associated with the intra-valley trions during \textasciitilde 25 ps, while the valley polarization of the inter-valley trions exhibits a fast decay of \textasciitilde 4 ps. Moreover, we show that intrinsic dynamics associated with the two types of trions can only be observed when they are excited resonantly. The exceptionally robust valley polarization associated with resonantly created intravalley trions in monolayer WSe2 may be exploited for applications of valleytronic applications such as the valley Hall effect.