Ultrafast dynamics of bright and dark positive trions for valley polarization in monolayer WSe2

Abstract

Optical excited-state dynamics of bright and dark positively charged excitons (positive trions) in one-layer (1L-) $\mathrm{WS}{\mathrm{e}}_{2}$ were studied by a combination of time- and polarization-resolved photoluminescence and pump-probe spectroscopy. Bright positive trions showed ultrafast relaxation to dark positive trions (\ensuremath{\sim}1 ps) and to ground state (10 \ifmmode\pm\else\textpm\fi{} 5 ps), while dark positive trions showed slow decay to the ground state (\ensuremath{\sim}300 ps) at 10 K. Under valley selective excitation with circularly polarized light, the valley relaxation time of the bright trions was 1.5 \ifmmode\pm\else\textpm\fi{} 0.5 ns at 10 K, which is much longer than typical exciton valley relaxation times of 10 ps. This insight into the slower valley relaxation dynamics of positive trions suggests that the electron-hole exchange interactions that are dominant in neutral exciton valley relaxation, are suppressed by the presence of additional holes.