Ultrafast dynamics of bright and dark positive trions for valley polarization in monolayer WSe_2

摘要

Optical excited-state dynamics of bright and dark positively charged excitons (positive trions) in one-layer (1L-) WSe2 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 (similar to 1 ps) and to ground state (10 +/- 5 ps), while dark positive trions showed slow decay to the ground state (similar to 300 ps) at 10 K. Under valley selective excitation with circularly polarized light, the valley relaxation time of the bright trions was 1.5 +/- 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.