Long-Lived Valley Polarization of Intravalley Trions in Monolayer ${\mathrm{WSe}}_2$

We investigate valley dynamics associated with trions in monolayer tungsten diselenide (${\mathrm{WSe}}_2$) using polarization resolved two-color pump-probe spectroscopy. When tuning the pump and probe energy across the trion resonance, distinct trion valley polarization dynamics are observed as a function of energy and attributed to the intravalley and intervalley trions in monolayer ${\mathrm{WSe}}_2$. We observe no decay of a near-unity valley polarization associated with the intravalley trions during $\sim 25\mathrm{ps}$, while the valley polarization of the intervalley trions exhibits a fast decay of $\sim 4\mathrm{ps}$. Furthermore, we show that resonant excitation is a prerequisite for observing the long-lived valley polarization associated with the intravalley trion. The exceptionally robust valley polarization associated with resonantly created intravalley trions discovered here may be explored for future valleytronic applications such as valley Hall effects.

Figure 1

(a) Possible configurations of charged and neutral bright excitons in ${\mathrm{WSe}}_2$. (b) Differential transmission spectrum of ${\mathrm{WSe}}_2$ (dots). The black line is an interpolation curve, serving as a guide to the eye. The solid Gaussians illustrate the spectral position of the exciton and the two trion (inter- and intravalley) resonances. The spectral positions of probe energies for data in Figs. 2, 3 and 3 (dashed colored lines), and the pump energy for Fig. 3 (gray line) are also illustrated.

Figure 2

Co- (solid dots) and cross-polarized (open dots) signals recorded in resonant excitation experiments across the trion resonance, at probe energies of (a) 1726.8 meV, (b) 1722 meV, and (c) 1719.6 meV. (d) Valley polarization for the measurements displayed in (a) and (c).

Figure 3

Co- (solid dots) and cross-polarized (open dots) signals recorded in nonresonant excitation experiments for pumping at the exciton resonance and probing at (a) 1724.4 meV, (b) 1722 meV, and (c) 1719.6 meV. (d) Valley polarization for the measurements displayed in (a) and (c).

Figure 4

Schematic of the suggested valley polarization dynamics, exciton and trion conversion processes, and their respective time scales as measured in the experiment. Dashed lines suggest that such processes are possible in principle but do not compete favorably with other faster processes.