Phonon-mediated intervalley relaxation of positive trions in monolayer ${\mathrm{WSe}}_2$

The valley relaxation mechanism of the bright state of positively charged excitons (bright positive trions) in monolayer (1L-) ${\mathrm{WSe}}_2$ is studied through polarization- and time-resolved photoluminescence and transient reflection measurements. A long valley relaxation time, exceeding 100 ps, is observed for positive trions at low temperature, which is notably prolonged when compared with the characteristic excitonic valley relaxation time of 10 ps. With increasing temperature, the relaxation time decreases to a few ps. This finding implies that phonon-mediated intervalley scattering is important for the relaxation process of the valley-polarized bright state of positive trions.

Figure 1

(a) Valley configurations of the bright state of a positively charged exciton (bright positive trion) in the $K$ valley in 1L-${\mathrm{WSe}}_2$. The red and green lines denote the spin-up and spin-down conduction/valence bands, respectively. The $-K$ bright trion state represents the time reversal of the schematics. (b) Helicity-resolved PL spectra for a gate voltage of $-40\mathrm{V}$ under ${\sigma }_{+}$ cw excitation at different temperatures. The red (black) curves show the PL spectra for the ${\sigma }_{+}\left({\sigma }_{-}\right)$ component. (c) Temperature dependence of the valley polarization of bright positive trions for a gate voltage of $-40\mathrm{V}$ (red circles). The green curve is the valley polarization calculated from Eqs. (1) and (3).

Figure 2

(a) PL decay traces of positive trions under pulsed excitation at various temperatures for a gate voltage of −40 V (red circles), and fitting profiles obtained from the convolution of the double-exponential decay with the IRF (green curves). The black curve is the IRF. (b) Pump-probe traces near the B transition at several temperatures (red circles). The solid green curves show fitting results. The inset shows a schematic of the pump and probe configuration in 1L-${\mathrm{WSe}}_2$. (c) Temperature dependence of the decay times determined from the PL decays (red circles) and transient differential reflectance (blue triangles) measurements. The solid curve shows the decay of bright trions based on Eq. (2). (d) Schematic of the intravalley relaxation of positive trions.

Figure 3

Temperature dependence of the valley relaxation time for a gate voltage of −40 V (red circles). The solid green curve is the valley relaxation time calculated from Eq. (3). The inset presents a schematic of the relaxation process of the valley-polarized bright state of positive trions in 1L-${\mathrm{WSe}}_2$.