Valley dynamics probed through charged and neutral exciton emission in monolayer ${\mathrm{WSe}}_2$

Optical interband transitions in monolayer transition metal dichalcogenides such as ${\mathrm{WSe}}_2$ and ${\mathrm{MoS}}_2$ are governed by chiral selection rules. This allows efficient optical initialization of an electron in a specific K valley in momentum space. Here we probe the valley dynamics in monolayer ${\mathrm{WSe}}_2$ by monitoring the emission and polarization dynamics of the well-separated neutral excitons (bound electron-hole pairs) and charged excitons (trions) in photoluminescence. The neutral exciton photoluminescence intensity decay time is about 4 ps, whereas the trion emission occurs over several tens of ps. The trion polarization dynamics shows a partial, fast initial decay within tens of ps before reaching a stable polarization of $\approx 20\%$, for which a typical valley polarization decay time of the order of 1 ns can be inferred.

Figure 1

(a) Investigated sample structure, (b) reflectivity measurements at $T=4$ K, and (c) PL emission of the neutral exciton $X^0$, the trion (T), and localized states as a function of temperature.

Figure 2

(a) Optical interband selection rules for monolayer ${\mathrm{WSe}}_2$ according to Ref. [18]. (b) Investigated sample structure. (c) Laser polarization ${\sigma }^{+}$ and $E_{\mathrm{Laser}}=1.893$ eV. PL emission of ML ${\mathrm{WSe}}_2$ at $T=4$ K, and the $X^0$, trion, and localized states are marked. Black (red): ${\sigma }^{+}(\sigma -)$ polarized. (d) Linear laser polarization X. Green (blue) corresponds to linear X (linear Y) polarized emission. (e) Streak camera image of TRPL total intensity showing different emission times for $X^0$, trion, and localized states. Blue (red) corresponds to zero (10 000) counts.

Figure 3

Time-resolved photoluminescence. $T=4$ K. Normalized PL dynamics (total peak intensity) in log scale for $X^0$, trion, L1, and L2. The lines show fits with exponential decays with typical times of $33\pm 5$ ps for the second decay time of $X^0$ (initial time too short to be fitted) and $32\pm 2$ ps for L1 and $80\pm 6$ ps for L2. Trion fitted with biexponential decay, with $18\pm 2$ and $30\pm 3$ ps characteristic decay times. The small peaks superimposed on the $X^0$ and trion dynamics come from laser reflections marked by arrows.

Figure 4

Time-resolved photoluminescence. $T=4$ K. Laser polarization ${\sigma }^{+}$. (a) Left axis: $X^0$ PL emission (in log scale) copolarized (black) and cross-polarized (red) with respect to the excitation laser as a function of time. Right axis: Circular polarization degree of the PL emission. As the PL intensity decays very quickly, we clearly observe a periodic signal of laser reflections, marked by arrows. (b) Same as (a), but for trion emission. The polarization is well reproduced by a biexponential decay using an initial, fast decay time of ${\tau }_1=12$ ps and a long decay time ${\tau }_2=1$ ns (solid green line). Lower bounds (dotted orange line for ${\tau }_2=150$ ps) and upper bounds (dotted purple line for ${\tau }_2=3$ ns) of the slow decay are shown. (c) Same as (a), but for localized exciton emission L1.