Carrier and Polarization Dynamics in Monolayer ${\mathrm{MoS}}_2$

In monolayer ${\mathrm{MoS}}_2$, optical transitions across the direct band gap are governed by chiral selection rules, allowing optical valley initialization. In time-resolved photoluminescence (PL) experiments, we find that both the polarization and emission dynamics do not change from 4 to 300 K within our time resolution. We measure a high polarization and show that under pulsed excitation the emission polarization significantly decreases with increasing laser power. We find a fast exciton emission decay time on the order of 4 ps. The absence of a clear PL polarization decay within our time resolution suggests that the initially injected polarization dominates the steady-state PL polarization. The observed decrease of the initial polarization with increasing pump photon energy hints at a possible ultrafast intervalley relaxation beyond the experimental ps time resolution. By compensating the temperature-induced change in band gap energy with the excitation laser energy, an emission polarization of 40% is recovered at 300 K, close to the maximum emission polarization for this sample at 4 K.

Figure 1

The time-resolved photoluminescence of an $A$ exciton. (a) Laser pulse (blue line) and PL emission (black line) intensity at $T=4\mathrm{K}$ detected at the maximum of an $A$-exciton PL $E_{\mathrm{Det}}=1.867\mathrm{eV}$ as a function of time. Inset: Chiral optical selection rules in 1 ML ${\mathrm{MoS}}_2$ (b) $T=4\mathrm{K}$, $E_{\text{Laser}}=1.965\mathrm{eV}$, $E_{\mathrm{Det}}=1.867\mathrm{eV}$. Laser polarization ${\sigma }^{+}$. Left axis: ${\sigma }^{+}$ (${\sigma }^{-}$) polarized PL emission intensity presented in black (red) as a function of time. Right axis: Circular polarization degree during exciton emission (blue hollow squares: excitation power $P_{\text{Laser}}\simeq 550\mu \mathrm{W}/\mu {\mathrm{m}}^2$; green full squares: 0.01 $P_{\text{Laser}}$), error bars take into account the uncertainty in time origin $\mathrm{\Delta }t\sim 0.7\text{ps}$. (c) Same as (b) but for $T=300\mathrm{K}$, $E_{\text{Laser}}=1.937\mathrm{eV}$, $E_{\mathrm{Det}}=1.828\mathrm{eV}$.

Figure 2

The time-resolved photoluminescence (a) $T=200\mathrm{K}$, $E_{\text{Laser}}=1.968\mathrm{eV}$, $E_{\mathrm{Det}}=1.83\mathrm{eV}$. $A$-exciton PL intensity as a function of time plotted in log scale to emphasize the weak slow component that appears when raising $T$. The small peak at about 50 ps is due to a laser reflection in the setup. Inset: cw-PL spectrum at $T=4\mathrm{K}$ showing the energy of the intrinsic $A$-exciton emission and localized exciton emission. (b) $T=4\mathrm{K}$, $E_{\text{Laser}}=1.968\mathrm{eV}$, $E_{\mathrm{Det}}=1.775\mathrm{eV}$. The localized exciton PL intensity as a function of time (black). Blue line: Fit with a single exponential decay (${\tau }_{loc}\simeq 125\text{ps}$). Inset: The polarization-resolved emission of an $A$ exciton and localized exciton which spectrally slightly overlap.

Figure 3

The PL polarization as a function of laser excitation energy (a) $T=4\mathrm{K}$, $P_{\text{Laser}}\simeq 550\mu \mathrm{W}/\mu {\mathrm{m}}^2$ detected on the $A$-exciton PL maximum $E_{\mathrm{Det}}=1.867\mathrm{eV}$. The $A$-exciton emission (blue) is shown. (b) Same as (a) but for $T=300\mathrm{K}$, $P_{\text{Laser}}\simeq 950\mu \mathrm{W}/\mu {\mathrm{m}}^2$, $E_{\mathrm{Det}}=1.828\mathrm{eV}$.