Robust optical emission polarization in MoS${}_2$ monolayers through selective valley excitation

We report polarization resolved photoluminescence from monolayer ${\mathrm{MoS}}_2$, a two-dimensional, noncentrosymmetric crystal with direct energy gaps at two different valleys in momentum space. The inherent chiral optical selectivity allows exciting one of these valleys, and close to 90$\%$ polarized emission at 4 K is observed with 40$\%$ polarization remaining at 300 K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9 T indicating robust, selective valley excitation.

Figure 1

(a) Measured PL circular polarization degree of a 1 ML MoS${}_2$ as a function of temperature. Low energy peak ($\mathit{L}$) not detectable for $T>175$ K. Dotted lines are a guide to the eye. (b) PL spectra copolarized (red/light gray) and cross polarized (blue/dark gray) with respect to the ${\sigma }^{+}$ polarized excitation laser (HeNe at 1.96 eV). At 300 K only the high energy peak $\mathit{A}$ is detectable. Black curves indicate fit to the data with Gaussians. (c) Same as (b) but at $T=125$ K. Both $\mathit{L}$ and $\mathit{A}$ peaks are detectable and the polarization degree is extracted by fitting them separately (dashed lines). (d) Same as (c) but at $T=4$ K; the (green) right axis shows the circular polarization. PL intensity drops as a function of temperature; counts in (b)–(d) are normalized for better visibility.

Figure 2

(a) Selection rules for optical dipole transitions in a simple single particle picture in $K_{+}$ and $K_{-}$ valleys at zero magnetic field, adapted from Refs. 8, 9, 10; states are represented as $|L,M\rangle$ for valence states, and $|\uparrow \rangle$ ($|\downarrow \rangle$) stand for electron spin up (down). For simplicity, the $k$-linear term in the CB is neglected, and the $\mathit{A}$($\mathit{B}$) VBs for $K_{+}$ and $K_{-}$ are degenerate in energy. Local trigonal symmetry of electron states at $K_{\pm }$ points is $C_{3h}$ in Koster notation (Ref. 24), with states labeled using corresponding group representations (b). PL spectra at $T=4$ K copolarized (red/light gray) and cross polarized (blue/dark gray) with respect to the ${\sigma }^{+}$ polarized highly nonresonant excitation laser at 2.33 eV. Transitions $\mathit{L}$ and $\mathit{A}$ are visible in addition to the $\mathit{B}$ exciton. The small periodic signal variations are due to optical interference in the setup. The (green) right axis shows the PL circular polarization.

Figure 3

(a) Comparison of PL spectra of a single MoS${}_2$ ML recorded for a transverse magnetic field $B_T=0$, 4.5, and $8.5$ T under otherwise identical experimental conditions using HeNe laser at 1.96 eV. Inset: Experimental geometry. (b) Measured PL circular polarization degree of the PL as a function of the applied transverse magnetic field $B_T$ for $\mathit{A}$ exciton at $T=4$ K (squares) and at $T=100$ K (circles).