Raman-like resonant secondary emission causes valley coherence in CVD-grown monolayer $\mathrm{Mo}{\mathrm{S}}_2$

Monolayer transition metal dichalcogenides are promising materials for “valleytronics.” They have band gaps at energy-degenerate $K$ and $K^{\prime }$ valleys with opposite spins. Due to the lack of inversion symmetry, electron-hole pairs can be selectively created at $K$ or $K^{\prime }$ valleys by circularly polarized photons. In addition, linearly polarized light excitation creates the coherent superposition of exciton valley states, referred to as the generation of valley coherence. In this study we performed polarization resolved photoluminescence and resonant Raman spectroscopy of CVD-grown monolayer $\mathrm{Mo}{\mathrm{S}}_2$. We found that the lowest exciton photoluminescence becomes polarized, indicating the effective generation of valley polarization and valley coherence due to the resonant effect, accompanied by a drastic change of the polarization selection rule of Raman scattering. These results were theoretically explained from the viewpoint of the selection rules of resonant Raman scattering. We conclude that the Raman-like resonant second-order optical process should be the main mechanism of valley coherence.

Figure 1

(a) Photoluminescence excitation (PLE) spectrum of monolayer $\mathrm{Mo}{\mathrm{S}}_2$ (1L-$\mathrm{Mo}{\mathrm{S}}_2)$ at 34 K. Detection energy is set to 1.62–1.67 eV (black arrow). Red and green arrows represent the excitation energies used in this study. (b) Polarization-unresolved photoluminescence spectrum with 2.33 eV excitation (green arrow). Data around 1.7 eV are not shown because there are strong components derived from sapphire substrate in this energy region. Four Gaussian distributions by fitting and the sum of them (gray curve) are also shown. (c) Circular and (d) linear polarization-resolved photoluminescence spectra of monolayer $\mathrm{Mo}{\mathrm{S}}_2$ with 1.96 eV excitation (red arrow). Red curve shows co-circular (co-linear) component and blue curve shows anticircular (cross-linear) component. Polarization configurations are represented in accordance with Porto notation.

Figure 2

(a) Emission spectra of the A exciton and Raman scattering with various temperatures with 1.96 eV excitation (indicated by black arrow). Higher energy (anti-Stokes) components are multiplied by a hundred for clarity. (b) Arrhenius plot of intensity ratio between anti-Stokes and Stokes components for 150 and $200\mathrm{c}{\mathrm{m}}^{-1}$ energy shift. Solid lines are eye guides. (c) Stokes components multiplied by $A_0\exp (-E_{\mathrm{shift}}/k_{B}T)$ (solid curves) versus inverse of energy shift and anti-Stokes components (dotted curves).

Figure 3

Polarization-resolved (a) and (b) resonant and (g) and (h) nonresonant Raman scattering spectra of monolayer $\mathrm{Mo}{\mathrm{S}}_2$. Incident laser is (a) circularly polarized, 1.96 eV, (b) linearly polarized, 1.96 eV, (g) circularly polarized, 2.33 eV, and (h) linearly polarized, 2.33 eV. Red curve shows co-circular (co-linear) component and blue curve shows anticircular (cross-linear) component. Normalized intensity of (c), (d), (i), and (j) Raman scattering and (e), (f), (k), and (l) A exciton photoluminescence (PL) as a function of detection angle.

Figure 4

Schematics of A exciton photoluminescence and Raman scattering with (a) and (b) nonresonant excitation and (c) and (d) resonant excitation. Solid and dashed lines represent real exciton states and virtual intermediate states, respectively. Dotted arrows indicate the energy relaxation involving scattering processes causing decoherence.