Fig. 1. (Color online) (a) AFM micrography of the monolayer WSe₂ grown on sapphire substrate, the inset is the height profile of the monolayer MBE-WSe₂. (b) Room temperature PL spectra of the as-grown monolayer WSe₂ on sapphire and sapphire substrate itself. (c) The PL spectra of MBE-WSe₂ monolayer as a function of temperature. (d) PL spectrum and Gaussian fit of MBE-WSe₂ monolayer measured at 10 K.

Fig. 2. (Color online) (a) Normalized transient differential reflectivity (ΔR/R) and (b) TRKR signals, with respect to the peak values measured at various probe energies in resonance with A exciton (A) and trion (T), respectively, for MBE-WSe₂ monolayer. Here pump energies are all set at 1.879 eV, and the measured temperature is 10 K. The inset in (b) shows the normalized TRKR response measured under excitation of right- (σ⁺, red trace) and left- (σ⁻, blue trace) circularly polarized pump beam, with the probe energy to be resonant with A exciton.

Fig. 3. (Color online) (a) Temperature-dependent TRKR responses and their fittings (solid lines) probed for trion, pump and probe photon energies are set to be 1.879 and 1.722 eV, respectively. (b) Temperature-dependent valley relaxation time deduced from results in (a). (c) TRKR measured at various pump energies with a fixed probe energy of 1.710 eV at 10 K. The solid lines are fitting results. (d) The extracted long valley lifetime τ_v3 as a function of pump wavelength at 10 K.

Fig. 4. (Color online) Schematics of the bright and dark trion states in WSe₂. (a) A bright singlet trion and (d) a bright triplet trion; when optically excited in theK valley of n-doped WSe₂. (b), (c), (e), and (f) illustrate the possible conversion channels from bright to dark trions under phonon- or defect-assisted scattering.