We show that the light-matter interaction in monolayer WSe_{2} is strongly enhanced when the incoming electromagnetic wave is in resonance with the energy of the exciton states of strongly Coulomb bound electron-hole pairs below the electronic band gap. We perform second harmonic generation (SHG) spectroscopy as a function of laser energy and polarization at T=4  K. At the exciton resonance energies we record an enhancement by up to 3 orders of magnitude of the SHG efficiency, due to the unusual combination of electric dipole and magnetic dipole transitions. The energy and parity of the exciton states showing the strong resonance effects are identified in 1- and 2-photon photoluminescence excitation experiments, corroborated by first principles calculations. Targeting the identified exciton states in resonant 2-photon excitation allows us to maximize k-valley coherence and polarization.

Optical interband transitions in monolayer transition metal dichalcogenides such as WSe 2 and 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 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 4ps, 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 ≈ 20%, for which a typical valley polarization decay time larger than 1ns can be inferred. This is a clear signature of stable, optically initialized valley polarization.

In monolayer MoS2, 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.

We study the neutral exciton energy spectrum fine structure and its spin dephasing in transition metal dichalcogenides such as MoS2. The interaction of the mechanical exciton with its macroscopic longitudinal electric field is taken into account. The splitting between the longitudinal and transverse excitons is calculated by means of the both electrodynamical approach and k ·p perturbation theory. This long-range exciton exchange interaction can induce valley polarization decay. The estimated exciton spin dephasing time is in the picosecond range, in agreement with available experimental data.PACS numbers: 71.70.Gm,72.25.Rb,78.66.Li Introduction. Monolayers (MLs) of transition metal dichalcogenides, in particular, MoS 2 form a class of novel two-dimensional materials with interesting electronic and optical properties. The direct band gap in these systems is realized at the edges of the Brillouin zone at points K + and K − .1 Strikingly, each of the valleys can be excited by the radiation of given helicity only.2-4 Recent experiments have indeed revealed substantial optical orientation in ML MoS 2 related to selective excitation of the valleys by circularly polarized light.5-8 Strong spin-orbit coupling in this material lifts the spin degeneracy of electron and hole states even at K + and K − points of the Brillouin zone resulting in relatively slow spin relaxation of individual charge carriers, which requires their intervalley transfer.9-13 However recent time-resolved measurements revealed surprisingly short, in the picosecond range, transfer times between valleys.14,15 This could be due to excitonic effects which are strong in transition metal dichalcogenides 16,17 Although individual carrier spin flips are energetically forbidden, spin relaxation of electron-hole pairs can be fast enough owing to the exchange interaction between an electron and a hole forming an exciton, 18,19 in close analogy to exciton dephasing in quantum wells.

We report polarization resolved photoluminescence from monolayer MoS2, a two-dimensional, non-centrosymmetric 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 4K is observed with 40% polarization remaining at 300K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9T indicating robust, selective valley excitation.

We perform photoluminescence experiments at 4 K on two different transition metal diselenide monolayers, namely MoSe2 and WSe2 in magnetic fields Bz up to 9 T applied perpendicular to the sample plane. In MoSe2 monolayers the valley polarization of the neutral and the charged exciton (trion) can be tuned by the magnetic field, independent of the excitation laser polarization. In the investigated WSe2 monolayer sample the evolution of the trion valley polarization depends both on the applied magnetic field and the excitation laser helicity, while the neutral exciton valley polarization depends only on the latter. Remarkably we observe a reversal of the sign of the trion polarization between WSe2 and MoSe2. For both systems we observe a clear Zeeman splitting for the neutral exciton and the trion of about ±2 meV at Bz ∓ 9 T. The extracted Landé-factors for both exciton complexes in both materials are g ≈ −4.

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We investigate exciton dynamics in 2H-WSe2 bilayers in time-resolved photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for both the direct exciton with $\tau_{D}$ ~ 3 ps and the indirect optical transition with $\tau_{i}$ ~ 25 ps. For temperatures between 4 to 150 K $\tau_{i}$ remains constant. Following polarized laser excitation, we observe for the direct exciton transition at the K point of the Brillouin zone efficient optical orientation and alignment during the short emission time $\tau_{D}$. The evolution of the direct exciton polarization and intensity as a function of excitation laser energy is monitored in PL excitation (PLE) experiments.Comment: 4 pages, 3 figure