Direct measurement of exciton valley coherence in monolayer WSe2

In crystals, energy band extrema in momentum space can be identified by a valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier, analogous to electronic charge or spin1–4. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides5–7. However, the valley coherence time—a crucial quantity for valley pseudospin manipulation—is di cult to directly probe. In this work, we use two-dimensional coherent spectroscopy to resonantly generate and detect valley coherence of excitons (Coulomb-bound electron–hole pairs) in monolayer WSe2 (refs 8,9). The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron–hole exchange interaction provides an important decoherence mechanism in addition to exciton population recombination. This work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications. Group-VI transition metal dichalcogenides (TMDs) with 2H structure (for example, MX2, M = Mo, W; X = S, Se) are a particularly intriguing class of semiconductors when thinned down to monolayers6,7. The valence and conduction band extrema are located at both K and K points at the corners of the hexagonal Brillouin zone, as illustrated in Fig. 1a. The degenerate K and K points are related to each other by time reversal symmetry and give rise to the valley degree of freedom (DoF) of the band-edge electrons and holes. Strong Coulomb interactions lead to the formation of excitons with remarkably large binding energies due to the heavy effective mass and reduced dielectric screening inmonolayer TMDs (refs 10–12). An exciton as a bound electron–hole pair inherits the valley DoF. Because of valley-dependent optical selection rules, they can be excited only by σ (σ) circularly polarized light at the K(K) valley. Owing to its close analogy to spin4, the valley DoF can be considered as a pseudospin represented by a vector S on the Bloch sphere (Fig. 1b). The out-of-plane component Sz and in-plane component Sx ,y describe the valley polarization and the coherent superposition of exciton valley states, respectively. After optical initialization, valley depolarization and decoherence are manifested by a reduction in the magnitudes of Sz and Sx ,y , respectively. The ability to coherently manipulate spins and pseudospins is at the heart of spintronics and valleytronics; however, previous investigations have focused mainly on the creation and relaxation of valley polarization using non-resonant photoluminescence (PL) or pump/probe spectroscopy techniques13–19. Optical excitation close to the lowest energy exciton resonance leads to nearly 100% valley polarization in monolayer TMDs such as MoS2 (refs 13,15,20). Time-resolved PL spectroscopy has revealed a fewpicosecond valley polarization decay time, possibly limited by the temporal resolution of the technique17. Experiments based on pump/probe spectroscopy reported similar timescales; however, these measurements may be difficult to interpret owing to the fact that only the incoherent exciton population dynamics are probed, which can be sensitive to scattering between optically bright and dark excitons21. Even more intriguing are experiments that seem to show that exciton valley coherence—the coherent superposition of excitons in K and K valleys manifested as linearly polarized luminescence—is preserved in PL following non-resonant linearly polarized optical excitation8,9,22. However, steady-state PL does not reveal the timescale of the valley coherence dynamics. Directlymeasuring the timescale over which quantum coherence in the valley pseudospin DoF is preserved remains an outstanding challenge in the field of valleytronics. Exciton valley coherence is a type of non-radiative quantum coherence, that is, coherence between states that are not dipole coupled. Probing exciton valley coherence therefore requires measurements that go beyond traditional linear spectroscopy techniques. In this paper, we examine exciton valley coherence dynamics in monolayer WSe2 using polarization-resolved optical two-dimensional coherent spectroscopy (2DCS; ref. 23). Using a sequence of laser pulses resonant with the exciton transition, we initialize and probe exciton valley coherence and find that it decays after ∼100 fs. The coherence time is faster than the exciton population recombination lifetime—also occurring on a sub-picosecond timescale—indicating the presence of additional valley decoherence channels. Following earlier work, we identify the electron–hole exchange interaction as an important decoherence mechanism in addition to exciton recombination24–26. Calculations taking the exchange interaction and the momentum-space distribution of excitons into account reproduce the measured valley coherence dynamics. We examine monolayer WSe2 flakes ∼20 μm in lateral size grown on a sapphire substrate using chemical vapour deposition (see Supplementary Note 1 and Supplementary Fig. 1; ref. 27). Steady-state PL measurements are first performed to identify the exciton resonance and confirm that a high degree of valley polarization can be achieved. Circularly polarized PL spectra are shown in Fig. 2a for circularly polarized excitation tuned to 660 nm (1,879meV). The spectra feature a high-energy peak