Direct Identification of Valley Coherence and Its Manipulation in Monolayer Two-Dimensional Semiconductor

Monolayer two-dimensional semiconductors are endowed with valley degrees of freedom due to broken inversion symmetry and strong spin–orbit coupling. The excited states in the K and K′ valleys possess information on valley pseudospin, which is useful for quantum-state manipulation in the valley pseudospin Bloch sphere. However, since intervalley decoherence occurs during light emission, directly probing valley coherence in the time domain has remained challenging. Herein, we have demonstrated the direct measurement of intervalley coherence between K and K′ valley excitons using polarized interferometry in a monolayer WSe 2 device. The experimental results showed that the intervalley coherence time is in the range of 200–300 fs, depending on the excitation power and temperature. Moreover, an asymmetric modulation of the intervalley coherence time as well as the degree of linear polarization was observed under electron and hole carrier doping by applying an external bias voltage, demonstrating an extended valley coherence time reaching 400 fs under high electron doping. The underlying mechanism of the polarity and carrier doping dependence of valley decoherence between K and K′ valley excitons was elucidated. The results presented herein offer a viable approach for directly probing valley coherence in the time domain and elucidating the underlying mechanism of the decoherence process toward quantum-state manipulation of valley pseudospin.