Nonlinear all-optical coherent manipulation and read-out of valleys in atomically thin semiconductors

The need for faster and more efficient methods to write and read information remains high, while electronics has reached limits regarding speed and miniaturization. Light switching presents an ideal candidate for both, high speed and low consumption logic devices: fast and broadband all-optical modulation via nonlinear optics has been already shown in atomically thin materials, such as graphene and transition metal dichalcogenides (TMDs) [1,2]. A further advantage of TMDs for information processing is the availability of the valley degree of freedom. Due to their honeycomb lattice structure and time-reversal symmetry, TMDs have energy degenerate, but non-equivalent valleys at the K/K^′ points of the Brillouin zone. An imbalance between the K/K^′ valleys, called valley polarization (VP), can be used to read, write and store information at the nanoscale. Excitons can be selectively excited (write state), either via one-photon or two-photon absorption (TPA) [3], due to specific valley selection rules within the dipole approximation (see Fig. 1). The detection of VP (read) is mainly based on photoluminescence (PL), which has the disadvantages of being invasive and recording an average VP over a long time scale. Recently, second harmonic generation (SHG) has been proposed as a novel ultrafast and non-destructive method, thus overcoming both drawbacks [4]. The polarization of the emitted SHG rotates depending on the magnitude of the VP.