Photocarrier-Induced Active Control of Second-Order Optical Nonlinearity in Monolayer MoS₂

Atomically thin transition metal dichalcogenides (TMDs) in their excited states can serve as exceptionally small building blocks for active optical platforms. In this scheme, optical excitation provides a practical approach to control light-TMD interactions via the photocarrier generation, in an ultrafast manner. Here, it is demonstrated that via a controlled generation of photocarriers the second-harmonic generation (SHG) from a monolayer MoS₂ crystal can be substantially modulated up to ≈55% within a timeframe of ≈250 fs, a set of performance characteristics that showcases the promise of low-dimensional materials for all-optical nonlinear data processing. The combined experimental and theoretical study suggests that the large SHG modulation stems from the correlation between the second-order dielectric susceptibility χ⁽²⁾ and the density of photoexcited carriers in MoS₂ . Indeed, the depopulation of the conduction band electrons, at the vicinity of the high-symmetry K/K' points of MoS₂ , suppresses the contribution of interband electronic transitions in the effective χ⁽²⁾ of the monolayer crystal, enabling the all-optical modulation of the SHG signal. The strong dependence of the second-order optical response on the density of photocarriers reveals the promise of time-resolved nonlinear characterization as an alternative route to monitoring carrier dynamics in excited states of TMDs.