Ultrafast Phase-Control of the Nonlinear Optical Response of 2D Semiconductors

Nonlinear optical phenomena, such as sum-frequency generation (SFG) and four-wave mixing (FWM), play a central role in various applications ranging from spectroscopy, laser pulse characterization, and design to all-optical switching. Monolayer semiconducting transition metal dichalcogenides (TMDs) feature particularly strong nonlinear light-matter interactions which result from the large oscillator strength of tightly bound excitons. Here, we tune the spectral phase of a broadband 12 fs laser pulse resonant with the first excitonic transition in monolayer WSe2 and MoSe2 to coherently control the nonlinear signal intensities via the phase of the interacting optical fields. We find that stretching the laser pulse compared to the transform-limited case allows for an enhancement of FWM intensities by a factor of ∼2. For low excitation densities the corresponding optimal spectral phase profile can be predicted by a classical model based on the measured linear absorption spectrum of the TMD monolayer without free parameters. For increasing excitation densities, however, the influence of excitonic resonances vanishes. Excitation-induced dephasing of the resonance together with the onset of the Mott transition at high excitation densities inhibit coherent population control at room temperature. In contrast to FWM, SFG cannot be enhanced by phase-shaping as compared to the transform-limited pulse. Instead, SFG appears to be unaffected by the first excitonic resonance, which we attribute to the dominating role of higher-energy bands in this process.