Monolayer semiconductors are an emerging platform for strong nonlinear light–matter interactions that are enhanced by the giant oscillator strength of tightly bound excitons. Little attention has been paid to the impact of excitonic resonances on the temporal dynamics of such nonlinearities, since harmonic generation and optical wave mixing are generally considered instantaneous processes. We find that a significant time difference, ranging from −40 to +120 fs, is necessary between two light pulses for optimal sum-frequency generation (SFG) and four-wave mixing (FWM) to occur from monolayer WSe₂ when one of the pulses is in resonance with an excitonic transition. These resonances involve both band-edge A excitons and high-lying excitons that comprise electrons from conduction bands far above the bandgap. Numerical simulations in the density-matrix formalism rationalize the distinct dynamics of SFG and FWM. The interpulse delays for maximal SFG and FWM are governed primarily by the lifetime of the one-photon and two-photon resonant states, respectively. The method therefore offers an unconventional probe of the dynamics of excitonic states accessible with either one-photon or two-photon transitions. Remarkably, the longest delay times occur at the lowest excitation powers, indicating a strong nonlinearity that offers exploration potential for excitonic quantum nonlinear optics.

单层半导体是一种新兴的强非线性光物质相互作用平台，通过紧密束缚激子的巨大振荡器强度增强。由于谐波产生和光波混合通常被认为是瞬时过程，因此很少关注激子共振对这种非线性的时间动力学的影响。_{我们发现，两个光脉冲之间需要一个显着的时间差，范围从 -40 到 +120 fs，以便从单层 WSe 2}发生最佳和频生成 (SFG) 和四波混频 (FWM)当其中一个脉冲与激子跃迁共振时。这些共振涉及带边 A 激子和高位激子，这些激子包括来自远高于带隙的导带的电子。密度矩阵形式的数值模拟使 SFG 和 FWM 的不同动态合理化。最大 SFG 和 FWM 的脉冲间延迟主要分别由单光子和双光子谐振状态的寿命决定。因此，该方法为单光子或双光子跃迁可访问的激子态动力学提供了一种非常规的探索。值得注意的是，最长的延迟时间发生在最低的激发功率下，这表明强烈的非线性为激子量子非线性光学提供了探索潜力。