Mechanical exfoliation (ME) and chemical vapor deposition (CVD) MoS₂ monolayers have been extensively studied, but the large differences of nonlinear optical performance between them have never been clarified. Here, we prepared MoS₂ monolayers using ME and CVD methods and investigated the two-photon absorption (TPA) response and its saturation. We found that the TPA coefficient of the ME monolayer was about (1.88 ± 0.21) × 10³ cm/GW, nearly two times that of the CVD one at (1.04 ± 0.15) × 10³ cm/GW. Furthermore, we simulated and compared the TPA-induced optical pulse modulation in multilayer cascaded structures, which is instructive and meaningful for the design of optical devices such as a beam shaper and optical limiter.

Questions hovering over the modulation of bandgap size and excitonic effect on nonlinear absorption in two-dimensional transition metal dichalcogenides (TMDCs) have restricted their application in micro/nano optical modulator, optical switching, and beam shaping devices. Here, degenerate two-photon absorption (TPA) in the near-infrared region was studied experimentally in mechanically exfoliated MoS2 from single layer to multilayer. The layer-dependent TPA coefficients were significantly modulated by the detuning of the excitonic dark state (2p). The shift of the quasiparticle bandgap and the decreasing of exciton binding energy with layers were deduced, combined with the non-hydrogen model of excitons in TMDCs and the scaling rule of semiconductors. Our work clearly demonstrates the layer modulation of nonlinear absorption in TMDCs and provides support for layer-dependent nonlinear optical devices, such as optical limiters and optical switches.