fig10

Figure 10. Exciton-mediated SHG and TPA in monolayer TMDs. (A) SHG image of a MoS₂ thin film under 800 nm excitation; (B) SHG intensity as a function of excitation power measured at the marked position in (A), yielding a slope of approximately 2 and confirming the second-order nonlinear origin of the signal; (C) extracted second-order susceptibility χ² of monolayer and trilayer MoS₂ as a function of excitation photon energy, overlaid with linear absorption spectra, demonstrating resonant enhancement of SHG when the two-photon energy matches excitonic transitions. (A-C) are adapted with permission^[26]. Copyright 2013, American Physical Society; (D) photographs of WS₂ and MoS₂ films with varying thicknesses (monolayer to multilayer), illustrating the systematic layer-dependent platform; (E) open-aperture Z-scan traces of monolayer WS₂ films under 1030 nm femtosecond excitation, revealing pronounced TPA behavior below the linear bandgap; (F) extracted TPA coefficients as a function of layer number, highlighting the dramatic enhancement of nonlinear absorption in the monolayer limit. (D-F) are adapted with permission^[27]. Copyright 2015, American Chemical Society; (G) schematic illustration of the exciton-assisted TPA mechanism at the K valley in monolayer MoS₂, where the two-photon energy is nearly resonant with the two-photon dark excitonic state, enabling strong nonlinear absorption; (H) layer-resolved micro-I-scan measurements of MoS₂. (G and H) are adapted with permission^[28]. Copyright 2019, Optica Publishing Group.