Spectral Dispersion of Effective Second-Order Susceptibility in Monolayer WS ₂ , WSe ₂ , MoS ₂ , and MoSe ₂ on Bare Glass: A Systematic Comparison for Nonlinear Photonics

Monolayer transition metal dichalcogenides are promising nonlinear optical materials for metasurfaces, integrated photonics, and multiphoton imaging, but their second-order nonlinear response is highly sensitive to excitation wavelength, substrate, and passivation environment. As a result, reported values of second-order nonlinear susceptibility are difficult to compare directly. Here, we provide a self-consistent bare glass benchmark of the effective second-order susceptibility dispersion, |χ eff 2 (λ)|, for mechanically exfoliated WS 2, WSe 2, MoS 2, and MoSe 2 monolayers. Using calibrated femtosecond second-harmonic microscopy over an 800–900 nm pump range, with photoluminescence verification of monolayer quality, we compare the four materials under identical preparation and fixed laboratory polarization measurement conditions. The tungsten-based monolayers show an increasing nonlinear response with pump energy within most of the measured range, whereas the molybdenum-based monolayers show the opposite trend and a weaker overall response under these bare glass substrate conditions. These material-dependent dispersions are consistent with C-type excitonic and band-nesting resonances and are qualitatively supported by density functional theory calculations. Comparison with previous broadband reports further shows that substrate and passivation conditions can shift resonances and alter the effective nonlinear response. This work establishes an internally consistent benchmark for selecting transition metal dichalcogenides and excitation wavelengths in glass- and silica-compatible nonlinear photonic, meta-optical, and multiphoton imaging platforms.