We have investigated strong optical nonlinearity of monolayer MoS2(1–x)Se2x across the exciton resonance, which is directly tunable by Se doping. The quality of monolayer alloys prepared by chemical vapor deposition is verified by atomic force microscopy, Raman spectroscopy, and photoluminescence analysis. The crystal symmetry of all of our alloys is essentially D 3h , as confirmed by polarization-dependent second-harmonic generation (SHG). The spectral structure of the exciton resonance is sampled by wavelength-dependent SHG (λ = 1000–1800 nm), where the SHG resonance red-shifts in accordance with the corresponding optical gap. Surprisingly, the effect of compositional variation turns out to be much more dramatic owing to the unexpected increase of B-exciton-induced SHG, which indeed dominates over the A-exciton resonance for x ≥ 0.3. The overall effect is therefore stronger and broader SHG resonance where the latter arises from different degrees of red-shift for the two exciton states. We report the corresponding absolute SHG dispersion of monolayer alloys, χ(2), as a function of Se doping. We believe that our finding is a critical step toward engineering highly efficient nonlinear optical van der Waals materials working in a broader performance range.

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In this study, we utilized picosecond pulses from an Nd:YAG laser to investigate the nonlinear optical characteristics of monolayer MoSe 2 . Two-step growth involving the selenization of pulsed-laser-deposited MoO 3 film was employed to yield the MoSe 2 monolayer on a SiO 2 /Si substrate. Raman scattering, photoluminescence (PL) spectroscopy, and atomic force microscopy verified the high optical quality of the monolayer. The second-order susceptibility χ (2) was calculated to be ß50 pm V −1 at the second harmonic wavelength λ SH G ß810 nm, which is near the optical gap of the monolayer. Interestingly, our wavelength-dependent second harmonic scan can identify the bound excitonic states including negatively charged excitons much more efficiently, compared with the PL method at room temperature. Additionally, the MoSe 2 monolayer exhibits a strong laser-induced damage threshold ß16 GW cm −2 under picosecond-pulse excitation . Our findings suggest that monolayer MoSe 2 can be considered as a promising candidate for high-power, thin-film-based nonlinear optical devices and applications.

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