Impact of Selenium Doping on Resonant Second-Harmonic Generation in\nMonolayer MoS<sub>2</sub>

We have investigated strong optical\nnonlinearity of monolayer MoS_{2(1–<i>x</i>)}Se_2<i>x</i> across the exciton resonance, which\nis directly tunable by Se doping. The quality of monolayer alloys\nprepared by chemical vapor deposition is verified by atomic force\nmicroscopy, Raman spectroscopy, and photoluminescence analysis. The\ncrystal symmetry of all of our alloys is essentially <i>D</i>_3<i>h</i>, as confirmed by polarization-dependent\nsecond-harmonic generation (SHG). The spectral structure of the exciton\nresonance is sampled by wavelength-dependent SHG (λ = 1000–1800\nnm), where the SHG resonance red-shifts in accordance with the corresponding\noptical gap. Surprisingly, the effect of compositional variation turns\nout to be much more dramatic owing to the unexpected increase of <i>B</i>-exciton-induced SHG, which indeed dominates over the <i>A</i>-exciton resonance for <i>x</i> ≥ 0.3.\nThe overall effect is therefore stronger and broader SHG resonance\nwhere the latter arises from different degrees of red-shift for the\ntwo exciton states. We report the corresponding absolute SHG dispersion\nof monolayer alloys, χ⁽²⁾, as a function of Se doping.\nWe believe that our finding is a critical step toward engineering\nhighly efficient nonlinear optical van der Waals materials working\nin a broader performance range.