Impact of Selenium Doping on Resonant Second-Harmonic Generation in Monolayer MoS₂

We have investigated strong optical nonlinearity of monolayer MoS_{2(1–<i>x</i>)}Se_2<i>x</i> 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 <i>D</i>_3<i>h</i>, 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 <i>B</i>-exciton-induced SHG, which indeed dominates over the <i>A</i>-exciton resonance for <i>x</i> ≥ 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