Tunable second harmonic generation of monolayer MoS$_{2}$ by Se doping*

As a transition metal dichalcogenides whose bandgap becomes direct with inversion symmetry breaking in the monolayer limit, MoS$_{2}$ has been getting ample attention as next-generation nonlinear optic material for its strong optical nonlinear properties. In this study, we demonstrate the wavelength second harmonic generation tunability of monolayer Mo(S, Se)$_{2}$. Employing the two-zone furnaces system, we selenized as-grown monolayer MoS$_{2\, }$at different temperature. X-ray photoluminescence spectroscopy was used to confirm the chemical composition of selenized film. Photoluminescence spectra shows the red shift in optical bandgap from 1.83 to 1.53 eV as a function of concentration Se replacing S. Second harmonic generation characteristics were measured in reflection geometry using $p$s pulse from Nd:YAG laser. Applying the previous bulk model, we calculated that the maximum value of $\chi^{(2)\, }$varied from \textasciitilde 40 pm/V for pure MoS$_{2}$ to \textasciitilde 100 pm/V for pure MoSe$_{2.\, }$We believe that our findings along with the ability to stack different 2D materials will create stacked 2D heterostructure with high $\chi^{(2)\, }$over a wide range of wavelength from visible to NIR.