Strong optical nonlinearity of CVD-grownMoS2monolayer as probed by wavelength-dependent second-harmonic generation

Abstract

While noncentrosymmetric ${\mathrm{MoS}}_{2}$ monolayer is known to exhibit efficient second-harmonic generation (SHG), there is currently no agreement on its absolute nonlinear susceptibility ${\ensuremath{\chi}}^{(2)}$, varying over three orders of magnitude according to recent experiments. In order to resolve this conflicting issue, we have studied the nonlinear optical properties of ${\mathrm{MoS}}_{2}$ monolayer grown by chemical vapor deposition. The polycrystalline nature of the monolayer was directly probed by the SHG polarization dependence across the grain boundaries using femtosecond pulses. Broadband wavelength-dependent SHG response $(\ensuremath{\lambda}=1.1--2.0\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m})$ using picosecond pulses was studied by comparing the relative SHG counts of ${\mathrm{MoS}}_{2}$ to quartz and incorporating the structural and optical characteristics of the monolayer. Significant nonlinear optical dispersion gives rise to ${\ensuremath{\chi}}^{(2)}\ensuremath{\sim}430$ pm/V at 580 nm, where SHG is neither affected by any excitonic absorption/resonance nor by fundamental absorption. We also show that ${\ensuremath{\chi}}^{(2)}$ must be derived from a thin bulk (sheet) optical nonlinearity and that the previous measurements are in fact all consistent, together with our measurements and first-principle calculations.