Strong optical nonlinearity of CVD-grown MoS_2 monolayer as probed by wavelength-dependent second-harmonic generation

摘要

While noncentrosymmetric MoS_2 monolayer is known to exhibit efficient second-harmonic generation (SHG), there is currently no agreement on its absolute nonlinear susceptibility X~((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 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 (λ = 1.1-2.0μm) using picosecond pulses was studied by comparing the relative SHG counts of MoS_2 to quartz and incorporating the structural and optical characteristics of the monolayer. Significant nonlinear optical dispersion gives rise to X~((2)) ～ 430 pm/V at 580 nm, where SHG is neither affected by any excitonic absorption/resonance nor by fundamental absorption. We also show that X~((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.