Effects of Excitonic Resonance on Second and Third Order Nonlinear Scattering from Few-Layer MoS₂

Abstract

Nonlinear optical scattering from single- and few-layer MoS₂ contains important information about the orientation, inversion symmetry, and degree of interlayer coupling between the layers. We simultaneously map second harmonic generation (SHG) and four wave mixing (FWM) signals in chemical vapor deposition (CVD) grown 2H-phase MoS₂ from single to five layers. We tune the excitation wavelengths to compare cases where the nonlinear signals are on and off resonance with the <i>A</i>-exciton band. The SHG signal shows the expected 4-fold symmetry, however, the FWM signal depends on the incident laser polarization only, and is independent of the crystallographic orientation. We show using the symmetry of the χ⁽³⁾ tensor that this results from out of plane FWM dipoles. We explore the scaling of SHG and FWM signals with layer number on and off excitonic resonance When a nonlinear scattered signal overlaps with the <i>A</i> excitonic band, the scaling of the signals with layer number deviates from the expected values, due to the layer dependent red shift in the exciton absorption peak due to interlayer coupling. Finally we show that circularly polarized excitation significantly enhances nonlinear scattering which overlaps with the <i>A</i> excitonic band and indicates the presence of spin splitting of valence bands at the energy degenerate points (<i>K</i>, <i>K</i>′) of the Brillouin zone