Nonlinear optical susceptibility of two-dimensional WS₂ measured by hyper Rayleigh scattering

Hyper Rayleigh scattering (HRS) was used to measure the second-order nonlinear susceptibility, χ⁽²⁾, for liquid exfoliated WS₂ monolayers. To the best of our knowledge, it is the first reported application of the HRS technique to assess the bulk-like χ⁽²⁾ of a two-dimensional (2D) material. The concentration-dependent HRS signal indicated a 4.90±0.30×10^-25  esu first hyperpolarizability for 42 nm WS₂ monolayers under 1064 nm laser irradiation using para-nitroaniline as an external reference. The corresponding value of χxxx(2) was calculated to be 460±28  pm V^-1. This was within 46% of independent density functional theory predictions. Agreement with theory was improved over related microscopy-based approaches. These results support the use of HRS to evaluate 2D materials for nonlinear frequency mixing applications.