Characterization of the second- and third-harmonic optical susceptibilities of atomically thin tungsten diselenide

We report the first detailed characterization of the sheet third-harmonic optical susceptibility, χ⁽³⁾_s, of tungsten diselenide (WSe₂). With a home-built multiphoton microscope setup developed to study harmonics generation, we map the second and third-harmonic intensities as a function of position in the sample, pump power and polarization angle, for single- and few-layers flakes of WSe₂. We register a value of |χ⁽³⁾_s| ≈ 0.9 × 10^-28 m³ V^-2 at a fundamental excitation frequency of ℏω = 0.8 eV, which is comparable in magnitude to the third-harmonic susceptibility of other group-VI transition metal dichalcogenides. The simultaneously recorded sheet second-harmonic susceptibility is found to be |χ⁽²⁾_s| ≈ 0.7 × 10^-19 m² V^-1 in very good agreement on the order of magnitude with recent reports for WSe₂, which asserts the robustness of our values for |χ⁽³⁾_s|.