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, ${\chi }_s^{(3)}$, of tungsten diselenide (WSe${}_2$). With a home-built confocal 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 WSe${}_2$. We register a value of $\mathrm{\mid }{\chi }_s^{(3)}\mathrm{\mid }\approx$ 0.91 $\times$ 10${}^{-28}$ m${}^3$ V${}^{-2}$ at a fundamental excitation frequency of $\mathrm{\hslash }\omega$ = 0.8 eV, which is comparable in magnitude to the third-harmonic susceptibility of other group-VI transition metal dichalcogenides. The simultaneously recorded second-harmonic susceptibility is found to be $\mathrm{\mid }{\chi }_s^{(2)}\mathrm{\mid }\approx$ 0.70 $\times$ 10${}^{-19}$ m${}^2$ V${}^{-1}$ in very good agreement on the order of magnitude with recent reports for WSe${}_2$, which asserts the robustness of our values for $\mathrm{\mid }{\chi }_s^{(3)}\mathrm{\mid }$.