  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 $|\chi_{s}^{(3)}| \approx$
0.91 $\times$ 10$^{-28}$ m$^{3}$ V$^{-2}$ at a fundamental excitation frequency
of $\hbar\omega$ = 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 $|\chi_{s}^{(2)}| \approx$ 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
$|\chi_{s}^{(3)}|$.
