We report the observation of strong third harmonic generation from a macroscopic array of aligned ultralong single-wall carbon nanotubes (SWCNTs) with intense midinfrared radiation. Through power-dependent experiments, we determined the absolute valu
e of the third-order nonlinear optical susceptibility, $chi^{(3)}$, of our SWCNT film to be 5.53 $times$ 10$^{-12}$ esu, three orders of magnitude larger than that of the fused silica reference we used. Taking account of the filling factor of 8.75% for our SWCNT film, we estimate a $chi^{(3)}$ of 6.32 $times$ 10$^{-11}$ esu for a fully dense film. Furthermore, through polarization-dependent experiments, we extracted all the nonzero elements of the $chi^{(3)}$ tensor, determining the magnitude of the weaker tensor elements to be $sim$1/6 of that of the dominant $chi^{(3)}_{zzzz}$ component.
