Midinfrared third-harmonic generation from macroscopically aligned ultralong single-wall carbon nanotubes

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 value of the third-order nonlinear optical susceptibility ${\chi }^{\left(3\right)}$ 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 ${\chi }^{\left(3\right)}$ 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 ${\chi }^{\left(3\right)}$ tensor, determining the magnitude of the weaker tensor elements to be $\sim$1/6 of that of the dominant ${\chi }_{zzzz}^{\left(3\right)}$ component.

Figure 1

(a) Third-harmonic spectrum generated at 700 nm (left, black), and the fundamental spectrum (2100 nm, right, red). (b) Shift in third-harmonic wavelength (left, black) from 700 to 500 nm due to a shift in the fundamental from 2.1 to 1.5 $\mu$m (right, red). The SWCNTs are aligned parallel to the incident fundamental, and the induced third harmonic is polarized parallel to the fundamental.

Figure 2

Power dependence of the 700 nm third harmonic vs the 2100 nm fundamental for the (a) 1.6-$\mu$m-thick highly aligned SWCNT sample and the (b) 26.5-$\mu$m-thick fused silica reference. The third harmonic shows cubic power dependence with the incident fundamental, as shown by the slope of the log-log plot (red). The same third-harmonic power is produced by the SWCNT sample at an order of magnitude less fundamental power (see blue dashed lines).

Figure 3

Experimental and theoretical angular dependence for a THG signal polarized (a) parallel and (b) perpendicular to the fundamental, considering the ${\chi }^{\left(3\right)}$ tensor contribution relationship is $\alpha {\chi }_{zzzz}^{\left(3\right)}={\chi }_{zzxx}^{\left(3\right)}$. The theoretical fits (red) show $\varphi$ dependence for $\alpha \approx 1/6$. $\varphi =0$ corresponds to light polarization parallel to the nanotube axis.