By using femtosecond laser pulses at a wavelength range from 720 to 780 nm, we have observed absorptive and refractive nonlinearities in a film of multi-walled carbon nanotubes grown mainly along the direction perpendicular to the surface of quartz substrate. The Z-scans show that both absorptive and refractive nonlinearities are of negative and cubic nature in the laser irradiance range from a few to 300 GW/cm 2 . The magnitude of the third-order nonlinear susceptibility, χ, is of an order of 10 -11 esu. The degenerate pump-probe measurement reveals a relaxation time of ~ 2 ps. *
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