  Ultrafast degenerate pump-probe experiments performed on a free standing film
of double walled carbon nanotubes near the first metallic transition energy of
outer tube show ultrafast ($97 fs$) photobleaching followed by a photo-induced
absorption with a slow relaxation of $1.8 ps$. Femtosecond closed and open
aperture z-scan experiments carried out at the same excitation energy show
saturation absorption and negative cubic nonlinearity. From these measurements,
real and imaginary part of the third order nonlinear susceptibility are
estimated to be Re($\chi^{(3)}) \sim -2.2 \times 10 ^{-9} esu$ and
Im($\chi^{(3)}) \sim 1.1 \times 10 ^{-11} esu$.
