Anisotropic decay dynamics of photoexcited aligned carbon nanotube bundles

We have performed polarization-dependent ultrafast pump-probe spectroscopy of a film of aligned single-walled carbon nanotube bundles. By taking into account imperfect nanotube alignment, as well as anisotropic absorption cross sections, we quantitatively determined distinctly different photobleaching dynamics for polarizations parallel and perpendicular to the tube axis. For perpendicular polarization, we observe a slow (1.0--1.50.3em0exps) relaxation process, previously unobserved in randomly oriented nanotube bundles. We attribute this slower dynamics to the excitation and relaxation of surface plasmons in the radial direction of the nanotube bundles.