  In recent years, there have been conflicting reports regarding the ultrafast
photoconductive response of films of single walled carbon nanotubes (CNTs),
which apparently exhibit photoconductivities that can differ even in sign.
Here, we observe explicitly that the THz photoconductivity of CNT films is a
highly variable quantity which correlates with the length of the CNTs, while
the chirality distribution has little influence. Moreover, by comparing the
photo-induced change in THz conductivity with heat-induced changes, we show
that both occur primarily due to heat-generated modification of the Drude
electron relaxation rate, resulting in a broadening of the plasmonic resonance
present in finite-length metallic and doped semiconducting CNTs. This clarifies
the nature of the photo-response of CNT films and demonstrates the need to
carefully consider the geometry of the CNTs, specifically the length, when
considering them for application in optoelectronic devices.
