Charge dynamics in transparent single-walled carbon nanotube films from optical transmission measurements

We report on the transmission over a wide frequency range (from the far infrared through the visible) of pristine and hole-doped, free-standing carbon nanotube films at temperatures between 50 and 3000.3em0exK. Optical constants are estimated by Kramers-Kronig analysis of the transmittance. We see evidence in the far infrared for a gap below 100.3em0exmeV. Hole doping causes a shift of spectral weight from the first interband transition into the far infrared. Temperature dependence in both the doped and undoped samples is restricted to the far-infrared region.