  Electronic degrees of freedom and their coupling to lattice vibrations in
semiconductors can be strongly modified by doping. Accordingly, the addition of
surplus charge carriers to chirality-mixed carbon nanotube samples has
previously been found to give rise to a Drude-type plasmon feature as well as
Fanotype antiresonances in the far- to mid-infrared spectral range (FIR/MIR).
Here we investigate the FIR/MIR response of redox-chemically doped
semiconducting (6,5) carbon nanotubes (s-SWNTs). We find that, contrary to
expectations, the Drude-type plasmon shifts to lower wavenumbers with
increasing doping level. By means of Monte-Carlo simulations of the optical
response, we attribute this behavior to the confinement of excess charge
carriers at low doping levels and their progressive delocalization when
approaching degenerate doping. The coupling of vibrational modes to intraband
excitations in the doped s-SWNTs can be probed via a double resonance process
similar to that responsible for the Raman D-band. The resulting Fano
antiresonances shed new light onto the character and coupling of electronic and
vibrational degrees of freedom in these one-dimensional semiconductors.
