  We show that packed, horizontally aligned films of single-walled carbon
nanotubes are hyperbolic metamaterials with ultra-subwavelength unit cells and
dynamic tunability. Using Mueller-matrix ellipsometry, we characterize the
films' doping-level dependent optical properties and find a broadband
hyperbolic region tunable in the mid-infrared. To characterize the dispersion
of in-plane hyperbolic plasmon modes, we etch the nanotube films into
nanoribbons with differing widths and orientations relative to the nanotube
axis, and we observe that the hyperbolic modes support strong light
localization. Agreement between the experiments and theoretical models using
the ellipsometry data indicates that the packed carbon nanotubes support bulk
anisotropic responses at the nanoscale. Self-assembled films of carbon
nanotubes are well suited for applications in thermal emission and
photodetection, and they serve as model systems for studying light-matter
interactions in the deep subwavelength regime.
