  For facile manipulation of polarization states of light for applications in
communications, imaging, and information processing, an efficient mechanism is
desired for rotating light polarization with a minimum interaction length.
Here, we report giant polarization rotations for terahertz (THz)
electromagnetic waves in ultrathin (~45 nm), high-density films of aligned
carbon nanotubes. We observed polarization rotations of up to ~20{\deg} and
~110{\deg} for transmitted and reflected THz pulses, respectively. The amount
of polarization rotation was a sensitive function of the angle between the
incident THz polarization and the nanotube alignment direction, exhibiting a
`magic' angle at which the total rotation through transmission and reflection
becomes exactly 90{\deg}. Our model quantitatively explains these giant
rotations as a result of extremely anisotropic optical constants, demonstrating
that aligned carbon nanotubes promise ultrathin, broadband, and tunable THz
polarization devices.
