  Anisotropy is ubiquitous in solids and enhanced in low-dimensional materials.
In response to an electromagnetic wave, anisotropic absorptive and refractive
properties result in dichroic and birefringent optical phenomena both in the
linear and nonlinear optics regimes. Such material properties have led to a
diverse array of useful polarization components in the visible and
near-infrared, but mature technology is non-existent in the terahertz (THz).
Here, we review several novel types of anisotropic material responses observed
in the THz frequency range, including both linear and circular anisotropy,
which have long-term implications for the development of THz polarization
optics. We start with the extreme linear anisotropy of macroscopically aligned
carbon nanotubes, arising from their intrinsically anisotropic dynamic
conductivity. Magnetically induced anisotropy will then be reviewed, including
the giant Faraday effects observed in semiconductors, semimetals, and
two-dimensional electron systems.
