  By measuring the polarization changes in THz, infrared, and visible radiation
over an extended energy range (3-2330 meV), we observe symmetry-breaking in
cuprate high temperature superconductors over wide energy, doping, and
temperature ranges. We measure the polarization rotation and ellipticity of
transmitted radiation though thin films as the sample is rotated. We observe a
two-fold rotational symmetry in the Faraday angle, which is associated with
linear dichroism (LD) and occurs when electromagnetic radiation polarized along
one direction is absorbed more strongly than radiation polarized in the
perpendicular direction. Such polarization anisotropies can be generally
associated with symmetry breakings. We measure the amplitude of the LD signal
and study its temperature, energy, and doping dependence. The LD signal shows a
resonant behavior with a peak in the few hundred meV range, which is coincident
with the mid-infrared optical feature that has been associated with the
formation of the pseudogap state. The strongest LD signal is found in
under-doped films, although it is also observed in optimally- and over-doped
samples. The LD signal is consistent with nematic charge ordering as well as
novel magnetoelectric effects.
