  An ordered phase showing remarkable electronic anisotropy in proximity to the
superconducting phase is now a hot issue in the field of
high-transition-temperature superconductivity. As in the case of copper oxides,
superconductivity in iron arsenides competes or coexists with such an ordered
phase. Undoped and underdoped iron arsenides have a magnetostructural ordered
phase exhibiting stripe-like antiferromagnetic spin order accompanied by an
orthorhombic lattice distortion; both the spin order and lattice distortion
break the tetragonal symmetry of crystals of these compounds. In this ordered
state, anisotropy of in-plane electrical resistivity is anomalous and difficult
to attribute simply to the spin order and/or the lattice distortion. Here, we
present the anisotropic optical spectra measured on detwinned BaFe2As2 crystals
with light polarization parallel to the Fe planes. Pronounced anisotropy is
observed in the spectra, persisting up to an unexpectedly high photon energy of
about 2 eV. Such anisotropy arises from an anisotropic energy gap opening below
and slightly above the onset of the order. Detailed analysis of the optical
spectra reveals an unprecedented electronic state in the ordered phase.
