  In this Letter, both the manipulation of valley-polarized currents and the
optical-like behaviors of Dirac fermions are theoretically explored in
polycrystalline graphene. When strain is applied, the misorientation between
two graphene domains separated by a grain boundary can result in a mismatch of
their electronic structures. Such a discrepancy manifests itself in a strong
breaking of the inversion symmetry, leading to perfect valley polarization in a
wide range of transmission directions. In addition, these graphene domains act
as different media for electron waves, offering the possibility to modulate and
obtain negative refraction indexes.
