  Asymmetric transmission of circularly polarized waves is a well-established
property of lossy, anisotropic, two-dimensionally chiral patterns. Here we show
that asymmetric transmission can be observed for oblique incidence onto any
lossy periodically structured plane. Our results greatly expand the range of
natural and artificial materials in which directionally asymmetric transmission
can be expected making it a cornerstone electromagnetic effect rather than a
curiosity of planar chiral metamaterials. Prime candidates for asymmetric
transmission at oblique incidence are rectangular arrays of plasmonic spheres
or semiconductor quantum dots, lossy double-periodic gratings and planar
metamaterial structures.
