  As a basic optical element, circular polarizer plays significant roles in
signal transmission, measurements and life science microscopy.
Three-dimensional (3D) chiral structures have been thought to be necessary to
realize circular polarizers. Here we demonstrate theoretically and
experimentally for the first time that a high-efficiency circular polarizer
could be realized by a single layer of planar 2D chiral structure. Our proposed
circular polarizer is based on unidirectional polarization conversion instead
of circular polarization stop bands. Since two-dimensional planar structures
present obvious advantage for fabrication and integration on chip, the proposed
circular polarizer is of great interest in integrated optics and microscopy. It
provides a novel scheme to manipulate polarizations of light wave, as well as
Terahertz wave and microwave.
