Large Chiroptical Effects in Planar Chiral Metamaterials

Chiroptical effects, characterized by different optical responses for left- (LCP) and right- handed circularly polarized light (RCP), are powerful and valuable tools in optics with wide applications in polarization resolved imaging and sensing. Previously observed strong chiroptical effects are limited to metamaterials with complex three-dimensional chiral structures at the sub-wavelength scale. Although asymmetrical transmission of LCP and RCP have been investigated in planar chiral metasurfaces, the observed weak chiroptical effects result from anisotropic Ohmic dissipation of the metal constituents. Here, we demonstrate by theory and proof-of-concept experiments that large difference in transmittances of LCP and RCP can be attained in a single-layer planar chiral metamaterial with a sub-wavelength thickness. Without violating the reciprocity and mirror symmetry, the strong chiroptical effect, independent of dielectric loss, arise from a novel mechanism of multimode interference. The described effect may lead to new gateway towards chiral manipulations of light and chiral optical devices.