Chiral Mirrors

Conventional mirrors convert right-handed circularly polarized waves (RCP) into left-handed circularly polarized waves (LCP) and vice versa, see Fig. 1(a-c). Here we demonstrate a novel type of chiral mirror that will reflect one circular polarization without changing its handedness, while absorbing waves of the opposite handedness, see Fig. 1(d-f). This functionality, which fundamentally relies on planar chirality, is enabling for circularly polarized cavities and lasers, detection of circularly polarized electromagnetic waves and optical activity measurements with resonantly enhanced sensitivity. Key for the realization of such polarization selective RCP or LCP mirrors is a chiral structure to enable different responses to waves of opposite handedness. In principle, conventional 3D-chiral media can exhibit different absorption for waves of opposite handedness, however, for a reflecting device, handedness reversal upon reflection would undo any discrimination between RCP and LCP during the second pass through the 3Dchiral medium. In fact, what is needed is a medium that reverses its chiral response with the handedness reversal of the wave upon reflection in order to amplify the polarization selectivity, rather than eliminating it. This peculiar property is associated with 2D chirality (planar chirality). In contrast to a 3D-chiral helix, which has the same sense of twist when observed from opposite ends, a flat 2D-chiral spiral reverses its sense of twist for observation from opposite sides. The same is true for any pattern that cannot be superimposed with its mirror image without being lifted out of its plane and such patterns are said to be 2D-chiral. We realize polarization preserving mirrors for one circular polarization by placing a 2D-chiral metamaterial in front of a conventional mirror. Fig. 1(f) shows proof-of-principle experiments in the microwave part of the spectrum, where an array of 2D-chiral split rings has been placed in front of a metal mirror. The metadevice reflects RCP without polarization change, and it absorbs LCP around 5.7 GHz. The opposite response is observed for split rings of the opposite handedness (i.e. the enantiomer or “mirror image structure”). The novel chiral metadevice acts as handedness-preserving mirror, reflective circular polarizer and polarization selective perfect absorber. Furthermore, it offers in principle unlimited polarization contrast and thus it exhibits the ultimate 2D-chiral optical effect (avoiding fundamental limitations of 2D-chiral metasurfaces).