Broadband transparent chiral mirrors: Design methodology and bandwidth analysis

Chiral mirrors are a class of metamaterials that reflect circularly polarized light of a certain helicity in a handedness-preserving manner, while absorbing circular polarization of the opposite handedness. However, most absorbing chiral mirrors operate only in a narrow frequency band, as limited by the causality principle. Instead of absorbing the undesired waveform, here we propose a transparent chiral mirror that allows undesired waves to pass through. In particular, the handedness-preserving band of the transparent chiral mirror is free of the causality limit, thus enabling broadband functionality. Furthermore, since electromagnetic waves outside the handedness-preserving band may transmit through the proposed chiral mirror, the reflected wave contains only circular polarization components of a certain handedness over a wide frequency range, which is favored in many applications. Moreover, the scheme is lossless and scalable. To realize the proposed transparent chiral mirror, we apply an array of helical microstructures in a two-dimensional square lattice. Traditionally, this kind of structure has been used as a circular polarizer but we apply it instead in a reflective mode. Our work provides a bandwidth analysis of chiral mirrors, and paves the way to new opportunities for creating broadband chiral metamaterials with handedness-preserving properties.