Advances on broadband and resonant chiral metasurfaces

Chirality describes mirror symmetry breaking in geometric structures or certain physical quantities. The interaction between chiral structure and chiral light provides a rich collection of means for studying the chirality of substances. Recently, optical chiral metasurfaces have emerged as planar or quasi-planar photonic devices composed of subwavelength chiral unit cells, offering distinct appealing optical responses to circularly polarized light with opposite handedness. The chiroptical effects in optical metasurfaces can be manifested in the absorption, scattering, and even emission spectra under the circular polarization bases. A broadband chiroptical effect is highly desired for many passive chiral applications such as pure circular polarizers, chiral imaging, and chiral holography, in which cases the resonances should be avoided. On the other hand, resonant chiroptical responses are particularly needed in many situations requiring strong chiral field enhancement such as chiral sensing and chiral emission. This article reviews the latest research on both broadband and resonant chiral metasurfaces. First, we discuss the basic principle of different types of chiroptical effects including 3D/2D optical chirality and intrinsic/extrinsic optical chirality. Then we review typical means for broadband chiral metasurfaces, and related chiral photonic devices including broadband circular polarizers, chiral imaging and chiral holography. Then, we discuss the interaction between chiral light and matter enhanced by resonant chiral metasurfaces, especially for the chiral bound states in the continuum metasurfaces with ultra-high quality factors, which are particularly important for chiral molecule sensing, and chiral light sources. In the final section, the review concludes with an outlook on future directions in chiral photonics.