Rational design of maximum chiral dielectric metasurfaces

Contemporary metasurfaces manifest unprecedented strong optical chirality, which is desired in quantum informatics, telecommunications, microscopy, and biomedical sensing. In this chapter, we review recent progress in dielectric chiral metasurfaces, paying special attention to their advantages associated with low absorption losses and high-quality resonances stemming from Mie-type resonances of individual dielectric particles. We review general restrictions, implied by the reciprocity and symmetry for different metasurface types, and identify the regimes of ultimate maximum-selective interaction with circularly polarized light. To reveal how such regimes can be realized in practice, we use phenomenological coupled mode theory (CMT), which points out specific properties of eigenstates and their combinations required for a metasurface to perform as a chiral helicity-preserving mirror, maximum-chiral absorber, or a lossless maximum-chiral filter. For each regime, we present exemplary metastructures and verify that they perform in full accordance with analytical CMT predictions. Employing the concept of bound states in the continuum greatly simplifies the optimization of metasurfaces and allows designing those exhibiting ultra-narrow bands of maximum optical chirality prospective for diverse linear and nonlinear optical applications.