Bidirectional invisibility in Kramers-Kronig optical media.

A broad class of planar dielectric media with complex permittivity profiles that are fully invisible, for both left and right incidence sides, is introduced. Such optical media are locally isotropic, non-magnetic, and belong to the recently discovered class of Kramers-Kronig media [Nat. Photonics9, 436 (2015)], i.e., the spatial profiles of the real and imaginary parts of the dielectric permittivity are related each other by a Hilbert transform. The transition from unidirectional to bidirectional invisibility, and the possibility to realize sharp reflection above a cut-off incidence angle, are also discussed.