  We propose an effective route to fully control the phase of plane waves
reflected from electrically (optically) thin sheets. This becomes possible
using engineered artificial full-reflection layers (metamirrors) as arrays of
electrically small resonant bi-anisotropic particles. In this scenario, fully
reflecting mirrors do not contain any continuous ground plane, but only arrays
of small particles. Bi-anisotropic omega coupling is required to get asymmetric
response in reflection phase for plane waves incident from the opposite sides
of the composite mirror. It is shown that with this concept one can
independently tailor the phase of electromagnetic waves reflected from both
sides of the mirror array.
