  In planar metamaterial lenses, the focal point moves with the frequency. Here
it is shown numerically that this movement can be controlled by properly
engineering the dimensions of the metamaterial-based phase shifters that
constitute the lens. In particular, such lenses can be designed to exhibit
unusual chromatic aberration with the focal length increasing, rather than
decreasing, with the frequency. It is proposed that such an artificial
"reverse" chromatic aberration may optimize the transverse resolution of
millimeter wave diagnostics of plasmas and be useful in compensating for the
natural "ordinary" chromatic aberration of other components in an optical
system. More generally, optimized chromatic aberration will allow to
simultaneously focus on several objects located at different distances and
emitting or reflecting at different frequencies.
