calling metamaterials periodic structures is NOT tautological: a great many periodic structures (as in all most all of them) are not metamaterials, and metamaterials are created by repeating metalic small metalic structures close enough to each other that certain wavelengths of EM radiation see perfect magnetic conductors (which are the same as perfect electric conductors, but a quarter-wavelength away.) this required periodicity is inherently narrow-band, because only a select range of wavelengths will be subject to the quarter-wavelength condition. ...the long and short of it is that you don't get a cloak of invisibility, because metamaterials are just periodic structures and as such are very much narrow band.