Harry Potter's cloak of invisibility spans roughly one million wavelengths of the visible light. A metamaterial implementing it would need to have an attenuation constant of less than 1 dB/m. A paper by Urzhumov and Smith, published recently in the top physics journal, Physical Review Letters, suggests that this may be achievable with photonic crystals composed of optically transparent dielectric materials.This Letter was selected as a cover article for the October 15 issue of Phys. Rev. Lett.(2010).
Transformation Optics with Photonic Band Gap Media, Urzhumov, Yaroslav A., and Smith D. R. , Physical Review Letters, 10/2010, Volume 105, Issue 16, (2010).