Robust Control of a Multifrequency Metamaterial Cloak Featuring Intrinsic Harmonic Selection

Experimental verifications of cloaking, which can make objects invisible, have long been limited to single-frequency operation for relatively small objects. In this paper, we demonstrate a metamaterial cloak enabling multifrequency operation for an electrically large object, which can be robustly controlled based on an intrinsic harmonics selection condition. By designing an inhomogeneous isotropic refractive index that collimates the scattered wave path into forward direction rather than bending the propagating wave around the object exactly, the harmonics selection condition for multifrequency cloaking is naturally derived by matching the phase delay induced from the cloak medium to 2π harmonics. Then, the cloaking harmonics are controlled robustly by adjusting the size of the hidden object, which determines the phase delay inside the cloak without any extrinsic schemes such as actively controllable metamaterials. From the merit of the isotropic nature, the proposed cloak is fabricated in the three-dimensional space and verified experimentally from the reduced electromagnetic scatterings of a copper double cone. The proposed cloak employs a distinct manner, which may open a new paradigm to design multiband apparatuses such as stealth or camouflage.