Experimental demonstration of a nonmagnetic metamaterial cloak at microwave frequencies

Metamaterials have paved the way to unprecedented control of the electromagnetic field. The conjunction with space coordinate transformation has led to a “relativity inspired” approach for the control of light propagation. “Invisibility cloak” is the most fascinating proposed device. However, the realized structures up to now used a graded “metamagnetic” so as to achieve the cloaking function. Artificial magnetism is still very challenging to obtain in optics despite the currently promising building blocks, not suited for optical cloaking. We report here the experimental demonstration of a nonmagnetic cloak at microwave frequencies by direct mapping of the magnetic field together with the experimental characterization of a cloak in free space configuration. The diameter of the concealed region is as big as 4.4 in wavelength units, the biggest reported experimentally so far. The principle can be scaled down to optical domain while keeping the compatibility with current nanofabrication technologies.

Figure 1

(Color online). (a) Unit cell. (b) The dimensions of a typical square SRR are: $L=3.6\text{mm}$, $w=0.3\text{mm}$ and copper thickness $\text{t}=35\mu \text{m}$. The SRRs gap g and ${\text{l}}_{\theta \text{i}}$, are the only varying parameters. ${\text{l}}_{\theta \text{i}}$ linearly decreases from the outer to the inner boundary of the cloak (Ref. 28). (c) Picture of a radial stripes and corresponding relevant effective parameters.

Figure 2

(Color online). (a) Assembling of the cloaking device from metamaterials based SRRs stripes in the silicone mold. (b) Realized metamaterial cloaking device (c) Two dimensional view of the experimental setup. (d) Picture of a portion of the experimental setup with the loop antenna mapping the magnetic field at the bottom surface of the cloak.

Figure 3

(Color online). Real part of the measured magnetic field output from the horn antenna in free space (a) with the metallic cylinder alone (b) and with the cloak surrounding the metallic cylinder (d). Finite element simulation exciting the cloak by the appropriate optical excitation (Comsol Multiphysics) with the reduced set of parameters presented in Eq. (2) is reported for comparison (c). In all cases, the 11 GHz wave travels from bottom to top.