Thin film invisibility cloak leading to 3D camoflage and invisibility and near field sensors

Demonstration of an ultralow profile cloak for scattering suppression of a finite-length rod in free space – Researchers present the first experimental realization and verification of a three-dimensional stand-alone mantle cloak designed to suppress the total scattering of a finite-length dielectric rod of moderate cross-section. Mantle cloaking has been proposed to realize ultralow-profile conformal covers that may achieve substantial camouflage, transparency and high-performance non-invasive near-field sensing. Here, we realize and verify a mantle cloak for radio-waves. We report an extensive campaign of far- and near-field free-space measurements demonstrating that conformal cloaks can indeed produce strong scattering suppression in all directions and over a relatively broad bandwidth of operation.

Main results. We have experimentally verified a new route to render a 3D object standing in free space invisible to radio waves, without requiring a bulk metamaterial cover. We have instead applied the ‘mantle cloaking’ technique, showing that ordinary 3D objects can be cloaked in all directions and from all observer’s positions using a single, ultrathin patterned ‘metasurface’ conformal to the object. We applied this technique to an 18 cm cylindrical tube, covering it with a flexible metasurface realized with copper tape on a polycarbonate cover. The metasurface consisted of a mesh of vertical and circular stripes, providing the required inductive surface impedance such that, when excited by an external wave, it would support a scattering response that is ‘opposite’ to the one of the object, canceling out most of the scattered waves. The cloak showed optimal functionality at 3.7 GHz over a moderately broad bandwidth. Our experiment shows that this approach may not only make the cloak realization and design easier, but may also achieve larger scattering suppression and broader bandwidths compared to bulk metamaterial covers.

Wider implications. In principle this technique may be extended to visible frequencies; in fact metasurfaces are easier to realize than metamaterials in optics. However, the object size that can be efficiently cloaked with this method scales with the wavelength, so when applied to optical frequencies we may be able to efficiently stop the scattering of only micrometer-sized objects. Still, we have envisioned various exciting cloaking applications for small objects—i.e., non-invasive near-field imaging devices, optical nanotags and nanoswitches—as well improving the absorption efficiency of nanoparticles. These may provide great benefits for biomedical and optical devices.

(left) Experimental setup for the far-field measurement of the cloaked cylinder. (right) Near-field measurements comparing the uncloaked cylinder, the cloaked cylinder and a free-space measurement at the design frequency. The top row refers to illumination at normal incidence, the bottom row to oblique illumination at 30 degrees off the axis.

We have reported here the first experimental verification of mantle cloaking for a 3D finite-length dielectric cylinder of moderate cross-section. We have shown good scattering suppression over a broad range of viewing angles and over a moderate frequency range using a simple ultrathin patterned metascreen tailored to suppress the dominant scattering order. Higher-order multipoles can contribute to the residual scattering and it is foreseen that the use of multiple mantle layers or an asymmetric tailoring of the cloak may be optimized to suppress more scattering orders, especially for larger objects. The observed scattering reduction was not limited to far-field observers, but it was also verified right around the cloaked object. As demonstrated in this work, cloaking in the very near-field by means of an equivalent surface reactance guarantees strong scattering suppression for arbitrary wavefronts, showing that the conformal metascreen cloak is resilient to complex and non-ideal phase patterns. Combined with the field penetration inside the cloak, these results pave the way to realizing not only 3D conformal camouflaging and invisibility, but also a practical scheme for non-invasive high-performance near-field sensors. The ease of fabrication of our design is very appealing, especially at microwave frequencies, as opposed to other more complex cloaking strategies, and the realized prototype tested in this work also shows a strong invariance to manufacturing and measurement imperfections as well as realistic losses.

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