Tuesday, November 20, 2012

Invisibility Cloak Hides Objects Of Different Sizes

A metamaterial invisibility cloak that can adapt to hide different sized objects is demonstrated in Nature Communications this week. The findings represent a useful advance for more practical applications of metamaterial cloaking.

According to Kyoungsik Kimm Yonsei University, Seoul, Republic of Korea,  "the ability to render objects invisible with a cloak that fits all objects and sizes is a long-standing goal for optical devices. Invisibility devices demonstrated so far typically comprise a rigid structure wrapped around an object to which it is fitted. Here we demonstrate smart metamaterial cloaking, wherein the metamaterial device not only transforms electromagnetic fields to make an object invisible, but also acquires its properties automatically from its own elastic deformation."

Photographs of the sample used in our experiment with an additional triangle patch of 125-mm base and 65-mm height.
 Experimental sample of non-auxetic smart metamaterial cloak.
Credit Nature Communications

Metamaterials have already been shown to hide objects from electromagnetic waves by manipulating the light so that it appears to have not interacted with anything. However, these cloaks need to be redesigned and rebuilt if the shape of the object changes. Kyoungsik Kim and colleagues now present a smart metamaterial that is able to adapt to changes in the object shape, so that a range of objects may be hidden by one cloak.

 The cloak is based on elastic materials, which enable it to deform around the object. At the same, the deformation alters its properties to maintain invisibility. The team build and demonstrate cloaks for objects whose height varies over around 10mm, using microwave frequency light incident for a range of angles. They report that the object remains well hidden in all cases.

The demonstrated device is a ground-plane microwave cloak composed of an elastic metamaterial with a broad operational band (10–12 GHz) and nearly lossless electromagnetic properties. The metamaterial is uniform, or perfectly periodic, in its undeformed state and acquires the necessary gradient-index profile, mimicking a quasi-conformal transformation, naturally from a boundary load. This easy-to-fabricate hybrid elasto-electromagnetic metamaterial opens the door to implementations of a variety of transformation optics devices based on quasi-conformal maps.

These smart metamaterial cloaks provide a new avenue to explore adaptable, real-world applications of cloaking that are not limited by the objects they hide.

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