Cloaking With Metamaterials

In October 2006, A team of British and U.S. scientists had demonstrated a breakthrough physical phenomena, then only known to science fiction; the world’s first working “invisibility cloak”. The team, led by Professor Sir John Pendry, created a small device about 12 cm across that had the intrinsic property of redirecting microwave radiation around it, rendering it almost invisible to microwaves.

What made this demonstration particularly remarkable was that this characteristic of microwave invisibility was not derived from the chemical composition of the object but rather the structure of its constituent materials. The team had demonstrated the cloaking properties of a meta-material.

WHAT ARE THEY

A metamaterial is a material purposely engineered to possess one or more properties that are not possible with traditional, naturally occurring materials. Radiation can be bent, amplified, absorbed or blocked in a manner that far supersedes what is possible with conventional materials.

PROPERTIES OR REFRACTION

The refractive index of a material varies with the radiation’s wavelength, which in turn also causes the angle of the refraction to vary. Every known natural material possesses a positive refractive index for electromagnetic waves. Metamaterials however, are capable of negative refraction.

HOW REFRACTION IS CONTROLLED

Permittivity is a measure of how much a material polarizes in response to an applied electric field while magnetic permeability is the measure of magnetization that a material obtains in response to an applied magnetic field. As an electromagnetic wave propagates through the metamaterial, each unit responds to the radiation and the collective results of these interactions creates an emergent material response to the electromagnetic wave that supersedes what is possible with natural materials.

FIRST CONCEPTS

The first mention of the properties of metamaterials was in 1904, with the conceptualization of negative wave propagation by British mathematician Horace Lamb and British physicist Arthur Schuster. Veselago’s research included producing methods for predicting the phenomena of refraction reversal, in which he coined the term left-handed materials.

ARTIFICIAL DIELECTRICS

From this, the development of artificial dielectrics during the 1950s and 1960s, began to open up new ways to shape microwave radiation, especially for radar antennae design. Artificial dielectrics are composite materials made from arranged arrays of conductive shapes or particles, supported in a nonconductive matrix. Similar to metamaterials, artificial dielectric are designed to have a specific electromagnetic response, behaving as an engineered dielectric material.

FIRST METAMATERIALS

Pendry’s expertise in solid state physics had led him to be contracted by Marconi Materials Technology in order to explain the physics of how their naval stealth material actually worked. Pendry had discovered that the microwave absorption of the material did not come from the chemical structure of the carbon it was made from but rather the long, thin shape of the fibers. He had figured out how to manipulate a materials electric and magnetic response, effectively allowing for a method to engineer how electromagnetic radiation moves through a material.

SUPERLENS

By late 2000, Pendry had proposed the idea of using metamaterials to construct a superlens. Pendry theorized that one could be developed employing the negative refractive index behavior of a metamaterial. However, in practice, this proved to be an incredibly difficult task due to the resonant nature of metamaterials. By 2003, Pendry’s theory was first experimentally demonstrated at microwave frequencies, by exploiting the negative permittivity of metals to microwaves.

CLOAKING

Composed of 21 alternating sheets of silver and a glasslike substance, the material, referred to as a fishnet, causes light to bend in unusual ways as it moves through the alternating layers. What made this particularly notable was that it operated on a wider band of radiation than their previous attempts.

FUTURE OF CLOAKING

Despite the ongoing research and relative success with microwave radiation, to date optical cloaking still remains elusive due to the technical challenges of manipulated light within a metamaterial Light moving through materials typically gets ab­sorbed until, at some point, the energy of the radiation falls off, making it a challenge to guide it’s propagation in a useful way.