Metamaterials: How to become invisible

Metamaterials exhibit physical features that do not exist in nature, such as the ability to render things invisible, alter radiation, and even cause light to cease. Structure-wise, they allow for never-before-possible effects and applications.

Just how does one go about making oneself invisible? You can hide from prying eyes by donning a cloak. You are not in Tolkien’s Middle-earth with the ring of invisibility; rather, you are in a metamaterials research lab in particle physics. What are metamaterials and how do they act this way? The key to understanding how metamaterials work is in their structure, which is composed of small, repeating fundamental units that affect the transmission of light and other radiation in a manner similar to that of a conventional crystal. Metamaterials are able to control radiation in a physically novel manner because of the tiny size and unique form of these units.

What is metamaterial?

With metamaterials, it seems like practically anything is feasible, from ultrathin lenses to camouflage cloaks to sound absorbers. It’s no surprise that research and development in the field of these man-made, individualized structures are thriving at the moment. But what explains their strange abilities? And how may they be put to use?

Artificial medium, often arranged in a periodic fashion, with unexpected electromagnetic characteristics are referred to by the relatively crude phrase “metamaterials,” a combination of the word “material” and the prefix “meta,” which in ancient Greek meant “beyond.”

The majority of metamaterials are quite unremarkable. Some are composed of metal, while others are made of silicon or even plastic, so even their composition is nothing out of the ordinary. However, they cause incomprehensible behavior in electromagnetic radiation, including light. 

Metamaterials light
Because of their unique structure, metamaterials can bend light in “impossible” ways. (Getty Images)

Manipulation of light

In the case of some metamaterials, for instance, the beam’s direction, phase, and polarization can be altered to the point where the light is effectively compelled to go backwards. The substance causes the beam to be refracted in the opposite direction from what would be expected of a typical material. This means that a concave convergent lens constructed from this metamaterial would actually scatter rather than concentrate light. Light would be focused by a lens that diverges, against the principles of physics.

This seemingly contradictory behavior is made feasible by the fact that such metamaterials have a negative refractive index. Radiation incident upon this substance is hence not bent back toward the perpendicular but instead bent back beyond the perpendicular. Before the 1968 theoretical prediction by Russian scientist Viktor Veselago, the idea that such materials may exist was nothing more than a thought experiment. Because naturally occurring negative refractive indices were unknown. The first time they were seen was in man-made materials.

The negative refraction

negative refraction
On the right is the effect of metamaterial on light.

The negative refraction predicted by the Russian scientist Victor Veselago 50 years ago is one such property. While we learned in high school that a lens is convergent if it is convex (curved), he proved that a flat lens can be used to focus light too. There must be a negative value in the magnetic permeability and electrical permittivity for the refractive index to be negative. This seems to be impossible to do in reality, if for no other reason than that common materials (metals, glassware, plastics) are not magnetic.

But with a metamaterial consisting of a periodic network of parallel metallic wires (which presents a negative permittivity at low frequency) and a periodic network of small metallic loops called “split ring resonators,” which presents a negative permeability around a resonance frequency, English physicist John Pendry demonstrated in 2000 that this is indeed possible.

Some 16 years ago, Pendry developed a proposal for an invisibility cloak based on concentric layers of split ring resonators where the waves would travel along curved routes.

Micro or nano etching is the current method of producing metamaterials. The dielectric portion of the metamaterial is formed by copper fibers imprinted on glass fibers. Fundamentally, they serve microwaves (at frequencies of a few gigahertz).

Making them function in the visible spectrum (frequencies of a few hundred terahertz), where metals become extremely absorbent, is one of the technical obstacles. This makes the metamaterial opaque, which is not ideal for the creation of a flat lens or an invisibility cloak.

About five years ago, scientists started wondering whether the science of electromagnetic metamaterials might be applied to seismic waves. At first glance, the comparison between light in nano or micro-structured materials and mechanical waves in structured soils appears very far-fetched. Indeed, the so-called mass and spring models provide the basis for this transcription.

Light and Earth waves

Transforming the physical characteristics of space is the common denominator between electromagnetic and seismic metamaterials, and it is this technology that enables the invisibility effect. The basic idea is straightforward: modifying the coordinates in the equations controlling wave propagation results in a highly heterogeneous anisotropic material with intricate interference events.

For almost 20 years, Belgian physicist André Nicolet has advocated for the employment of the space transformation approach to facilitate the numerical solution of electromagnetic problems in unbounded medium (by transforming coordinates to shrink distances) or twisted guides (helical coordinates leading to a problem of an untwisted guide that is made of a heterogeneous anisotropic medium).

In 2006, Michael Pendry proposed using a solid disk in conjunction with a cloak to achieve invisibility. The cloak would be constructed from a heterogeneous anisotropic medium, which would cause light traveling through it to be deflected. Below is an illustration of this study in the fields of hydrodynamics, acoustics, and electromagnetism using a 8-inch-diameter (20 cm) aluminum invisibility cloak created in 2008 at the Fresnel Institute.

A shield against tsunamis or a cloak that makes waves invisible. This structure, which is machined into an aluminum disk 8 inches in diameter, is covered with tiny studs. It has a rim that is 4 inches (10 cm) in diameter. The employed metamaterial acts like an anisotropic fluid, causing the waves to deflect away from the focal point. Using a structure like this may be an innovative approach to stopping coastal erosion or shielding offshore infrastructure like oil rigs from waves.

Cloak prototype for seismic waves.

Tests with waves (8–15 Hz), sound waves (3–7 kHz), and microwaves (3–7 GHz) on an 8-inch-diameter metamaterial composed of 0.40-inch-high (1 cm) aluminum studs structured in concentric layers showed promising results. The lack of a field in the cloak’s core is seen on the experimental magnetic field map (top left), suggesting that this approach may be scaled up to create a large-scale seismic protection system (CNRS infographic, top right).

The term “invisible” is often used to describe an item that does not alter or just slightly alters the surrounding wave field. Either the item itself or the apparatus (which is called a “cloak”) around it has the ability to rebuild the wave field as if it were not existing in the region investigated (which is called “transparency”).

An invisibility cloak deflects waves away from a specific area by punching a hole in the space’s metric, but realizing such precise control over the paths of waves at a civil engineering scale is still a formidable issue.

Seismic metamaterials

Structured soil is an ancient concept. For “soft” soils, some civil engineers recommend using wooden piles due to their superior effective bearing capacity. This confirms the feasibility of a seismic cape for civil engineering, which, if wrapped around a building’s footings, could significantly dampen the local effect of an earthquake or of anthropogenic sources like vibrating machines used in public works and industrial equipment, at least for some frequencies.

In 2012, tests were conducted on a prototype cloak to verify the similarity between electromagnetic metamaterials and structured flooring. Surface waves (known as Rayleigh waves) could be converted into volume waves, which were absorbed by the soil and thus did not return to the source.


By Bertie Atkinson

Bertie Atkinson is a history writer at Malevus. He writes about diverse subjects in history, from ancient civilizations to world wars. In his free time, he enjoys reading, watching Netflix, and playing chess.