Win the Race for Invisibility

Professor John Pendry, a physicist at Imperial College London, launched what has become a metamaterial revolution. In the late 1990s, he discovered something radical: He could change an object’s material properties just by changing its internal structure, without altering its chemical or molecular makeup. Pendry suggested that building a complex lattice structure could allow manipulation of an object’s magnetic and electrical fields. Theoretically, this could allow engineers to design materials that could control their interactions with the electromagnetic spectrum, including visible light.

Pendry’s theory gave rise to a new field of basic material research: metamaterials and metasurfaces. These  materials achieve properties that exceed previously accepted natural and synthetic limitations.¹ Sensational forecasts for one application appear already to have come true. In 2006, David Smith and fellow researchers at Duke University announced creation of a proof of concept for an invisibility cloak. It used metamaterials to mask itself from microwave radiation.² The prospect of masking systems and weapons from detection—making them invisible to sensors—now looms.

In previous eras, major innovations in warfare depended on the development and application of new materials and designs. Wood-hulled sailing ships evolved into iron-plated, steam-powered warships. Coal-burning steam power gave way to diesel, gas-turbine, nuclear, and hybrid-electric propulsion. Lightweight aluminum replaced heavy wood in airframes, as airfoils changed shape to improve supersonic performance. Each of these evolutions depended on a new material or design whose properties overcame the limitations of the old, while remaining within the boundaries of long-established scientific laws.

This time is different. Metamaterials and surfaces do not face the same incremental limitations. In some cases, they have altered conventional understanding of the laws of physics. For society and the warfighter alike, metamaterials and surfaces offer a wide range of near- and long-term technological possibilities, as well as risks. Most concerning for the military, these opportunities are equally available to the United States and its potential adversaries, and those adversaries are working hard to take advantage of U.S. advances. (See sidebar, above.)

The Basics

An aircraft, ship, submarine, or weapon covered by a metamaterial or metasurface lattice can be extremely difficult for an adversary to identify against the surrounding air and water. But how?

Metamaterials derive their physical, electrical, and magnetic properties from their structure. Those that contribute to electromagnetic and acoustic invisibility are best conceptualized as a complex lattice of either microchips (metamaterials) or uniquely shaped and layered composite and/or natural components (metasurfaces). Typically, the lattice elements have a scale of up to a few hundred nanometers—billionths of a meter. Because metamaterials can dynamically reshape their associated electrical and magnetic fields, they can alter how various energy waves (radar, microwave, infrared, visible, and acoustic) interact with them. That is, they change how various forms of energy are absorbed, reflected, dispersed, refracted, and amplified. Thus, metamaterials control energy in radical new ways. 

Basic physics is the foundation of metamaterial design.³ When an energy wave travels through a medium (e.g., space, air, water, glass), crosses a boundary, and interacts with another medium, the wave refracts (bends). Think of light bending as it passes through a prism. This phenomenon is known as Snell’s Law. When this wave encounters an object, parts of the wave are either reflected, dispersed, or absorbed. For example, when light hits a car, some is absorbed, but what reflects lets us see the car and tells us what color it is. Because most sensors measure reflection and/or absorption of emitted energy, achieving near-zero reflection and absorption would be the endstate of perfect stealth technology—all the sensor energy emitted is returned, except for expected environmental losses.⁴ 

Metamaterials can control absorption and refraction of energy around an object thanks to several unique properties. One of the most surprising discoveries related to metamaterials is that they can have a negative “refractive index,” which is a way of describing how much an energy wave changes speed and direction when traveling through a specified material. Until 1967, it was believed all real materials must have positive refractive indices. This characteristic allows metamaterials to controllably refract waves in the opposite direction of naturally occurring materials and mediums. Another property concerns how the shape, size, and configuration of unit cells determine their permittivity (electric) and permeability (magnetic)—how an electric or magnetic field interacts with its environment.⁵ By changing these fields, metamaterials and metasurfaces can achieve continuously variable properties throughout their structures. This trait allows them to redirect and disperse energy waves in different ways throughout the material. 

The Challenge of Scale

Metamaterials remain a relatively new technology. They have existed for just 20 years yet have made tremendous advances. While their potential is immense, scaling the technology is difficult. Design of metamaterials requires nanoscale components to manipulate wavelengths in the infrared and visible portions of the electromagnetic spectrum. This limitation challenges engineers’ ability to move its applications beyond radio, micro-, and infrared waves into the longer wavelengths that make up the visible spectrum. 

A given layer of metamaterial lattice can control and/or manipulate only a specific wavelength; different materials must be layered to impact a broader spectrum. But layering increases the complexity and overall thickness of the material, which adversely affects application. Industry will need to develop new capabilities in custom circuit board printing to meet design requirements.

Applications

Communications. By increasing electromagnetic spectrum agility and redundancy, metamaterial-based antennas can help ensure command and control in support of electromagnetic maneuver warfare. Such antennas reduce size, increase power output, improve directionality, and increase the range of frequencies possible for a single antenna. By replacing conventional radio antennas with electronically scanned arrays (also known as phased arrays), the joint force can introduce advanced communication capabilities to small units and groups. The antennas’ increased power and directionality allow metamaterial designs to overcome full-spectrum jamming capabilities, at least partially. In essence, they “buy back” portions of the spectrum from the adversary. Rather than depending on a directional satellite connection, these antennas can “catch” transmissions from any angle. Further, they allow for rapid, nearly continuous shifts in frequency to provide additional antijam capability. This type of control across the electromagnetic spectrum would be a crucial mission enabler. 

Optics. Metamaterials challenge the optical diffraction limit of materials. The diffraction limit is the minimum angular separation required between two points for a sensor to distinguish them. Traditionally, diffraction—the bending of energy waves around corners, like ocean waves around a moored boat—occurs as they pass through the sensor’s aperture. Metamaterial superlenses overcome this limitation by forcing the energy wave to bend in new ways, controlling the dispersion through negative refraction.⁶ Increasing the optical resolution of an electro-optical (EO) or infrared (IR) device allows for increased fidelity for collection at longer ranges and across larger swaths of the battlespace. More important, these sensors can be miniaturized for installation on smaller manned and unmanned systems. They also provide the capability to observe nanoscale structures visually. An optical device that could do so would have significant applications in the rapid detection of chemical, biological, and radiological threats or even toxic gas leaks on ships. From troops on the ground to aircraft and ships, the improvement in optics would substantially increase battlespace awareness, decision-making, and safety. 

Weapons. For precision-guided weapons with EO or IR seekers, improved resolution could allow for target identification and discrimination at extended ranges, increasing the effectiveness of smart weapons. It also promises to improve the precision of current U.S. defensive systems, including Standard, Patriot, ground-based interceptor, Hellfire (AGM-114), and Sidewinder (AIM-9) missiles. Beyond seekers, metasurface missile shrouds could change the next generation of ordnance, allowing it to avoid detection by integrated air-defense systems or aircraft-—a significant potential application. (Designs of these seekers and shrouds will need to be open and modular to account for adversary metamaterial and metasurface advances.) The combined improvement of weapon sensors and design will make the use of metamaterials to cloak or camouflage ships, aircraft, and submarines even more important. 

Acoustics. Initial research on metamaterials focused on electromagnetic phenomena, but more recent research has investigated metamaterial applications in acoustics.⁷ The physics of sound propagation in water are more challenging than those in air, thanks to water’s density and limited ability to be compressed. Nonetheless, researchers at Pennsylvania State University in 2018 revealed that they had designed a three-foot tall, perforated steel pyramid that was effectively invisible to wideband sonar frequencies—an “acoustic ground cloak” that can dampen sound emissions or even mask an object entirely.⁸ In the undersea domain, the ability to avoid detection depends on defeating active and passive sonar. The metamaterial sound dampening would provide the possibility of masking self-noise of both ships and submarines. Mechanical metamaterials also offer similar opportunities not only to mask acoustics but also to store energy and manage thermal footprints. Like stealth for aircraft, acoustic masking of ships, submarines, mines, or unmanned underwater vehicles would provide a significant warfighting advantage by reinvigorating surprise as a tenet of warfare. This type of dampening technology is already in prototype and could soon be tested in the field.

Advance to Win

With cross-domain and cross-spectrum uses, metamaterials offer a multitude of opportunities to dominate the battlefield. Their wide application to platforms, sensors, and weapons speaks to metamaterials’ transformative potential. The challenge is to keep pace with any adversary whose resolve reflects a national mission to dominate the field of metamaterials. The United States must invigorate its own defense enterprise to meet this challenge. In particular, the Department of Defense must engage war-fighters, scientists, and engineers across industry, government, and higher education to seize the frontier of metamaterial applications while building sustainable capacity. By integrating end-users, prototyping capabilities, developing and releasing incremental capabilities, and creating modular, open systems, the Navy and the Department of Defense can achieve agility in the field of metamaterial applications and increase the lethality of the force. These applications must be tightly controlled, with a close eye on adversaries. The future of these materials is now.⁹