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The Wright Flyer Papers

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AIR UNIVERSITY

AIR COMMAND AND STAFF COLLEGE

The Cheshire Jet

Harnessing Metamaterials to Achieve 

an Optical Stealth Capability

Timothy E. Beers

Major, USAF

Air Command and Staff College

Wright Flyer Paper No. 44

Air University Press

Air Force Research Institute

Maxwell Air Force Base, Alabama

July 2010
ii

Disclaimer

Opinions, conclusions, and recommendations expressed or implied within are 

solely those of the authors and do not necessarily represent the views of Air University, the United States Air Force, the Department of Defense, or any other US 

government agency. Cleared for public release: distribution unlimited.

This Wright Flyer Paper and others in the series are 

available electronically at the Air University Research 

Web site http://research.maxwell.af.mil. and the AU 

Press Web site http://aupress.au.af.mil.
iii

Foreword

It is with great pride that Air Command and Staff College presents another in a series of award winning student 

research projects from our academic programs that reach 

nearly 11,000 students each year. As our series title indicates, we seek to promote the sort of imaginative, forwardlooking thinking that inspired the earliest aviation pioneers, and we aim for publication projects which combine 

these characteristics with the sort of clear presentation 

that permits even the most technical topics to be readily 

understood. We sincerely hope what follows will stimulate 

thinking, invite debate, and further encourage today’s air 

war fighters in their continuing search for new and better 

ways to perform their missions—now and in the future.

ANTHONY J. ROCK

Brigadier General, USAF

Commandant

v

Preface

In truth, this paper came about by chance. I stumbled 

across metamaterials while researching another technology. 

I was fascinated, finding myself quickly drawn into this exciting new realm of science and technology. Something 

about invisibility must ring a primeval note. Who among us 

has never daydreamed of being invisible? Who among us 

never laughed in amusement while playing peek-a-boo with 

an infant who believes that something somehow vanishes, 

or ceases to exist because he or she cannot see it? As the 

saying goes, “Out of sight, out of mind.” Invisibility is a veritable wellspring, finding mention in religion, mythology, 

and popular culture worldwide. Interestingly enough, it is 

usually villains and our enemies that we imbue with this 

ability. Perhaps no single thought creates more fear in our 

hearts than the adversary who catches us unaware, or one 

who can deliver an attack we cannot anticipate.

The last time I sat in a physics class was 14 years ago. 

That said, I never could have understood this topic, nor ultimately have written this paper, without the patient guidance, 

direction, and insights of Professors Steven A. Cummer from 

Duke University and Nicholas X. Fang from the University 

of Illinois. Their enthusiasm for the subject is nothing short 

of contagious and their ability to explain such a technical 

concept as cloaking is equally awe-inspiring.

A very special thanks to my family for their saint-like 

forgiveness toward my countless vanishing acts while I 

tackled this topic.

vii

Abstract

Laboratory research that melds physics with materials 

science has ventured into the design of matter with 

unique electromagnetic characteristics and response 

functions. This new class of ordered composites—known 

as metamaterials—exhibits exceptional, unnatural properties derived from their structure rather than from their 

composition. These materials promise to provide significant advances in stealth and survivability technology, 

both of which are highly applicable for future Air Force 

capabilities against a counter-low observable, directed 

energy–equipped integrated air defense system.

This paper addresses the question, “Will metamaterials 

facilitate an operationally feasible and significant optical 

stealth capability for the US Air Force?” To answer this 

question, the author’s research is directed at the advances 

and development patterns of optical band metamaterials; 

specifically, it addresses the leading indicators of frequency, bandwidth, and energy loss. Following that, a 

backcasting futures technique helps uncover the obstacles 

of metamaterial durability, suitability, and manufacturability. This paper concludes with a 20-year timeline for optical 

band metamaterial capabilities and applications.

This paper highlights the need for continued Department of Defense (DOD) funding and pursuit of metamaterial “cloaking” technology. The near-term objective of such 

a program should be the development of infrared laser 

protection and the reduction of the optical signature of 

currently fielded systems and their follow-ons.

1

Why Metamaterials?

Any sufficiently advanced technology is indistinguishable from magic.

—Arthur C. Clarke

 Profiles of the Future

1 October 2029

The bright midday sun glints off the East China Sea. 

Several miles above the water, a sleek aircraft streaks 

across the cloudless azure sky. Its revolutionary skin, an 

advanced engineered electromagnetic shell, makes the aircraft impossible to detect by radar and infrared energy. 

Even light seems to curve around the aircraft’s structure, 

making it seem smaller than it really is and shrouding it in 

a distorted, shadowy effect. The navigation system indicates 

that the initial point is rapidly approaching and, soon thereafter, the aircraft will enter the heavily defended airspace 

surrounding the enemy’s operations center. Already the 

self-protection sensors are detecting the probing energy of 

the hostile integrated air defense system. Based on these 

emissions, and further confirmed by the enemy electronic 

order of battle database, the shell reconfigures, tuning itself 

to maximize protection from the numerous incoming frequencies. With a push of a button, the aircraft’s skin is 

pumped with energy. Instantly the vehicle vanishes from 

sight; light bends around the craft like water flowing around 

a stone. The enemy does not expect the audacity of a daylight strike. In minutes, the world’s first invisible jet will 

make history.

Is this scenario mere science fiction, or is it reality waiting to happen? Humans have always dreamed of the power 

of invisibility. In Greek mythology, the hero Perseus used 

an enchanted helmet to disappear, allowing him to approach unnoticed and slay the Gorgon Medusa.1 Invisibility 

was thus an equalizer, the ancient equivalent of a force 

multiplier, allowing man to battle a terrible monster. Another character that many might recognize is the fictional 

Cheshire Cat, who could appear and disappear at will, or 
2

even disappear gradually until nothing but a grin remained.2

More recently, invisibility and cloaking have been made 

popular through countless movies, books, video games, and 

television shows that rationalize the ability through magic 

and alien technology. Science, however, may now be presenting 

this ability as fact, rather than a mysterious phenomenon.

Materials science promises to provide significant advances 

in cloaking technology that will be highly applicable for future 

Air Force capabilities. Recent laboratory research in physics 

has ventured into the design of materials with unique electromagnetic (EM) characteristics and response functions.3 This 

new class of ordered composites, known as metamaterials, 

exhibits exceptional, unnatural properties derived from their 

structure rather than from their composition.

Why does structure, in the context of order and scale, 

matter? Take, for example, a single, tiny quartz crystal. A 

small amount of light reflects from its surface, but for all 

practical purposes, the crystal is transparent. Place a handful of these same crystals together and all the little reflections from the many scattering surfaces result in a pile of 

white sand that is no longer transparent. However, if the 

crystals were all the same size, shape, and arranged in an 

orderly fashion, it would be possible to guide light in distinct patterns through and around them. In this way metamaterials affect EM waves by having meticulously patterned 

design features, smaller than the wavelength with which 

they interact, to guide energy in a precise way.

In the near future, it will be possible to dictate the optical 

constants of objects using engineered nanostructures. These 

materials are able to suppress the scattering of energy and 

vary its indexes of refraction, thus curving, even sharply 

turning, the path of EM waves. This feat effectively renders 

the object invisible to the designed wavelengths and has already been demonstrated in the laboratory with microwaves 

at frequencies as high as 16 gigahertz (GHz).4 Achieving this 

phenomenon within the broad optical spectrum of light is 

possible. Furthering metamaterial efforts ensures an optical 

stealth capability, thus increasing aircraft survivability, specifically against a counter-low observable (LO), directed energy (DE)-equipped integrated air defense system (IADS).5

This paper provides research for the required advances in 
3

metamaterial science to facilitate an operationally feasible 

and significant optical stealth capability.

Understanding Optical Stealth

By design, most combat aircraft employ some form of low 

visibility. To date, this essential facet of comprehensive 

stealth has been achieved through geometric design, 

isoluminosity,and painting aircraft to blend into the environment.6 For instance, high-altitude U-2 reconnaissance 

planes are painted black to match the dark background 

sky. B-2 stealth bombers fly at night to minimize visual 

signature and are also painted black. Likewise, fighter aircraft, like the F-22, are painted “air superiority grey” to better 

blend in with the daytime sky. In addition, bright reflections 

from cockpit glass or other smooth surfaces are minimized 

using special coatings. These efforts are commonly referred to 

as elements of visual stealth. What has yet to be achieved, 

however, is the broader goal of optical stealth.7

Today’s Air Force has an impressive portfolio of attack 

capabilities but lacks sufficient numbers of stealthy aircraft 

that can survive in a dangerous antiaccess environment, 

operating effectively around the clock, prosecuting critical 

targets.8 The future will only exaggerate this predicament. 

As stated in the 2009 Capstone Concept for Joint Operations, 

“We will need to develop new capabilities and change the capacities of existing ones. . . . We will need to develop new technologies and adapt existing ones to new missions.”9 To ensure 

victory against any future counter-LO, DE-equipped IADS, the 

United States must embrace new disruptive technology.

In 2006 the National Research Council published a report entitled Future Air Force Needs for Survivability. This 

report described the various factors that determine the signature characteristics of an aircraft and its weapons systems, the status of the technologies involved, and the Air 

Force’s goals for future strike systems. The report concluded by highlighting the longer-term research and development programs needed for air vehicles in the 2025 timeframe and beyond. The report stated,

Visual signature sources can provide foreign 

elements an additional source to enable alert, 
4

detection, and cueing . . . provides both airborne 

and ground-based threat system operators with 

valuable knowledge that can then be applied in 

the effective operation of their systems. . . . Visual 

signature control continues to be an area that 

should be considered for future development.10

(emphasis added)

Just as radar signature reduction represented a revolution in military operations, the same must be done for the 

optical spectrum. Operating LO aircraft only at night is no 

longer an option. Furthermore, advanced weapons, those 

possessing antiaccess and area-denial capabilities, are increasingly proliferating among less developed states and 

nonstate actors.11 Metamaterial cloaking represents the new 

fog of war, the superior covertness required to “penetrate 

and persist” in, or near, heavily defended airspace.12

How Cloaking Works

To make something invisible, the nature of visibility must 

be understood. Electromagnetic radiation can have wavelengths from thousands of kilometers to a tiny fraction of the 

size of an atomic nucleus. Specifically, and of interest to this 

discussion, visible light consists of relatively high-frequency, 

short-wavelength emissions: 1014–1015 hertz and 400–750 

nanometers, respectively. We see objects in this spectrum 

because light is absorbed by and reflected off matter, which 

in turn hits our eyes (see figure 1).

Figure 1. Pathways of light
5

These waves are absorbed and emitted by the individually 

charged particles in atoms, molecules, or materials. As light 

passes through air or matter, its electric field polarizes the 

medium, and the wave is slowed due to the magnetic field 

created. In turn, this decrease in speed bends the light 

more directly into the material. Refraction describes the 

way light bends when it transits from one medium boundary 

into another. The factor by which light slows down is known 

as the material’s index of refraction.13

By convention, both electrical permittivity (the ability of 

a material to transmit or permit an electric field) and magnetic permeability (the ability of a material to acquire magnetization) are defined relative to the corresponding qualities of a vacuum, expressed as equal to one.14 Metamaterials, 

however, display different EM wave propagation properties. 

When either of these values is negative or smaller than one, 

this offers a means of manipulating refraction.15 In fact, 

these nanofabricated composite materials use deep subwavelength-scale features to engineer optical constants to 

the point of reversing the natural direction of light so that it 

flows against energy. This is in seeming contradiction to 

what we understand to be normal behavior.16

Soviet physicist Victor G. Veselago postulated this phenomenon more than 40 years ago. His theoretical research 

speculated on a material that was “tuned” using electronic 

components to manipulate its electric and magnetic properties, thereby producing a negative refraction.17 Though the 

concept was in place, researchers lacked the computing 

power and material engineering techniques to make this 

a reality.

Over the past decade, however, science has conquered 

these hurdles and proved that metamaterials can cloak 

objects. For example, in 2006, Duke University physicists 

designed and arranged tiny electromagnetic cells to manipulate microwaves and precisely steer them around a 

four-inch diameter cylinder and then shift them back 

onto their original path. Imagine this as water flowing 

around a smooth rock and rejoining downstream. In essence, the concealed volume appeared to have the properties of free space when viewed externally, neither scattering waves nor imparting a shadow in the transmitted 

field. Neither the cylinder, nor the cloak shell, was ever 
6

“seen” by the microwaves. Figure 2 depicts the results of 

this experiment, showing the relatively unaffected wave 

fronts and ray lines.18

It is important to differentiate between current lowobservable stealth, which minimizes the size and clarity of 

signatures, making an object difficult but not impossible to 

detect, and what “cloaking” technology offers: complete invisibility for the designed wavelength.19 Though these early 

experiments were performed at microwave frequencies, it 

was not long before scientists downscaled these structures 

to tackle optical frequencies.

In August 2008, University of California (UC), Berkeley researchers constructed a “fishnet” material that achieved a 

negative refraction of light in the near-infrared (IR) range.20

This was accomplished by creating nanoscale current loops—

in essence a series of circuits—which respond together in opposition to that of the magnetic field from the incoming light. 

Figure 3 depicts two ponds, one exhibiting normal positive 

refraction and one with negative refraction.21

When sunlight travels from air to water in the first pond, 

it slows and bends toward the direction that is perpendicular 

to the boundary layer between the media. A practical consequence of this refraction is that the fish appears to be at 

only 75 percent of its actual depth.22 In the case of the second pond, a fish swimming under water would instead appear inverted in the air above the water’s surface due to 

negative refraction.

Figure 2. Duke University’s 2006 microwave cloak
7

It is important to note that cloaking is not synonymous 

with negative refraction. Cloaking requires a calculated and 

designed nonuniform variance in refraction along an object’s 

surface. Light, or any other EM energy desired, must be 

guided around the shell in curved lines, effectively leaving a 

void in space. Nonetheless, this paper largely addresses negative index of refraction, as it serves as an observable milestone and a proof of concept in science’s ability to steer and 

otherwise manipulate light, which is essential for achieving a 

cloaking capability.

Despite recent advances, current metamaterials remain 

poor at conducting visible light’s shorter wavelengths. This 

means light waves lose energy as they travel around an object, slightly disturbing them, thus preventing unaffected 

emergence on the other side. In other words, the object 

would not be cloaked, but rather appear as a spot or shadow. 

Likewise, the techniques used for fabrication of lab samples 

are limited to test sizes measured in square inches. Further 

work delving into mass-producible, active energy material 

will enable the possibility of completely hiding objects from 

visible light.23

“Why is optical band cloaking relevant today?”24 Following 

the dramatic performance of US stealth aircraft during Operation Desert Storm, many nations sought counter-LO 

technology. Advances in radar, target processing, track correlation, and passive detection all diminish the veil of low 

observables. As such, it is only a matter of time before current stealth is inadequate protection.25

Figure 3. Observing positive and negative refractions
8

Emergence of Directed Energy Weapons

Defense analysts postulate that China is pursuing novel 

concepts to defeat American technology with directed energy weapons (DEW).26 These “new-concept weapons” likely 

include lasers, microwaves, and particle-beam weapons for 

both air and space defense missions—all developed to negate anything from low–altitude, unmanned aerial vehicles to high-flying satellites through blinding, degradation, or destruction.

During the last four and a half decades, increases in laser power and suitability have set the foundation for DEWs 

to enter and eventually dominate battlespace engagements.27 According to experts at the Air Force Research 

Laboratory (AFRL), the next two decades will likely bring 

about additional advancements in beam quality, power, 

compactness, and magazine depth.28 Furthermore, DEWs 

will benefit from more accurate beam steering and tracking, 

resulting in faster “kills” against operators, sensors, and 

vehicles at greater distances.29

Despite these threats and the added mission difficulty 

they present, the 2006 Quadrennial Defense Review directed 

the Air Force to increase its 2025 penetrating component by 

a factor of five.30 With stealth’s marginalization looming on 

the horizon, new technology is needed to achieve this goal 

and assure US access into denied areas. As noted in AFRL’s 

Focused Long Term Challenges (FLTC) for Assured Operations in High Threat Environments, the DOD will require 

specially designed and/or equipped systems with unique 

defensive capabilities to go against DE-IADS.31

It is in this hostile environment that metamaterial cloaks 

will prove their worth, as sensors and weapons alike are 

susceptible to metamaterials’ unique characteristics and response functions. Imagine laser beam energy being directed 

around an aircraft, like water around a stone, neither detecting nor damaging it. Figure 4 depicts the EM spectrum, 

highlighting current high-power microwave (HPM) and laser 

weapons ranges for reference.32 Note that international 

metamaterials research has exploded over both realms.

DOD senior leadership must maximize metamaterial 

technology development for two main reasons: first, to 

maintain airpower relevance in the aforementioned future 
9

The Obstacles Ahead: 

Environmental Scanning

August 2008 was an exciting time for metamaterials research, as scientists at UC Berkeley engineered 3-D materials 

that reversed the natural direction of near-IR light using 

optical magnetism. Previous 3-D materials had reported 

negative refraction but only in the much lower frequency 

range of microwaves. Similarly, earlier optical frequency 

materials had been limited to a 2-D, single monolayer of 

artificial atoms that relied on the physics of resonance to 

achieve negative refraction. This remarkable development, 

the creation of “bulk”33 optical metamaterials with wider 

spectral ranges and minimized energy loss for all incident 

angles, was an important step toward the practical application of optical-cloaking technology.34 To understand where 

research is heading, one must examine the record of metamaterials, especially the rapid burst of accomplishments 

made in nanotechnology over the past decade.

Metamaterial history began in 1967, when Veselago, a 

physicist at Moscow’s Russian Academy of Sciences, ponFigure 4. Electromagnetic Spectrum

counteraccess environments and, second, to stay a step 

ahead of near-peer adversaries. This research will characterize the recent advances in metamaterials and highlight both 

the benefits and challenges to the US Air Force obtaining an 

optical stealth capability. Conclusions drawn from this research will help shape DOD investments in metamaterials.
10

dered whether matter interacts with the magnetic field of 

light. The following year, he published a paper postulating 

the electrodynamics of “left-handed” materials with simultaneously negative dielectric permittivity and magnetic permeability.35 The theory would go largely unnoticed until 

1992, when John Pendry, of Imperial College’s Blackett 

Laboratory in London, while working on radar-absorbing 

materials for the Marconi Company, came across Veselago’s 

work. Seven years later, he was successful in creating 

splint-ring resonators (SRR) that exhibited negative magnetic permeability, magnetized in a direction opposite to 

that of the applied magnetic field, at specific resonance 

frequencies. The following year, his subsequent article in 

Physics Review ignited major research activities worldwide.

In 2001 Rodger Walser, from the University of Texas, 

Austin, coined the term “metamaterial” referring to an artificial composite that achieved performance beyond conventional limitations. Shortly thereafter, researchers at the 

Defense Advanced Research Projects Agency established 

their own metamaterials research program and expanded 

the definition as follows:

Metamaterials are a new class of ordered composites 

that exhibit exceptional properties not readily 

observed in nature. These properties arise from 

qualitatively new response functions that are: (1) 

not observed in the constituent materials and (2) 

result from the inclusion of artificially fabricated, 

extrinsic, low dimensional inhomogeneities.36

Later in 2001, David R. Smith, of Duke University’s Pratt 

School of Engineering, combined an array of SRRs and 

negative electrical permittivity metallic wires, creating a 

wedge-type structure comprised of a double negative composite metamaterial to experimentally demonstrate negative refraction at microwave frequencies.37

By 2003 numerous research groups, including Boeing 

Phantom Works and the Massachusetts Institute of Technology’s Media Laboratory, had confirmed the initial results 

of Smith’s negative refraction.38 In February 2005, Andrea 

Alu and Nader Engheta at the University of Pennsylvania 

announced that plasmons could be used to suppress scat-
11

tering by resonating at specific wavelengths, thus cancelling 

out the energy coming from an object.39 In May 2006,Pendry 

suggested that, theoretically, metamaterials could make an 

invisibility cloak.40 Five months later, Smith demonstrated 

the first working two-dimensional cloak, which deflected microwave beams around a five-inch wide cylinder. The material, comprised of copper rings and wires patterned into 

fiberglass, allowed the concealed volume, plus the cloak, to 

appear as free space when viewed externally.

In January 2007, researchers at the US Department of 

Energy’s Ames Laboratory developed a metamaterial with a 

negative refraction index at the red end of the visible spectrum.41 In April 2007, Purdue University engineers announced a theoretical design for an optical cloak. The following month, California Institute of Technology researchers 

announced obtaining negative refraction in the blue-green 

portion of visible light.42 In August 2008, UC Berkeley researchers created a 10-layer fishnet metamaterial structure 

fabricated on a metal-dielectric stack using focused ion 

beam milling, achieving a negative index of refraction for IR 

wavelengths ranging from 1,475 to 1,775 nanometers (nm).43

This structure is depicted in Figure 5.44

Figure 5. Schematic and scanning electron microscope image of 

fishnet structure

The researchers also developed a material composed of 

parallel silver nanowires embedded inside porous aluminum oxide that achieved negative refraction of red light 

wavelengths as short as 660 nm.45 This was the first dem-
12

onstration of bulk media bending visible light backwards.46

Later that month, Dr. Richard Hammond, a theoretical 

physicist at the US Army’s Research Office, stated that the 

“proof of principle” regarding the unprecedented level of 

control of light had been met: “Similar to general relativity . . . 

the space for light can also be bent in an almost arbitrary 

way.”47

In October 2008, Duke researchers covered a small, onesquare-inch rounded bump with a metamaterial cloak and 

shined it with a 13–16 GHz microwave beam. Normal curved 

material would have scattered the beam, but the metamaterial bump instead reflected the waves back toward the 

source, just as a flat surface would do, successfully hiding 

the object from observation.48 This newest cloak measured 

four inches in thickness and was made up of more than 

10,000 individually arranged pieces. Researchers expect 

the cloak will function equally as well at frequencies as low 

as one GHz and as high as 18 GHz.49

In a Discovery Channel interview corresponding with this 

research being published, Smith stated that an invisibility 

cloak for visible light could be made by June 2009 and that 

it is only “a matter of coupling the right matter to the right 

device.”50 Even with the progress made, further exploration 

of the implementation of metamaterials for DOD applications will require significant improvements in several existing properties. This notion funnels this paper toward its 

second futures research methodology.

Leading Indicators: Advances and Development

Leading indicators rely on specific factors to announce 

the approach of an event. As a futures methodology, it has 

the benefit of correlating results, performance, and proficiency. Specifically, this research focuses on three measurable parameters required to advance metamaterial cloaking 

capability: operating frequency, bandwidth, and energy 

loss.51 The obvious appeal of these leading indicators is that 

they may signal, or predict, when optical stealth will occur 

before it actually does.
13

Metamaterial Frequency Progress

As one should gather from the brief history of metamaterials, 

progress in this field is rapid, and the scaling of structures 

from radio to near IR wavelengths, spanning seven orders 

of magnitude in frequency, occurred in the first seven 

years.52 Figure 6 depicts this growth as a solid line. Also 

shown on the graph, as a dashed line, is the trended expanse 

of metamaterial research over the next eight years.53

Figure 6. Span of metamaterial frequency research

As with any discovery, one would expect a “leveling off” of 

the growth curve until some subsequent breakthrough in 

either theory or technology. If the trend line is accurate, 

negative refraction of violet light is likely to occur in 2010, 

with the same happening for ultraviolet (UV) by 2014.54

Frequency progress alone, however, does not constitute a 

viable optical cloak capability.

Metamaterial Bandwidth Progress

Bandwidth is equally essential for an operationally significant cloak. Invisibility at a single or narrow frequency is 

unlikely to be of much value, unless an adversary has only 
14

a single sensor or weapon. Unfortunately, current metamaterial capabilities are “narrow” in optical frequency 

bandwidth. This is understandable, as the majority of metamaterial research has been directed toward achieving negative 

refraction around a specific frequency and not necessarily 

a broad-spectrum capability. To date, the widest range has 

occurred between 780 and 660 nm, expressed as a bandwidth of 70,000 GHz, which occurred in the red portion of 

light.55 While this is an impressive feat, a broadband optical 

cloak would require a far greater accomplishment.

The challenge with bandwidth is that it has a logarithmic 

relation to frequency. For example, one could blanket all 

radio and television signals with a one GHz bandwidth. By 

comparison, to cover the entire IR spectrum from far to 

near one would require a bandwidth of one million GHz. 

Tackling the visible spectrum would entail an additional 

half million GHz bandwidth. Consequently, enveloping the 

span of the UV spectrum would demand an immense 100 

million GHz bandwidth. An optical cloak covering the nearIR through the near-UV would require a 700,000 GHz 

bandwidth.56 Further complicating this accomplishment, 

the cloak’s bandwidth would need to be instantaneous, 

working on all frequencies equally well, all the time.

Unfortunately, sufficient data does not exist to predict when 

this might occur. As previously mentioned, most research has 

reported optimal negative index frequencies and not the range 

over which they occurred. Assuming a relatively constant research effort and a bandwidth growth of 35,000 GHz a year, it 

will take approximately 17 years to cover the optical spectrum 

and achieve a broadband cloak.57 The data points available for 

analysis are depicted in Figure 7.58

Equally discouraging, it is unlikely that a single material 

will achieve such a broad-spectrum feat any time in the 

near future. Dynamic control over metamaterial properties 

is nontrivial, and the desired manipulation of refraction 

only exists in some finite frequency range, determined by 

the geometry of the nanostructure.59 The answer may exist 

with wavelength tunable structures or “club sandwich” 

stacking of various materials working on different portions 

of the EM spectrum. Another solution for tunability in the 

optical range may involve incorporating electro-optically active materials, such as liquid crystals, into metamaterial 
15

structures.60 In either case, creating tunable structures in 

which field intensity is used for dynamic control of transmission properties will require extensive nonlinear studies. 

Thus, bandwidth may very well present the single greatest 

hurdle for the operational use of optical cloaking.

Metamaterial Energy Loss

Material energy loss is another significant hurdle that 

must be overcome to realize a feasible and significant optical stealth capability. In terahertz (THz) or greater frequencies, this loss can contribute significantly to increased scattering. Furthermore, materials themselves have a tendency 

to absorb portions of EM radiation’s power.61 Obviously, 

this is important because absorption would degrade cloak 

performance. Such shortcomings place heavy constraints on 

the usability of current materials.62 The UC Berkeley fishnet 

structure reflected approximately 35 percent of incident radiation per micrometer.63 By comparison, a standard windowpane reflects only about 4 percent of incident radiation and 

yet is still considered hardly invisible.

Figure 7. Span of metamaterial bandwidth research
16

In fact, most researchers are rather guarded when 

pressed to cite explicit losses for optical metamaterials because such losses are significantly high. Nonetheless, Vlad 

Shalaev, a professor at Purdue University’s Birck Nanotechnology Center, introduced the term “figure of merit” 

(FOM) to describe them. He defines FOM as the magnitude 

of the real part of refractive index divided by the imaginary 

part of refractive index.64 This means the higher the FOM, 

the less attenuation per wavelength. To date, reported figures 

of merit have ranged from 1.3 to 3.5 for IR negative index 

metamaterial. How does this convert into something more 

tangible? The reciprocal of FOM is equal to attenuation in 

units of nepers per radian.65 Further translated into units 

of decibels per wavelength, the attenuation of propagation 

is equal to 54.6637 divided by the FOM.

In other words, an FOM of 3.0 means waves experience a 

loss of about 18 decibels (dB) per wavelength as they travel 

though the metamaterial. By comparison, radio frequency 

(RF) band cloaking materials suffer losses of only 0.5 dB 

per wavelength.66 Thus, with present technology, it is difficult to make an optical metamaterial through which waves 

can travel more than a few microns.

These findings surmise that isotropic 3-D bulk metamaterial 

designs, with low absorption and high transmission qualities, are required for optical cloaking applications. Any 

metal-dielectric structure will suffer from an imperfect 

cloak, however, due to the loss associated with its inclusions. Further research is therefore needed to design metamaterials with lower propagation losses or those that concentrate the electromagnetic fields after refraction has 

taken place.67 Likewise, the development of an active medium, one that “pumps” energy toward the surface of the 

material, is a promising means to compensate for the losses 

observed in metallic structures.68 A likely component of 

such media would be quantum dots or wells.69 This concept appears both reasonable and feasible within a 20-

year horizon.70

Backcasting: What are the Metamaterial Hurdles?

Backcasting represents a strategic approach toward developing technology. This method begins with a successful 
17

outcome, in this case an operationally significant and feasible optical cloak, and then predicts backwards what milestones must occur to make it a reality. The benefits of backcasting are that it does not limit the range of options, stifle 

creativity, or project current shortcomings into the future. 

Backcasting typically involves four main steps. First, define 

the future goal. Second, analyze the technological and 

physical characteristics of a path that would lead toward 

that goal. Third, evaluate the path in terms of physical, 

technological, and socioeconomic feasibility and policy implications. Fourth, brainstorm ways this desired end-point 

can be achieved, working backwards to the present.71 For sake 

of brevity, this paper uses a minimalist approach to backcasting and focuses more heavily on milestone analysis.

Define the Goal

Assuming metamaterials will span the breadth of the optical band by the year 2014 and that both bandwidth and 

energy losses will be tackled during following decade, an 

optical cloaking capability should be available around 2025. 

For metamaterials to be operationally significant and feasible, they must provide a needed utility—in this case, 

cloaking—without being overly detrimental toward form 

factor or aircraft functionality.

Analyze the Technological and Physical Characteristics

Material must be taken from the lab and placed in the 

hands of the engineers who will marry it to an operational 

system. For this to occur, the material must be somewhat 

resilient, and must be able to be cut, bent, molded, or 

shaped. They must withstand heating and cooling, and otherwise fare well within the environment for which they are 

intended. The material need not be easily manufactured, 

but it should be able to be repaired. Weight and thickness 

cannot be prohibitive to flight, nor can the material interfere with aerodynamics and laminar flow.72 The material 

should be strong enough to bear loads and withstand g-forces. 

It should experience graceful versus rapid degradation and 

have a multiuse service life. The material should not reduce 

one signature while raising another. With these factors in 

mind, the resounding consequence is that metamaterials 
18

will need to be comparable in almost every way to contemporary aircraft composites, while at the same time providing the unique EM response function of cloaking.

Evaluate the Path

Socioeconomic feasibility enforces the notion that metamaterials must be resilient and reusable. Furthermore, 

materials should be safe to handle and disposed of easily. 

In other words, highly toxic materials are less likely to be 

pursued due to their environmental-economic impacts.

What about metamaterial research itself? Dispersed, unclassified research conducted in parallel at government 

laboratories and college universities promotes intellectual 

sharing while imbuing a sense of competition and reducing 

costs. At a certain level of technology maturation, however, 

industry must take the baton and, using its larger budgets 

and scales of production, present an operational capability.

It is at this systems level that research is driven toward 

classification, thereby safeguarding cloaking capability and 

technology. Policy implications also raise an interesting 

question. Once a cloaking capability is operational, does 

one announce it, hoping it acts as a deterrent, or keep it a 

secret, waiting for the opportune time to use it? Both cloaking research and application policy are likely to be shaped 

by future events and the global political landscape. Perceived 

need, however, will often trump all other considerations.

Brainstorm to Achieve the Desired End Point

In essence, backcasting asks the question, “What must we 

consider today to achieve a decisive optical stealth capability 

in the future?” As summarized by the numerous concerns 

previously listed, the answer is that one must overcome the 

challenges of metamaterial durability, suitability, and manufacturability. The next section of this paper addresses and 

analyzes these issues.

Tackling Limitations within 

an Operational Context

Because metamaterials research is still in its first decade, all reported findings have taken place in controlled 
19

laboratory settings. Therefore, when discussing limitations, 

one must consider not only the hurdles directly ahead but 

also the ones farther down the track.

As mentioned previously, the conceptual requirements 

for a metamaterial cloaking capability lie ahead. Functional 

requirements lie even farther ahead but must be addressed 

in the present to better focus research dollars and time. Specifically, this paper addresses the challenges of durability 

and suitability. 

Durability

A convenient definition of durability is the quality of a 

material to be useful even after an extended period of use. 

To date, metamaterials are generally characterized as lightweight, thin, fragile, and largely metal. As one moves 

through microwaves and into the IR and visible light, wavelengths become shorter, and metamaterial structure is 

forced to the nanoscale. While RF cloaks are upwards of 

one-tenth of a meter in thickness in the GHz frequency 

bands, cloaking at visible frequencies dictates materials on 

the order of one micron, or one millionth of a meter, in 

thickness.73 By comparison, the average piece of paper is 

about 80 microns thick. This is great news for the engineer 

worried about weight, but bad news for the one worried 

about material ruggedization. The laws of physics and the 

desired range of cloaked wavelengths will ultimately dictate 

metamaterial thickness. Weight and strength, however, may 

be negotiable design characteristics.

In April 2008, researchers at Los Alamos National Laboratory published their findings citing their success with THz 

frequency metamaterials fabricated by stacking multiple layers of thin but flexible substrates.74 For a material to be 

useful to aircraft, it must be able to withstand vibration, 

acceleration, and loading. Furthermore, such material must 

be able to be molded or shaped according to aerodynamic 

or functional need. Thus, flexible or conformable metamaterials are essential to any cloaking application. Fabrication 

techniques using rigid or fragile substrates, such as silicon, 

are clearly unfit.75 Polyimide fillers, on the other hand, have 

been utilized in multilayer far-IR frequency metamaterials 

with promising results.76 In fact, polyimides have been 
20

widely used for photonic and electronic devices for many 

years because of their high electrical and thermal stability. 

Their flexibility, durability, and relatively low refractive index and absorption also make them favorable as a THz 

metamaterial substrate.77 Los Alamos researchers verified 

that a substrate stacking fabrication approach could produce an effective bulk 3-D, durable, and conformable metamaterial. This represents an important step in realizing 

functional optical cloaking devices.

Though further research is needed to create durable optical cloaking metamaterial shells, this eventuality is definitely closer, mostly due to the previously mentioned bulk 

3-D designs developed by the UC Berkeley and Los Alamos 

National Laboratory research teams.78 True durability, 

however, will not be measured in the lab but in realistic 

environmental settings by the maintainers and operators.

Suitability

This paper defines suitability as the level of adaption required to produce a feasible and operationally significant 

metamaterial cloak. Specifically, this research investigates 

the environmental limitations and cloaking shell properties 

of current metamaterials. The US Air Force test and evaluation community acknowledges that suitability has a large 

impact on effectiveness, and thus designates an acquisition 

objective of fielding operationally suitable systems. Furthermore, it dictates that suitability should be assessed from 

early development through fielding.79 Thus, when leaving the 

laboratory and entering the realm of airpower, one has to 

contend with the conditions of moisture and temperature.

Environmental Limitations. Weather is likely to have a 

considerable impact on cloak employment for the foreseeable 

future. Clouds and fog consist of tiny, transparent water droplets suspended in air that scatter light. This is germane to the 

topic at hand because if droplets were allowed to accumulate 

on the surface of a cloak, light would be scattered instead of 

deflected, thereby reducing the cloak’s effectiveness. Furthermore, studies indicate that surface perturbations greater than 

one-tenth of the desired wavelength could be significant to 

scatter incident waves.80 This means that any cloak shell would 

have to be kept immaculately clean or imbued with some self-
21

cleaning or aerosol repelling capability. Though not impossible, 

this does add another element for consideration.

If one cannot repel water vapor or aerosols,81 one must avoid 

them, which dictates flying above the troposphere, a height of 

about 12 kilometers (km) or 39,000 feet. 82 The water vapor 

mixing ratio reaches its minimum at around 16 km or 52,000 

feet. Maximum cloak effectiveness may require the clear sky 

conditions of this medium- to high-altitude regime. Furthermore, a cloak-equipped aircraft would need timely and accurate weather information as well as the ability to adjust its flight 

profiles to avoid cloudy or precipitous conditions. As seen in 

figure 8, an added benefit of an increase in altitude is a

decrease in ambient temperature.83 Why this is both important 

and beneficial is not intuitive.

Figure 8. Mean atmospheric temperature and water vapor mixing ratio

20

15

10

5

0

100

200

300

500

850

-60 -50 -40 -30 -20 -10 0

0.001 0.01 0.1 1 10

10 20

1013

100

Stratosphere

Troposphere

Tropopause

Height above the surface (kilometers)Atmospheric Pressure (millibars)

Water Vapor Mixing Ratio (grams per kilogram)

Temperature (degrees Centigrade)

Temperature

Water Vapor Mixing Ratio
22

Metamaterials are basically nanoscale current loops. In 

other words, they are a series of circuits and therefore susceptible to thermal noise.84 Hence, the optical conductivity of 

metals, and thus cloak efficiency, could increase by a factor 

of six if ambient temperature is decreased from standard 

room down into the cryogenic range85 As seen in the figure 

above, flying at 16 km, or roughly 52,000 feet, one could 

decrease the ambient temperature to around -55 degrees 

Celsius. This is far from the realm of cryogenic temperatures, but it nevertheless has the impact of reducing transmission loss by a factor of four.86

These factors illustrate that initial metamaterial cloaking 

may be relegated to the environmental “sweet spot” between 

45,000 and 65,000 feet, where temperature and water vapor 

are minimized. This does not mean that an optical cloak 

would not work in weather or at the higher temperatures of 

lower altitudes. It simply means that in those environments, 

it might not be perfect and may require further advances in 

technology. This raises the fundamental question: “Just 

how well does an optical cloak really need to work?”

Acceptable Level of Performance. When discussing acceptable levels of imperfection, it is important to acknowledge 

that initial optical cloaks will likely perform at discrete sets 

of frequencies. Even the next iteration may only work 

reasonably well, certainly not perfectly, at a wider frequency 

range.87 However, contrary to popular belief, a cloak does 

not have to be perfect. The Air Force test and evaluation 

community states that for a system to be suitable, it must 

be reliable enough to accomplish the mission.88 Thus, perhaps simply reducing one’s visual signature is a success. 

Imagine optically shrinking a KC-135-sized aircraft down to 

that of an F-16 or smaller!

An imperfect optical cloak may not completely hide an object but instead distort its image or shrink its observed size 

down to that of a spot. The desired result may be obtained by 

simply degrading the enemy’s capability or shaking trust 

and confidence in their systems. In this regard, initial optical 

metamaterials will be much like current-day stealth. Furthermore, cloaks may intentionally be designed with some 

level of imperfection to facilitate the integration of aircraft 

sensors, communication, and navigation systems.89
23

Likewise, unless one is only concerned about cloaking 

gliders or dirigibles, breaks in the shell will be required for 

inlets and exhausts. Interestingly enough, complete cloak 

coverage is, in fact, not necessary. Even a partial cloak can 

still reduce the overall scattering to a wide angle of observation. Similarly, an increase in the physical/electrical size of 

an object does not lead to an increase in the cloak’s overall 

scattering cross-section. It is the percentage, and not the 

actual amount, of surface area covered that determines 

effectiveness. Research indicates that with only 50 percent 

coverage, about a 90 percent reduction in observed spherical 

surface could be accomplished.90

Cloak design is also not restricted to symmetry or shape.91

Thus, a cloak may only need to work well enough to deny the 

enemy the opportunity to establish a targetable first look. 

With this in mind, the linchpin of cloak effectiveness may well 

be determined by the feasibility and cost of manufacturing.

Manufacturability

Optical band metamaterials, as a whole, are challenging 

to make. One must ask, then, “What is the feasibility of 

mass-producing 3-D metamaterials?” Clearly, the challenge 

exists to develop a means to overcome fabrication challenges 

and allow manufacturing on a large scale. To get an idea of 

the scale of this task, in mid-2006 a lab’s typical metamaterial output, in a single batch, might only fill a coffee cup.92

Output rate is largely determined by the complexities of 

material being created. Unfortunately, optical metamaterials 

are extremely intricate. These materials rely on nanoscale 

technology—in essence, designing and building electronics 

in which every atom and chemical bond is specified precisely.93 With tolerances less than 10 nm when approaching 

the UV, fabrication of optical band materials requires strict 

synthesis, patterning, and/or direct assembly.

Nevertheless, some progress has been made in metamaterial manufacturing. Direct laser writing through 

chemical vapor deposition is a promising technique for fabricating large-scale materials.94 Other successful techniques include electron beam nanolithography and monolithically growing materials on a single substrate using 
24

multilayer processing.95 Further studies on advantageous 

bottom-up, “self-assembly” fabrication need to be conducted.96

Contrary to the economics of high technology, however, 

metamaterials need not be expensive or time consuming to 

manufacture. Duke University’s October 2008 microwave 

cloak was cheap and easily producible. Using a transformation optics approach, researchers developed a systematic 

algorithm that “vastly speeds the metamaterial cloak-design 

process and makes the design of complex media possible.”97

The researchers used hobby-level circuit boards at a cost of 

about one dollar per square inch to cloak a bump. At an 

equivalent rate, one could cloak both the top and bottom 

sides of an F-16’s wings for $86,400.98 According to Smith, 

“If you were to commercialize this technology, it would cost 

next to nothing.”99 Even more impressive, it took Smith and 

his colleagues only about nine days to design and implement the experiment.100

With the investigation of challenges complete, one must 

analyze the information at hand to better understand the 

time frames and applications of optical metamaterials.

Analysis

Technology is created from existing knowledge and often 

through measured iterations, with each intermediate step 

conferring an advantage of its own. For these reasons, the 

next two decades are likely to see three distinct generations 

of optical metamaterials.

The next four years, 2010–14, will be categorized as Generation One: specific use and narrow frequency bandwidth, 

on the order of 250,000 GHz. This iteration will be centered 

around the near-IR radiation band and able to cover about a 

quarter of the IR spectrum. Cloaking will be imperfect and 

require maximized flight profiles. Generation One, in point of 

fact, will be more of a shield than a cloak, designed to protect 

an aircraft’s critical systems against a weapon, such as IR 

lasers, or degrade an enemy sensor, such as an IR laser rangefinder. Generation One will also be appliqué in nature, a metamaterial armor with the ability to be married to a wide range 

of systems, though cost and weight may be prohibitive.101

The subsequent five to 15 years, 2015–24, will be categorized as Generation Two: limited use but with a bandwidth 
25

approaching the entire optical frequency range, roughly 

300 to 3,000 THz. This will require several tunable materials 

working in concert. Cloaking will still be imperfect but more 

effective over a wider environmental range. Generation Two 

will be an improved shield, designed to reduce optical signature and provide protection from numerous laser frequencies. Materials will be a mix of appliqué and those imbedded within an aircraft’s skin/structure. “Shield Plus” 

will be found on a wide range of systems, though weight 

may still be prohibitive in some cases.

The last 15- to 25-year period, 2025–34, will be categorized as Generation Three: wide use and broadband frequency range from IR to UV provided by a single, though 

layered, active material. Cloaking will be perfected, thus 

achieving optical stealth as well as DE protection in most 

environments. These materials will be integral to aircraft 

design and function as both skin and structure. Weight is 

no longer prohibitive, but cost may drive production to relatively few numbers of such aircraft.

Implications

Would you tell me, please, which way I ought to 

go from here? That depends a good deal on where 

you want to get to, said the Cat.

—Lewis Carroll

 Alice’s Adventures in Wonderland

Laboratory research has melded physics with materials 

science and designed matter with unique EM characteristics and response functions. Metamaterials exhibit exceptional, unnatural properties; namely, the ability to manipulate EM waves in ways previously only dreamed about. 

Cloaking is a proven phenomenon with broad applications. 

Thus, metamaterials promise significant technological advances in stealth and survivability across the EM spectrum, from radio through UV frequencies. The Air Force 

will require this capability against a counter-LO, DEequipped IADS.
26

The near-term reality is that optical band cloaks/

shields will be designed for specific purposes and possess 

relatively narrow capabilities. Ironically, this may serve 

to defeat many sensors and weapons. Because of atmospheric windows and the availability of suitable components, laser and IR systems are built at specific wavelengths. The following table depicts several such systems 

and their operating parameters.102

Table 1. Sampling of active IR and laser devices

PURPOSE WAVELENGTH 

(nm)

FREQUENCY 

(THz) BAND

Laser Dazzler 532 563.9 Visible 

(Green)

Aiming Laser 

(Sight) 633 473.9 Visible(Red)

Laser Illuminator 830 361.4 Near-IR

Laser Target 

Designator 1,064 282.0 Near-IR

High-energy, 

Chemical Oxygen 

Iodine Laser

 1,316 228.0 Near-IR

Laser Range 

Finder 1,540 194.8 Near-IR

Laser Detection 

and Ranging 10,600 28.3 IR

Infrared Line 

Scan 12,000 25.0 IR

Today, albeit under laboratory conditions, any one of 

these devices could be degraded or defeated by a metamaterial. 

Furthermore, technology is advanced enough that a single 

material is close to being able to work against three out of 

four near-IR devices simultaneously. With further research, 

resources, and time, a single metamaterial cloak may very 

well be able to elude or defeat all of these and countless 

other sensors and weapons.

Adapted from Guercio, Sabatini, and Vignola, “Eye-Safety Analysis;” Crane, “New Laser 

Technologies;” Jane’s Electro-Optic Systems 2008-2009; and http://www.optotronics 

.com/laser-dazzlers.php.
27

Relevance in Low-Technology Scenarios

This paper has addressed the benefits of optical band 

cloaking in a high-tech DE-IADS environment. But what 

about the other end of the conflict spectrum? Does cloaking 

promise any advantages in low-intensity, counterinsurgency 

engagements? The answer is a resounding yes.

A modern military, one heavily dependent on technology, 

is quick to forget that the most prolific battlespace threat, 

and in many ways the toughest to beat, is still the human 

eye. Likewise, the US military has yet to fully recognize the 

increasing threat of aircraft being tracked and engaged by 

electro-optical and IR means; namely, laser-beam riding or 

IR-guided missiles and visually aimed weapons such as antiaircraft artillery, rocket-propelled grenades (RPG), and small 

arms. During operations in both Somalia and more recently 

in Iraq, an alarming number of rotary wing aircraft were 

damaged or downed by RPGs and small arms.

Similarly, both the lethality and proliferation of nonradarguided surface-to-air missiles (SAM) have been on the rise. 

During the course of Operation Desert Storm, approximately 80 percent of US fixed-wing aircraft losses were attributed to IR SAMs. Moreover, they account for a staggering 90 percent of all aircraft lost in combat in the past 15 

years.103 Equally sobering is that IR seekers have everimproving counter-countermeasure capabilities that seriously degrade the effectiveness of flares and jammers. 

Reducing optical signature through metamaterials may 

present the single best means of combating the low end of 

the threat spectrum.104 In fact, metamaterial cloaking in 

the IR and visible bands will allow persistent intelligence, 

surveillance, and reconnaissance (ISR) and rapid attack operations against a wide range of air defense forces.

Disruptive Capability in the Hands of the Enemy

US Joint Forces Command’s Joint Operating Environment 

2008 sets the stage for a grim possibility. “Advances in 

technology will continue at an exponential pace as they 

have over the past several decades. Some pundits have 

voiced worries the United States will lose its lead as the 

global innovator in technology or that an enemy could make 

technological leaps that would give it significant advantages 
28

militarily.”105 With this in mind, this paper would be remiss 

if it did not ask the question, “What could an enemy do with 

this technology?”

Invisibility, much like stealth, has the unique ability to 

convey a sense of invulnerability or invincibility, highly 

desirable for both striking first and creating fear. Imagine 

an enemy that can frustrate our ISR efforts by not just 

camouflaging by cloaking their high-value targets. An adversary that we cannot find, fix, or track presents quite a 

dilemma. Imagine the panic of finding an enemy we cannot engage because it can defeat our laser designators/

range finders.

Recommendations

Is optical invisibility just a gimmick, or does it offer a 

highly desirable and essential warfighting capability? Even 

skeptics are apt to offer a nod of support. This author concedes the point that invisibility is an advantage only when 

your adversary is not expecting you. Furthermore, it is “only 

of fleeting value at best if your adversary knows that you are 

there.”106 Thus, optical stealth does not diminish the value 

of radar or acoustic stealth. As a result, optical cloaking 

may only be seen as a more perfected form of camouflage.

Future aircraft, however, will need special capabilities, beyond current stealth properties, to survive in a DE-IADS environment. Furthermore, the future may not be as far off as one 

would hope. In December 2008, Boeing successfully acquired, 

tracked, and shot down an unmanned aerial vehicle with a 

truck mounted laser.107 Fortunately, metamaterials are a 

promising avenue to shield systems from disruptive radio 

waves, electric and magnetic fields, and laser weapons.108

This paper sought to answer the question, “Will metamaterials facilitate an operationally feasible and significant 

optical stealth capability for the US Air Force?” The answer 

is a qualified yes, assuming the United States establishes 

itself at the forefront of this science and funds and conducts further research that eventually proves its worth during operational testing. As a senior advisor at the National 

Science Foundation so aptly stated, “A keystone of US defense posture includes maintaining a strong science and 

technology R&D [research and development] program in order 
29

to have leading edge technologies available for timely weapons 

development as required.”109

Areas for continued DOD research in the pursuit of metamaterial cloaking technology were highlighted in this paper. 

Specifically, the near-term objective should focus on developing laser protection and reducing the optical signature 

of currently fielded systems.

Metamaterials represent an emerging technology with 

far-reaching and broad potential that may well usher in an 

era of “EM dominance.” Just as Perseus used an enchanted 

helmet to sneak up on and slay Medusa, metamaterials 

represent a practical way to defeat the DE-IADS monster by 

negating its deadly gaze. Based on frequency trends and 

assumptions made about bandwidth and material energy 

loss, this paper predicts a broadband optical cloaking capability, in the laboratory at least, by the year 2025.

1 October 2029 Continued

Having delivered a devastating blow to the enemy’s operations center and DE defense sites, the sole aircraft created 

an opportunity for the lesser capable follow-on forces to mass 

and prosecute the remaining bulk of the attack plan. The 

lethal first strike would wreak havoc on the IADS for several 

hours, a duration estimated to span the entire succinct but 

brutal conflict. The aircraft had only been visible for a brief 

instant, when the weapons bay doors opened to dispatch its 

deadly payload. Aircrew morbidly referred to this as “seeing 

the Cheshire’s smile.”

Safely back over the East China Sea, the optical cloak 

was disengaged. The now visible aircraft gave a slight wing 

roll to correct to course, revealing the subdued roundel of 

the People’s Liberation Army Air Force. The United States 

and its Taiwanese allies had just been introduced to the 

devastating capability of the next revolution in military 

affairs—metamaterial cloaking.110

Conclusion

Metamaterial cloaking represents the pinnacle of electronic protection and the next generation of stealth.111 This 

technology has a broad and demonstrated application of 
30

concept that stretches from radio to UV frequencies. Hence, 

it should be an integral element of future Air Force systems. 

Though it might be two decades before invisibility becomes 

a reality, it would be in the DOD’s best interest to further 

metamaterial research and consider safeguards against 

possible misuse. Cloak-endowed aircraft will possess improved covertness and survivability against optical-based 

defense systems and contribute to a much-needed daylight 

stealth attack capability. Hence, optical cloaking represents 

a clear combat advantage.

Initially equipping small numbers of various weapon systems with metamaterial will represent a significant and viable 

“Silver Bullet Force,” protected against, and capable of engaging and destroying long-range advanced DE-IADS. In doing 

so, the Air Force will be able to penetrate and reign uncontested in the battlespace. Should laser weapons become the 

IADS long-range “shooters” of choice, the Air Force may very 

well find that survival requires a majority of its air- and spacebased assets to possess an optical-cloaking capability.

As with any disruptive technology, the fundamental issue will be how affordably, quickly, and effectively one can 

incorporate metamaterials not only into concepts, doctrine, 

and approach to war but into the actual units and commands that will require those technologies.110 In the future, 

someone will develop a suitable broad-spectrum cloak. 

Merely hoping that it will belong to the United States is not 

a prudent strategy.

Notes

(All notes appear in shortened form. For full details, see the appropriate entry in 

the bibliography.)

1. Once a beautiful maiden, Medusa enraged the goddess Athena, who 

transformed her hair into serpents and made her face so terrible to behold that 

the mere sight of it would turn a man to stone. http://www.greekmythology 

.com/Myths/Creatures/Medusa/medusa.html. 

2. Carroll, Alice’s Adventures in Wonderland.

3. Smith, “What are Electromagnetic Metamaterials?”

4. Liu et al., “Broadband Ground-Plane Cloak,” 366.

5. Joint Publication (JP) 3-13.1, Electronic Warfare, I-8. Directed energy

(DE) is an umbrella term covering technologies that produce a beam of 

concentrated electromagnetic (EM) energy or atomic or subatomic particles. 

A DE weapon is a system using such technology as a direct means to damage or destroy adversary equipment, facilities, and personnel. DE applica-
31

tions easily fit into traditional energy weapons (EW) roles. For example, a 

laser designed to blind or disrupt optical sensors are, in EW terms, electronic attack. A visor or goggle designed to filter out the harmful wavelength 

or laser light is electronic protection.

6. Smith, “Weapons of the Future Are Here.” Isoluminosity is the phenomenon that occurs when different objects displaying the same brightness 

seem indistinguishable from each other. In the 1940s, the US Navy developed operation “Yahootie,” the first practical attempt to create an invisible 

aircraft. A string of bright lights was placed on the wings and propeller hubs 

and crews adjusted the lights to match the natural background light behind 

the aircraft, masking it against the sky.

7. Visual light plus elements of the near-infrared (IR) and near-ultraviolet 

(UV) spectrum, expressed as wavelengths from roughly 1,000 to 100 nanometers.

8. Hebert, “Great Expectations,” 32.

9. DOD, Capstone Concept for Joint Operations, 15 January 2009, iv.

10. National Research Council, Future Air Force Needs for Survivability, 52.

11. DOD, Capstone Concept, 4.

12. Hebert, “Great Expectations,” 33.

13. Bloomfield, How Things Work, 524–25, 527. Index of refraction is 

defined as the ratio of the speed of light in a vacuum to that in a material. It 

also quantifies how strongly a material reflects light on its surface.

14. Hecht, “Photonic Frontiers: Negative-Index-Materials.”

15. Fang to the author, e-mail.

16. Krowne, The “Electronics and Physics.”

17. Shalaev, “Optical negative-index metamaterials,” 41.

18. Schuring et al., “Metamaterial Electromagnetic Cloak,” 977–79.

19. Duke University, “David Smith.”

20. Goshen, “Invisibility cloak moves from fantasy.”

21. Yang, “Invisibility shields one step closer.”

22. Voland, “ ‘Invisible’ materials bend light.”

23. Duke University, “David Smith.”

24. The science behind cloaking can be applied to the entire EM spectrum. However, the scope of this paper is merely to discuss the benefits of 

doing so in what the author refers to as the “optical” bands, roughly 300 to 

3,000 terahertz (THz), which includes near-IR and near-UV radiation.

25. Thill, “Penetrating the Ion Curtain,” 12.

26. Fitzgerald, “China Plans to Control Space;” Pillsbury, An Assessment 

of China’s Anti-Satellite, 12; and Thomas, Cyber Silhouettes, 235.

27. Thill, “Penetrating the Ion Curtain,” ix.

28. Air Force Research Laboratory (AFRL), “Focused Long Term Challenges (FLTC),” 3.5, 3.6, 4.1, and 5.2.

29. Thill, “Penetrating the Ion Curtain,” 11.

30. Hebert, “Great Expectations,” 34.

31. AFRL, “FLTC,” 5, states the capability vision for achieving mission objectives with impunity against full-spectrum threats, from antiaccess integrated 

air defense system to cyber: anticipate threats and avoid through stealth and 

deception, detect and defeat through defenses, survive the attack through 

adaptive and passive protection, and recover from threat effects.
32

32. Wikipedia, “Electromagnetic Spectrum;” high-power microwave and 

laser weapons highlights added by Thill, “Penetrating the Ion Curtain,” 5; and 

metamaterial research material added by author.

33. Material thickness greater than 10 times the size of a wavelength 

of light.

34. Yao et al., “Optical Negative Refraction,” 930.

35. Defense Advanced Research Projects Agency (DARPA), “Negative 

Index Materials.”

36. Smith, “What are Electromagnetic Metamaterials,” 1.

37. Ozbay, “The Magical World of Photonic Metamaterials,” 23.

38. Padilla, Basov, and Smith, “Negative refractive index materials,” 32.

39. Alu and Engheta, “Achieving transparency with plasmonic coatings;” 

and Dawson, “More information on Surface Plasmons.” Quanta of waves are 

produced by collective effects of large numbers of electrons in matter when 

the electrons are disturbed from equilibrium.

40. Sanderson, “Unexpected tricks of light,” 364.

41. Ames Laboratory, “Metamaterials found to work for visible light.”A 

wavelength of 780 nanometers (nm), or expressed as a frequency, 385 THz.

42. Lezec, Dionne, and Atwater, “Negative Refraction at Visible Frequencies,” 430 (wavelengths of 476 to 514 nm expressed as a frequency of 630 

THz); and Yang, “Invisibility shields one step closer,” alternating layers of 

silver and magnesium fluoride.

43. Ozbay, “The Magical World of Photonic Metamaterials,” 25.

44. Yang, “Invisibility shields one step closer.”

45. Expressed as a frequency of 454 THz.

46. Yang, “Invisibility shields one step closer.”

47. Kyzer, “Army research on invisibility,” blogger’s roundtable discussion 

on the development of metamaterials.

48. Liu et al., “Broadband Ground-Plane Cloak,” 366.

49. Ibid., 366, 368.

50. Bland, “Invisibility Cloak Closer than Ever.”

51. DARPA, “Negative Index Materials.”

52. Padilla, Basov, and Smith, “Negative refractive index materials,” 

28–29.

53. In order to predict metamaterial development, the author chose a 

polynomial trendline, as it best relates the process to an engineering and 

manufacturing endeavor. New scientific discovery, however, would likely 

produce logarithmic growth. Omitted from the graph was data lower than 

one GHz frequency. Data points obtained from numerous sources, but 

largely from Padilla, Basov, and Smith, “Negative refractive index materials;” 

and Shalaev, “Optical negative-index metamaterials.”

54. Bohren and Huffman, Absorption and Scattering of Light by Small Particles, 256–57. This prediction was based upon the assumption that any and 

all metamaterial research, such as negative index of refraction, can leverage 

cloaking applicability. In effect, any knowledge is knowledge for the cause. 

Progression below 330 nm wavelengths promises to be difficult as all freeelectron metals exhibit transparency near this plasma frequency. Light in UV 

region is strongly absorbed because the photon energy matches energy differences between filled and empty electron energy levels.

55. University of California, Berkeley, August 2008.
33

56. Wavelengths from 1,000 nm to 330 nm, respectively.

57. Between 2007 and 2008, the bandwidth growth appears to have 

been about 24,000 GHz. The assumption is that at some point in the near 

future, increasing bandwidth will become the aim of research efforts and 

that technology and science will drive added material functionality.

58. Data points from numerous sources, but largely from Padilla, Basov, 

and Smith, “Negative refractive index materials;” and Shalaev, “Optical 

negative-index metamaterials.”

59. Shadrivov and Kivshar, “Nonlinear Effects in Left-Handed Metamaterials,” 331, 332.

60. Shalaev, “Optical negative-index metamaterials,” 46.

61. Cummer et al., “Full-wave simulation of electromagnetic cloaking 

structures,” 4.

62. Fang to the author, e-mail.

63. Valentine et al., “Three-Dimesional optical metamaterials,” 378–79.

64. Cummer to the author, e-mail. The imaginary part of the index controls the exponential field decay rate in relation to distance, while the real part 

of the index controls wavelength. Thus the ratio of the two, expressed as field 

decay per wavelength, is inversely proportional to figure of merit.

65. A neper is a logarithmic unit of ratio, used to express gain, loss, and 

relative values. Like a decibel, it is a logarithmic scale unit, the difference 

being that instead of using base-10 logarithms, the neper uses base e 

(~2.71828). The neper is often used to express ratios of voltage and current 

amplitudes in electrical circuits, whereas the decibel is used to express 

power ratios. http://www.microwaves101.com/encyclopedia/neper.cfm.

66. Cummer to the author, e-mail. This translates into a FOM of 108.

67. Plumridge, Steed, and Philips, “Negative refraction in anisotropic 

waveguides,” 20,548–55.

68. Ozbay, “The Magical World of Photonic Metamaterials,” 27.

69. Paschotta, Encyclopedia of Laser Physics and Technology. A quantum 

well is a thin material structure which confines electrons in the dimension 

perpendicular to the layer surface, whereas the movement in the other dimensions is not restricted. The confinement has profound effects on the density of 

states for the confined particles. Similarly, quantum dots are small structures 

that confine particles in all three dimensions and are considered a kind of artificial atom where the energy levels can be adjusted by design (e.g., by controlling the quantum dot dimensions or the material composition). 

70. Cummer to the author, e-mail.

71. Victoria Department of Sustainability, “Backcasting.”

72. Both considerations are largely dependent on what one wants to cloak 

and at what frequencies. Regarding the optical band, this is unlikely to be a 

cause for concern. Frequency has an inverse relation to material thickness; the 

higher the desired frequency, the thinner the cloak material. As such, optical 

band metamaterials will be no thicker than a piece of paper. Furthermore, 

metamaterials could replace aircraft paint. Thus, the issue of weight is null.

73. Cummer et al., “Full-wave simulations,” 3.

74. Azad et al., “Flexible Quasi-Three-Dimensional Terahertz Metamaterials,” 1.

75. Ibid.
34

76. A polyimide is a polymer of carbon-nitrogen monomers. Known for 

thermal stability, good chemical resistance, and excellent mechanical 

properties, it can be combined with graphite or fiberglass for increased 

flexural strength. Polyimides also exhibit high tensile strength and very low 

tendency to deform permanently during continuous use at temperatures of 

450°F (232°C) and for short excursions as high as 900°F (482°C). Furthermore, polyimides are not affected by commonly used solvents and oils and 

resist weak acids. Such polymers are often used for flexible cables, high 

temperature adhesives, and mechanical stress buffers. From various 

sources, including http://www2.dupont.com/Kapton/en_US/index.html; 

http://www.professionalplastics.com/POLYIMIDE; and http://www.nasa 

.gov/home/hqnews/2008/dec/HQ_08333_Commercial_Invention.html.

77. Azad et al., “Flexible Quasi-Three-Dimensional Terahertz Metamaterials,” 1.

78. Voland, “‘Invisible’ materials bend light.”

79. Hamilton, address.

80. Fang to the author, e-mail; and Cummer et al., “Full-wave simulation,” 4. For sunlight, this means a surface irregularity of greater than 56 nm 

could cause scattering. By comparison, the average thickness of a human 

hair is 90,000 nm.

81. Bohren and Huffman, Absorption and Scattering of Light, 434 and 

436. Atmospheric aerosols refer to the solid and liquid particles in the 

earth’s atmosphere, excluding those of water in clouds, fog, and rain. Sizes 

range from 0.1 to 10 micrometers and include surface dust, sea spray, and 

sulfur compounds. Although very tenuous and highly variable, they act as 

condensation nuclei and alter the optical properties of clouds. Aerosols are 

of military importance, especially in the applications of space-based visible 

and IR surveillance and laser-guided weapons.

82. The troposphere is the lowest portion of Earth’s atmosphere, containing 

approximately 75 percent of its mass and nearly all the water vapor and aerosols. The average height of the troposphere is about six nm in the middle latitudes, deeper at the equator (11 nm), and shallower near the poles (4 nm). 

http://www.metoffice.gov.uk/education/secondary/teachers/

atmosphere.html.

83. American Geophysical Union, “Water Vapor in the Climate System.”

84. Also referred to as Johnson noise, it is the random white noise generated by thermal agitation of electrons in a conductor or electronic device and 

is proportional to the absolute temperature of the conductor. The thermal 

noise level is the limiting minimum noise any circuit can attain at a given 

temperature. http://www.sengpielaudio.com/calculator-noise.htm. This corresponds to 20 and -150 degrees Celsius, respectively.

85. Fang to the author, e-mail.

86. The formula for calculating noise voltage is √4kBT∆ƒR, where kB is 

the Boltzmann constant, T is the absolute temperature in Kelvin, Δf is the 

bandwidth, and R is the resistance of the circuit element.

87. Cummer to the author, e-mail.

88. Hamilton, address.

89. Interestingly enough, cloaking theory suggests that EM energy 

should be able to penetrate cloaks from the inside out while redirecting 

that same energy from coming in. This would suggest that cloaks may 
35

need some blanking capability or that any sensors and communications 

will have to operate out of cloak band.

90. Fang to the author, e-mail.

91. Rahm et al., “Design of Electromagnetic Cloaks,” 91.

92. Markey, “Invisibility Cloak Possible,” 2.

93. Hall, Nanofuture, 21.

94. Ozbay, “The Magical World of Photonic Metamaterials,” 25 and 26.

95. Ibid., 23; and Azad, “Flexible Quasi-Three-Dimensional Terahertz 

Metamaterials,” 1.

96. Park and Kim, “Negative-Index Materials,” 910.

97. Liu et al., “Broadband Ground-Plane Cloak,” 366.

98. An F-16 has a wing area of 300 square feet, or total surface area of 

roughly double that.

99. Smith, quoted in Bland, “Invisibility Cloak Closer than Ever.”

100. Bland, “Invisibility Cloak Closer than Ever.”

101. As stated previously, the thickness of a cloak has no relation to the 

size of the object being hidden. Thickness only depends upon the frequency 

of the EM energy that is to be negated. As such, an optical band cloak would 

be paper-thin for both a Boeing 747 and a hummingbird. The added weight 

might be negligible for the jumbo jet while disastrous for the bird.

102. From various sources, including http://www.optotronics.com/laser

-dazzlers.php; Guercio, Sabatini, and Vignola, “Eye-Safety Analysis,” 1; Crane, 

“New Laser Technologies;” and Jane’s Electro-Optic Systems 2008–2009.

103. http://www.globalsecurity.org/military/systems/aircraft/systems/

ircm.htm; Erwin, “U.S. warplanes vulnerable,” 26; and Puttré, “Facing the 

Shoulder-Fired Threat,” 39.

104. As an aircraft travels through air, the very molecules that comprise the atmosphere “rub” against its skin, generating friction, and ultimately heat and IR radiation. A cloaking shell would also be susceptible 

to this phenomenon. Specifically designed metamaterials, however, could 

redirect or greatly absorb this radiation, to avoid the watchful eyes of 

passive detectors.

105. US Joint Forces Command (USJFCOM), Joint Operating Environment 2008, 37.

106. Smith, “Blueprint for Invisibility,” 6.

107. “The Monitor News,” Journal of Electronic Defense, 24.

108. Cho, “High-Tech Materials,” 1,120.

109. Dr. Mihail C. Roco, senior advisor for nanotechnology, National 

Science Foundation, quoted in Berbube, Nano-Hype, 105.

110. How realistic is this “surprise” ending? Though the probability of 

such an aggressive Chinese action may be low, the technical aspects of such 

a feat is possible based upon two facts. First, China has been on the leading 

edge of metamaterial research. Duke University’s January 2009 cloaking 

research was supported by InnovateHan Technology, the National Science 

Foundation of China, the National Basic Research Program of China, and 

the National Science Foundation of Jiangsu Province, China. Second, if the 

gross domestic product alone directly translates into military power, in the 

2030s China would have the capacity to afford military forces equal or 

superior to current US capabilities. Latter point is from USJFCOM, Joint 

Operating Environment 2008, 28.
36

111. JP 3-13.1, Electronic Warfare, I–4; and USJFCOM, Joint Operating 

Environment 2008, 50. Electronic protection is the subdivision of electronic 

warfare involving actions taken to protect personnel, facilities, and equipment 

from any effects of friendly or enemy use of the electromagnetic spectrum that 

degrades; neutralizes, or destroys friendly combat capability. Examples include spectrum management, electromagnetic hardening, emission control, 

and use of wartime reserve modes.
37

Abbreviations

ABL airborne laser

AFRL Air Force Research Laboratory

COIL chemical oxygen iodine laser

dB decibel

DE directed energy

DEW directed energy weapon

EM electromagnetic

FLTC Focused Long Term Challenges

FOM figure of merit

GHz gigahertz

HPM high power microwave

IADS integrated air defense system

IR infrared radiation

ISR intelligence, surveillance, and

 reconnaissance

km kilometer

LO low observable

nm nanometer

R&D research and development

RF radio frequency

RPG rocket-propelled grenade

SAM surface-to-air missile

SRR splint-ring resonator

THz terahertz

UC University of California

UV ultraviolet

 

39

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