Negative index metamaterials

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While developing new lenses for next-generation sensors, researchers have crafted a layered material that causes light to refract, or bend, in a manner not occurring naturally

Negative index metamaterials are metamaterials which have the capability to direct and regulate waves due to their negative refractive index.^[1] Metamaterial broadly refers to any synthetic material with unusual refractive properties, in this context. The application of negative index metamaterials involves blending objects with the environment, and this application is being actively developed in the laboratory.^[1][2][3] This on-going research is funded by several US government agencies, including DARPA, the United States Air Force and the United States Navy.^[4] The theory of negative index metamaterials was originally outlined in 1968,^[5] and it was confirmed by experiment in 2001.^[1][2] As a practical demonstration of the associated capabilities, a small object was hidden from view, appearing to be empty space, using negative index metamaterials in 2006.^[6] Furthermore, in the news, as early as May 2006, there was discussion about using negative index materials for military applications, at radar frequencies.^[7] For example, the beam would be directed around the electromagnetically secreted object, returned to their original trajectory, appearing as empty sky.^[6]. Hence, in this instance, this science shares the same technological goal as stealth aircraft, such as the F-117, F-22, and B-2.^[6][7] The need for stealth capabilities has a long history in the development of military aircraft, which began with cold war tensions.^[8][9] However, their "invisibility" to radars is achieved at a high cost via delicate coatings, which are easily damaged and require regular maintenance. ^[8][9] Besides, the stealth technology is bound to certain aircraft shapes and materials.^[10] In contrast, negative index metamaterials would allow manipulation of the electromagnetic spectrum, in order to envelop the object so that it cannot be seen or detected — all that would be seen is its surroundings.^[3][11][12]

One example of today's stealth technology is active camouflage, which is derived from optical camouflage.^[13] It uses a video camera that takes moving frames of the background and projects the image onto a special reflective cloth using external projection, creating the illusion of the object blending with its surroundings.^[13] Arbitrary control of electromagnetic fields using negative index metamaterials does not require the sharp angles built into an aircraft to disperse radar beams,^[10] However, it also does not require a complex set of equipment, such as camera, projector, computer, and reflective cloth.^[13] Currently, "blending in" has been achieved only with small objects, and only in several frequency ranges within the electromagnetic spectrum. ^[1][14] In the future, concealment might be achieved across the entire electromagnetic spectrum, from the lowest radio frequencies, through microwaves, to the visible light. ^[1][6] This technology might not be limited to military applications but has projected applications for everyday life.^[3][15]

[edit] Scientific background

[edit] Controlling electromagnetic fields

A diagram of the electromagnetic spectrum, showing characteristic type, wavelength, frequency and the black body emission temperature of the electromagnetic spectrum.

A hot road mirage, "fake water" on the road, is a common example of how light bends naturally, when there are different refractive indexes caused by layered temperature differences in the air.

Beam of light passing through a medium with changing positive refractive index.

Refractive index is a mathematical term (description) of the effect transparent substances have on light.^[16][17] When discussing or describing refractive index within the sciences of optics and physics, it is represented by, and interchangeable with, the letter n. ^[2][16][17] Most of the time, when light interacts with two substances, such as air and water at once, the refraction and refractive index do not vary^[2][16][17] However, with certain materials, known as dielectrics, the refractive index can be variable. ^[3] When the refractive index varies, the shortest path for light rays are curved lines, rather than straight lines.^[3] This can happen because, according to Fermat's principle, light rays take the shortest optical paths in dielectric media.^[2][3] Dielectrics are poor conductors of electrical current, and lessen the force between two electrical charges. However, the stored energy is increased.^[18] An example of a dielectric is the insulating material between the metallic plates of a capacitor.

A varying refractive index, n , can be demonstrated within the common dielectric medium known as air. ^[16] Optical illusions occur because of this phenomenon, as with a mirage, in which light rays are bent to produce a displaced image of distant objects.^[3][19] Examples are the illusion of water on the road on a hot day, or a distant cool blue lake appearing in a desert.^[19] When a material can be controlled that has the capability to control the intended direction of electromagnetic waves (e.g. light waves),^[6] it is possible to bend rays around a hole, and recombine them in precisely the same direction as they entered the material.^[3] There would be no observable effect of this phenomena. It would appear to an observer as if light was propagating across empty space. An object placed in the hole would not be visible.^[3] The material is a type of optical transmission medium.^[1][20]

• Transmission medium is a material substance (solid, liquid or gas) which can propagate (electromagnetic) waves. For example, electromagnetic radiation can be transmitted through an optical medium, such as optical fiber, or through twisted pair wires, coaxial cable, or dielectric-slab waveguides. In optics, the electromagnetic field is controlled and directed, however design is constrained by the choice of the materials.^[21][2]

New synthetic transmission media, called negative index metamaterials, can have properties that are dependent on their interaction with the electromagnetic field.^[2]. This is because negative index metamaterials can create a negative refractive index, among other possibilities.^[5] Mathematically, the square of the index of refraction, n² = ɛ µ, is given by the product of the electric permittivity^{[note 1]} ɛ and the magnetic permeability^{[note 2]} µ of the medium ^[5]. If both permittivity and permeability are negative, the resulting refractive index is negative as well.^[5][14]

In addition, negative index metamaterials have a design flexibility that allows for control and direction of the permittivity and permeability values in 2-dimensional space, taking positive or negative values as desired.^[2][22]. In this instance, two-dimensional space is referred to because the object is hidden only on the same plane as the line of sight to the hidden object. If the object is viewed from above or below, i.e. the third dimension, then it can be seen. The design flexibility allows scientists to govern ɛ and µ at any desired points within the material, and at any desired scales of gradient.^[22] This had not been possible prior to the fabrication of metamaterials because natural materials most often have uniform electromagnetic fields.^[23] According to researchers in 2006, "The design flexibility of [negative index metamaterials] can be used to achieve new electromagnetic devices, and now [negative index metamaterials] enable a new paradigm for the design of electromagnetic structures." For example, the construction of a device, discussed below, which can hide a small cylinder making its location appear empty. This is accomplished in the microwave (8.5 GHz) range. ^[6] The capability of a negative refraction^[22][24] is an example of a property which is not found in nature, but is developed through the construction of metamaterials.^[17][22] With these new synthetic materials, electric displacement fields, magnetic field intensities, focus of rays, dispersion of frequencies, angles of deflection, and flow around an object by these fields, ending with return to their original angles of trajectory — all these properties can be controlled. In principle, this covers all emanations of electromagnetic fields, at any wavelength. In practice, however, limitations have allowed only parts of the electromagnetic spectrum to be used to conceal an object. ^[6]

[edit] Orthogonal coordinate systems

Orthogonal coordinates — Cartesian plane as it transforms from rectangular to curvilinear coordinates

A thought experiment involving this powerful new capability is to construct a mesh material with dielectric and magnetic properties,^[6] and then embed any quantity of sources into the mesh material. Each source is effectively a point where innumerable choices of values of permittivity ɛ and permeability µ can be assigned, throughout the mesh material. ^[6] With eg. 100 embedded sources, imagine using this mesh material to be able to move a uniform electric field in a chosen direction – left, right, down, up, forward or back, to avoid an object, for example.^[6] Next, imagine this mesh material as flexible so that now the field can be stretched and turned at will. This is the control of an electric field, as it is being moved, stretched or "distorted" at will. ^[6] Imagine that the stretching and turning field is followed and recorded by a detector. The record can now be used to develop mathematical equations which describe, in the mathematical realm, what just happened.^[6] The equations are then used to produce a representative image of what just happened on a grid that is known as a Cartesian plane. The Cartesian plane represents the embedded sources as points. ^[6] However, since the sources exist in three dimensions, three axes must be represented. Hence, each point exists as a set of Cartesian x, y, and z coordinates. ^[6] Assign each point a letter to differentiate it from any other point for visual purposes. Then each point (source) can be represented as unique and existing in three dimensions, represented by three axes. For example beginning with point "a" a mathematical representation would be a(x, y, z) ; followed by b(x, y, z) ; then c(x, y , z).; and so on until all 100 points are defined in this way. As the field is moved it actually curves by some amount and perhaps stretches by some amount. In the realm of mathematics the x, y, and z coordinates of each point dynamically transform. The new coordinates, as they dynamically move, must somehow be distinguished from the original representation that uses x, y, and z. ^[6] Hence x, y, and z become x’, y’, and z’, whose values change each time the field is turned back and forth or stretched up and down. Furthermore, x’, y’, and z’ values change “over time”, and are “continuous”, and represented by Curvilinear coordinates. The transformations are now shown in an Orthogonal coordinate system, x', y', and z'. ^[6] Electric permittivity ɛ, and the magnetic permeability µ scale to agree with the new coordinate system. The formulas are kept simplified, and Maxwell's equations remain consistent.^[6]

Continuing with the thought experiment, which can be verified mathematically, the transformations are used to conceal an object of arbitrary size.^[6] Any observers are kept unaware of the concealed object. The object is hidden from view with use of a negative index metamaterial which deflects the electromagnetic waves that would have struck the object. At almost the same instant, the electromagnetic waves are guided around the object, and returned to their original trajectory. ^[6] In addition, no EM radiation can get into the concealed volume of space, and no EM radiation can emanate out. "Any radiation attempting to penetrate the secure volume [such as radar] is smoothly guided around, by the system, to emerge travelling in the same direction as if it had passed through an empty volume of space. An observer concludes that the secure volume is empty, but we are free to hide an object in the secure space." ^[6] Theoretically, this is the perfect electromagnetic shield because the hidden object can be any size or shape. All the work is done by manipulating electromagnetic fields.^[6]

Annulus diagram

For example, a sphere is chosen as the concealed object with radius r₁. The shielded area r is expressed as the annulus r₁ < r < r₂. By compressing the field, the intended transformations are achieved. Mathematically, it can be shown that this method excludes all fields from the central region, and no fields can emanate from the central region.^[6]

There are issues to be dealt with to achieve electromagnetic shielding. ^[6] One issue, related to ray tracing, is the anisotropic effects of the material on the electromagnetic rays entering the "system". Parallel bundles of rays, headed directly for the center are abruptly curved and, along with neighboring rays, are forced into tighter and tighter arcs. This is due to rapid changes in the now shifting and transforming ɛ' µ'. ^[6] The second issue is that, while it has been discovered that the negative index metamaterials are capable of working within the parameters of the anisotropic effects and the continual shifting of ɛ' and µ', the values for ɛ' and µ' cannot be very large or very small. The third issue is that these negative index metamaterials are currently unable to achieve broadband capabilities.^[6] This is because the rays must curve around the "concealed" sphere, and therefore have longer trajectories than traversing free space. However, the rays must arrive around the other side of the sphere in phase with the beginning radiation. If this is happening then the phase velocity exceeds the velocity of light in a vacuum. (Note, this does not violate the laws of physics). And, with a required absence of frequency dispersion, the group velocity will be identical with phase velocity. In the context of this experiment, group velocity can never exceed the velocity of light, hence the experimental parameters are effective for only one frequency.^[6]

[edit] Dispersion

Color	Wavelength	Frequency
violet	380–450 nm	668–789 THz
blue	450–495 nm	606–668 THz
green	495–570 nm	526–606 THz
yellow	570–590 nm	508–526 THz
orange	590–620 nm	484–508 THz
red	620–750 nm	400–484 THz

A common example of dispersion is the refraction of white light into the visible spectrum. Each visible color has its own frequency, wavelength, and refractive index. The visible spectrum is part of the much broader electromagnetic spectrum, which comprises frequencies of light not visible to the human eye. Dispersion applies to any frequency of the whole electromagnetic spectrum. The table to the right represents the very narrow band of the visible spectrum. On either side of this is infrared (below red) and ultraviolet (above violet). Microwave frequencies (e.g. radar) are below infrared light.

In the sciences of physics and optics refractive index is represented as n and, wavelength is represented by λ. For visible light, most transparent materials (e.g. glasses) have:

A gaseous, liquid, or solid material which disperses electromagnetic waves can exhibit unusual and even exotic behavior.^[25] With substances, in any of the three (classical) states of matter, that have a positive refractive index, this results in a positive phase velocity.^[25] However, group velocity can become negative, and exceed the velocity of light c, in the particular case of anomalous dispersion.^[20] As a result, a burst of a laser's pulse will appear to exit the rear side of the negative index metamaterial before the laser pulse appears to enter the material.^[20][25] However the speed of transmitting information is always limited to c.^[25][26]

[edit] Optical conformal mapping and ray tracing in transformation media

A comparison of refraction in a negative index metamaterial to that in a conventional material having the same, but positive refractive index. The incident beam 8 enters from air and refracts in a normal (8') or metamaterial (-8').

The goal then is to create no discernible difference between a concealed volume of space and the propagation of electromagnetic waves through empty space.^[3] It would appear that achieving a perfectly concealed (100%) hole, where an object could be placed and hidden from view, is not probable. The problem is the following: in order to carry images, light propagates in a continuous range of directions. ^[3] The scattering data of electromagnetic waves, after bouncing off an object or hole, is unique compared to light propagating through empty space, and is therefore easily perceived. ^[3] Light propagating through empty space is consistent only with empty space. This includes microwave frequencies.^[3]

Although mathematical reasoning shows that perfect concealment is not probable because of the wave nature of light, this problem does not apply to electromagnetic rays, i.e., the domain of geometrical optics. Imperfections can be made arbitrarily, and exponentially small for objects that are much larger than the wavelength of light.^[3]

Mathematically, this implies n < 1, because the rays follow the shortest path and hence in theory create a perfect concealment. In practice, a certain amount of acceptable visibility occurs, as noted above. ^[3] The range of the refractive index of the dielectric (optical material) needs to be across a wide spectrum to achieve concealment, with the illusion created by wave propagation across empty space. ^[3] These places where n < 1 would be the shortest path for the ray around the object without phase distortion. Artificial propagation of empty space could be reached in the microwave-to-terahertz range. ^[3] In stealth technology, impedance matching could result in absorption of beamed electromagnetic waves rather than reflection, hence, evasion of detection by radar. ^[3] These general principles can also be applied to sound waves, where the index n describes the ratio of the local phase velocity of the wave to the bulk value. Hence, it would be useful to protect a space from any sound sourced detection. This also implies protection from sonar. ^[3] Furthermore, these general principles are applicable in diverse fields such as electrostatics, fluid mechanics, classical mechanics, and quantum chaos.^[3]

Mathematically, it can be shown that the wave propagation is indistinguishable from empty space where light rays propagate along straight lines. The medium performs an optical conformal mapping to empty space.^[3]

[edit] Experimental verification of a negative index of refraction

Left-handed metamaterial array configuration, which was constructed of copper split-ring resonators and wires mounted on interlocking sheets of fiberglass circuit board. The total array consists of 3 by 20×20 unit cells with overall dimensions of 10×100×100 mm.^[2][27]

In 2000, R. A. Shelby, D. Smith, and S. Schultz constructed a prism composed of metamaterials (negative index metamaterials) to experimentally test for negative refractive index.^[1] Before this experiment, negative refractive index was shown to be possible mathematically.^[15] According to a press release on March 21, 2000, this class of material, negative index metamaterials, a left-handed composite material, had never been produced before the prism experiment.^[15]

Refraction is one of the most elementary electromagnetic phenomena. When a beam strikes the interface between two materials, at an arbitrary angle, another beam is produced, "refracted" at an angle which is dependent on the refractive indices of the two materials.^[1] The phase of the incident beam matches the phase of the transmitted beam.^[1] Therefore, referring to the Snell's law, the transmitted beam and the incident beam are never on the same side of the normal. ^[1] Hence, upon entering a naturally occurring material from air, n > 1.^[1] From the knowledge and experience in optics, developed over centuries, there are now a wide variety of lenses which range over a broad spectrum, from radio to optical.^[28][29]

In 1968, Victor Veselago hypothesized the negative refractive index which resulted in a refracted ray occurring on the same side as the normal, instead of the opposite side of the normal.^[5][30] He was the first to articulate, that when a material has both negative permittivity and permeability, the resulting refractive index is negative as well. He deduced this from the equation n² = ɛ µ. ^[5] From 1968 until 1999 this theory was dormant until John Pendry took a second look,^[30] in 1999, realizing he could construct

Pendry made this proposal after analyzing ferrites, which he deemed to be too heavy for his requirements, and, which may not have desirable mechanical properties, and had only uniform electromagnetic fields throughout the material.^[30]

Veselago first articulated the term Left Handed Substances for which, Shelby et al. use the nomenclature Left Handed Material (LHM). LHMs exhibit electromagnetic properties that are usually opposite to conventional or natural media. This includes negative refractive indices. Veselago showed that a negative refractive index does not violate the laws of physics. This was been echoed by Shelby et al. who state that the exhibited properties of LHMs result in optical and other results that are nonintuitive. In March 2000, they implemented Pendry's proposal with the metamaterial prism to actually demonstrate negative refraction.^[5][23][30]

Copper split-ring resonators and wires mounted on interlocking sheets of fiberglass circuit board. A split-ring resonator consists of an inner square with a split on one side embedded in an outer square with a split on the other side. The split-ring resonators are on the front and right surfaces of the square grid and the single vertical wires are on the back and left surfaces.^[2][27]

For the prism experiment,^[31] a metamaterial was fabricated from a combination of copper rings (split ring resonators) and wires and created a pattern of multiple cells, etched onto a fiberglass circuit boards, in an array of two dimensions. The split ring resonators were used to create negative magnetic permeability and the electric wires were used to create negative electric permittivity. The boards were then cut and assembled. A prism was cut out of this structure to conduct the experiment;^[31] to see if this matched Veselago's conclusions and Pendry's proposal.^[1][2]

The index of refraction was determined by first measuring the deflection of the polarized microwave beam as it passed through the prism, then measuring the propagation as it exited the second surface, (the refraction interface). A microwave waveguide/power meter assembly, which was used as the detector, was rotated in an arc of 1.5° steps, to measure the exit angle. At each step, the detector recorded the transmitted power spectrum over the entire X-band range, from 8 to 12 GHz.^[32][33]

A similarly shaped Teflon sample was used as the control, which refracted the microwaves to positive angles. The microwaves were then refracted to the negative (angle) side of the normal using the metamaterial prism.^[34] The NSF and the Department of Energy financed the development of this new class of left-handed metamaterial. This experiment was conducted at Duke University. This could be applied in the areas of microwave transmissions, antennae design, and optical components.^[15]

[edit] Metamaterial electromagnetic concealment at microwave frequencies

The next step, then, is to actually conceal an object by controlling electromagnetic fields. ^[35] Therefore, the demonstrated and theoretical ability for controlled electromagnetic fields has opened a new field, transformation optics.^[36] The nomenclature is derived, in part, from coordinate transformations used for this technology. This demonstration is based on previous theoretical prescriptions, along with the accomplishment of the prism experiment.^[35] One possible application of transformation optics and materials is electromagnetic cloaking for the purpose of rendering a volume or object undetectable to incident radiation, including radiated probing.^[35]

This demonstration, for the first time, of actually concealing an object with electromagnetic fields, also uses the method of prescribed spatial variation (an effect of embedding electromagnetic sources in the metamaterial).^[22][37]

As discussed earlier, the fields produced by the metamaterial are compressed into a shell (coordinate transformations) surrounding the now concealed volume.^[6][35] Earlier this was supported theory; this experiment demonstrated the effect actually occurs.^[35] Maxwell's equations are scalar when applying transformational coordinates, ^[6] only the permittivity tensor and permeability tensor are affected, which then become spatially variant, and directionally dependent along different axes.^[35] The researchers state:

Before the actual demonstration, the experimental limits of the transformational fields were computationally determined, in addition to simulations, as both were used to determine the effectiveness of the cloak.^[35]

A month prior to this demonstration, the results of an experiment to spatially map the internal and external electromagnetic fields of negative refractive metamaterial was published in September 2006. ^[22] This was innovative because prior to this the microwave fields were measured only externally.^[22] In this September experiment the permittivity and permeability of the microstructures (instead of external macrostructure) of the metamaterial samples were measured, as well as the scattering by the two-dimensional negative index metamaterials.^[22] This gave an average effective refractive index, which results in assuming homogeneous metamaterial.^[22]

Employing this technique for this experiment, spatial mapping of phases and amplitudes of the microwave radiations interacting with metamaterial samples was conducted. The performance of the cloak was confirmed by comparing the measured field maps to simulations.^[35]

For this demonstration, the concealed object was a conducting cylinder at the inner radius of the cloak. As the largest possible object designed for this volume of space, it has the most substantial scattering properties. The conducting cylinder was effectively concealed in two dimensions. ^[35]

[edit] Limitations of current stealth aircraft technology

The F-117A Nighthawk stealth plane

The F-117 Nighthawk was the first operational combat stealth aircraft and was designed to fulfill the need for an "aircraft capable of attacking high value targets without being detected by enemy radar".^[38] It fulfilled expectations throughout its life of service between its first operational flight in 1981, until its retirement in 2008.^[38][39][40] For example, a group of F-117's flew over 1,270 sorties,^[38][40] over four nights in Operation Desert Storm.^[40] These flew low into combat past what was at the time the "world's most densely concentrated network of air defenses", which would direct and deliver thousands of anti-aircraft guns and surface-to-air missiles.^[40] Wing leaders expected losses of 50%. However, engineers gave positive assurances.^[40] On the fourth night, at the conclusion of their part in the campaign, "not one F-117 received battle damage."^[38][40] The F-117 demonstrated the effectiveness of today's stealth technology.^[8][40]

The F-117s effectively neutralized the Iraq Integrated Air Defense system within the first few minutes of the war, and gave control of the skies, to Coalition Forces. However, they were limited to night flying with clear skies.^[41] Several decades from now,^[42] with a fully developed electromagnetic cloaking capability, ^[35] there would be no limitation to night flying. It could not be visually observed during daylight hours because the propagation of controlled electromagnetic waves would make it appear as if no aircraft were present.^[35] Several decades from ^[42] now radar would not be a concern at both night and day hours^[7] because with electromagnetic cloaking capability, the fighter would potentially return no signal to enemy radar ^[7] because the intersecting beam would be shifted around in the direction of the shielding rays.^[3] Furthermore, several decades from now, ^[42] since cloaking would have been achieved through manipulation of the electromagnetic spectrum, development of any type of aircraft desired or required becomes possible. ^{[3][6][7][35]}

When in service, the F-117 had a specialized coating on its outer skin known as radar absorbing material (RAM). This is effective in attenuating any incident radar beam thus avoiding detection. However, an air bubble in the coating is enough for radar detection.^[10] The physical design consisting of unconventional edges and angles also dampens the return signal. The windshield is coated with a special film to hide the pilot's helmet from radar. However, compared to most conventional military aircraft which are constructed to be aerodynamically stable^[10] the F-117's design makes the craft unstable. It is difficult to keep the F-117 straight and level during flight.^[10] A computer is needed to maintain stability.^[10] With an electromagnetic shielding capability, it would be possible to fly either aerodynamically stable or aerodynamically unstable military aircraft, as desired or required, because the cloak is in the control of electromagnetic waves.^[3][6][35]

F-22A Raptor stealth fighter

The F-22 Raptor stealth fighter experiences similar problems with its specialized RAM coating. In 2003 it was observed that the coating process takes 15 days.^[43] Then the skin is meticulously inspected for flaws, which can cause the F-22 Raptor to become visible on radar. ^[43] In addition, any contamination on uncoated surfaces can affect the cohesion of the coat, and it then must be partially or fully reapplied.^[43] In July 2009, it has been reported that, while in service, the F-22 requires more than 30 hours of maintenance for every hour in the skies, and the RAM coating is the principal cause.^[8] Similar to the B-2 stealth bomber, the coating is key to the craft's stealth capability, and, as with the B-2, the coating is vulnerable to rain and abrasions.^[8][9] The ratio of maintenance hours to flight hours means that F-22 flies at a rate of $44,000 per hour.^[8]

Furthermore, each F-22 requires hand-crafted components, which exemplifies a general flaw, or limitation, of the Stealth program.^[8] There are often multiple chains of supply lines for specialized components for the B-2, and F-22 stealth aircraft. For example, "the B-2 (stealth bomber)...used dozens of different materials, each with its own supply chain, and required its own maintenance, and handling training."^[44] Moreover, on the B-2, some of the materials can be restored only slowly, and laboriously.^[44] Low-observable materials have to be removed and replaced at the slightest sign of damage.^[44] These hurdles for the F-22, and B-2 aircraft helps to create large expenditures when compared to conventionally built aircraft. For example the F-16 F-16C/D has a unit cost of US$ 18.8 million (1998 dollars).^[45] This compares to a unit cost of US$ 42.6 M for the F-117, and US$ 137.5 million unit cost for the F-22 Raptor. ^[46] Conventional aircraft have of the availability of interchangeable parts, and do not require special RAM coating.^[8][9][44] With electromagnetic shielding, radar absorption would be in the electromagnetic fields, as the beam is directed around the secreted object, returned to their original trajectory, appearing as empty sky.^[6]. When mature, this cloaking capability could utilize conventionally produced aircraft or other types.^[7] This is because as early as May 2006 there was discussion about military applications, in the news.^[7]John Pendry, a pioneer in this science, stated that this technology could be available to the military within 18 months at radar frequencies.^[7] A Pendry and Smith research paper presents theoretical methods of cloaking for radar. This was posted on Science Express, in May 2006, in advance of print publication in the journal Science.^[7]

[edit] Stealth ship technology limitations

The U.S. Navy's Sea Shadow (IX-529)

A stealth ship is a ship which employs stealth technology construction techniques in an effort to ensure that it is harder to detect by one or more of radar, visual, sonar, and infrared methods. These techniques borrow from stealth aircraft technology, although some aspects such as wake and acoustic signature reduction are unique to stealth ships' design.

First revealed in 1993, the Sea Shadow is built to be undetectable by sonar, radar, and infrared detectors. The shape is so different that as it turns it appears to be a trapezoid, then after turning some more it appears like "a jumbled gemstone". After turning even more it appears to be a "truncated letter." ^[47] Several decades from now,^[42] with a fully developed electromagnetic shielding capability, ^[35] outfitting ships with unconventional designs, such as this would no longer be necessary. It could not be visually observed during daylight hours because the propagation of controlled electromagnetic waves would make it appear as if no ship were present.^[35] Several decades from ^[42] now detection by sonar, radar, or infared would not be a concern at both night and day hours^[7] because with electromagnetic cloaking capability, the ship would potentially return no signal ^[7] because the intersecting beam would be shifted around in the direction of the shielding rays, utilizing conformal mapping to empty space. ^[3] Furthermore, several decades from now, ^[42] since cloaking would have been achieved through manipulation of the electromagnetic spectrum, development of any type of ship of any configuration becomes possible.^{[3][6][7][35]}

[edit] More research into this technology

Other accomplishments related to electromagnetic manipulation followed. In early 2007, a wave expansion method was analyzed which would create a scattering field to ideally conceal an object, or a person, by developing a 2D cylindrically shaped, electromagnetic shielding field. ^[48] Later in 2007, a mathematical improvement in the cylindrical shielding to produce an electromagnetic "wormhole", analyzed in three dimensions.^[49]

[edit] Broadband ground-plane cloak

If a transformation to orthogonal coordinates is applied to Maxwell's equations in order to conceal a perturbation on a flat conducting plane, then an object can be hidden underneath the perturbation.^[50] An automated process, guided by a set of algorithms, was used to construct a metamaterial consisting of thousands of elements. each with its own geometry. "The ground-plane cloak can be realized with the use of nonresonant metamaterial elements, resulting in a structure having a broad operational bandwidth (covering the range of 13 to 16 gigahertz in our experiment) and exhibiting extremely low loss." Developing the algorithm allowed the manufacturing process to be automated, which resulted in fabrication of the metamaterial in nine days. The previous device used in 2006 is rudimentary in comparison, and the manufacturing process required four months in order to create the device.^[51]

[edit] Proposed applications

• This technology can be used in other applications, such as wireless communications, defense and radar. When the technology is fully developed, it may be possible to prevent detection of objects by infrared, radar and radio waves. Structures that would otherwise disrupt wireless signals could route the signals around the structure, improving reception nearby. As the technology progresses, with materials that enable more and more precise control of the electromagnetic field, advanced lenses could be developed for cameras, and unwanted reflections from objects could be reduced. It could improve microwave antennae design, and optical components.^[15][51][52]

• Collaborating researchers from University College London, Helsinki University of Technology, the University of Rochester and the University of Washington stated, in 2009, that:

...some possible applications [with] electromagnetic wormholes include the creation of invisible fiber optic cables, for example for security devices, and scopes for MRI-assisted medical procedures for which metal tools would otherwise interfere with the magnetic resonance images. The invisible optical fibers could even make three-dimensional television screens possible in the distant future. The effectiveness and implementation of [this technology] in practice, however, are dependent on future developments in the design, investigation, and production of metamaterials.^[53]

• "Configurations of geometrical optical designs are now possible that could not be realized by positive index materials."^[1] Any material that exhibits the property of negative refractive index, a property not observed in naturally occurring materials, will have a variety of practical applications, such as beam steerers, modulators, band-pass filters, and lenses permitting subwavelength point source focusing."^[1]

• "Our work has relevance to exotic lens design and to the cloaking of objects from electromagnetic fields."^[6]

• "With (optical conformal mapping) the use of modern metamaterials, practical demonstrations of such devices may be possible. The method developed here can also be applied to escape detection by other electromagnetic waves or sound."I^[3] In addition, "With these artificial dielectrics, invisibility could be reached for frequencies in the microwave-to-terahertz range. In contrast, stealth technology is designed to make objects of military interest as black as possible to radar where, using impedance matching , electromagnetic waves are absorbed without reflection, i.e., without any echo detectable by radar. Recently, nanofabricated metamaterials with custom-made plasmon resonances have been demonstrated that operate in the visible range of the spectrum and may be modified to reach invisibility."^[3]

• "Acoustic 'superlens' could mean finer ultrasound scans - Negative refraction — Devices based on such acoustic metamaterials could produce ultra-sharp medical scans, more detailed seismic maps, and even earthquake-resistant buildings. Conventional lenses cannot focus on anything smaller than roughly half the wavelength of light or sound they use. That is because they cannot recover and focus weak scattered waves that are needed to "see" the really small features. Thanks to their odd refractive properties, though, metamaterial superlenses can — so the acoustic superlens designed by Guennea and colleagues could focus on details that are invisible to today's equipment. Such a superlens could be used at a variety of scales. "You can build one to see a foetus," says Guenneau, "or something underground". Imaging underground features requires long-wavelength sound that consequently gives poor resolution, an acoustic superlens could help.^[54]

• "Earthquake protection — Guenneau's group also showed that a checkerboard array of their halved ring-shapes can stop sound waves passing. This pattern could be used to damp noise or vibrations on a wide range of scales, from tiny electronic or mechanical components to large buildings protected from the earthquake shear waves using columns with the metamaterial incorporated within. Although the work is so far mathematical, the team is confident its acoustic metamaterial can be built. The model is based on the known properties of silica and the voids cut into it are accompanied by stiff bars with properties similar to carbon nanotubes."^[54]

• September 26th, 2008 - A team of physicists has shown that its possible to make a type of dam that acts as an invisibility cloak which hides off-shore platforms from water waves and tsunamis. The collaboration of physicists is from the Centre National de la Recherche Scientifique (CNRS) and Aix-Marseille Universite in France and the University of Liverpool in England. They have conducted laboratory experiments showing that it’’s possible to make type of dike that acts as an invisibility cloak that hides off-shore platforms from water waves. Laboratory experiments show that obstacles arranged in fluids in certain patterns can effectively make objects they surround invisible to waves. Tsunami invisibility cloaks wouldn't make structures disappear from sight, but they could manipulate ocean waves in ways that makes off-shore platforms, and possibly even coastlines and small islands, effectively invisible to tsunamis.^[55]

• January 10, 2008- "Duke University engineers will reveal Friday (Jan. 11) details of an acoustic cloak fabricated from metamaterials that they claim can render objects invisible to sonar. If this works then we will have unprecedented control to hide from the effects or to enhance the effects of sound and other waves in all kinds of material. Submarines invisible to sonar would extend the security of nuclear weapons deployed on submarines against future sensing technology. Even if one side had nanotechnology it would take a lot to find invisible to sonar stealth submarines that were carrying nuclear weapons. Extending the deterrent of nuclear weapons makes for a more militarily stable future world. Further, the engineers claim that the technique proves that waves can be redirected around objects in different media, opening up the possibility of improving the acoustics in concert halls by cloaking structural beams from sound waves in air. It may even be possible to redirect seismic waves around buildings [making them earthquake resistant], or ocean waves around ships."^[56]

• "July 7, 2009 - Earthquake cloak: Adapting Optical Invisibility Techniques for Earthquake Shockwave Resistant Buildings Correcting article: There are several papers on cloaking buildings from earthquake waves. The new theoretical cloak comprises a number of large, concentric rings made of plastic fixed to the Earth's surface. The stiffness and elasticity of the rings must be precisely controlled to ensure that any surface waves pass smoothly into the material, rather than reflecting or scattering at the material's surface." "When waves travel through the cloak they are compressed into tiny fluctuations in pressure and density that travel along the fastest path available. By tuning the cloak's properties, that path can be made to be an arc that directs surface waves away from an area inside the cloak. When the waves exit the cloak, they return to their previous, larger size." Unlike some of the optical invisibility cloaks that have been studied in physics labs in recent years, the new cloak is "broadband", meaning that it can divert waves across a range of frequencies."^[57]

[edit] Institutional research

The research in this field has diffused out into the American government science research departments, including the US Naval Air Systems Command, US Air Force, and US Army. Many scientific institutions are involved including:

Funding for research into this technology is provided by the following American agencies:^[4]

• Air Force Research Laboratory
• Defense Advanced Research Projects Agency
• Director of Central Intelligence
• National Geospatial-Intelligence Agency
• Naval Air Systems Command
• Office of Naval Research

Through this research, it has been realized that developing a method for controlling electromagnetic fields can be applied to escape detection by radiated probing, or sonar technology, and to improve communications in the microwave range; that this method is relevant to superlens design and to the cloaking of objects within and from electromagnetic fields.^[3]

[edit] In the news

On October 20, 2006, the day after Duke University achieved enveloping and "disappearing" an object in the microwave range, the story was reported by Associated Press.^[59] Media outlets covering the story included USA Today, MSNBC's Countdown With Keith Olbermann: Sight Unseen, The New York Times with Cloaking Copper, Scientists Take Step Toward Invisibility, (London) The Times with Don't Look Now—Visible Gains in the Quest for Invisibility, Christian Science Monitor with Disappear Into Thin Air? Scientists Take Step Toward Invisibility, Australian Broadcasting, Reuters with Invisibility Cloak a Step Closer, and the (Raleigh) News & Observer with 'Invisibility Cloak a Step Closer.^[59]

On November 6, 2006, the Duke University research and development team was selected as part of the Scientific American best 50 articles of 2006.^[60]

[edit] See also

• Acoustic metamaterials
• Metamaterial
• Metamaterial antennas
• Perfect lens
• Photonic metamaterials
• Photonic crystal
• Terahertz metamaterials
• Tunable metamaterials

[edit] EM interactions

• Bloch wave
• EM radiation
• Electron mobility
• Permeability (electromagnetism)*
• Permittivity*
• Permittivity of free space
• Wavenumber
• Photo-dember
• Impedance

[edit] Notes

[edit] References

[edit] External links

• Manipulating the Near Field with Metamaterials Slide show, with audio available, by Dr. John Pendry, Imperial College, London
• F-117 Nighthawk Stealth Strike Aircraft Elegant in flight