Metamaterial

Negative-index 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 mm × 100 mm × 100 mm (0.39 in × 3.94 in × 3.94 in).^[1][2]

Metamaterials (from the Greek word "meta-", μετά- meaning "beyond") are materials engineered to have properties that have not yet been found in nature.^[3] They are made from assemblies of multiple elements fashioned from composite materials such as metals or plastics. The materials are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence. Metamaterials derive their properties not from the properties of the base materials, but from their newly designed structures. Their precise shape, geometry, size, orientation and arrangement gives them their smart properties capable of manipulating electromagnetic waves: by blocking, absorbing, enhancing, bending waves, to achieve benefits that go beyond what is possible with conventional materials.

Appropriately designed metamaterials can affect waves of electromagnetic radiation or sound in a manner not observed in bulk materials.^[4][5][6] Those that exhibit a negative index of refraction for particular wavelengths have attracted significant research.^[7][8][9] These materials are known as negative-index metamaterials.

Potential applications of metamaterials are diverse and include optical filters, medical devices, remote aerospace applications, sensor detection and infrastructure monitoring, smart solar power management, crowd control, radomes, high-frequency battlefield communication and lenses for high-gain antennas, improving ultrasonic sensors, and even shielding structures from earthquakes.^{[10][11][12][13]} Metamaterials offer the potential to create superlenses. Such a lens could allow imaging below the diffraction limit that is the minimum resolution that can be achieved by a given wavelength. A form of 'invisibility' was demonstrated using gradient-index materials. Acoustic and seismic metamaterials are also research areas.^[10][14]

Metamaterial research is interdisciplinary and involves such fields as electrical engineering, electromagnetics, classical optics, solid state physics, microwave and antennae engineering, optoelectronics, material sciences, nanoscience and semiconductor engineering.^[5]

History

Explorations of artificial materials for manipulating electromagnetic waves began at the end of the 19th century. Some of the earliest structures that may be considered metamaterials were studied by Jagadish Chandra Bose, who in 1898 researched substances with chiral properties. Karl Ferdinand Lindman studied wave interaction with metallic helices as artificial chiral media in the early twentieth century.

Winston E. Kock developed materials that had similar characteristics to metamaterials in the late 1940s. In the 1950s and 1960s, artificial dielectrics were studied for lightweight microwave antennas. Microwave radar absorbers were researched in the 1980s and 1990s as applications for artificial chiral media.^[5]

Negative-index materials were first described theoretically by Victor Veselago in 1967. He proved that such materials could transmit light. He showed that the phase velocity could be made anti-parallel to the direction of Poynting vector. This is contrary to wave propagation in naturally-occurring materials.^[15]

John Pendry was the first to identify a practical way to make a left-handed metamaterial, a material in which the right-hand rule is not followed. Such a material allows an electromagnetic wave to convey energy (have a group velocity) against its phase velocity. Pendry's idea was that metallic wires aligned along the direction of a wave could provide negative permittivity (dielectric function ε < 0). Natural materials (such as ferroelectrics) display negative permittivity; the challenge was achieving negative permeability (µ < 0). In 1999 Pendry demonstrated that a split ring (C shape) with its axis placed along the direction of wave propagation could do so. In the same paper, he showed that a periodic array of wires and rings could give rise to a negative refractive index. Pendry also proposed a related negative-permeability design, the Swiss roll.

In 2000, Smith et al. reported the experimental demonstration of functioning electromagnetic metamaterials by horizontally stacking, periodically, split-ring resonators and thin wire structures. A method was provided in 2002 to realize negative-index metamaterials using artificial lumped-element loaded transmission lines in microstrip technology. In 2003, complex (both real and imaginary parts of) negative refractive index^[16] and imaging by flat lens^[17] using left handed metamaterials were demonstrated. By 2007, experiments that involved negative refractive index had been conducted by many groups.^[4][13] At microwave frequencies, the first, imperfect invisibility cloak was realized in 2006.^{[18][19][20][21][22]}

Electromagnetic metamaterials

An electromagnetic metamaterial affects electromagnetic waves incident on it via structural features that are smaller than the wavelength. To behave as a homogeneous material accurately described by an effective refractive index, its features must be much smaller than the wavelength.

For microwave radiation, the features are on the order of millimeters. Microwave frequency metamaterials are usually constructed as arrays of electrically conductive elements (such as loops of wire) that have suitable inductive and capacitive characteristics. One microwave metamaterial is a suitably scaled split-ring resonator.^[6][7]

Photonic metamaterials, nanometer scale, manipulate light at optical frequencies. To date, subwavelength structures have shown only a few, questionable, results at visible wavelengths.^[6][7] Photonic crystals and frequency-selective surfaces such as diffraction gratings, dielectric mirrors and optical coatings exhibit similarities to subwavelength structured metamaterials. However, these are usually considered distinct from subwavelength structures, as their features are structured for the wavelength at which they function and thus cannot be approximated as a homogeneous material. However, material structures such as photonic crystals are effective in the visible light spectrum. The middle of the visible spectrum has a wavelength of approximately 560 nm (for sunlight). Photonic crystal structures are generally half this size or smaller, that is <280 nm.

Plasmonic metamaterials utilize surface plasmons, which are packets of electrical charge that collectively oscillate at the surfaces of metals at optical frequencies.

Frequency selective surfaces (FSS) can exhibit subwavelength characteristics and are known variously as artificial magnetic conductors (AMC) or High Impedance Surfaces (HIS). FSS display inductive and capacitive characteristics that are directly related to their subwavelength structure.^[23]

Negative refractive index

A comparison of refraction in a left-handed metamaterial to that in a normal material

Almost all materials encountered in optics, such as glass or water, have positive values for both permittivity ε and permeability µ. However, metals such as silver and gold have negative permittivity at shorter wavelengths. A material such as a surface plasmon that has either (but not both) ε or µ negative is often opaque to electromagnetic radiation. However, anisotropic materials with only negative permittivity can produce negative refraction due to chirality.

Although the optical properties of a transparent material are fully specified by the parameters εr and µr, refractive index n is often used in practice, which can be determined from $\scriptstyle n =\pm\sqrt{\epsilon_\mathrm{r}\mu_\mathrm{r}}$. All known non-metamaterial transparent materials possess positive εr and µr. By convention the positive square root is used for n.

However, some engineered metamaterials have εr < 0 and µr < 0. Because the product εrµr is positive, n is real. Under such circumstances, it is necessary to take the negative square root for n.

Video representing negative refraction of light at uniform planar interface.

The foregoing considerations are simplistic for actual materials, which must have complex-valued εr and µr. The real parts of both εr and µr do not have to be negative for a passive material to display negative refraction.^[24][25] Metamaterials with negative n have numerous interesting properties:^[5][26]

• Snell's law (n₁sinθ₁ = n₂sinθ₂), but as n₂ is negative, the rays are refracted on the same side of the normal on entering the material.
• Cherenkov radiation points the other way.
• The time-averaged Poynting vector is antiparallel to phase velocity. However, for waves (energy) to propagate, a –µ must be paired with a –ε in order to satisfy the wave number dependence on the material parameters $k = \omega\sqrt{\mu \epsilon}$.

Negative index of refraction derives mathematically from the vector triplet E, H and k.^[5]

For plane waves propagating in electromagnetic metamaterials, the electric field, magnetic field and wave vector follow a left-hand rule, the reverse of the behavior of conventional optical materials.

Classification

Electromagnetic metamaterials are divided into different classes, as follows:^[4][5][27]

Negative index

In negative-index metamaterials (NIM), both permittivity and permeability are negative, resulting in a negative index of refraction. These are also known as double negative metamaterials or double negative materials (DNG). Other terms for NIMs include "left-handed media", "media with a negative refractive index", and "backward-wave media".^[4]

In optical materials, if both permittivity ε and permeability µ are positive, wave propagation travels in the forward direction. If both ε and µ are negative, a backward wave is produced. If ε and µ have different polarities, waves do not propagate.

Mathematically, quadrant II and quadrant IV have coordinates (0,0) in a coordinate plane where ε is the horizontal axis, and µ is the vertical axis.^[5]

To date, only metamaterials exhibit a negative index of refraction.^[4][26][28]

Single negative

Single negative (SNG) metamaterials have either negative relative permittivity (εr) or negative relative permeability (µr), but not both. They act as metamaterials when combined with a different, complementary SNG, jointly acting as a DNG.

Epsilon negative media (ENG) display a negative εr while µr is positive.^[4][26] Many plasmas exhibit this characteristic. For example, noble metals such as gold or silver are ENG in the infrared and visible spectrums.

Mu-negative media (MNG) display a positive εr and negative µr.^[4][26] Gyrotropic or gyromagnetic materials exhibit this characteristic. A gyrotropic material is one that has been altered by the presence of a quasistatic magnetic field, enabling a magneto-optic effect. A magneto-optic effect is a phenomenon in which an electromagnetic wave propagates through such a medium. In such a material, left- and right-rotating elliptical polarizations can propagate at different speeds. When light is transmitted through a layer of magneto-optic material, the result is called the Faraday effect: the polarization plane can be rotated, forming a Faraday rotator. The results of such a reflection are known as the magneto-optic Kerr effect (not to be confused with the nonlinear Kerr effect). Two gyrotropic materials with reversed rotation directions of the two principal polarizations are called optical isomers.

Joining a slab of ENG material and slab of MNG material resulted in properties such as resonances, anomalous tunneling, transparency and zero reflection. Like negative-index materials, SNGs are innately dispersive, so their εr, µr and refraction index n, are a function of frequency.^[26]

Bandgap

Electromagnetic bandgap metamaterials (EBM) control light propagation. This is accomplished either with photonic crystals (PC) or left-handed materials (LHM). PCs can prohibit light propagation altogether. Both classes can allow light to propagate in specific, designed directions and both can be designed with bandgaps at desired frequencies.^[29][30] The period size of EBGs is an appreciable fraction of the wavelength, creating constructive and destructive interference.

PC are distinguished from sub-wavelength structures, such as tunable metamaterials, because the PC derives its properties from its bandgap characteristics. PCs are sized to match the wavelength of light, versus other metamaterials that expose sub-wavelength structure. Furthermore, PCs function by diffracting light. In contrast, metamaterial does not use diffraction.^[31]

PCs have periodic inclusions that inhibit wave propagation due to the inclusions' destructive interference from scattering. The photonic bandgap property of PCs makes them the electromagnetic analog of electronic semi-conductor crystals.^[32]

EBGs have the goal of creating high quality, low loss, periodic, dielectric structures. An EBG affects photons in the same way semiconductor materials affect electrons. PCs are the perfect bandgap material, because they allow no light propagation.^[33] Each unit of the prescribed periodic structure acts like one atom, albeit of a much larger size.^[4][33]

EBGs are designed to prevent the propagation of an allocated bandwidth of frequencies, for certain arrival angles and polarizations. Various geometries and structures have been proposed to fabricate EBG's special properties. In practice it is impossible to build a flawless EBG device.^[4][5]

EBGs have been manufactured for frequencies ranging from a few gigahertz (GHz) to a few terahertz (THz), radio, microwave and mid-infrared frequency regions. EBG application developments include a transmission line, woodpiles made of square dielectric bars and several different types of low gain antennas.^[4][5]

Double positive medium

Double positive mediums (DPS) do occur in nature, such as naturally occurring dielectrics. Permittivity and magnetic permeability are both positive and wave propagation is in the forward direction. Artificial materials have been fabricated which combine DPS, ENG and MNG properties.^[4]

Bi-isotropic and bianisotropic

Categorizing metamaterials into double or single negative, or double positive, normally assumes that the metamaterial has independent electric and magnetic responses described by ε and µ. However, in many cases, the electric field causes magnetic polarization, while the magnetic field induces electrical polarization, known as magnetoelectric coupling. Such media are denoted as bi-isotropic. Media that exhibit magnetoelectric coupling and that are anisotropic (which is the case for many metamaterial structures^[34]), are referred to as bi-anisotropic.^[35][36]

Four material parameters are intrinsic to magnetoelectric coupling of bi-isotropic media. They are the electric (E) and magnetic (H) field strengths, and electric (D) and magnetic (B) flux densities. These parameters are ε, µ, κ and χ or permittivity, permeability, strength of chirality, and the Tellegen parameter respectively. In this type of media, material parameters do not vary with changes along a rotated coordinate system of measurements. In this sense they are invariant or scalar.^[5]

The intrinsic magnetoelectric parameters, κ and χ, affect the phase of the wave. The effect of the chirality parameter is to split the refractive index. In isotropic media this results in wave propagation only if ε and µ have the same sign. In bi-isotropic media with χ assumed to be zero, and κ a non-zero value, different results appear. Either a backward wave or a forward wave can occur. Alternatively, two forward waves or two backward waves can occur, depending on the strength of the chirality parameter.

In the general case, the constitutive relations for bi-anisotropic materials read $\bold{D} = \epsilon \bold{E} + \xi \bold{H},$ $\bold{B} = \zeta \bold{E} + \mu \bold{H},$ where $\epsilon$ and $\mu$ are the permittivity and the permeability tensors, respectively, whereas $\xi$ and $\zeta$ are the two magneto-electric tensors. If the medium is reciprocal, permittivity and permeability are symmetric tensors, and $\xi=-\zeta^T=-i \kappa^T$, where $\kappa$ is the chiral tensor describing chiral electromagnetic and reciprocal magneto-electric response. The chiral tensor can be expressed as $\kappa=\frac{1}{3}Tr(\kappa) I+N+J$, where $Tr(\kappa)$ is the trace of $\kappa$, I is the identity matrix, N is a symmetric trace-free tensor, and J is an antisymmetric tensor. Such decomposition allows us to classify the reciprocal bianisotropic response and we can identify the following three main classes: (i) chiral media ($Tr(\kappa) \neq 0, N \neq 0, J=0$), (ii) pseudochiral media ($Tr(\kappa) = 0, N \neq 0, J=0$), (iii) omega media ($Tr(\kappa) = 0, N = 0, J \neq 0$). Generally the chiral and/or bianisotropic electromagnetic response is a consequence of 3D geometrical chirality: 3D chiral metamaterials are composed by embedding 3D chiral structures in a host medium and they show chirality-related polarization effects such as optical activity and circular dichroism. The concept of 2D chirality also exists and a planar object is said to be chiral if it cannot be superposed onto its mirror image unless it is lifted from the plane. On the other hand, bianisotropic response can arise from geometrical achiral structures possessing neither 2D nor 3D intrinsic chirality. Plum et al. ^[37] investigated extrinsic chiral metamaterials where the magneto-electric coupling results from the geometric chirality of the whole structure and the effect is driven by the radiation wave vector contributing to the overall chiral asymmetry (extrinsic electromagnetic chiralilty). Rizza et al. ^[38] suggested 1D chiral metamaterials where the effective chiral tensor is not vanishing if the system is geometrically one-dimensional chiral (the mirror image of the entire structure cannot be superposed onto it by using translations without rotations).

Chiral

Chiral metamaterials are constructed from chiral materials in which the effective parameter k is non-zero. This is a potential source of confusion as the metamaterial literature includes two conflicting uses of the terms left- and right-handed. The first refers to one of the two circularly polarized waves that are the propagating modes in chiral media. The second relates to the triplet of electric field, magnetic field and Poynting vector that arise in negative refractive index media, which in most cases are not chiral.

Wave propagation properties in chiral metamaterials demonstrate that negative refraction can be realized in metamaterials with a strong chirality and positive ε and μ.^{[39] [40]} This is because the refractive index has distinct values for left and right, given by

$n = \sqrt{\epsilon_r\mu_r} \pm \kappa$

It can be seen that a negative index will occur for one polarization if κ > √εrµr. In this case, it is not necessary that either or both εr and µr be negative for backward wave propagation.^[5]

FSS based

Frequency selective surface-based metamaterials block signals in one waveband and pass those at another waveband. They have become an alternative to fixed frequency metamaterials. They allow for optional changes of frequencies in a single medium, rather than the restrictive limitations of a fixed frequency response.^[41]

Other types

Elastic

These metamaterials use different parameters to achieve a negative index of refraction in materials that are not electromagnetic. Furthermore, "a new design for elastic metamaterials that can behave either as liquids or solids over a limited frequency range may enable new applications based on the control of acoustic, elastic and seismic waves."^[42] They are also called mechanical metamaterials.

Acoustic

Acoustic metamaterials control, direct and manipulate sound in the form of sonic, infrasonic or ultrasonic waves in gases, liquids and solids. As with electromagnetic waves, sonic waves can exhibit negative refraction.^[14]

Control of sound waves is mostly accomplished through the bulk modulus β, mass density ρ and chirality. The bulk modulus and density are analogs of permittivity and permeability in electromagnetic metamaterials. Related to this is the mechanics of sound wave propagation in a lattice structure. Also materials have mass and intrinsic degrees of stiffness. Together, these form a resonant system and the mechanical (sonic) resonance may be excited by appropriate sonic frequencies (for example audible pulses).

Structural

Structural metamaterials provide properties such as crushability and light weight. Using projection micro-stereolithography, microlattices can be created using forms much like trusses and girders. Materials four orders of magnitude stiffer than conventional aerogel, but with the same density have been created. Such materials can withstand a load of at least 160,000 times their own weight by over-constraining the materials.^[43][44]

A ceramic nanotruss metamaterial can be flattened and revert to its original state.^[45]

Nonlinear

Metamaterials may be fabricated that include some form of nonlinear media, whose properties change with the power of the incident wave. Nonlinear media are essential for nonlinear optics. Most optical materials have a relatively weak response, meaning that their properties change by only a small amount for large changes in the intensity of the electromagnetic field. The local electromagnetic fields of the inclusions in nonlinear metamaterials can be much larger than the average value of the field. Besides, remarkable nonlinear effects have been predicted and observed if the metamaterial effective dielectric permittivity is very small (epsilon-near-zero media).^[46][47][48] In addition, exotic properties such as a negative refractive index, create opportunities to tailor the phase matching conditions that must be satisfied in any nonlinear optical structure.

Frequency bands

Terahertz

Terahertz metamaterials interact at terahertz frequencies, usually defined as 0.1 to 10 THz. Terahertz radiation lies at the far end of the infrared band, just after the end of the microwave band. This corresponds to millimeter and submillimeter wavelengths between the 3 mm (EHF band) and 0.03 mm (long-wavelength edge of far-infrared light).

Photonic

Photonic metamaterial interact with optical frequencies (mid-infrared). The sub-wavelength period distinguishes them from photonic band gap structures.^[49][50]

Tunable

Tunable metamaterials allow arbitrary adjustments to frequency changes in the refractive index. A tunable metamaterial expands beyond the bandwidth limitations in left-handed materials by constructing various types of metamaterials.

Plasmonic

Plasmonic metamaterials exploit surface plasmons, which are produced from the interaction of light with metal-dielectrics. Under specific conditions, the incident light couples with the surface plasmons to create self-sustaining, propagating electromagnetic waves known as surface plasmon polaritons.

Applications

Metamaterials are under consideration for many applications.^[51] Metamaterial antennas are commercially available.

In 2007, one researcher stated that for metamaterial applications to be realized, energy loss must be reduced, materials must be extended into three-dimensional isotropic materials and production techniques must be industrialized.^[52]

Antennas

Metamaterial antennas are a class of antennas that use metamaterials to improve performance.^[13][53][54] Demonstrations showed that metamaterials could enhance an antenna's radiated power.^[13][55] Materials that can attain negative permeability allow for properties such as small antenna size, high directivity and tunable frequency.^[13]

Absorber

A metamaterial absorber manipulates the loss components of metamaterials' permittivity and magnetic permeability, to absorb large amounts of electromagnetic radiation. This is a useful feature for photodetection^[56] and solar photovoltaic applications.^[57] Loss components are also relevant in applications of negative refractive index (photonic metamaterials, antenna systems) or transformation optics (metamaterial cloaking, celestial mechanics), but often are not utilized in these applications.

Skin depth engineering can be used in metamaterial absorbers in photovoltaic applications as well as other optoelectronic devices, where optimizing the device performance demands minimizing resistive losses and power consumption, such as photodetectors, laser diodes, and light emitting diodes.^[58]

Superlens

A superlens is a two or three-dimensional device that uses metamaterials, usually with negative refraction properties, to achieve resolution beyond the diffraction limit (ideally, infinite resolution). Such a behaviour is enabled by the capability of double-negative materials to yield negative phase velocity. The diffraction limit is inherent in conventional optical devices or lenses.^[59][60]

Cloaking devices

Metamaterials are a potential basis for a practical cloaking device. The proof of principle was demonstrated on October 19, 2006. No practical cloak exists.^{[61][62][63][64][65][66]}

Seismic protection

Seismic metamaterials counteract the adverse effects of seismic waves on man-made structures.^[10][67][68]

Sound filtering

Metamaterials textured with nanoscale wrinkles could control sound or light signals, such as changing a material's color or improving ultrasound resolution. Uses include nondestructive material testing, medical diagnostics and sound suppression. The materials can be made through a high-precision, multi-layer deposition process. The thickness of each layer can be controlled within a fraction of a wavelength. The material is then compressed, creating precise wrinkles whose spacing can cause scattering of selected frequencies.^[69][70]

Theoretical models

All materials are made of atoms, which are dipoles. These dipoles modify light velocity by a factor n (the refractive index). In a split ring resonator the ring and wire units act as atomic dipoles: the wire acts as a ferroelectric atom, while the ring acts as an inductor L, while the open section acts as a capacitor C. The ring as a whole acts as an LC circuit. When the electromagnetic field passes through the ring, an induced current is created. The generated field is perpendicular to the light's magnetic field. The magnetic resonance results in a negative permeability; the refraction index is negative as well. (The lens is not truly flat, since the structure's capacitance imposes a slope for the electric induction.)

Several (mathematical) material models frequency response in DNGs. One of these is the Lorentz model, which describes electron motion in terms of a driven-damped, harmonic oscillator. The Debye relaxation model applies when the acceleration component of the Lorentz mathematical model is small compared to the other components of the equation. The Drude model applies when the restoring force component is negligible and the coupling coefficient is generally the plasma frequency. Other component distinctions call for the use of one of these models, depending on its polarity or purpose.^[4]

Three-dimensional composites of metal/non-metallic inclusions periodically/randomly embedded in a low permittivity matrix are usually modeled by analytical methods, including mixing formulas and scattering-matrix based methods. The particle is modeled by either an electric dipole parallel to the electric field or a pair of crossed electric and magnetic dipoles parallel to the electric and magnetic fields, respectively, of the applied wave. These dipoles are the leading terms in the multipole series. They are the only existing ones for a homogeneous sphere, whose polarizability can be easily obtained from the Mie scattering coefficients. In general, this procedure is known as the "point-dipole approximation", which is a good approximation for metamaterials consisting of composites of electrically small spheres. Merits of these methods include low calculation cost and mathematical simplicity.^[71][72]

Institutional networks

MURI

The Multidisciplinary University Research Initiative (MURI) encompasses dozens of Universities and a few government organizations. Participating universities include UC Berkeley, UC Los Angeles, UC San Diego, Massachusetts Institute of Technology, and Imperial College in London, UK. The sponsors are Office of Naval Research and the Defense Advanced Research Project Agency.^[73]

MURI supports research that intersects more than one traditional science and engineering discipline to accelerate both research and translation to applications. As of 2009, 69 academic institutions were expected to participate in 41 research efforts.^[74]

Metamorphose

The Virtual Institute for Artificial Electromagnetic Materials and Metamaterials ”Metamorphose VI AISBL” is an international association to promote artificial electromagnetic materials and metamaterials. It organizes scientific conferences, supports specialized journals, creates and manages research programs, provides training programs (including PhD and training programs for industrial partners); and technology transfer to European Industry.^[75][76]

See also

• Artificial dielectrics—macroscopic analogues of naturally occurring dielectrics that came into use with the radar microwave technologies developed between the 1940s and 1970s.
• METATOY (Metamaterial for rays)—composed of super-wavelength structures, such as small arrays of prisms and lenses and can operate over a broad band of frequencies
• Magnonics
• Metamaterials (journal)
• Metamaterials Handbook
• Metamaterials: Physics and Engineering Explorations

References

1. Shelby, R. A.; Smith D.R.; Shultz S.; Nemat-Nasser S.C. (2001). "Microwave transmission through a two-dimensional, isotropic, left-handed metamaterial" (PDF). Applied Physics Letters 78 (4): 489. Bibcode:2001ApPhL..78..489S. doi:10.1063/1.1343489. Archived from the original (PDF) on June 18, 2010.
2. Smith, D. R.; Padilla, WJ; Vier, DC; Nemat-Nasser, SC; Schultz, S (2000). "Composite Medium with Simultaneously Negative Permeability and Permittivity" (PDF). Physical Review Letters 84 (18): 4184–7. Bibcode:2000PhRvL..84.4184S. doi:10.1103/PhysRevLett.84.4184. PMID 10990641. Archived from the original (PDF) on June 18, 2010.
3. http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1368916&tag=1
4. Engheta, Nader; Richard W. Ziolkowski (June 2006). Metamaterials: Physics and Engineering Explorations. Wiley & Sons. pp. xv, 3–30, 37, 143–150, 215–234, 240–256. ISBN 978-0-471-76102-0.
5. Zouhdi, Saïd; Ari Sihvola; Alexey P. Vinogradov (December 2008). Metamaterials and Plasmonics: Fundamentals, Modelling, Applications. New York: Springer-Verlag. pp. 3–10, Chap. 3, 106. ISBN 978-1-4020-9406-4.
6. Smith, David R. (2006-06-10). "What are Electromagnetic Metamaterials?". Novel Electromagnetic Materials. The research group of D.R. Smith. Archived from the original on July 20, 2009. Retrieved 2009-08-19.
7. Shelby, R. A.; Smith, D. R.; Schultz, S. (2001). "Experimental Verification of a Negative Index of Refraction". Science 292 (5514): 77–79. Bibcode:2001Sci...292...77S. doi:10.1126/science.1058847. PMID 11292865.
8. Pendry, John B. (2004). "Negative Refraction" (PDF). Contemporary Physics (Princeton University Press) 45 (3): 191–202. Bibcode:2004ConPh..45..191P. doi:10.1080/00107510410001667434. ISBN 0-691-12347-0. Retrieved 2009-08-26.
9. Veselago, V. G. (1968). "The electrodynamics of substances with simultaneously negative values of [permittivity] and [permeability]". Soviet Physics Uspekhi 10 (4): 509–514. Bibcode:1968SvPhU..10..509V. doi:10.1070/PU1968v010n04ABEH003699.
10. Brun, M.; S. Guenneau; and A.B. Movchan (2009-02-09). "Achieving control of in-plane elastic waves". Appl. Phys. Lett. 94 (61903): 1–7. arXiv:0812.0912. Bibcode:2009ApPhL..94f1903B. doi:10.1063/1.3068491.
11. Rainsford, Tamath J.; D. Abbott; Abbott, Derek (9 March 2005). Al-Sarawi, Said F, ed. "T-ray sensing applications: review of global developments". Proc. SPIE. Smart Structures, Devices, and Systems II (Conference Location: Sydney, Australia 2004-12-13: The International Society for Optical Engineering). 5649 Smart Structures, Devices, and Systems II (Poster session): 826–838. Bibcode:2005SPIE.5649..826R. doi:10.1117/12.607746.
12. Cotton, Micheal G. (December 2003). "Applied Electromagnetics" (PDF). 2003 Technical Progress Report (NITA – ITS) (Boulder, CO, USA: NITA – Institute for Telecommunication Sciences). Telecommunications Theory (3): 4–5. Retrieved 2009-09-14.
13. Alici, Kamil Boratay; Özbay, Ekmel (2007). "Radiation properties of a split ring resonator and monopole composite". Physica status solidi (b) 244 (4): 1192–1196. Bibcode:2007PSSBR.244.1192A. doi:10.1002/pssb.200674505.
14. Guenneau, S. B.; Movchan, A.; Pétursson, G.; Anantha Ramakrishna, S. (2007). "Acoustic metamaterials for sound focusing and confinement". New Journal of Physics 9 (11): 399. Bibcode:2007NJPh....9..399G. doi:10.1088/1367-2630/9/11/399.
15. Veselago, V. G. (1968) [Russian text 1967]. "The electrodynamics of substances with simultaneously negative values of ε and μ". Sov. Phys. Usp. 10 (4): 509–14. Bibcode:1968SvPhU..10..509V. doi:10.1070/PU1968v010n04ABEH003699.
16. AIP News, Number 628 #1, March 13Physics Today, May 2003, 2003 Press conference at APS March Meeting, Austin, Texas, March 4, 2003, New Scientist, v.177, p.24, 15 March 2003
17. "Imaging by Flat Lens using Negative Refraction", P. V. Parimi, W. T. Lu, P. Vodo, and S. Sridhar, Nature, 426, 404 (2003).
18. Kock, W. E. (1946). "Metal-Lens Antennas". IRE Proc. 34 (11): 828–836. doi:10.1109/JRPROC.1946.232264.
19. Kock, W.E. (1948). "Metallic Delay Lenses". Bell. Sys. Tech. Jour. 27: 58–82. doi:10.1002/j.1538-7305.1948.tb01331.x.
20. Caloz, C.; Chang, C.-C. and Itoh, T. (2001). "Full-wave verification of the fundamental properties of left-handed materials in waveguide configurations" (PDF). J. Appl. Phys. 90 (11): 11. Bibcode:2001JAP....90.5483C. doi:10.1063/1.1408261.
21. Eleftheriades, G.V.; Iyer A.K.; and Kremer, P.C. (2002). "Planar Negative Refractive Index Media Using Periodically L-C Loaded Transmission Lines". IEEE Transactions on Microwave Theory and Techniques 50 (12): 2702–2712. Bibcode:2002ITMTT..50.2702E. doi:10.1109/TMTT.2002.805197.
22. Caloz, C. and Itoh, T. (2002). "Application of the Transmission Line Theory of Left-handed (LH) Materials to the Realization of a Microstrip 'LH line'". IEEE Antennas and Propagation Society International Symposium 2: 412. doi:10.1109/APS.2002.1016111. ISBN 0-7803-7330-8.
23. Sievenpiper, Dan; et al. (November 1999). "High-Impedance Electromagnetic Surfaces with a Forbidden Frequency Band" (PDF). IEEE Transactions on Microwave Theory and Techniques 47 (11): 2059–2074. Bibcode:1999ITMTT..47.2059S. doi:10.1109/22.798001. Archived from the original (Free PDF download. Cited by 1,078) on July 19, 2011. Retrieved 2009-11-11.
24. R. A. Depine and A. Lakhtakia (2004). "A new condition to identify isotropic dielectric-magnetic materials displaying negative phase velocity". Microwave and Optical Technology Letters 41 (4): 315–316. doi:10.1002/mop.20127.
25. A. Voznesenskaya, D. Kabanova,"ANALYSIS OF RAY TRACING THROUGH OPTICAL SYSTEMS WITH METAMATERIAL ELEMENTS", Scientific and Technical Journal of Information Technologies, Mechanics and Optics, Volume 5, Number 12, p. 5, 2012
26. Eleftheriades, George V.; Keith G. Balmain (2005). Negative-refraction metamaterials: fundamental principles and applications. Wiley, John & Sons. p. 340. ISBN 978-0-471-60146-3.
27. Pendry, John B.; David R. Smith (June 2004). "Reversing Light: Negative Refraction" (PDF). Physics Today (American Institute of Physics) 57 (June 37): 2 of 9 (originally page 38 of pp. 37–45). Bibcode:2004PhT....57f..37P. doi:10.1063/1.1784272. Retrieved 2009-09-27.
28. Alù, Andrea and; Nader Engheta (January 2004). "Guided Modes in a Waveguide Filled With a Pair of Single-Negative (SNG), Double-Negative (DNG), and/or Double-Positive (DPS) Layers" (Free PDF download. Cited by 123). IEEE Transactions on Microwave Theory and Techniques 52 (1): 199–210. Bibcode:2004ITMTT..52..199A. doi:10.1109/TMTT.2003.821274. Retrieved 2010-01-03.
29. Engheta, Nader; Richard W. Ziolkowski (June 2006). Metamaterials: physics and engineering explorations (added this reference on 2009-12-14.). Wiley & Sons. pp. 211–221. ISBN 978-0-471-76102-0.
30. Valentine, J.; Zhang, S.; Zentgraf, T.; Ulin-Avila, E.; Genov, D. A.; Bartal, G.; Zhang, X. (2008). "Three-dimensional optical metamaterial with a negative refractive index". Nature 455 (7211): 376–379. doi:10.1038/nature07247. PMID 18690249.
31. Pendry, JB (2009-04-11). "Metamaterials Generate Novel Electromagnetic Properties" (This is a seminar – lecture series lasting one semster – fall 2009.). UC Berkeley Atomic Physics Seminar 290F Main. Retrieved 2009-12-14. External link in |work= (help)
32. Chappell, William leads the IDEA laboratory at Purdue University (2005). "Metamaterials". research in various technologies. Retrieved 2009-11-23.
33. Edited by Soukoulis, C. M. (May 2001). Photonic Crystals and Light Localization in the 21st Century (Proceedings of the NATO Advanced Study Institute on Photonic Crytals and Light Localization, Crete, Greece, June 18–30, 2000 ed.). London: Springer London, Limited. pp. xi. ISBN 0-7923-6948-3.
34. Marques, Ricardo; Medina, Francisco; Rafii-El-Idrissi, Rachid (2002-04-04). "Role of bianisotropy in negative permeability and left-handed metamaterials" (Free PDF download of this article is linked to this reference.). Physical Review B 65 (14): 144440–1. Bibcode:2002PhRvB..65n4440M. doi:10.1103/PhysRevB.65.144440. Retrieved 2009-10-20.
35. Rill, M. S.; et al. (2008-12-22). "Negative-index bianisotropic photonic metamaterial fabricated by direct laser writing and silver shadow evaporation". Optics Letters 34 (1): 19–21. doi:10.1364/OL.34.000019. PMID 19109626.
36. Kriegler, C. E.; et al. (submitted 2009-03-31 to be published 2010-04). "Bianisotropic photonic metamaterials" (download free PDF article.). IEEE journal of selected topics in quantum electronics 999 (2): 1–15. doi:10.1109/JSTQE.2009.2020809. Retrieved 2009-10-20. Check date values in: |date= (help)
37. E. Plum,X.-X. Liu, V. A. Fedotov, Y. Chen, D. P. Tsai, and N. I. Zheludev (2009). "Metamaterials: Optical Activity without Chirality". Phys. Rev. Lett. 102: 113902. doi:10.1103/physrevlett.102.113902.
38. C. Rizza, Andrea Di Falco, Michael Scalora, and Alessandro Ciattoni (2015). "One-Dimensional Chirality: Strong Optical Activity in Epsilon-Near-Zero Metamaterials". Phys. Rev. Lett. 115: 057401. Bibcode:2015PhRvL.115e7401R. doi:10.1103/PhysRevLett.115.057401.
39. Wang, Bingnan; et al. (November 2009). "Chiral metamaterials: simulations and experiments" (PDF). J. Opt. Soc. Am. A 11 (11): 114003 (10pp). Bibcode:2009JOptA..11k4003W. doi:10.1088/1464-4258/11/11/114003. Retrieved 2009-12-24.
40. S. Tretyakov, A. Sihvola and L. Jylhä (2005). "Backward-wave regime and negative refraction in chiral composites". Photonics and Nanostructures Fundamentals and Applications 3 (2–3): 107–115. arXiv:cond-mat/0509287. Bibcode:2005PhNan...3..107T. doi:10.1016/j.photonics.2005.09.008.
41. Capolino, Filippo (October 2009). "32". Theory and Phenomena of Metamaterials. Taylor & Francis. pp. 32–1. ISBN 978-1-4200-5425-5.
42. Page, John (2011). "Metamaterials: Neither solid nor liquid". Nature Materials 10 (8): 565–6. Bibcode:2011NatMa..10..565P. doi:10.1038/nmat3084. PMID 21778996.
43. Szondy, David (June 22, 2014). "New materials developed that are as light as aerogel, yet 10,000 times stronger". Gizmag. Retrieved April 2015.
44. Fang, Nicholas. "Projection Microstereolithography" (PDF). Department of Mechanical Science & Engineering, University of Illinois. Retrieved April 2015.
45. Fesenmaier, Kimm. "Miniature Truss Work". Caltech. Retrieved 23 May 2014.
46. A. Ciattoni, C. Rizza and E. Palange (2010). "Extreme nonlinear electrodynamics in metamaterials with very small linear dielectric permittivity". Phys. Rev. A 81: 043839. doi:10.1103/PhysRevA.81.043839.
47. M. A. Vincenti, D. de Ceglia, A. Ciattoni, and M. Scalora (2011). "Singularity-driven second- and third-harmonic generation at epsilon-near-zero crossing points". Phys. Rev. A 84: 063826. doi:10.1103/PhysRevA.84.063826.
48. A. Capretti, Y. Wang, N. Engheta, and L. Dal Negro (2015). "Enhanced third-harmonic generation in Si-compatible epsilon-near-zero indium tin oxide nanolayers". Opt. Lett. 40: 1500. doi:10.1364/OL.40.001500.
49. Rüdiger Paschotta (2008-18). "Photonic Metamaterials". Encyclopedia of Laser Physics and Technology. I & II. Wiley-VCH Verlag. p. 1. Retrieved 2009-10-01. Check date values in: |date= (help)
50. Capolino, Filippo (October 2009). Applications of Metamaterials. Taylor & Francis, Inc. pp. 29–1, 25–14, 22–1. ISBN 978-1-4200-5423-1. Retrieved 2009-10-01.
51. Oliveri, G.; Werner, D.H.; Massa, A. (2015). "Reconfigurable electromagnetics through metamaterials - A review". Proceedings of the IEEE 103 (7): 1034–1056. doi:10.1109/JPROC.2015.2394292.
52. DOE /Ames Laboratory (2007-01-04). "Metamaterials found to work for visible light" (Costas Soukoulis discusses some metamaterials). HTML. Retrieved 2009-11-07.
53. Enoch, Stefan; Tayeb, GéRard; Sabouroux, Pierre; Guérin, Nicolas; Vincent, Patrick (2002). "A Metamaterial for Directive Emission". Physical Review Letters 89 (21): 213902. Bibcode:2002PhRvL..89u3902E. doi:10.1103/PhysRevLett.89.213902. PMID 12443413.
54. Siddiqui, O.F.; Mo Mojahedi; Eleftheriades, G.V. (2003). "Periodically loaded transmission line with effective negative refractive index and negative group velocity". IEEE Transactions on Antennas and Propagation 51 (10): 2619–2625. Bibcode:2003ITAP...51.2619S. doi:10.1109/TAP.2003.817556.
55. Wu, B.-I.; W. Wang, J. Pacheco, X. Chen, T. Grzegorczyk and J. A. Kong; Pacheco, Joe; Chen, Xudong; Grzegorczyk, Tomasz M.; Kong, Jin Au (2005). "A Study of Using Metamaterials as Antenna Substrate to Enhance Gain" (PDF). Progress in Electromagnetics Research (MIT, Cambridge, MA, USA: EMW Publishing) 51: 295–328. doi:10.2528/PIER04070701. Archived from the original (PDF) on September 6, 2006. Retrieved 2009-09-23.
56. W. Li and J. Valentine, "Metamaterial Perfect Absorber Based Hot Electron Photodetection," Nano Letters, 14(6), 3510-3514. (2014)
57. A. Vora, J. Gwamuri, N. Pala, A. Kulkarni, J.M. Pearce, and D. Ö. Güney, "Exchanging ohmic losses in metamaterial absorbers with useful optical absorption for photovoltaics," Sci. Rep. 4, 4901 (2014). doi:10.1038/srep04901 arxiv preprint
58. Wyatt Adams, Ankit Vora, Jephias Gwamuri, Joshua M. Pearce, Durdu Ö. Guney. Controlling optical absorption in metamaterial absorbers for plasmonic solar cells. Proc. SPIE 9546, Active Photonic Materials VII, 95461M (August 31, 2015); doi:10.1117/12.2190396
59. Pendry, J. B. (2000). "Negative Refraction Makes a Perfect Lens". Physical Review Letters 85 (18): 3966–9. Bibcode:2000PhRvL..85.3966P. doi:10.1103/PhysRevLett.85.3966. PMID 11041972.
60. Fang, N.; Lee, H; Sun, C; Zhang, X (2005). "Sub-Diffraction-Limited Optical Imaging with a Silver Superlens". Science 308 (5721): 534–7. Bibcode:2005Sci...308..534F. doi:10.1126/science.1108759. PMID 15845849.
61. "First Demonstration of a Working Invisibility Cloak". Office of News & Communications Duke University. Archived from the original on July 19, 2009. Retrieved 2009-05-05.
62. Schurig, D.; et al. (2006). "Metamaterial Electromagnetic Cloak at Microwave Frequencies". Science 314 (5801): 977–80. Bibcode:2006Sci...314..977S. doi:10.1126/science.1133628. PMID 17053110.
63. "Experts test cloaking technology". BBC News. 2006-10-19. Retrieved 2008-08-05.
64. "Engineers see progress in creating 'invisibility cloak'".
65. A. Alu and N. Engheta (2005). "Achieving transparency with plasmonic and metamaterial coatings". Phys. Rev. E 72: 016623. arXiv:cond-mat/0502336. Bibcode:2005PhRvE..72a6623A. doi:10.1103/PhysRevE.72.016623.
66. Richard Merritt, "Next Generation Cloaking Device Demonstrated: Metamaterial renders object 'invisible'" Archived February 20, 2009, at the Wayback Machine.
67. Johnson, R. Colin (2009-07-23). "Metamaterial cloak could render buildings 'invisible' to earthquakes". EETimes.com. Retrieved 2009-09-09.
68. Barras, Colin (2009-06-26). "Invisibility cloak could hide buildings from quakes". New Scientist. p. 1. Retrieved 2009-10-20.
69. "Wrinkled metamaterials for controlling light and sound propagation". KurzweilAI. 2014-01-28. Retrieved 2014-04-15.
70. Rudykh, S.; Boyce, M. C. (2014). "Transforming Wave Propagation in Layered Media via Instability-Induced Interfacial Wrinkling". Physical Review Letters 112 (3). Bibcode:2014PhRvL.112c4301R. doi:10.1103/PhysRevLett.112.034301.
71. Shore, R. A.; Yaghjian, A. D. (2007). "Traveling waves on two- and three-dimensional periodic arrays of lossless scatterers". Radio Science 42 (6): RS6S21. Bibcode:2007RaSc...42.6S21S. doi:10.1029/2007RS003647.
72. Li, Y.; Bowler, N. (2012). "Traveling waves on three-dimensional periodic arrays of two different magnetodielectric spheres arbitrarily arranged on a simple tetragonal lattice". IEEE Transactions on Antennas and Propagations 60 (6): 2727–2739. Bibcode:2012ITAP...60.2727L. doi:10.1109/tap.2012.2194637.
73. MURI metamaterials, UC Berkely (2009). "Scalable and Reconfigurable Electromagnetic Metamaterials and Devices" (Link to web site). Retrieved 2009-12-08.
74. U.S. Department of Defense, Office of the Assistant Secretary of Defense (Public Affairs) (2009-05-08). "DoD Awards $260 Million in University Research Funding". DoD. Archived from the original on March 2, 2010. Retrieved 2009-12-08.
75. Tretyakov, Prof. Sergei; President of the Association; Dr. Vladmir Podlozny; Secretary General (2009-12-13). "Metamorphose" (See the "About" section of this web site for information about this organization.). Metamaterials research and development. © 2009 Metamorphose VI. Retrieved 2009-12-13.
76. de Baas, A. F.; J. L. Vallés (2007-02-11). "Success stories in the Materials domain" (PDF). Metamorphose (European Commission Directorate-General for Research. Directorate G – Industrial technologies. Unit G3 ‘Value – added Materials’.). Networks of Excellence Key for the future of EU research: 19. Retrieved 2009-12-13. External link in |publisher= (help)