The Next Wave | Vol

Vol. 20 | No. 3 | 2014 Forecasting faster, more powerful, and more secure technology At a Glance | Pointers | Spinouts This Z antenna tested at the National Institute of Standards and Technology is smaller than a standard antenna with comparable properties. Its high eciency is derived from the “Z element” inside the square that acts as a metamaterial, greatly boosting the signal sent over the air. The square is 30 millimeters on a side. Innovation in materials science: Electromagnetic metamaterials Jane E. Heyes | Nathaniel K. Grady | Diego A. R. Dalvit | Antoinette J. Taylor aterial properties aect the propagation found in nature, metamaterials have the potential of electromagnetic (EM) waves in to aid in the creation of ultrathin planar lenses, Mprofound ways, which has allowed for superresolution microscopes, compact antennas, devices ranging from eyeglasses to radar to ber- faster computer chips, and surfaces that radically optic cables. However, there are a limited number alter or cloak the EM signature of an object (e.g., an of responses found in natural materials. How can invisibility cloak). the range of possibilities be expanded? Enter EM e limitations of natural materials are a major metamaterials. obstacle that must be overcome to meet the ever- EM metamaterials are composites built increasing demand for faster, lighter, cheaper, and from a structured combination of conductors, more compact devices, making metamaterials semiconductors, and insulators. e individual an important and timely tool for meeting future features make up an ordered array smaller than technology needs. the wavelengths of radiation they are designed to aect, so the EM wave responds to the overall Background combination of these individual structures as if it were an eectively homogeneous material. Two fundamental EM properties of matter are the By providing eective material properties not electric permittivity (ε) and permeability (μ). In The Next Wave | Vol. 20 No. 3 | 2014 191 Innovation in materials science: Electromagnetic metamaterials all naturally occurring materials at all EM wave- the microwave regime [3]. ey demonstrated that lengths, these two values are never simultaneously EM waves (e.g., light) are able to propagate in such negative. Knowing the values of these two param- composite metamaterials with simultaneously nega- eters, it is possible to calculate a number of dierent tive eective values of the constitutive parameters ε properties of the material, including the speed of and μ—that is, with a negative index of refraction. propagation, propensity to absorb energy, ability to ese are the kind of hypothetical “substances” reect, and possibly the eects on polarization. that Veselago had speculated about in the past. e speed of propagation, inversely proportional In his paper, Veselago predicted several funda- to the refractive index, describes how a wave’s path mental phenomena occurring in or in association will change when it moves from one medium to with such substances, including the characteristic another at an oblique angle. When a wave travels frequency dispersion, negative index of refraction, from a lower index medium into a higher one, it reversal of Snell’s law, focusing with a at slab, and bends closer to the line normal to the interface reversal of Doppler eect and Cherenkov radia- between the two media, with the inverse true for a tion—all of which have now been experimentally wave moving from a higher index medium into a observed using metamaterials. lower one. us, the study of metamaterials began with the Over 40 years ago, Victor Veselago predicted exploration of materials with a negative refractive that a negative index of refraction would result in index. However, the bulk of research has diverged light bending in the opposite direction from what into dierent specialties, and now many dierent is expected [1], but no natural materials have a kinds of devices are studied over many decades of negative refractive index. In 1999, John Pendry, the EM spectrum. e expanded breadth of re- a pioneer in metamaterials, worked on reducing search has yielded discoveries of new phenomena the electrical plasma frequency in metal wires and including seminal proof-of-concept demonstra- created an articial magnetic response via metal- tions of superresolution in optical imaging, perfect lic split-ring resonators (SRRs), illustrated in gure metamaterial absorbers, EM invisibility or cloaking, 1, a key theoretical step in creating a negative and transformation optics. Figure 2 shows examples refractive index [2]. of metamaterials [4]. David Smith and colleagues were the rst to demonstrate composite metamaterials, using a combination of plasmonic-type metal wires and an SRR array to create a negative ε and negative μ in FIGURE 2. These example metamaterials are composites built from a structured combination of conductors, semiconductors, and insulators. Image adapted by permission from Macmillan Publishers Ltd: Nature Photonics, Soukoulis CM, Wegener M, “Past achievements and future challenges in the development of three-dimensional photonic metamaterials,” FIGURE 1. A schematic illustration of a split-ring resonator. doi: 10.1038/nphoton.2011.154, 2011 [4]. The Next Wave | Vol. 20 No. 3 | 2014 202 Innovation in materials science: Electromagnetic metamaterials While the initial work primarily focused on three-dimensional metamaterials, recent eorts (a) x(E) (b) focusing on explicitly considering two-dimensional z(k) t exp( y(H) 12 ie planar metamaterials (i.e., metasurfaces) and con- 12) w i ) exp(i r exp( ĳ12 `) ceptually focusing on modifying boundary condi- 12 r = -1 r 23 ) 21exp( tions at interfaces have proven extremely fruitful. l substrate exp(ie21 iĳ ) ground plane t21 21 For example, the generalized laws of refraction and d spacer the interference theory of perfect absorbers/anti- p air reection coatings, both derived from investigat- ing how the boundary conditions at the interface (c) 1.0 between two materials can be radically altered by spacer thickness ( m): metasurfaces, have led to rapid advances in the 0.8 ȝ 4 area of planar optics. Despite being a more recent 7 innovation, metasurfaces are likely to rapidly reach 0.6 10 13 signicant commercial relevance due to their being 16 readily fabricated using widespread conventional 0.4 lithography techniques and being easier to integrate Absorptance directly onto existing detectors or sources. 0.2 Metamaterial absorbers, emitters, 0.0 0.6 0.8 1.0 1.2 1.4 and antireection coatings Frequency (THz) In 2007, researchers developed a metamaterial FIGURE 3. (a) This illustrated metamaterial perfect absorber capable of absorbing all of the light that strikes consists of a metal cross-resonator array, dielectric spacer, it—a perfect absorber—representing one of the metal ground plane, and substrate. (b) This diagram shows most important applications of metamaterials [5]. the interference model of metamaterial perfect absorp- (c) As a function of frequency ω, a material’s eec- tion. This graph shows absorptance in the decoupled metamaterial absorber using the interference model for 1/2 tive impedance, dened as Z(ω) = [μ(ω)/ε(ω)] various spacer thicknesses. The inset graph is a simulation changes. At a particular frequency, the impedance of absorptance when treating the whole metamaterial absorber as a coupled system. Image adapted by permission matches the free-space impedance (Z0), and there- fore reection is minimized. In metamaterials with from The Optical Society: Optics Express, Chen H, “Interfer- ence theory of metamaterial perfect absorbers,” doi: 10.1364/ simultaneous electrical and magnetic resonances, OE.20.007165, 2012 [6]. both the eective permittivity ε(ω) and permeability μ(ω), are highly frequency dependent and can be tailored independently, making it much easier to refraction into the realm of practical devices, as achieve a high-reection state. If the metamaterial discussed in more detail below. also achieves high loss, resulting in low transmis- Metamaterial perfect absorbers typically consist sion, then near-unity absorption can occur. of a subwavelength resonator array backed with a Additional eorts in understanding the underly- metal ground plane and are separated with a dielec- ing physics responsible for the impedance match- tric spacer, as illustrated in gure 3(a). Compared ing and perfect absorption are also under way. to conventional absorption screens, the overall Researchers at Los Alamos National Laboratory thickness of a metamaterial absorber is much small- (LANL) recently composed an interference theory er than the operation wavelength. [6] (see gure 3c) and explained the observed Currently, work in this eld is focused on perfect absorption and antiparallel surface currents creating multiband and broadband metamaterial in two metallic layers. is theoretical advance absorbers. ese typically employ multilayered also led to the development of highly ecient metamaterials or unit cells containing structures ultrathin planar polarization rotators and brought resonating at dierent frequencies. the eciency of structures exhibiting generalized The Next Wave | Vol. 20 No. 3 | 2014 213 Innovation in materials science: Electromagnetic metamaterials Metamaterial-based absorbers are expected to to code information into the polarization state, increase energy conversion eciency in photovolta- actively controllable wave plates are highly desired ics and solar-thermal energy harvesting systems. to modulate the beam polarization. e design of nanostructured

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