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

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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].

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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 investigating 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 therefore 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

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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 “black” superab- However, there are still many challenges in the sorbers from materials comprised only of lossless development of polarimetric devices. Birefringent dielectric materials and highly reective noble crystals work well for short wavelengths in the vis- metals represents a new research direction. For ible and near-infrared regimes but become bulkier example, Harry Atwater at the California Institute and costlier to fabricate or have other undesirable of Technology is currently investigating metal–in- properties, such as being so or hydroscopic, for sulator–metal stack-based metamaterial absorbers longer wavelengths. ey also suer from a narrow with the intention of increasing the eciency of operating bandwidth, a problem sometimes amelio- photovoltaic or thermovoltaic cells [7]. rated with complex fabrication. Polymers are also Regarding the terahertz frequency range, there widely used in polarimetric applications; however, are also eorts to produce narrowband terahertz they are not suitable for longer wavelengths due to sources with relative high output power [8]. Liu high absorption. et al. found that the emissivity (i.e., the ability of Scientists in Antoinette Taylor’s group at LANL a material’s surface to emit heat as radiation) and carried out research on improving the eciency absorptivity of a surface follows Kirchho’s law of and bandwidth of linear polarization converters thermal radiation of blackbody [9]. ese results [11]. ey have recently demonstrated metama- may have a great eect on controlling thermal terial polarization converters that are capable of signatures emitted from an object. For example, the rotating the linear polarization to its orthogonal outer surface of a hot object can be coated with a direction over a very broad bandwidth with high designed metamaterial to control the emissivity in eciency. a narrow spectral frequency which will deviate the natural thermal blackbody spectrum. is concept For a metamaterial linear polarization converter also has the potential to enable the creation of high- operating in reection, their experimental results eciency incandescent light sources [10] and play a have shown that over the frequency range from key role in thermophotovoltaics. 0.65 to 1.87 terahertz (THz) the cross-polarized reection carries more than 50% of the incident Polarization control EM power and the copolarized reection power is less than 14%. Between 0.73 and 1.8 THz, the e polarization state is one of the basic properties cross-polarized reected power is higher than 80%, of EM waves conveying valuable information that is and at frequencies near 0.76 THz and 1.36 THz, the important in transmitting signals and making sen- copolarized reected power is less than 1%. Simi- sitive measurements. In fact, EM polarization has larly, the same principles applied to polarization greatly aected our daily life for products as simple conversion in transmission leads to a device with as sunglasses to high-tech applications including conversion eciency greater than 50% from 0.52 radar, laser technology, ber-optic communica- to 1.82 THz and a maximum eciency of 80% at tions, liquid-crystal displays, and three-dimensional 1.04 THz. is design is expected to be applicable movies. Similar to controlling the EM wave inten- to wavelengths ranging from microwaves through sity, manipulating the polarization states enables infrared light with straightforward scaling of the many applications, and its importance should not geometry and appropriate choice of materials. be underestimated. e metamaterial approaches are versatile and As such, there has been a long history in the can avoid the restriction of needing materials with development of numerous devices for manipula- intrinsic birefringence or the small optical activ- tion of EM polarization states, including polarity of natural materials. e remaining challenge izers, half-wave plates, and quarter-wave plates. is then how to further expand the bandwidth and Conventional approaches include using gratings, prove the operation of high-eciency polarimetric birefringent crystals, and Brewster plates. In order devices at a broad range of wavelengths.

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Flat optics: Anomalous refraction and reection (a) Ordinary Refraction (b) Anomalous Refraction One recent breakthrough in metamaterials is the demonstration of generalized laws of reection and refraction [12]. e textbook laws state that q the direction of travel is determined by both the t refractive indices and angle of incidence in the case of refraction and solely on the angle of incidence in the case of reection. e recent generalized laws expand these concepts to explain how the wave’s path changes when there is a phase discontinuity FIGURE 4. (a) This diagram demonstrates ordinary refrac- on the interface. When the discontinuity imposes a tion. (b) This diagram demonstrates ecient anomalous constant phase gradient from 0 to 2π with uniform refraction [11]. The sample consists of a metal wire grating, energy amplitude on waves propagating through spacer layer, metamaterial layer designed to impose a phase the interface, the outbound wavefront (which is gradient, a second spacer layer, and a second wire grating, all normal to the phase gradient) travels in a direction encapsulated in polyimide. other than that determined by the traditional laws of reection and refraction. eciencies at some frequencies [11]. Compu- Such a constant gradient of phase discontinuity tational simulations suggest that similar design was experimentally realized (identical scattering principles can be used to create a device capable of amplitude of the resonators was also required and anomalous reection. realized), and consequently, anomalous reection and transmission were observed in the mid-in- Going one step beyond simply turning a beam, frared [12] and visible [13] regimes. is demon- Vladimir Shalaev’s group at Purdue University stration may nd important applications, such as recently succeeded in creating a at lens based on direction control of light [14], wavefront shaping exploiting the phase shi from dierent resonator [15], and at metalens design [16, 17]. structures on a planar metamaterial surface [18]. A conventional lens works because its varying e main challenge with these devices is that the thickness creates a phase change across an incom- regularly reected and refracted beams carry most ing wave front. Flat lenses instead use an array of of the incident EM energy, and the intensities of the resonators with dierent phase responses to achieve anomalously reected and transmitted beams are the same feat with a thickness well below the wave- much weaker than the regular beams. is issue is length of the EM wave. By putting resonators with fundamentally associated with the use of a single- dierent phase shis in precisely spaced concentric layered metasurface. e anomalous reection and circles, the researchers were able to reproduce the transmission critically rely on the cross-polarization relative intensity distribution of a lens, although the coupling in anisotropic metamaterials, which is throughput was only on the order of 10%. Continu- weak for such a single-layered metamaterial. ing research in this area will lead to more ecient Using the Fabry-Pérot-like multiple reection and more specialized designs, providing greater interference and layering principles from their functionality by allowing optical elements to be polarization converter design, Antoinette Taylor’s more easily integrated into devices, especially in ap- team at LANL was able to overcome this limita- plications where mass and volume are an issue. tion on eciency (see gure 4), demonstrating anomalous refraction with over 50% intensity Metamaterials in wireless antennas transmission across most of the 1.0 to 1.4 THz band with transmission peaks over 60%, while the usual Antennas are crucial elements for microwave refraction direction showed intensity transmissions wireless communication and wireless devices. e below 20% and approaching negligible transmission major goals of wireless technology are to reduce the

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antenna size, increase the radiation eciency, in- allows for engineering the gain and bandwidth of crease the bandwidth of operation, and increase the an antenna. For example, researchers found that the gain/directivity. e applications of metamaterials gain and bandwidth of a microwave patch antenna in wireless technology are mainly divided into three could be increased by introducing a metamaterial categories: (a) where bulk metamaterials are used to slab on top of a patch antenna [21]. e double- enhance the performance of antenna [19–21], (b) negative-index or negative-index metamaterial where the designs of the antennas are inspired by coupled to the antenna via a near-eld interaction, the unit cell of the metamaterial [22], and (c) where which increased both the operational bandwidth a gain element is integrated into the metamaterial and antenna gain. unit cells in the form of self-oscillating metamate- Recently, there have been eorts to make an- rial emitters, or metamitters [23]. tennas based on a single metamaterial unit cell; It is well known that an electrically small electric such an antenna is termed a meta-antenna. In this dipole antenna is an inecient radiator because it approach, a single metamaterial element is excited has a very small radiation resistance while simulta- either by a small monopole or by a loop antenna via neously having a very large capacitive reactance. It near-eld parasitic coupling. e combined system thus introduces a large impedance mismatch to any shows unusually improved radiation eciency and realistic power source and prevents the microwave allows tuning of the input impedance. Ziolkowski energy from feeding into the antenna eciently. To et al. experimentally demonstrated a metamaterial- obtain a high overall eciency, considerable eort inspired electrically small antenna that reached must be expended on creating a matching network an overall radiation eciency of 80% without a that forces the total reactance to zero by introduc- matching network [22]. ing a very large inductive reactance and tunes the Self-resonating metamaterial antennas, or eective input resistance of the antenna to match a metamitters, are metamaterial structures in which a 50-ohm source. Generally, this matching method source of gain has been integrated into the individ- utilizes passive lumped elements. Because of the ual metamaterial elements. For example, a tunnel very large reactance values involved, these matched diode biased into its negative dierential resistance resonant systems generally have very narrow band- regime can be integrated into the gap of an SRR widths, imperfect eciencies, and high tolerance (see gure 5), which acts as both a tank circuit and requirements for their fabrication. an ecient antenna [23]. In addition to working Metamaterials oer a unique opportunity to as a self-oscillating transmitter, these devices have match the reactance of the small antenna without a exhibited a range of nonlinearities, such as fre- matching network. To achieve impedance match- quency mixing, frequency pulling, and bistability, ing, the EM properties of the volume adjacent to suggesting they may be useful as elements of highly the antenna are modied using metamaterials con- compact detectors. While commercial gallium cepts. For example, if the antenna is capacitive, then arsenide (GaAs) tunnel diodes are limited to 12 an inductive metamaterial shell is used to make the gigahertz, advanced resonant tunneling diodes will overall reactance zero. Ziolkowski and Erentok pro- allow similar metamitter designs to operate into posed using a spherical shell made out of negative the terahertz. permittivity metamaterials to increase the radiation Active metamaterial concepts, discussed below, of the dipole and monopole antenna [19]. enable dynamic tuning of the frequency response, Metamaterials also provide an ecient way to beam steering, and variable focusing of the emit- obtain directive emission from an antenna. Enoch ted radiation. For example, Kymeta is working et al. demonstrated directive emission from a toward the commercial production of a metamate- monopole antenna when it was embedded in a rial broadband microwave antenna that uses active metamaterial slab [20], illustrating the ability of metamaterials to electronically steer a radio fre- metamaterials to change the radiation properties of quency beam so that it stays locked onto a satellite simple antennas. Using a metamaterial superstrate, while in motion without any moving parts [24]. rather than fully embedding the antenna, also

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already xed. Due to the highly dispersive proper- (a) (b) ties, the operational bandwidth of metamaterials is 2 mm oen very narrow; this static nature makes it ben- ecial for some applications but dicult to use for 4 mm broadband or tunable applications. us, dynami-

2 cm cally and actively tunable metamaterials are highly desirable to enhance functionality. Metamaterials derive their behavior from 1.5 mm combinations of eective permittivity and eective permeability that arise as a consequence of averaging over the behavior of a set of meta-atoms. It is frequently useful to view this subwavelength meta-atom as an inductive-capacitive circuit whose (c) 220 resonance frequency and amplitude are determined 200 by the eective capacitance and the inductance pro- vided by the unit cell. erefore, tunability can be

) 180 obtained by changing the values of the capacitance 160 and/or inductance through extrinsic or intrinsic stimuli. However, directly changing the proper- 140 ties of metallic elements is dicult except in a few DC bias (mV 120 exotic cases, such as when a superconductor or 100 graphene is being used instead of a normal metal. 80 Instead, dynamic metamaterials are usually ob- 2.390 2.395 2.400 2.405 2.410 tained by integrating semiconductors or a dielectric Frequency (GHz) material whose properties can be altered either through optical or electrical excitation [25]. For ex- FIGURE 5. (a) This diagram and (b) photograph show an ample, changing the metamaterial substrate aects individual metamitter element. (c) This graph shows the output of the metamitter showing frequency tuning with dier- both the resonance strength (via substrate losses) ent applied direct current biases. Image reprinted from [23]. and the resonance frequency (via the substrate’s dielectric constant). e functionality in active/ dynamic metamaterials is essentially determined by modifying the metamaterial substrate or by Electrically small antennas will improve the incorporating materials into critical regions of the performance of cell phones, personal digital as- resonant elements. sistants, and Wi-Fi interfaces in laptops. Indeed, a few examples have already shipped in large-volume Active metamaterials are of particular interest at consumer devices, including Wi-Fi routers made terahertz frequency from the device point of view. by Netgear. Wireless personal health monitoring Researchers at LANL demonstrated many of these systems and miniature wireless sensors for sen- concepts for realistic device applications by inte- sor network applications will greatly benet from grating semiconductors in the metamaterial designs the integration of compact ecient antennas and [25]. Optical illumination was used to dynamically could be a promising commercial application change the resonance amplitude by photoexciting of metamaterials. the carriers in the metamaterial’s unit cell, which damped the resonance because of the increased Active control of metamaterials loss in the capacitor gap. In another demonstration, Hou-Tong Chen demonstrated the frequency In general, aer a metamaterial structure is fabricat- redshiing in a metamaterial that had silicon semi- ed on a substrate, its resonance strength, frequency, conductor pads; upon photoexcitation, the pads and the relative phase of the individual elements are became conductive and thus increased the overall

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capacitance. Very recently, a similar approach was application of a reverse voltage bias, which in turn demonstrated by another group to show the reso- tunes the resonant amplitudes. is technique has nance blueshiing by using optical excitation [26]. been applied to realize terahertz electrical modulators and phase shiers. Tunable terahertz metama- Another approach to control the EM properties terials have also been realized using a microelectro- of individual meta-atoms remotely using light is mechanical systems structure. to integrate varactor diodes into each meta-atom, which could then be controlled by a nearby light- Currently, the terahertz technology is suering emitting diode (LED) [27]. In the absence of light, from the lack of compact sources and detectors. incoming microwaves would reect o this SRR However, the development of the quantum cas- array like a at mirror. By increasing the bright- cade terahertz laser might be the rst-generation ness of selected LEDs, the angle of reection could terahertz device that might benet from the meta- be altered. e array could even focus or defocus material-based active terahertz modulators. In this microwaves, as if it were a parabolic mirror. case, the modulator could be designed to match the frequency of terahertz radiation. At optical frequen- Electrically controllable terahertz metamaterials, cies, reconguration of negative-index metamateri- shown in gure 6, were demonstrated by Hou-Tong als has been proposed and designed by controlling Chen at LANL [28]. Room-temperature electri- the magnetic resonance via tuning the permittivity cally switchable metamaterials were rst created by of the embedded anisotropic liquid crystals [29]. fabricating planar metallic metamaterials on a thin Such a structure has recently been experimentally n-doped GaAs layer. e gold SRRs and n-GaAs investigated by inltrating shnet metamaterials form a Schottky diode structure, enabling control of with nematic liquid crystals [30]. Experimental the charge carriers in the metamaterial split gaps by results showed a signicant change in the optical transmission with a moderate laser power. A group of researchers led by Antoinette Taylor Bias at LANL demonstrated ultrafast switching of the H negative-index optical metamaterials using all opti- Ohmic E k Schottky cal switching [31]. eir device consisted of metallic shnet structures with a thin amorphous silicon layer in between. e optical excitation allowed photo-induced carrier injection, which dynamically tuned the resonance behavior of the metamaterials. e device was able to modulate at the communication wavelength with a speed of one terabit per second. e planar design of such a device can be Split gap fabricated easily using the existing deep ultraviolet photolithography technique. Depletion n-GaAs A signicant portion of the metamaterial research is committed to the development of such SI-GaAs active devices. While advantageous for some applications, the signicant energy loss and narrow bandwidth of operation of metamaterials are a detriment for many other applications. e lack of FIGURE 6. Electrically switchable terahertz metamaterials were rst created by fabricating planar metallic metamateri- compact devices has always been a roadblock for als on a thin n-doped GaAs layer. The gold SRRs and n-GaAs practical applications of terahertz technology, in- form a Schottky diode structure, enabling control of the dicating that this area will likely benet most from charge carriers in the metamaterial split gaps by application the emergence of active metamaterials devices. For of a reverse voltage bias, which in turn tunes the resonant amplitudes. Image is adapted from [28]. example, these kinds of terahertz modulators might emerge as an integrated part of resonant tunneling

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diode-based terahertz sources. e fabrication of work spanned a range of experimental and theo- active terahertz modulators is compatible with retical investigations into the fundamental nature current high-volume microfabrication methods; and application of plasmons in metallic nanostruc- therefore, commercialization does not require any tures including linear and nonlinear spectroscopy extra infrastructure development. An electrically of individual nanoparticles, surface-enhanced controllable phase modulator is another metamate- spectroscopy, surface-enhanced uorescence, rial application that is likely to enable the operation nanoparticle-enhanced photovoltaics, and solar- of terahertz scanners without moving parts in the thermal energy harvesting. Following his PhD, midterm (i.e., 4–8 year) timescale. he studied the guiding of light on the nanoscale using plasmons propagating on metal nanowires Conclusions as a postdoctoral student at the Institute of Phys- ics, Chinese Academy of Sciences in Beijing. He is Metamaterials are a promising technology on the currently a postdoctoral student at LANL where verge of transitioning from pure laboratory research he is studying terahertz, microwave, and infrared to commercial applications. Metamaterial absorb- metamaterials for ultrathin at optics, the genera- ers are likely to have a signicant eect on the tion of high-intensity terahertz waves, and nonlin- areas of EM signature manipulation, solar energy, ear terahertz spectroscopy. and thermophotovoltaics. Electrically small anten- Dr. Diego Dalvit is a technical sta scientist in nas, articial ferrites, and metamitters will have a the eory Division of LANL. He leads the LANL signicant eect on the miniaturization of wire- theory team working on modeling and simulation less communications devices. Metamaterial-based, of light-matter interactions in metamaterials, nano- ultrathin, lightweight optics will aect areas rang- photonics, and Casimir physics. His other areas ing from radar to terahertz and infrared imaging of expertise are in quantum information science and possibly communications, optical microscopy, and technology, including decoherence, measure- and lithography. ment, and control of open quantum systems. He In addition to the discussed applications, elec- has been a visiting scholar at the Ecole Normale tromagnetic metamaterials have inspired analogous Superieure and the French National Center for Sci- eorts to control other wave phenomena, includ- entic Research at the German Academic Exchange ing the emerging eld of acoustic metamaterials. Service. Dalvit has also been a LANL director's Preliminary demonstrations indicate that acoustic postdoctoral fellow. metamaterials may lead to signicant advances in Dr. Antoinette (Toni) Taylor is the leader of ultrasound and sonar imaging resolution, sound the Materials Physics and Applications Division isolation, and acoustical cloaking. In summary, at LANL. Prior to this position, she was director metamaterials are a rapidly advancing, dynamic of the Center for Integrated Nanotechnologies, a area of research with oen surprising discoveries joint Sandia/LANL Nanoscience Research Center routinely emerging. In some areas, notably compact funded through the Oce of Basic Energy Sciences. antennas, metamaterials are rapidly maturing into a Her research interests include the investigation of commercially relevant technology. ultrafast dynamical nanoscale processes in materials, the development of novel optical functional- About the authors ity using metamaterials, and the development of novel optics-based measurement techniques for Jane E. Heyes is a research technologist on the understanding of new phenomena. She has Antoinette Taylor's ultrafast optics team at the published over 300 papers in these areas, written Center for Integrated Nanotechnologies at Los three book chapters, and edited ve books. She is Alamos National Laboratory (LANL). She holds a a former director-at-large of the Optical Society of bachelor's and a master's degree in electrical engi- America (OSA), topical editor of the Journal of the neering from Stanford University. Optical Society B: Optical Physics, and a member Nathaniel Grady received his PhD in applied of the National Academies' Board of Physics and physics from Rice University in 2010. His doctoral Astronomy Solid State Science Committee. Taylor

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has also chaired the National Academies’ Commit- tee on Nanophotonics Applicability and Accessibil- der Körper für Wärme und Licht.” Annalen der ity. Currently, she is a Chair of the Division of Laser Physik und Chemie. 1860;109:275–301. doi: 10.1002/ andp.18601850205. [English translation, “On the rela- Science of the American Physical Society, and the tion between the radiating and the absorbing powers OSA representative on the Joint Council of Quan- of dierent bodies for light and heat.” Philosophical tum Electronics. She is a LANL laboratory fellow Magazine. 1860;4(20):1–21]. and a fellow of the American Physical Society, the [10] Kim Y, Lin S, Chang ASP, Lee J, Ho K. “Analysis OSA, and American Association for the Advance- of photon recycling using metallic photonic crystal.” ment of Science. In 2003, Taylor won the inaugural Journal of Applied Physics. 2007;102(6):063107. doi: Los Alamos Fellow’s Prize for Outstanding Leader- 10.1063/1.2779271. ship in Science and Engineering. [11] Grady N, Heyes J, Chowdhury DR, Zeng Y, Taylor AJ, Dalvit DAR, Chen H. “Terahertz metamaterials for linear polarization conversion and anomalous refraction.” Science. 2013;340(6138):1304–1307. doi: 10.1126/ science.1235399. References [12] Yu N, Genevet P, Kats MA, Aieta F, Tetienne J, [1] Veselago VG. “e electrodynamics of substances Capasso F, Gaburro Z. “Light propagation with phase with simultaneously negative values of ε and μ.” discontinuities: Generalized laws of reection and re- Soviet Physics Uspekhi. 1968;10(4):509. doi: 10.1070/ fraction.” Science. 2011;334(6054):333–337. doi: 10.1126/ PU1968v010n04ABEH003699. science.1210713. [2] Pendry JB, Holden AJ, Robbins DJ, Stewart WJ. [13] Ni X, Emani NK, Kildishev AV, Boltasseva A, “Magnetism from conductors and enhanced nonlin- Shalaev VM. “Broadband light bending with plas- ear phenomena.” IEEE Transactions on Microwave monic nanoantennas.” Science. 2012;335(6067):427. doi: eory and Techniques. 1999;47(11):2075–2084. doi: 10.1126/science.1214686. 10.1109/22.798002. [14] Aieta F, Genevet P, Yu N, Kats MA, Gaburro Z, Ca- [3] Shelby RA, Smith DR, Schultz S. “Experimental passo F. “Out-of-plane reection and refraction of light verication of a negative index of refraction.” Science. by anisotropic optical antenna metasurfaces with phase 2001;292(5514):77–79. doi: 10.1126/science.1058847. discontinuities.” Nano Letters. 2012;12(3):1702–1706. doi: 10.1021/nl300204s. [4] Soukoulis CM, Wegener M. “Past achievements and future challenges in the development of three- [15] Genevet P, Yu N, Aieta F, Lin J, Kats MA, Blanchard dimensional photonic metamaterials.” Nature Photonics. R, Scully MO, Gaburro Z, Capasso F. “Ultra-thin plas- 2011;5(9):523–530. doi: 10.1038/nphoton.2011.154. monic optical vortex plate based on phase discontinuities.” Applied Physics Letters. 2012;100(1):013101. doi: [5] Landy NI, Sajuyigbe S, Mock JJ, Smith DR, Pa- 10.1063/1.3673334. dilla WJ. “Perfect metamaterial absorber.” Physical Review Letters. 2008;100(20):207402. doi: 10.1103/ [16] Chen X, Huang L, Mühlenbernd H, Li G, Bai PhysRevLett.100.207402. B, Tan Q, Jin G, Qiu C, Zhang S, Zentgraf T. “Dual- polarity plasmonic metalens for visible light.” Nature [6] Chen H. “Interference theory of metamaterial perfect Communications. 2012;3(11):1198. doi: 10.1038/ absorbers.” Optics Express. 2012;20(7):7165–7172. doi: ncomms2207. 10.1364/OE.20.007165. [17] Aieta F, Genevet P, Kats MA, Yu N, Blanchard R, [7] Aydin K, Ferry VE, Briggs RM, Atwater HA. Gaburro Z, Capasso F. “Aberration-free ultrathin at “Broadband polarization-independent resonant light lenses and axicons at telecom wavelengths based on absorption using ultrathin plasmonic super absorbers.” plasmonic metasurfaces.” Nano Letters. 2012;12(9):4932– Nature Communications. 2011;2(10):517. doi: 10.1038/ 4936. doi: 10.1021/nl302516v. ncomms1528. [18] Ni X, Ishii S, Kildishev AV, Shalaev VM. “Ultra- [8] Liu X, Tyler T, Starr T, Starr AF, Jokerst NM, thin, planar, Babinet-inverted plasmonic metalenses.” Padilla WJ. “Taming the blackbody with infrared Light: Science & Applications. 2013;2(4):e72. doi: metamaterials as selective thermal emitters.” Physical 10.1038/lsa.2013.28. Review Letters. 2011;107(4):045901. doi: 10.1103/ PhysRevLett.107.045901–045905. [19] Ziolkowski RW, Erentok A. “Metamaterial-based ef- cient electrically small antennas.” IEEE Transactions on [9] Kirchho G. “Über das Verhältnis zwischen dem Antennas and Propagation. 2006;54(7):2113–2130. doi: Emissionsvermöogen und dem Absorptionsvermögen 10.1109/TAP.2006.877179.

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