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Planar Photonics with MetasurfacesAlexander V. KildishevBirck Nanotechnology Center, Purdue University, kildishev@purdue.edu

Alexandra BoltassevaBirck Nanotechnology Center, Purdue University, aeb@purdue.edu

Vladimir M. ShalaevBirck Nanotechnology Center, Purdue University, shalaev@purdue.edu

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Kildishev, Alexander V.; Boltasseva, Alexandra; and Shalaev, Vladimir M., "Planar Photonics with Metasurfaces" (2013). Birck andNCN Publications. Paper 1373.http://dx.doi.org/10.1126/science.1232009

DOI: 10.1126/science.1232009, (2013);339 Science

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www.sciencemag.org SCIENCE VOL 339 15 MARCH 2013

REVIEW SUMMARY

School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA.

*Corresponding author. E-mail: shalaev@purdue.edu

ARTICLE OUTLINE

Nanoantenna-Array Metasurfaces

Hyperbolic Metasurfaces

Improved Plasmonic Materials

for Metasurfaces

RELATED ITEMS IN SCIENCE

J. B. Pendry, D. Schurig, D. R. Smith, Science 312, 1780 (2006). DOI: 10.1126/science.1125907

Z. Liu, H. Lee, Y. Xiong, C. Sun, X. Zhang, Science 315, 1686 (2007). DOI: 10.1126/science.1137368

N. I. Zheludev, Science 328, 582 (2010). DOI: 10.1126/science.1186756

C. M. Soukoulis, M. Wegener, Science 330, 1633 (2010). DOI: 10.1126/science.1198858

A. Boltasseva, H. A. Atwater, Science 331, 290 (2011). DOI: 10.1126/science.1198258

N. Engheta, Science 334, 317 (2011). DOI: 10.1126/science.1213278

N. Yu et al., Science 334, 333 (2011).

DOI: 10.1126/science.1210713

X. Ni, N. K. Emani, A. V. Kildishev, A. Boltasseva, V. M. Shalaev, Science 335, 427 (2012). DOI: 10.1126/science.1214686

O. Hess, K. L. Tsakmakidis, Science 339, 654 (2013). DOI: 10.1126/science.1231254

BACKGROUND READING

New physics of optical MMs expands the functionality of modern photonic devices. Reduction in thickness of the bulk MMs, along with the use of alternative plas-monic materials, gives a new class of advanced, ultra-thin optical elements: optical metasurfaces, which exhibit extended or completely new capabilities.

N. I. Zheludev, Y. S. Kivshar, Nat. Mater. 11, 917 (2012). DOI: 10.1038/nmat3431

G. V. Naik, J. Liu, A. V. Kildishev, V. M. Shalaev, A. Boltasseva, Proc. Natl. Acad. Sci. U.S.A. 109, 8834 (2012). DOI: 10.1073/pnas.1121517109

F. Aieta et al., Nano Lett. 12, 1702 (2012). DOI: 10.1021/nl300204s

J. Kim et al., Opt. Express 20, 8100 (2012). DOI: 10.1364/OE.20.008100

Planar Photonics with Metasurfaces

Alexander V. Kildishev, Alexandra Boltasseva, Vladimir M. Shalaev*

Background: Metamaterials (MMs) are smartly engineered structures with rationally designed, nanostructured building blocks that allow us to build devices with distinct responses to light, acoustic waves, and heat fl ows that are not attainable with naturally available materials.

Advances: The latest developments have shown that optical metasurfaces comprising a class of optical MMs with a reduced dimensionality can exhibit exceptional abilities for controlling the fl ow of light; achieve the anomalously large photonic density of states; and, similar to their bulk analog,

provide superresolution imaging. Such a planar photonics technol-ogy is expected to facilitate new physics and enhanced functional-ity for devices that are distinctly different from those observed in their three-dimensional MM coun-terparts. As a result, this technol-ogy will enable new applications in imaging, sensing, data storage, quantum information processing, and light harvesting.

Outlook: The recent progress in optical metasurfaces can address the major issues hampering the full-scale development of MM technology, such as high loss, cost-ineffective fabrication, and chal-lenging integration. The studies of new, low-loss, tunable plasmonic materials—such as transparent conducting oxides and intermetal-lics—that can be used as building blocks for metasurfaces will com-plement the exploration of smart designs and advanced switching capabilities. This progress in meta-surface design and realization will lead to novel functionalities and improved performance and may

result in the development of new types of ultrathin metasurface designs with unparalleled proper-ties, including increased operational bandwidths and reduced losses. These new designs would also be compatible with planar, low-cost manufacturing. In turn, these advances will lead to ultrathin devices with unprecedented functionalities, ranging from dynamic spatial light modulation to pulse shaping and from subwavelength imaging or sensing to novel quantum optics devices.

1289

Plasmonic metasurfaces at work. (A) A nanoantenna-array plas-monic metasurface used to experimentally demonstrate negative refraction (inset) and refl ection angles. The unit cell of a representative metasurface (blue) consists of eight gold V-shaped antennas repeated periodically. (B) A hyperbolic metasurface enhancing the radiation rate of quantum emitters. The metasurface is arranged of a thin subwave-length metallic grating deposited on a dielectric substrate.

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Cite this article as A. V. Kildishev et al., Science 339, 1232009 (2013). DOI: 10.1126/science.1232009

Published by AAAS

Planar Photonics with MetasurfacesAlexander V. Kildishev, Alexandra Boltasseva, Vladimir M. Shalaev*

Metamaterials, or engineered materials with rationally designed, subwavelength-scale buildingblocks, allow us to control the behavior of physical fields in optical, microwave, radio, acoustic,heat transfer, and other applications with flexibility and performance that are unattainable withnaturally available materials. In turn, metasurfaces—planar, ultrathin metamaterials—extendthese capabilities even further. Optical metasurfaces offer the fascinating possibility of controllinglight with surface-confined, flat components. In the planar photonics concept, it is the reduceddimensionality of the optical metasurfaces that enables new physics and, therefore, leads tofunctionalities and applications that are distinctly different from those achievable with bulk,multilayer metamaterials. Here, we review the progress in developing optical metasurfaces that hasoccurred over the past few years with an eye toward the promising future directions in the field.

With the recent advances in micro- andnanofabrication methods, one can nowcontrol the flow of light in a way that

was not possible before. Metamaterials (MMs)are engineered structures with rationally designed,nanostructured building blocks (“meta-atoms”).MMs allow us to build devices with responsesto light, acoustic waves, and heat flows that areunattainable with naturally available materials(1–3). In the artificial patterns of meta-atoms,the propagation of electromagnetic energy canbe defined by the spatial and spectral dispersionsof the effective dielectric and magnetic proper-ties. These synthetic structures offer the distinctpotential to guide and control the flow of electro-magnetic energy in an engineered optical space(2) and open the door to a number of applica-tions that were previously considered impossible(4). We are no longer constrained by the electro-magnetic response of natural materials and theirchemical compounds. Instead, we can tailor theshape and size of the structural units of a MM,tune the composition and morphology of thenanostructure, and achieve new, desired function-alities. The extraordinary properties of opticalMMs and transformation optics (TO) devices (2),which were conceived by MMs, enable a nega-tive refractive index, imaging with the nanometer-scale resolution, invisibility cloaks, efficient lightconcentrators, nano-optics and quantum informa-tion applications (1–4).

Optical metasurfaces comprise a class ofoptical MMs with a reduced dimensionality thatdemonstrate exceptional abilities for controllingthe flow of light beyond that offered by conven-tional, planar interfaces between two naturalmaterials (5). Such two-dimensional (2D) andquasi-2D MMs provide us with the distinct possi-bility to fully control light with planar (or nearly

planar) MM elements and, thus, to realize “planarphotonics.” Metasurfaces enable new physicsand phenomena that are distinctly different fromthose observed in their 3D counterparts. More-over, they are compatible with on-chip nano-photonic devices, which is of critical importancefor future applications in opto-electronics, ultra-fast information technologies, microscopy, im-aging, and sensing.

A metasurface structured on the subwave-length scale in the lateral directions can be de-terministic (i.e., periodic and aperiodic) orrandom. In practice, such a metasurface is rep-resented by a patterned metal-dielectric layerthat is very thin compared with the wavelengthof the incident light and is typically depositedon a supporting substrate. The functionality of adevice based on such a metasurface depends di-rectly on the effective, surface-confined, opticaldispersion. Effective optical properties, along withnonconventional far-field responses of ultrathinmetasurfaces, for example, have been found todeviate from classical reflection and refractionlaws (5, 6). Hence, the responses of metasur-faces cannot be inferred from the experimentalresponses for bulk materials. To design reliableflat photonic devices, a fundamental understand-ing of the extraordinary properties, as a functionof the lateral dimensional features and the struc-tural ordering, is required. There is a critical needto develop innovative theoretical, experimental,and fabrication approaches to unleash the powerof functional optical metasurfaces.

In a long-wavelength regime (from radioto terahertz waves), surface-confined metallicantenna arrays, or “metafilms” (7–9), containingmultiple antenna elements have already beensuccessfully used for communication applications(10–12) or as highly confined cavity resonators(13, 14). Similar to optical metasurfaces, the an-tenna elements in such “reflectarrays” (10) and“transmitarrays” (12) also act as phase-controllingresonators for manipulating the direction in whichradio ormicrowave signals are received or broad-cast. Nevertheless, the desired phase shifts in

the reflect- and transmitarrays are obtainedwiththe dimensions of resonant elements and arrayperiods of a magnitude comparable in size tothe incident or transmitted free-space wave-length (15).

The importance and power of planar pho-tonics was demonstrated earlier for the speciallydesigned case of planar chiral elements (16–19).The recently discovered generalized Snell’s lawsuggests a way toward ultimate control of lightpropagation (5). As demonstrated by Yu et al.(5), special nanoantenna-array metasurfaces cre-ate phase discontinuities for light propagatingthrough the interfaces and drastically change theflow of reflected and refracted light, as initiallydemonstrated for the mid-infrared (mid-IR) wave-length of 8 mm (5). This phenomenon has recentlybeen extended to the near-IR wavelength region(6), where it was also shown that the effect isrobust and broadband. With these new approaches,metasurfaces could be used to fully control alllight parameters, including frequency, phase, po-larization, momentum, and angular momentum(20–23). Metasurface-based optical vortex plates(21), aberration-free and ultrathin flat lenses, andaxicons at telecommunication wavelengths (24)have recently been demonstrated. Ni et al. alsoreported that extremely thin (30 nm) and verysmall (2 mm in radius) metalenses based on Babinetcomplementary nanoantennas (V-shaped slotsin a metal sheet) can be used for the extra-strongfocusing of light (with a focal length as short as2.5 mm) in the visible wavelength range (25). Inaddition, ultrathin terahertz planar lenses havealso been proposed (26).

Another recent demonstration showed that3D effects on light propagation can be obtainedwithout the need for complex inclusions in bulkMMs. Instead, planarized, broadband, bianiso-tropic MMs consisting of stacked nanorod arrayscan be used (27). These nanorod arrays containa tailored rotational twist and were shown toconstitute an ultrathin, broadband circular polar-izer that can be directly integrated within nano-photonic systems (27). Plasmonic metasurfaceshave also been proposed and realized as quarter-wave plates (28, 29). Metasurfaces can be usedto effectively couple propagating waves to sur-face waves (30), which could be of great impor-tance for on-chip nanophotonic applications.Kang et al. have also shown that thin, U-shapedaperture antennas can be used to completely con-vert circularly polarized light into its cross-polarizedcounterpart (31). As shown by Shu et al. (32),transformation media can generate optical beamswith desirable orbital angular momenta (OAM)[see also (33)]. Simon et al. have reported thatspecially engineered metasurface-based OAMstates can be used also for high-efficiency quan-tum cryptography and a new quantum-key dis-tribution protocol, exploiting, for example, therecursive properties of the Fibonacci sequence (34).Finally, it has been shown that so called fishnetand multilayer MM structures can be used for

REVIEW

School of Electrical and Computer Engineering and BirckNanotechnology Center, Purdue University, West Lafayette,IN 47907, USA.

*To whom correspondence should be addressed. E-mail:shalaev@purdue.edu

www.sciencemag.org SCIENCE VOL 339 15 MARCH 2013 1232009-1

computer-generated holograms (35, 36). Conven-tionally, phase holograms are made by changingeither the thickness or the refractive index of amaterial. In contrast, metasurfaces can create ultra-thin holograms that rely on subwavelength-sizedantennas to modulate the phase appropriately andcreate the hologram image.

We note that the control of the phase withmetasurfaces can be related, in some cases, tothe fundamental physics associated with thePancharatnam-Berry phase. This geometrical phaseappears when the light polarization follows ageodesic triangle on the Poincaré sphere (37, 38).The phase difference between the initial and finalstates differs in this case by an amount represent-ing half of the solid angle encompassing by thetriangle. It has been demonstrated that contin-uous, space-variant, subwavelength gratings canresult in this phase change, which is not due tooptical path differences but rather stems fromlocal changes in polarization, representing the geo-metrical space-domain Pancharatnam-Berry phase[see, for example, (39)]. The subwavelength, pe-riodic structure in this case behaves as a uniaxialcrystal. By controlling the local orientation ofsuch a grating, one can form space-variant waveplates. Based on these computer-generated, space-variant, subwavelength dielectric gratings, a fam-ily of Pancharatnam-Berry phase optical elementshas been demonstrated; this family includes blazeddiffraction gratings and polarization-dependent fo-cusing lenses (39), image-encrypting devices (40),vectorial vortices, and spiral phase elements pro-ducing helical beams with different topologicalcharges (41, 42), as well as near- and mid-IRholograms employing circularly polarized illu-mination (43). For example, in the design of theexperiments by Hasman et al. and Niv et al.(39, 42), for incident light with a wavelength of10.6 mm, the typical depth and period of a de-signed GaAs dielectric grating structure were~2 mm each. It was also shown that when plas-mons are involved, one can develop an impor-tant quantum weak measurement tool (44) andobserve various fundamental phenomena, suchas the plasmonic Aharonov-Bohm effect and theoptical spin Hall effect (45, 46).

To advance the emerging metasurface tech-nology further and enable real-life applicationsfor metasurfaces, there is a critical need to de-velop a new material platform that provides per-formance enhancements and new functionalitiescombined with manufacturing and integration ca-pabilities. To enable the large-scale fabrication andchip integration of metasurfaces, customizable,cost-effective, and semiconductor-compatible ma-terials are required (47). Metasurfaces that incor-porate such materials can contribute to a newclass of hybrid, chip-scale devices (48) that areexpected to revolutionize nanophotonic and opto-electronic circuitry through smart integration ofmultiple functions in the metallic, dielectric, andsemiconductor building blocks. Due to their 2Dnature that is advantageous for wafer-scale pro-cessing and complex integration, metasurfaces

hold great promise to win the race for the mosttechnologically important research among ar-tificially structured materials. We will focus onthree areas that we believe are important for thecurrent and future developments of the field: (i)nanoantenna-based metasurfaces, (ii) hyperbolicmetasurfaces (HMSs), and (iii) the use of newmaterial platforms for metasurfaces, enabling ageneration of switchable devices that could beintegrated with the existing semiconductor tech-nology into complex, multifunctional systems.

Nanoantenna-Array MetasurfacesBy engineering a phase discontinuity along an in-terface, one can fully steer light and accomplishunparalleled control of anomalous reflection and re-fraction described by the generalized Snell’s law (5)

sinðqtÞnt − sinðqiÞni ¼ l0∇F=2p ð1Þ

sinðqrÞ − sinðqiÞ ¼ n−1i l0∇F=2p ð2Þ

where qt, qi, and qr are the angles of refraction,incidence, and reflection, respectively; nt and niare the refractive indices of the two media at thetransmission and incident side, respectively, andl0 is the free-space wavelength. These expressionsindicate that a gradient in a phase discontinuity,∇F,along an interface can modify the direction of therays refracted and reflected by the device, andthis can be achieved in a very thin layer. Note that∇F is essentially an additional momentum con-tribution that is introduced by coupling to the nano-antennas (NAs) and breaking the symmetry at theinterface; hence, the light wave bends according-ly to conserve the momentum.

The degree of light control is illustrated inFig. 1. Metasurface functionalities are com-bined with low losses due to the fact that theyare ultrathin. This could lead to a variety of de-vices enabling extraordinary light modulationand control.

Metasurfaces with symmetry-breaking V-shapednanoantennas create abrupt phase shifts from 0to 2p in cross-polarized light and, thus, can beused for a number of planar optical elements.For example, to achieve the lensing functional-ity, the NAs can be arranged in concentric rings(24, 25). Along with their different shapes, theNAs can be distributed in such a way that theabrupt phase shifts cause the wave propagatingthrough the interface to experience a fully con-structive interference at a distance f. Therefore,the outgoing cross-polarized light is focused at afocal length f (24, 25). Such ultrathin metalensescan be designed to generate, for example, non-diffracting Bessel beams (24), which are extreme-ly important for laser machining, high-precisionposition alignment, and sensing. By using Babinet-complementary NAs, one can significantly in-crease the signal-to-noise ratio, which is the ratioof the cross-polarized light that experiences theanomalous refraction to the transmitted light withthe original polarization (25).

One of most interesting applications for meta-surfaces could be related to ultrathin, deeply sub-wavelength spatial light modulators (SLMs). It isimportant to note that such SLMs can also pro-vide ultrafast, dynamically controlled responses.

Metasurfaces can also be used as broadbandmid-IR chemical sensors. The mid-IR range isan important part of the spectrum, especially for

Metasurface

Substrate Metasurface

SubstrateNegative refraction from

Negative reflection from

λ = 1 µm to λ = 1.9 µm

λ = 1 µm to λ = 1.9 µm

θi= 30 deg θi= 65 deg

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C D

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Fig. 1. Nanoantenna-arraymetasurfaces. (A) Schematic view of a representative nanoantenna array.The unit cell of the plasmonic interface (blue) consists of eight gold V-shaped antennas with 40-nmarm width and 30-nm thickness, repeated with a periodicity of L in the x direction and L/8 in the ydirection. The phase delay for cross-polarized light increases along the x axis from right to left. (B)Scanning electron microscope images of the unit cells of four antenna arrays with different periodsfabricated on a single silicon wafer. (C and D) Negative refraction (qt) (C) and reflection (qr) (D) anglesare obtained experimentally for incident cross-polarized light with 30° and 65° incidence angles (qi),respectively, for the range of near-IR wavelengths (l) from 1.0 to 1.9 mm (6).

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chemical sensing, because many chemicals ex-hibit signature absorption spectra in the mid-IR.It is well known that plasmonic structures withtheir local-field enhancement can dramatically in-crease sensitivity and, thus, can be used for effi-cient sensing. With metasurfaces, one can furtherincrease the sensitivity. Unlike conventional plas-monic structures, which translate the change inthe local environment into the intensity of the de-tected light, metasurfaces can transform the localenvironmental changes into a shift in the angleof the anomalous reflection and refraction (Dqr,and Dqt, respectively) as depicted in Fig. 2A.The detection of angular shifts can be substan-tially more sensitive than the detection of smallintensity changes. Thus, metasurfaces could be anefficient way to produce ultrasensitive chemicalsensors in the mid-IR.

Because metasurfaces can precisely controlthe phase of the incident light at the nanoscale,they can also be used as building blocks for meta-structures with extremely large values of refrac-tive index in the optical range. Such materialswith an extra-large refractive index could findvery important applications in nanophotonics,super-resolution imaging, and sensing devices.Access to extreme refractive index values thatare unattainable with natural materials can beachieved by using the large phase changes overthe deeply subwavelength scale provided by asingle layer of a NA-based metasurface. With twoor more layers of such metasurfaces, one canslow down the light inside the layers by creatingadditional phase shifts in the propagation direction.Figure 2B illustrates how a multilayer of meta-surfaces can operate as a MM with an extra-largerefractive index. The right panel of Fig. 2B illus-trates the phase change accumulated by a beamof light propagating through a normal homoge-nous material (the green line) and through a MMconsisting of several metasurfaces (the red curve).The slope of each curve indicates the effective

refractive index. Because of the abrupt phasechanges due to metasurfaces, the beam in the MMcan accumulate phase much faster than in normalmaterials; hence, the MM provides a much largereffective refractive index than in normal materials.

In addition to the control of incident light,metasurfaces can also control and manipulate sur-face electromagnetic waves. Applications can berealized by designing metasurfaces that supportand transform surface waves. For example, cloak-ing of 3D objects using bulk MMs can be ex-tended to surface structures that cloak surfacediscontinuities from detection via propagating sur-face waves (e.g., surface plasmon polaritons). Asmentioned previously, metasurfaces allow oneto very effectively couple propagating waves tosurface waves (30). One can apply the transfor-mation optics approach to control the propaga-tion of surface-confined waves with metasurfaces.This extends the previously proposed TO ap-proach for the surface plasmon waves (49) tothe generalized TO concept for waves supportedby metasurfaces.

To advance the proposed functionalities further,it is important to add switching and modulationcapabilities to metasurface devices. Switchablemetasurfaces could be extremely important forapplications involving dynamic beam steering.One can employ phase-changing materials such aschalcogenide glass, transition-metal oxides (e.g.,vanadium oxide), and liquid crystals as possiblecandidates for switching. By precisely control-ling the phase change acquired by the beam pass-ing through a metasurface, one can also developan approach for advanced pulse shaping. Dynam-ically controlled pulse shaping can be enabledby combining ultrathin metasurfaces with phase-changing and nonlinear dielectrics.

Hyperbolic MetasurfacesHighly anisotropic hyperbolic metamaterials(HMMs), which are usually made of alternating

metal and dielectric layers or using metal wiresembedded in a dielectric host, are metallic inone direction and dielectric in the other direc-tions (Fig. 3) (50–57). In HMMs, light encoun-ters extreme anisotropy, causing its dispersionto become hyperbolic. This leads to dramaticchanges in the light’s behavior. For example,HMMs enable many exotic applications for nano-scale imaging and efficient light concentration(50–52, 58, 59). Additionally, HMMs can en-hance the density of states for photons within abroad spectral range (53–55, 60, 61). The ra-diative decay rate for light emitters is proportion-al to the photonic density of states (PDOS); thus,the radiative decay can be efficiently enhanced inHMMs. This recent discovery of a broad-bandsingularity in the PDOS for HMMs (61) couldrevolutionize PDOS engineering, enabling lightsources with dramatically increased photon ex-traction and, ultimately, leading to nonresonantsingle-photon sources, which could be used, forexample, for quantum communication protocolsand other quantum information applications (60).

However, bulk HMMs usually exhibit highloss. Moreover, it is difficult to place emitters in-side 3D HMMs to efficiently use the high PDOS.Thus, it is important to develop quasi-2D HMSsthat could offer important advantages over their3D counterparts because such HMSs would bechip-compatible and would typically have smalllosses that are relatively easy to compensate. Planarand ultrathin HMSs also offer straightforward de-vice fabrication and integration. Complementingthe distinct properties of the NA-based metasur-faces considered above, HMSs would also pro-vide broadband control over the PDOS.

One of the interesting surface phenomenathat holds promise for developing efficient HMSswith a number of applications in nano-optics andimaging is a Dyakonov surface wave. Dyakonovplasmon surface waves occur at the interfacebetween two media, at least one of which is

NA

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Fig. 2. Example applications of optical metasurfaces. (A) A meta-surface acting as a chemical sensor: The analyte molecules adsorb onto themetasurface, causing a shift in the resonance of the constituent NAs. Thisresults in shifts in the angle of the anomalously reflected and transmittedbeams (Dqr and Dqt, respectively). (B) Illustration of an extra-large refractive-

index MM consisting of three metasurfaces (left panel). The right panelshows the accumulated phase change for light propagating through anormal homogenous material (green line) and through a MM consistingof three metasurfaces (red curve). The slope of each curve indicates theeffective refractive index.

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anisotropic, and where one component of thepermittivity of either media has to be negative(56). These surface waves can be observed intwo different geometries. The first is a layer ofmetal covered with a uniaxial dielectric material.The second possibility is to employ an HMMlayer covered with an isotropic dielectric layer.Both of these configurations can support surfaceswaves exhibiting hyperbolic dispersion, whichcauses highly directional propagation of the sur-

face waves with unboundedwave vectors (limitedin magnitude only by the inverse of the size of thestructural unit) in a broad wavelength range. It isimportant to note that the PDOS in HMSs di-verges similarly to PDOS in their 3D counterparts.Naturally, the unbounded isofrequency curves ofHMSs immediately suggest a diverging, anoma-lously large PDOS. Importantly, though, the hy-perbolic dispersion can be achieved within a widespectral range; thus, the expected enhancement of

the emission rate onHMSs has a broad bandwidthsimilar to that of their 3D counterparts.

We note that the anomalously large PDOS inHMSs could open up the exciting possibility ofcreating a new generation of active, on-chip lightsources with dramatically increased photon ex-traction. Moreover, PDOS engineering achieva-ble in HMS could lead to novel laser designswith extraordinary properties such as wide-bandtunability and high directionality.

One of the interesting directions in this fieldis exploring how to use HMSs for quantum op-tical applications, such as the control of sponta-neous emission and “single-photon guns,” whichcould be very important for quantum informationapplications (60). As follows from the consider-ations above, one can expect emission enhance-ment from quantum emitters onHMSs. In contrastto bulk HMMs, quantum emitters can be placeddirectly on the interfaces of HMSs. In addition, thehyperbolic dispersion relation of the Dyakonovsurface waves implies that the emitted light on thesurface is highly directional, which will improvethe collection efficiency. The surface nature ofthis approach makes it inherently compatible withon-chip integration. Figure 4A illustrates an ex-ample of a geometry involving quantum emitterson a HMS.

Another important application for HMSs is asurface hyperlens for superresolution imaging,which was previously proposed and realized forthe 3D case (52, 58, 62). Figure 4, B and C, illus-trates the hyperlensing effect for HMSs consistingof metallic gratings on top of a dielectric substrate.Figure 4B shows a surface hyperlens without themagnification effect. Two scatterers on top of thegrating are separated by a subwavelength distanceand behave as sources. The surface waves areconfined in a preferred angular direction, alongwhich the wave vector of the Dyakonov plasmonsis perpendicular to the isofrequency curve. Forthe hyperlensing condition (when the hyperbolasof the isofrequency curves are flat), the wavesemerging from the two sources are parallel toeach other and will be resolvable at a distancefar away from the sources. The design can bereadily changed to produce a magnifying effect,

2 6 10 14

0.2

0.6

1

Pow

er (

a.u.

)

Di s t a n c ef r om HMM

kx /k0

λ/10λ/20

HMMDielectric

CBAkz

ky

ε x , εy > 0; ε z < 0Transverse-positive

ε x , εy < 0; ε z > 0Transverse-negative

kx

kz

ky

kx

z

y

x

Fig. 3. Metamaterials with hyperbolic dispersion. (A and B) Examples of 3D HMMs with extremeanisotropy [transverse positive (TP) and transverse negative (TN)] made of plasmonic nanowires in adielectric host (A) and alternating layers of metallic and dielectric materials (B). The sphere in the centerof (A) represents the dispersion relation in a conventional, isotropic material, whereas the two hyperbolicsurfaces describe the dispersion in a TP HMM. The dispersion for the TNHMM is shown in (B). The unboundedwave vectors in HMMs imply an extremely high effective refractive index and, thus, the possibility of super-resolution imaging. This also leads to a singularity in the density of states that occurs over the entirebandwidth where hyperbolic dispersion is achieved. kx and k0, the tangential components of thenormalized wave vector; ex, ey, and ez, the diagonal components of the permittivity tensor; l, thefree-space wavelength. (C) Simulated emission in an HMM (top) and power spectrum in a HMM ascompared to conventional dielectric (bottom) (54). a.u., arbitrary units.

Fig. 4. Hyperbolic metasurfaces. (A) Illustration of a HMS enhancingthe emission rate of quantum emitters on a metasurface consisting ofa metallic grating on a dielectric substrate. (B and C) Illustration of a

surface hyperlens without magnification (B) and with magnification (C).The two scatterers are on the top of the grating and have a subwavelengthseparation.

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as shown in Fig. 4C. A grating made of stripsthat become wider with distance from the sourceswill magnify the image of the sources. When thelight is directed in the opposite direction, thesame HMS designs can be employed as lightconcentrators. Such HMSs could focus light toa subwavelength-sized spot and could be usedfor efficient, on-chip light harvesting.

The enhancement in the PDOS provided byHMSs also enables the engineering of thermalradiation and near-field heat transport. Engineeringthe thermal radiation of emitters over a broad bandof mid-IR frequencies that is enabled by HMSsalso allows the efficient control over the flow ofheat, which is essential for many applications suchas surveillance, sensing, detection, and imaging.

Improved Plasmonic Materialsfor MetasurfacesThe search for novel, low-loss, tunable materialsthat can be used as building blocks for metasurfaceswill no doubt complement the exploration ofsmart designs and lead to new functionalities andimproved performance. Similar to other plasmonicsystems, one of the largest obstacles to over-come in metasurfaces is optical loss arising inthe metallic unit cells (47, 57). Metals such assilver and gold have traditionally been selected asthe plasmonic materials of choice because theyhave large free-electron concentrations and highelectrical conductivities. Although these metalswork well in the infrared and microwave spec-tral regions, they suffer from high losses arisingin part from interband transitions as the incidentlight approaches the technologically importanttelecommunication and visible wavelengths (57).One common approach to loss compensation isthe addition of a gain medium in or around theplasmonic material. This approach can partiallyor fully compensate the loss in metals, but theincorporation of such active materials is very chal-lenging (63). Often, the gain that can be providedby usual materials is barely enough to compensatethe losses. Hence, an efficient alternative approachis needed to realize extremely low-loss or loss-less devices. Another related fact limiting opticalMM applications is the large plasma frequencyof noble metals (~2000 THz) that cannot beadjusted (57). This is disadvantageous for anumber of applications such as hyperlenses, trans-formation optics, and epsilon-near-zero materials(materials where the real part of the effective di-electric permittivity approaches zero) that enablesubwavelength-resolution imaging, cloaking, anddirectional beaming, respectively. Ideally, formany applications, metals should not be too me-tallic (so that their permittivity would be com-parable in magnitude to that of dielectrics butopposite in sign) and should have an adjustableor tunable plasma frequency to enable controlla-ble or switchable devices. Moreover, noble metalsare not compatible with the established semi-conductor processing technologies, restricting MMand metasurface devices to the proof-of-conceptstage only.

Those issues can be addressed by developingnew material platforms that use low-loss, tunablematerials for metasurface devices with improvedperformance, new functionalities, switchability, andcompatibility with existing semiconductor tech-nologies. The recent search for new plasmonicmaterials (47, 57, 64–66) defined new, interme-diate carrier density materials as the best candi-dates that exhibit low loss, extraordinary tuning,and modulation capabilities and that are com-patible with standard semiconductor fabricationand integration procedures.

Recently, the most promising classes of newplasmonic materials have been outlined and in-clude materials such as inorganic ceramics [no-tably transparent conducting oxides (TCOs)(57, 64, 65) and transition metal nitrides (65, 66)],and the merits of these materials for differentapplications have been explored (57, 64–66).Heavily doped (1021 cm−3) TCOs, such as indiumtin oxide and zinc oxide doped with aluminum orgallium, have been shown to be promising plas-monic materials in the near-IR range (57, 64–66).In the visible range, intermetallics such as silicides,germanides, borides, and nitrides could serve asplasmonic materials (47). Naik et al. have dem-onstrated that the optical performance of goldcan be at least matched by that of TiN (65, 66).

It is also important to study the possibilityof incorporating tunability and switching and/ormodulation capabilities into metasurface systems.Along with phase-change materials like liquidcrystals, chalcogenide glasses, and transition-metaloxides (e.g., vanadium oxide), the usage of TCOsfor electro-optical switching of metasurface devicescan be explored. The addition of electro-opticalcapabilities to metasurface devices, particularlyin the infrared domain, can be optimized by ex-ploiting the shift of the electromagnetic responsedriven by an applied voltage. Feigenbaum andAtwater have shown that heavily doped semi-conductor oxides and TCOs support extraordinarytuning and modulation of their complex refrac-tive indices because their carrier concentrationscan be changed by orders of magnitude throughthe application of an electric field (67). It is alsoimportant to look into the possibility of usingthe strong optical nonlinear responses that canbe exhibited by the intermediate carrier densitymaterials (68) proposed above to add new, use-ful functionalities to metasurface devices.

The recent progress in optical metasurfacescould address the major issues—such as highloss, cost-ineffective fabrication, and challengingintegration—that are hampering the full-scaledevelopment of MM technology. This progresscan result in the development of new types ofultrathin metasurfaces with unparalleled proper-ties, including increased operational bandwidthsand reduced losses; these metasurfaces wouldalso be compatible with planar, low-cost manu-facturing. In turn, this development could lead tonovel, ultrathin devices that offer unprecedentedfunctionalities ranging from dynamic spatial lightmodulation to pulse-shaping and beam-steering

and from nanoscale-resolution imaging and sensingto novel quantum optics devices.

References and Notes1. V. G. Veselago, The electrodynamics of substances

with simultaneously negative values of e AND m. Sov.Phys. Usp. 10, 509 (1968) and references therein.doi: 10.1070/PU1968v010n04ABEH003699

2. J. B. Pendry, D. Schurig, D. R. Smith, Controllingelectromagnetic fields. Science 312, 1780 (2006)and references therein. doi: 10.1126/science.1125907;pmid: 16728597

3. C. M. Soukoulis, M. Wegener, Optical metamaterials—morebulky and less lossy. Science 330, 1633 (2010) andreferences therein. www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=21164003&dopt=Abstract doi: 10.1126/science.1198858; pmid: 21164003

4. N. I. Zheludev, Y. S. Kivshar, From metamaterials tometadevices. Nat. Mater. 11, 917 (2012) and referencestherein. doi: 10.1038/nmat3431; pmid: 23089997

5. N. Yu et al., Light propagation with phase discontinuities:Generalized laws of reflection and refraction. Science334, 333 (2011). doi: 10.1126/science.1210713pmid: 21885733

6. X. Ni, N. K. Emani, A. V. Kildishev, A. Boltasseva,V. M. Shalaev, Broadband light bending withplasmonic nanoantennas. Science 335, 427 (2012).doi: 10.1126/science.1214686; pmid: 22194414

7. E. F. Kuester, M. A. Mohamed, M. Piket-May, C. L. Holloway,Averaged transition conditions for electromagnetic fieldsat a metafilm. IEEE Trans. Antenn. Propag. 51, 2641(2003). doi: 10.1109/TAP.2003.817560

8. C. L. Holloway, M. A. Mohamed, E. F. Kuester,A. Dienstfrey, Reflection and transmission propertiesof a metafilm: With an application to a controllablesurface composed of resonant particles. IEEE Trans.Electromagn. Compat. 47, 853 (2005). doi: 10.1109/TEMC.2005.853719

9. Y. Zhao, N. Engheta, A. Alù, Homogenization ofplasmonic metasurfaces modeled as transmission-lineloads. Metamaterials 5, 90 (2011). doi: 10.1016/j.metmat.2011.05.001

10. D. M. Pozar, S. D. Targonski, H. D. Syrigos, Design ofmillimeter wave microstrip reflectarrays. IEEE Trans.Antenn. Propag. 45, 287 (1997). doi: 10.1109/8.560348

11. D. Sievenpiper, L. Zhang, R. F. J. Broas, N. G. Alexopolous,E. Yablonovitch, High-impedance electromagnetic surfaceswith a forbidden frequency band. IEEE Trans. MicrowaveTheory Tech. 47, 2059 (1999). doi: 10.1109/22.798001

12. C. G. M. Ryan et al., A wideband transmitarrayusing dual-resonant double square rings. IEEE Trans.Antenn. Propag. 58, 1486 (2010). doi: 10.1109/TAP.2010.2044356

13. M. Caiazzo, S. Maci, N. Engheta, A metamaterial surfacefor compact cavity resonators. IEEE Antenn. WirelessPropag. Lett. 3, 261 (2004). doi: 10.1109/LAWP.2004.836576

14. C. L. Holloway, D. C. Love, E. F. Kuester, A. Salandrino,N. Engheta, Sub-wavelength resonators: On the use ofmetafilms to overcome the l/2 size limit. IET MicrowaveAntenn. Propag. 2, 120 (2008). doi: 10.1049/iet-map:20060309

15. N. Engheta, Antenna-guided light. Science 334, 317(2011). doi: 10.1126/science.1213278; pmid: 22021846

16. A. Papakostas et al., Optical manifestations of planar chirality.Phys. Rev. Lett. 90, 107404 (2003).

17. S. L. Prosvirnin, N. I. Zheludev, Polarization effects in thediffraction of light by a planar chiral structure. Phys. Rev. E71, 037603 (2005).

18. S. L. Prosvirnin, N. I. Zheludev, Analysis of polarizationtransformations by a planar chiral array of complex-shapedparticles. J. Opt. A 11, 074002 (2009).

19. Y. Svirko, N. Zheludev, M. Osipov, Layered chiral metallicmicrostructures with inductive coupling. Appl. Phys. Lett.78, 498 (2001). doi: 10.1063/1.1342210

20. F. Aieta et al., Out-of-plane reflection and refractionof light by anisotropic optical antenna metasurfaceswith phase discontinuities. Nano Lett. 12, 1702 (2012).doi: 10.1021/nl300204s; pmid: 22335616

www.sciencemag.org SCIENCE VOL 339 15 MARCH 2013 1232009-5

REVIEW

21. P. Genevet et al., Ultra-thin plasmonic optical vortexplate based on phase discontinuities. Appl. Phys. Lett.100, 013101 (2012). doi: 10.1063/1.3673334

22. R. Blanchard et al., Modeling nanoscale V-shapedantennas for the design of optical phased arrays.Phys. Rev. B 85, 155457 (2012).

23. S. Larouche, D. R. Smith, Reconciliation of generalizedrefraction with diffraction theory. Opt. Lett. 37, 2391(2012). doi: 10.1364/OL.37.002391; pmid: 22739918

24. F. Aieta et al., Aberration-free ultrathin flat lenses andaxicons at telecom wavelengths based on plasmonicmetasurfaces. Nano Lett. 12, 4932 (2012). doi: 10.1021/nl302516v; pmid: 22894542

25. X. Ni, S. Ishii, A. V. Kildishev, V. M. Shalaev, Ultra-thin,planar, Babinet-inverted plasmonic metalenses. Light Sci.Appl. 10.1038/lsa.2013.28 (2013).

26. D. Hu et al., http://arxiv.org/abs/1206.7011v1 (2012).27. Y. Zhao, M. A. Belkin, A. Alù, Twisted optical

metamaterials for planarized ultrathin broadband circularpolarizers. Nat. Commun. 3, 870 (2012).

28. Y. Zhao, A. Alu, Manipulating light polarization withultrathin plasmonic metasurfaces. Phys. Rev. B 84, 205428(2011).

29. N. Yu et al., A broadband, background-free quarter-waveplate based on plasmonic metasurfaces. Nano Lett. 12,6328 (2012).

30. S. L. Sun et al., Gradient-index meta-surfaces as a bridgelinking propagating waves and surface waves. Nat. Mater.11, 426 (2012). doi: 10.1038/nmat3292; pmid:22466746

31. M. Kang, T. H. Feng, H. T. Wang, J. S. Li, Wave frontengineering from an array of thin aperture antennas.Opt. Express 20, 15882 (2012). doi: 10.1364/OE.20.015882; pmid: 22772278

32. W. X. Shu et al., Generation of optical beams with desirableorbital angular momenta by transformation media.Phys. Rev. A 85, 063840 (2012).

33. N. M. Litchinitser, Structured light meets structuredmatter. Science 337, 1054 (2012) and referencestherein. doi: 10.1126/science.1226204; pmid:22936768

34. D. S. Simon, N. Lawrence, J. Trevino, L. Dal Negro,A. V. Sergienko, http://arxiv.org/abs/1206.3548(2012).

35. B. Walther, C. Helgert, C. Rockstuhl, T. Pertsch,Diffractive optical elements based on plasmonicmetamaterials. Appl. Phys. Lett. 98, 191101 (2011).

36. S. Larouche, Y. J. Tsai, T. Tyler, N. M. Jokerst, D. R. Smith,Infrared metamaterial phase holograms. Nat. Mater. 11,450 (2012). doi: 10.1038/nmat3278; pmid: 22426458

37. M. V. Berry, The adiabatic phase and Pancharatnam’sphase for polarized light. J. Mod. Opt. 34, 1401 (1987).doi: 10.1080/09500348714551321

38. S. Pancharatnam, Generalized theory of interference,and its applications. Part I. Coherent pencils. Proc. IndianaAcad. Sci. 44, 247 (1956).

39. E. Hasman, V. Kleiner, G. Biener, A. Niv, Polarizationdependent focusing lens by use of quantizedPancharatnam–Berry phase diffractive optics. Appl. Phys.Lett. 82, 328 (2003). doi: 10.1063/1.1539300

40. G. Biener, A. Niv, V. Kleiner, E. Hasman, Geometricalphase image encryption obtained with space-variantsubwavelength gratings. Opt. Lett. 30, 1096 (2005).doi: 10.1364/OL.30.001096; pmid: 15943278

41. A. Niv, G. Biener, V. Kleiner, E. Hasman, Spiral phaseelements obtained by use of discrete space-variantsubwavelength gratings. Opt. Commun. 251, 306(2005). doi: 10.1016/j.optcom.2005.03.002

42. A. Niv, G. Biener, V. Kleiner, E. Hasman, Manipulation ofthe Pancharatnam phase in vectorial vortices. Opt.Express 14, 4208 (2006). doi: 10.1364/OE.14.004208;pmid: 19516574

43. U. Levy, H.-C. Kim, C.-H. Tsai, Y. Fainman, Near-infrareddemonstration of computer-generated hologramsimplemented by using subwavelength gratings withspace-variant orientation. Opt. Lett. 30, 2089 (2005).doi: 10.1364/OL.30.002089; pmid: 16127919

44. Y. Gorodetski et al., Weak measurements of light chiralitywith a plasmonic slit. Phys. Rev. Lett. 109, 013901(2012).

45. Y. Gorodetski, S. Nechayev, V. Kleiner, E. Hasman,Plasmonic Aharonov-Bohm effect: Optical spin as themagnetic flux parameter. Phys. Rev. B 82, 125433(2010).

46. N. Shitrit, I. Bretner, Y. Gorodetski, V. Kleiner,E. Hasman, Optical spin Hall effects in plasmonic chains.Nano Lett. 11, 2038 (2011). doi: 10.1021/nl2004835;pmid: 21513279

47. A. Boltasseva, H. A. Atwater, Low-loss plasmonicmetamaterials. Science 331, 290 (2011). doi: 10.1126/science.1198258; pmid: 21252335

48. P. Y. Fan et al., An invisible metal–semiconductorphotodetector. Nat. Photonics 6, 380 (2012).doi: 10.1038/nphoton.2012.108

49. Y. M. Liu, T. Zentgraf, G. Bartal, X. Zhang, Transformationalplasmon optics. Nano Lett. 10, 1991 (2010).doi: 10.1021/nl1008019; pmid: 20465268

50. D. R. Smith, D. Schurig, Electromagnetic wave propagationin media with indefinite permittivity and permeabilitytensors. Phys. Rev. Lett. 90, 077405 (2003).

51. X. D. Yang, J. Yao, J. Rho, X. B. Yin, X. Zhang, Experimentalrealization of three-dimensional indefinite cavities at thenanoscale with anomalous scaling laws. Nat. Photonics 6,450 (2012). doi: 10.1038/nphoton.2012.124

52. Z. Jacob, L. V. Alekseyev, E. Narimanov, Opticalhyperlens: Far-field imaging beyond the diffraction limit.Opt. Express 14, 8247 (2006). doi: 10.1364/OE.14.008247; pmid: 19529199

53. H. N. S. Krishnamoorthy, Z. Jacob, E. Narimanov,I. Kretzschmar, V. M. Menon, Topological transitionsin metamaterials. Science 336, 205 (2012).doi: 10.1126/science.1219171; pmid: 22499943

54. Z. Jacob et al., Engineering photonic density of statesusing metamaterials. Appl. Phys. B 100, 215 (2010).doi: 10.1007/s00340-010-4096-5

55. M. A. Noginov et al., Controlling spontaneous emissionwith metamaterials. Opt. Lett. 35, 1863 (2010). doi:10.1364/OL.35.001863; pmid: 20517443

56. Z. Jacob, E. E. Narimanov, Optical hyperspace for plasmons:Dyakonov states in metamaterials. Appl. Phys. Lett. 93,221109 (2008).

57. P. R. West et al., Searching for better plasmonicmaterials. Laser Photonics Rev. 4, 795 (2010) andreferences therein. doi: 10.1002/lpor.200900055

58. A. Salandrino, N. Engheta, Far-field subdiffraction opticalmicroscopy using metamaterial crystals: Theory andsimulations. Phys. Rev. B 74, 075103 (2006).

59. A. J. Hoffman et al., Negative refraction in semiconductormetamaterials. Nat. Mater. 6, 946 (2007). doi: 10.1038/nmat2033; pmid: 17934463

60. Z. Jacob, V. M. Shalaev, Plasmonics goes quantum.Science 334, 463 (2011). doi: 10.1126/science.1211736; pmid: 22034423

61. Z. Jacob, I. I. Smolyaninov, E. E. Narimanov, BroadbandPurcell effect: Radiative decay engineering withmetamaterials. Appl. Phys. Lett. 100, 181105 (2012).

62. Z. W. Liu, H. Lee, Y. Xiong, C. Sun, X. Zhang, Far-fieldoptical hyperlens magnifying sub-diffraction-limitedobjects. Science 315, 1686 (2007). doi: 10.1126/science.1137368; pmid: 17379801

63. S. M. Xiao et al., Loss-free and active opticalnegative-index metamaterials. Nature 466, 735 (2010).doi: 10.1038/nature09278; pmid: 20686570

64. G. V. Naik, J. J. Liu, A. V. Kildishev, V. M. Shalaev,A. Boltasseva, Demonstration of Al:ZnO as a plasmoniccomponent for near-infrared metamaterials. Proc. Natl.Acad. Sci. U.S.A. 109, 8834 (2012) and references therein.doi: 10.1073/pnas.1121517109; pmid: 22611188

65. G. V. Naik, J. Kim, A. Boltasseva, Oxides and nitrides asalternative plasmonic materials in the optical range.Opt. Mater. Express 1, 1090 (2011) and references therein.doi: 10.1364/OME.1.001090

66. G. V. Naik et al., Titanium nitride as a plasmonic materialfor visible and near-infrared wavelengths. Opt. Mater.Express 2, 478 (2012). doi: 10.1364/OME.2.000478

67. E. Feigenbaum, H. A. Atwater, Resonant guided wavenetworks. Phys. Rev. Lett. 104, 129 (2010).

68. J. A. Schuller et al., Plasmonics for extreme lightconcentration and manipulation. Nat. Mater. 9, 193(2010). doi: 10.1038/nmat2630; pmid: 20168343

Acknowledgments: We thank A. Sokolov, X. Ni, and G. Naikfor useful discussions. This work was partially supported by Air ForceOffice of Scientific Research grant FA9550-12-1-0024, U.S. ArmyResearch Office grant 57981-PH (W911NF-11-1-0359), NSFgrant DMR-1120923, and NSF award Meta-PREM no. 1205457.

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