University of Pennsylvania ScholarlyCommons Departmental Papers (ESE) Department of Electrical & Systems Engineering August 2008 Infrared and optical invisibility cloak with plasmonic implants based on scattering cancellation Mário G. Silveirinha Universidade de Coimbra Andrea Alù University of Pennsylvania, [email protected] Nader Engheta University of Pennsylvania, [email protected] Follow this and additional works at: https://repository.upenn.edu/ese_papers Recommended Citation Mário G. Silveirinha, Andrea Alù, and Nader Engheta, "Infrared and optical invisibility cloak with plasmonic implants based on scattering cancellation", . August 2008. Copyright 2008 American Physical Society. Reprinted in Physical Review B, Volume 78, Article 075107, August 2008, 7 pages. Publisher URL: http://dx.doi.org/10.1103/PhysRevB.78.075107 This paper is posted at ScholarlyCommons. https://repository.upenn.edu/ese_papers/447 For more information, please contact [email protected]. Infrared and optical invisibility cloak with plasmonic implants based on scattering cancellation Abstract In recent works, we have suggested that plasmonic covers may provide an interesting cloaking effect, dramatically reducing the overall visibility and scattering of a given object. While materials with the required properties may be directly available in nature at some specific infrared or optical frequencies, this is not necessarily the case for any given design frequency of interest. Here we discuss how such plasmonic covers may be specifically designed as metamaterials at terahertz, infrared, and optical frequencies using naturally available metals. Using full-wave simulations, we demonstrate that the response of a cover formed by metallic plasmonic implants may be tailored at will so that at a given frequency, it possesses the plasmonic-type properties required for cloaking applications. Keywords light scattering, metamaterials, optical conductivity, optical materials, plasmons Comments Copyright 2008 American Physical Society. Reprinted in Physical Review B, Volume 78, Article 075107, August 2008, 7 pages. Publisher URL: http://dx.doi.org/10.1103/PhysRevB.78.075107 This journal article is available at ScholarlyCommons: https://repository.upenn.edu/ese_papers/447 PHYSICAL REVIEW B 78, 075107 ͑2008͒ Infrared and optical invisibility cloak with plasmonic implants based on scattering cancellation Mário G. Silveirinha,1,2 Andrea Alù,1 and Nader Engheta1,* 1Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA 2Department of Electrical Engineering, Instituto de Telecomunicações, Universidade de Coimbra, 3030 Coimbra, Portugal ͑Received 18 March 2008; revised manuscript received 26 June 2008; published 11 August 2008͒ In recent works, we have suggested that plasmonic covers may provide an interesting cloaking effect, dramatically reducing the overall visibility and scattering of a given object. While materials with the required properties may be directly available in nature at some specific infrared or optical frequencies, this is not necessarily the case for any given design frequency of interest. Here we discuss how such plasmonic covers may be specifically designed as metamaterials at terahertz, infrared, and optical frequencies using naturally available metals. Using full-wave simulations, we demonstrate that the response of a cover formed by metallic plasmonic implants may be tailored at will so that at a given frequency, it possesses the plasmonic-type properties required for cloaking applications. DOI: 10.1103/PhysRevB.78.075107 PACS number͑s͒: 42.70.Ϫa, 78.66.Sq, 41.20.Jb I. INTRODUCTION analogous to their homogeneous ideal models. The goal of the present study is to extend the concepts In our previous works,1–5 we have theoretically demon- introduced in Ref. 5 to the IR and visible domains, properly strated that isotropic plasmonic materials with ͑relative͒ per- taking into account the fact that at terahertz, IR, and optical mittivity below unity may be used to drastically reduce the frequencies, metals have finite conductivity, well modeled to specific scattering of moderately sized obstacles. It was a good approximation, as Drude plasmas. We derive some proven theoretically that such materials may behave as “an- simple design formulas for a class of novel metamaterial tiphase” scatterers, which may effectively cancel out the di- cloaks that operate at infrared and optical frequencies based polar radiation from the obstacle, inducing in this way elec- on plasmonic parallel-plate implants in a dielectric host, and tromagnetic invisibility. This phenomenon is possible we demonstrate with full-wave simulations how such because the polarization currents induced in a material with parallel-plate metallic covers may effectively reroute the in- permittivity less than unity are in opposite phase with respect coming light and induce a cloaking effect at the desired IR or to the local electric field. optical frequency. Recently, other groups have suggested alternative cloak- ing ideas6–11 based on coordinate transformation theory, anomalous localized resonances, and other related concepts. II. METAMATERIAL DESIGN Such configurations in general require anisotropic and/or in- homogeneous layers, and rely on the response of resonant The cloaking effect described in Ref. 1 exploits the nega- metamaterials and artificial magnetism. On the contrary, the tive polarizability provided by scatterers made of materials transparency phenomenon proposed by our group1 simply with ␧ negative ͑ENG͒ or ␧ near zero ͑ENZ͒͑␧ representing requires uniform plasmonic materials with an isotropic re- the material permittivity͒. For typical designs,1–5 the required ͑ ͒ ␧ sponse. Moreover, this transparency mechanism does not value of the real part of the permittivity c of the plasmonic ␧ Ͻ ͕␧ ͖Ͻ ␧ ␧ rely on a resonant effect, and so it is less affected by losses cover lies in the range −10 0 Re c 0.5 0, with 0 being and may have good tolerance with respect to changes in the the free-space permittivity. Since noble metals behave essen- geometrical or material parameters of the involved objects.2 tially as ENG materials at infrared and optical frequencies, While isotropic plasmonic materials with the required they may be directly used for cloaking purposes.1 A problem, electromagnetic properties may be readily available in nature however, may arise in that for frequencies one or two de- at certain specific IR or optical frequencies,12 in general one cades below the plasma frequency of the material of interest, may need to synthesize these materials as metamaterials to the ͑real part of͒ permittivity of such metals, even though operate with the required electromagnetic properties at a de- negative, may have an absolute value orders of magnitude sired frequency. Such ideas were explored in our previous larger than the permittivity of vacuum. For example, work,5 where we have effectively demonstrated at micro- following the experimental data tabulated in Ref. 13 waves how it may be possible to emulate the behavior of low at IR frequencies, silver may be well characterized by a ␧ / ␧ ␧ ␻2 / ␻͑␻ ⌫͒ ␧ or negative permittivity materials in cloaks by using parallel- Drude model Ag 0 = ϱ − p +i , with ϱ =5.0, ␻ ␲ϫ ͓ ͔ ⌫ ␲ϫ ͓ ͔ plate metallic implants embedded in a dielectric host. p =2 2175 THz , and =2 4.35 THz . This yields ␧ Ϸ ͑ ͒␧ Namely, we have shown how to effectively design metama- Ag 470 −1+0.04i 0 at 100 THz, which is 2 orders of terial cloaks using perfectly electric conducting ͑PEC͒ im- magnitude larger than the typical values required for cloak- plants. In this sense, we had to properly take into account ing applications. both interface effects and actual granularity of the structured To overcome this inconvenience, mimicking our micro- material, demonstrating numerically that these metamaterial wave setup5 we suggest to embed silver implants in a dielec- cloaks may indeed provide a drastic scattering reduction, tric region ͑with positive ␧͒, so that the resulting composite 1098-0121/2008/78͑7͒/075107͑7͒ 075107-1 ©2008 The American Physical Society SILVEIRINHA, ALÙ, AND ENGHETA PHYSICAL REVIEW B 78, 075107 ͑2008͒ f [THz] 0 20 40 60 80 100 Ag T 0 0 y Host, h a silver -5 -1000 inc g E A Artificial x inc z -10 material -2000 H and g A -15 -3000 Artificial -20 material FIG. 1. ͑Color online͒ Geometry of a truncated ͑semi-infinite͒ -4000 planar composite material formed by a periodic array of nanolayers FIG. 2. ͑Color online͒ Permittivity as a function of frequency: of silver embedded in a dielectric host. The truncated sample is Blue lines ͑associated with left-hand side scale͒: structured mate- illuminated by an incoming TEx plane wave. rial; Black lines ͑associated with right-hand side scale͒: silver. The artificial material is formed by silver slabs with thickness material has a tailored electromagnetic response suitable for T=0.0374a which are spaced by a=360 nm and embedded in a cloaking. host material with ␧h =6.5. The geometry of the proposed layered structure in its pla- ␧ ␧ ͓ nar version is depicted in Fig. 1. It consists of a periodic tric material with permittivity h =6.5 0 SiC has similar ␧ ͑ ͔͒ array of planar silver slabs with permittivity Ag inserted in a properties around 100 THz Ref. 15 . These values were ␧ ͕␧ ͖ ␧ dielectric host material with permittivity h. The thickness of chosen to obtain Re eff =−3.0 0 at 100 THz, and they will each silver layer is T and the lattice constant is a. The effec-