ICOD: 1 December 2009 UNCLASSIFIED/f l'Olt err1e11ct 1111 8Pllr.f Intelligence Acquisition Threat Support Metamaterials for Aerospace Applications UNCLASSIFIED/ (FAR AEEICIA! UNCLASSIFIED//POlt OPPIClslaL tj!II! 8H~.Y Metamaterials for Aerospace Applications Prepared by: Administrative Note COPYRIGHT WARNING: Further dissemination of the photographs In this publicat1on is not authorized. This product is one in a series of advanced technology reports produced in FY 2009 under the Defense Intelligence Agency, l7b)<3):10 usc 424 ' !Advanced Aerospace Weapon System Applications (AAWSA) Pro ram. Comments or questions pertaining to this document should be addressed to (b 3):10 use 424;(b)(6) ., AAWSA Program Manager, Defense Intelligence Agency, ATTN: (b){3):1o use 424 Bldg 6000, Washington, UNCLASSIFIED/ (FAR OEEICI OP UGli SHlb'J.f UNCLASSIFIED//PBR: 8PPl81AL W61 IHll:i/ Applications to Sub-Diffraction Imaging: Super-Lens and Hyper-Lens .................. 6 Applications to Circuits and Waveguide Miniaturization: Slowing Down and Manipulating Electromagnetic Pulses {EMP) Using Advanced Metamaterials ....... 16 Nonlinear Non-Reciprocal Chiral Metamaterials: For Developing Novel Optical Figure 1. Example of a Metamaterial Component: The Magnetic Spllt Ring Figure 2. Example of Another Metamaterial Component: Electric Ring Resonator Figure 3. Geometry of Original Planar Metamaterial Unit Cells (OE1-OE6) and Their Figure 4. Recent Optical Metamaterlals for Telecommunication Wavelength and Mid-Infrared Indefinite Permittivity Material .......................................... 5 Figure 5. Schematic of The Super-lens With n=-1 Refractive Index Corresponding Figure 6. Schematic of the SiC-based Super-lens Which Is Imaging Sub-wavelength Figure 7. Theoretical Concepts (left panel) and Experimental Implementation {right panel) of an Optical Hyperlens Capable of Magnifying Sub- Diffraction Objects to Observable (larger than Size ................................. 9 Figure 8. Hyperlens Based on a Converging Array of Metal Wires ..... 10 Figure 9. Far-Field Super-lens (FSL) Based on an Indefinite Permittivity Metamaterial Placed Between the Object and the Image-Releasing Figure 10. Tomographic Multi-Beam Multi-Detector Holography of Sub-Wavelength Objects Using Indefinite Permittivity Medium (IPM) ........................... 12 Figure 11. First Experimental Demonstration of Propagating Sub-Diffraction Waves In the Indefinite Permittivity Medium (IPM) .......... 13 Figure 12. Schematic for 2-Beams/2-Detectors Interferometric Measurement Figure 13. Experlmental Setup for 2-Beams/ 2-Detectors Interferometric Measurement in the Lab and Preliminary Experimental Results .......... 15 UNCLASSIFIED/} F8fl 8PPl81AI. W&i Dtb>C UNCLASSIFIED/./FCA OFFICI.C.L WE& 8Hl!Tf Figure 14. Schematic of Pulse Compression In Magnetized Plasma ...................... 16 Figure 15. Trapped Rainbow: A Waveguide With Negative Index Core Can Stop Figure 17. True Multi-Layer Metamaterial With a Unit Cell Shown in Figure16: Radiative Antenna (Single Metal Strip} Coupled to a Dark Antenna Figure 18. "Perfect" Narrow-Band Microwave Absorber ....................................... 20 Figure 19. Wide-Angle Plasmonic Absorber Based on Negative Index Figure 20. Specific Design of a Wide-Angle Plasmonic Absorber Based on Negative Figure 21. Experimental Reflectivity vs. Wavelength and Theoretical Plot of Figure 22. Preliminary Attempts to Design a Better Absorber Using Complementary Figure 23. Engineering the Complex Reflectivity Coefficient Using the Concept of a Figure 24. Example of a Generic Chiral Metamaterial ........................................... 28 Figure 25. Example of Time-Irreversibility of Light Propagation Inside the Twisted Figure 26. THz Properties of an Electric Split Ring Resonator .. 31 UNCLASSIFIED//F&lil. 8fPl81.-t tt9E e .. c i UNCLASSIFIED,,Si&A 8SiSil&I IL UEE QIJL. Definition of Metamaterials A metamaterial is defined as an artificial medium whose properties {mechanical, optical, magnetic, or other) cannot be found in naturally-occurring materials. The emphasis of this study will be on electromagnetic and optical metamaterials. Such metamaterials can exhibit rather extreme properties, such as negative refractive index, which implies that both electric permittivity and magnetic permeability must be negative (Ii< 0 p < 0} (Reference 1}. Such metamaterials used to be called "left-handed" because of the unusual phase relationship between the electric and magnetic fields. Specifically, in most (positive index, including vacuum} media one uses the right-hand rule to define the relationsh!p between electric field ( E) magnetic field ( H ), and the propagation wavenumber (f ). The physical basis of the right-hand rule is that the direction of energy propagation defined by the Poynting vector S = cEx HI 41t and the direction of the phase velocity (defined by the wavenumber k) must coincide. That does not hold true for negative index rnetamaterials where the two directions are opposite, therefore, the left-handed relationship must hold for the three vectors. Nevertheless, the "left-handed" designation did not withstand the test of time because it was causing confusion and creating irrelevant allusions to helical (a.k.a. chiral) structures. Although chiral structures can indeed exhibit negative index behavior (Reference 2) chirality is not necessary. A typical metamaterial consists of resonant elements such as Split Ring Resonators (SRR}. An example of an SRR is shown in Figure 1. The main function of the SRR is to enable strong magnetic response of the structure. A simple empirical formula exists for the magnetic permeability of a metamaterial comprised of the SRRs: where m,.,, is the resonant frequency of the SRR, and Fis proportional to the volume filling factor of SRRs. It is noteworthy that SRRs are designed in such a way that it has a large capacitance. As the result, the resonant frequency of an SRR is small, (that is, the SRR-containing cell is very sub-wavelength). In the example shown in Figure 1 (taken from Reference 6), the unit cell operated at to/2,r = 10 GHz is A/10. In fact, the sub-wavelength size of the metamaterial is what distinguishes them from their close cousins: photonic crystals. By properly designing magnetic SRRs, it is possible to achieve any value of J.L for any given frequency. Special challenges exist for optical structures, though, as will be explained below. UNCLASSIFIEDhSF8118FFl&llzt ll81! 9HLI UNCLASSIFIED//POll 9PPllltlal: WOli IHJlai\f in-plane lattice parameters are a,,= az = 10/3 mm. The ring is square, with edge length I =3 mm and tracewidth w = 0.2 mm. The substrate is 381 m-thick Duroid 5870 (E = 2.33, td = 0.0012 at 10 GHz, where td is the loss tangent). The Cu film, from which the SRRs are patterned, is 17 m thick. The parameters rands are given in the table together with the associated value of {Reference 6) Electric properties of metamaterials can be similarly controlled. An example of a planar electrically-active metamaterial is shown in Figure 2. Figure 2. Example of Another Metamaterial Component: Electric Ring Resonator (ERR). This component provides tunable resonant electrfc response to the incident electromagnetfc field, and can be utilized for engineerfng the frequency-dependent dlelectric permittivity e(w) . Possible application: THz and microwave absorbers. (Reference 7) UNCLASSIFIEDf,<f&A 8FFIGltlali Ulii &fU!i.t UNCLASSIFIED//l'zOA 8FFl1ill11! kHH! 8HLY The electric response of such (or similar} metamaterial is given by w--(JJ; + lf'JJY where wR is the resonant frequency and y is the loss coefficient. Negative index metamaterials are by no means the only potentially useful metamedia. Several new concepts such as Indefinite Permittivity Metamaterials (IPM) (References 3, 4) and Epsilon-Near-Zero (ENZ) metamaterials (Reference 5) have recently emerged and found some exciting applications that will be reviewed below. IPMs can be used as ultra-compact spatial filters (both high-pass and low-pass) whereas ENR metamaterials can be,used for making sub-wavelength waveguides capable of coupling close to 100 percent of the incident radiation (Reference 8), as well as directing it around tight bends with negligible bending losses. Yet another class of planar metamaterials, complementary metamaterials (CMMs), has recently emerged (Reference 7). Instead of using metallic structures deposited on a substrate (left panel of Figure 3), CMMs consist of slits in the continuous metal screen (right panel of Figure 3). The shape of the slits coincides with that of the materials themselves. Such complementary metamaterials have been recently used for making epsilon-near zero waveguides (Reference 8). Figure 3. Geometry of Original Planar Metamaterial Unit Cells (OE1-0E6) and Their Complements (CE1-CE6). The polarization of normally lncident electromagnetlc radiation is configured as shown In OE1 and CEl for the original and complementary metamaterials, respectively. (Reference 9) UNCLASSIFIED//F8R: 'IFPl@lilit tt9! OICCI UNCLASSIFIED/,CF&R: &IPPlllillt ~DI! 8HLZ/ In general, metamaterials offer a new way of designing electromagnetic structures with arbitrary values of permittivity/permeability tensors, as well as other parameters (such as bi-anisotropy coefficient). In many instances, metamaterials enable us to considerably minimize sizes of resonators, transmission lines, and so forth. Such miniaturization is possible due to the resonant nature of the individual unit cells. Specifically, the structures shown in Figure 3 have high capacitance; therefore, their individual sizes are very sub-wavelength. That enables arrangement within sub- wavelength units that can be densely packed and result in strongly miniaturized components. It is this miniaturization that makes metamaterials interesting for aerospace application where small weight and size are essential. While the most spectacular progress in the field of electromagnetic metamaterials has so far occurred in the microwave range, it is the optical (visible, infrared, mid-infrared) spectral regions that hold most promise for revolutionary applications. Electromagnetic metamaterials have a tremendous potential for revolutionizing propagation, storage, and conversion of electromagnetic waves across the entire Electromagnetic Spectrum. In our opinion, the most exciting applications that are relevant for aerospace applications include energy harvesting, developing novel optical devices with unusual yet practically important capabilities (for example, non-reciprocal devices), enhancing the efficiency of nonlinear optical devices, developing novel imaging modalities capable of breaking the diffraction limit (for example, super-lenses, hyper-lenses, far field super-lenses), and developing novel lithographic techniques. Optical metamaterials are still a very new area. Just a handful of experimental demonstrations of multi-layer (truly bulk) optical metamaterials exist at the moment. Among the most recent ones are (a) demonstration of the negative index optical metamaterial at the telecommunications wavelength (Reference 10) that used the so- called fishnet structure shaped as a prism for demonstrating Snell's Law, and (b) demonstration of the Indefinite Permittivity Material (IPM) and negative refraction (which, in the context of anisotropic metamaterials, is not the same as negative refractive index) in the mid-infrared part of the spectrum (Reference 11). These structures have the distinction of being multi-layer (or bulk). Most previous examples of optical metamaterials have dealt with single or double-layer substances which cannot be, strictly speaking, characterized as metamaterials. The difficulty in obtaining strong magnetic activity in optical metamaterials has been explained in several recent reviews (References 12, 13). In a nutshell, the issue is that the magnetic moment of most structures (including atomic systems} is very small, much smaller than the electric moment. Therefore, it is difficult to observe any optical effects that can be clearly assigned to magnetic activity. This is especially true for the structures that are much smaller than one wavelength. Exceptions, such as artificially constructed split rings, are possible. However, such structures cannot be operated at very high frequency because of the excitation of electrostatic resonances (Reference 12). In other words, when the resonant frequency is too high (or the dielectric permittivity of a metal is.,not sufficiently large), electrostatfc resonances disrupt magnetic activity. More specifically, the energy inside and in the vicinity of a metamateri.al element (for example, Split Ring Resonator) becomes predominantly electrostatic, (that is, in the form of the kinetic energy of oscillating electrons). The recently described multi-layer fishnet (Reference 10) is not an exception: its unit cell (that is, the lateral period) is only one-half of the operating wavelength. UNCLASSIFIEDf,SF85l 8FFl&ICk W&i o,1bY UNCLASSIFIED//PSR err1e1.-t ltDf! UHLY \\':1,elff9:h !lfll Figure 4. Recent Optical Metamaterials for Telecommunication Wavelength A = 1.5 pm (Left and Middle) and Mid-Infrared IPM. The multi-layer fishnet Is made of silver films separated by a dielectric spacer. A focused ion beam was used to produce the prism-shaped fishnet. The IPM was obtained by depositing interleaved 80 nm layers of Ino_s3Gao.41As and Alo.4aino_o;2As. The layers, approximately 8.1 m thick, grown by molecular beam epitaxy on lattlcematched InP substrates. The InGaAs layers were uniformly doped to create different values of permittivity in alternating layers. (Reference 10 and 11) That is not to say that there is not ongoing theoretical and experimental work on designing optical metamaterials for practical applications. The author's research group at UT-Austin, has designed the first Plasmonic N~gative Index Metamaterials (P-NIM) super-lens (Reference 14 }, developed novel techniques for analyzing optical properties of plasmonic nanostructures, (including band-structure calculations of periodic nanostructures) (Reference 15) and quasi-static calculations of plasmonic resonances (Reference 16). The UT-Austin group has also designed a number of unique sub- wavelength P-NIMs in the optical part of the spectrum (References 14, 17, 18), and has recently published a review of optical P-NIMs (Reference 12). The group has also contributed to developing and experimentally implementing the concept of the "perfect (Reference 19) based on plasmonic/polaritonic materials. A perfect lens enables imaging of sub-wavelength objects in the infrared part of the spectrum, including objects buried under the surface. Also developed is a Wide-Angle "Perfect" Absorber of Mid-Infrared Radiation (WAPAMIR) (Reference 20) based on the negative index metamaterial whose impedance is perfectly matched to vacuum. Below is a concise summary of various topics/applications that are especially suitable for the aerospace industry. This study concentrates on the facility of metamaterials to miniaturize various optical and microwave components. Metamaterials can also be used for imaging very small (sub-wavelength) objects without resorting to costly and space- consuming near-field scanning optical microscopy. Also described are the ongoing efforts in the field to make extremely compact metamaterials-based lasers. Smaller lasers mean smaller weight and more room for other diagnostic devices and useful payload within the confines of a space vehicle. Applications of metamaterials to photon harvesting is especially fitting for advanced aerospace platforms because of the necessity to collect electromagnetic energy for battery recharging, diagnostic spectroscopy, and other vital functions of a space vehicle. Complementary Metamaterials for Energy Harvesting. Development of ultra- thin photovoltaic and thermo-photovoltaic cells is hampered by weak photon absorption in semiconductors. Metamaterials can modify absorption making it wavelength-selective (tunable), highly efficient, and, if desired, wide-angle. Recently a way has been found for creating quarter-wavelength resonators backed by leaky mirrors made out of CMMs. UNCLASSIFIED/ {FOB OEEICIOP I rss PIil.