DOW-UAP-D141, AAWSAP DIRD, Metamaterials for Aerospace Applications, April 2010

This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD surveys metamaterials, engineered structures designed to control electromagnetic waves in ways ordinary materials cannot, and argues that their main aerospace value lies in unusual optical and microwave properties together with significant component miniaturization. The report reviews possible applications including sub-wavelength imaging, compact waveguides and lasers, energy harvesting, tunable absorbers, nonreciprocal devices, and switchable materials, with particular emphasis on infrared and microwave uses for sensing, power management, and payload efficiency. It notes that many of the most ambitious applications depend on the practical output of a still-nascent field, especially in optical metamaterials, where only limited demonstrations had been achieved and fabrication remained a major constraint. The document presents metamaterials as a promising advanced materials field with credible niche applications and broader long-term potential.

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Defense
Intelligence
Reference
Document
Acquisition Threat Support
6 April 2010
ICOD: 1 December 2009
DIA-08-1004-006
Metamaterials for Aerospace
Applications
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Metamaterials for Aerospace Applications
Prepared by:
Acquisition Support Division (DW0-3)
Defense Warning Office
Directorate for Analysis
Defense Intelligence Agency
Author:
AAP Person 78
Administrative Note
COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized.
This product is one in a series of advanced technology reports produced in FY 2009
under the Defense Intelligence Agency, Defense Warning Office's Advanced Aerospace
weapon System Applications (AAWSA) Program. Comments or questions pertaining to
J
this document should be addressed to JAAP Person 1 AAWSA Program
Manager, Defense Intelligence Agency, ATTN: CLAR/DWO-3, Bldg 6000, Washington,
DC 20340-5100.
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Contents
Definition of Metamaterials .................................................................................... 1
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
Metamaterials for Energy Harvesting ................................................................... 20
Nonlinear Non-Reciprocal Chiral Metamaterials: For Developing Novel Optical
Isolators and "One-Way" Microwave Mirrors ....................................................... 27
Tunable Switchable Metamaterials....................................................................... 30
Summary and Conclusions ................................................................................... 31
References ........................................................................................................... 31
Figures
Figure 1. Example of a Metamaterial Component: The Magnetic Split Ring
Resonator (SRR) Design .......................................................................... 2
Figure 2. Example of Another Metamaterial Component: Electric Ring Resonator
(ERR)......................................................................................................... 2
Figure 3. Geometry of Original Planar Metamaterial Unit Cells (OE1-OE6) and Their
Complements (CE1-CE6) ......................................................................... 3
Figure 4. Recent Optical Metamaterials for Telecommunication Wavelength and
Mid-Infrared Indefinite Permittivity Material .......................................... 5
Figures. Schematic of The Super-lens With n=-1 Refractive Index Corresponding
to ( Surrounded by Vacuum ..................................................................... 7
Figure 6. Schematic of the SiC-based Super-lens Which is Imaging Sub-wavelength
Holes Buried Under the SiO2 Layer.......................................................... 8
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
Grating .................................................................................................. 12
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
and Numerical Simulation................................................................... 14
Figure 13. Experimental Setup for 2-Beams/2-Detectors Interferometric
Measurement in the Lab and Preliminary Experimental Results .......... 15
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Figure 14. Schematic of Pulse Compression in Magnetized Plasma ...................... 16
Figure 15. Trapped Rainbow: A Waveguide With Negative Index Core Can Stop
Light.................................................................................................... 17
Figure 16. "Plasmonic Molecule" Exhibiting EIT ................................................... 18
Figure 17. True Multi-Layer Metamaterial With a Unit Cell Shown in Figure16:
Radiative Antenna (Single Metal Strip) Coupled to a Dark Antenna
(Two Perpendicular Metal Bars) .......................................................... 19
Figure 18. "Perfect" Narrow-Band Microwave Absorber....................................... 20
Figure 19. Wide-Angle Plasmonic Absorber Based on Negative Index
Metamaterial ....................................................................................... 21
Figure 20. Specific Design of a Wide-Angle Plasmonic Absorber Based on Negative
Index Metamaterial Operating at A= 1550 nm ..................................... 22
Figure 21. Experimental Reflectivity vs. Wavelength and Theoretical Plot of
Reflectivity Contours ........................................................................... 23
Figure 22. Preliminary Attempts to Design a Better Absorber Using Complementary
MetaMaterials (U-shaped C-MM) ......................................................... 25
Figure 23. Engineering the Complex Reflectivity Coefficient Using the Concept of a
MetaMirror.......................................................................................... 26
Figure 24. Example of a Generic Chiral Metamaterial ........................................... 28
Figure 25. Example of Time-Irreversibility of Light Propagation Inside the Twisted
Fiber Core............................................................................................ 29
Figure 26. THz Properties of an Electric Split Ring Resonator .............................. 31
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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
( £ < 0 µ < 0) (Reference 1). Such meta materials used to be called "left-handed"
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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
E) H),
rule to define the relationsh[p between electric field ( magnetic field ( and the
propagation wavenumber ( f ). The physical basis of the right-hand rule is that the
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direction of energy propagation defined by the Poynting vector =cEx HI 47Z' and the
direction of the phase velocity (defined by the wavenumber k) must coincide. That
does not hold true for negative index metamaterials 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:
(1)
where is the resonant frequency of the SRR, and F is proportional to the volume
{J)M
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 = 10 GHz is A/10. I n fact, the
{J)j27Z"
sub-wavelength size of the meta material is what distinguishes them from their close
cousins: photonic crystals. By properly designing magnetic SRRs, it is possible to
achieve any value of µ for any given frequency. Special challenges exist for optical
structures, though, as will be explained below.
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I. r
C I.Lr
I 0.260 1.654 0.003
2 0.254 1.677 .023
3 0.-45 1.71 .052
4 0. 0 1.771 .0
0.20 I. 25 .1..0
6 0.190 I. 6 .1
7 0.17 1.9 I . I
0.148 2.027 .220
9 0.129 2.110 .2 0
10 0.116 2. 0.279
r1yun: .L, cxc1mp1t: u, i:I r111::cc1mc1c11::nc11 1.,umpun 11::nc; 1 n - ,.,_,..,_., _ -,-........,. _____.,___, ,---··, _ __ ,,.... .. ,~
in-plane lattice parameters are av= a,= 10/3 mm, The ring is square, with edge length I =3 mm and tracewidth w
= 0.2 mm. The substrate is 381 µm-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 µm thick. The parameters rand s are given in the
table together with the associated value of µ,. (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
electric response to the incident electromagnetic field, and can be
utilized for engineering the frequency-dependent dielectric
permittivity &(w) . Possible application: THz and microwave absorbers,
(Reference 7)
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The electric response of such (or similar) metamaterial is given by
(2)
where is the resonant frequency and r is the loss coefficient.
wR
Negative index metamaterials are by no means the only potentially useful metamedia.
Several new concepts such as Indefinite Permittivity Metamaterials (1PM) (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).
CE1 CE4
CE2 CE5
CE3 CE6
Figure 3. Geometry of Original Planar Metamaterial Unit Cells (OE1-0E6) and Their Complements
(CE1-CE6}. The polarization of normally incident electromagnetic radiation is configured as shown in OEl
and CEl for the original and complementary metamaterials, respectively. (Reference 9)
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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