Defense Intelligence Reference Document Metamaterials For Aerospace Applications

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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 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.
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