Multifunctional Materials - Biomimetics: Biologically Inspired Technologies - page 312

12.2.1 Electromagnetic Functionality

Recent advances in electromagnetic (EM) metamaterials have provided an opportunity to change
and tune the dielectric constant as well as the index of refraction of the material over a range of
useful frequencies. Electromagnetic metamaterials are artificially structured media with unique and
distinct EM properties that are not observed in naturally occurring materials. A variety of meta-
materials with striking EM properties have been introduced, most notably those with a negative
refractive index (NRI). NRI is associated with a medium of simultaneously negative electric
permittivity, « and the magnetic permeability, m . There are no known conventional materials
with such exceptional properties. Recently, Smith et al. (2000a,b) at UCSD have produced a
medium with effective « and m that are measured to be simultaneously negative. Later, Smith
et al. performed a Snell's law experiment on a similar metamaterial wedge sample, and demon-
strated the negative refraction of a microwave beam (Shelby et al., 2001). Thus they showed that
their medium does indeed possess an NRI, that is, it is a negative index material (NIM). Such a
property has been hypothesized by Veselago who termed the medium ''left-handed'' (Veselago,
1968). The work on controlling the dielectric constant and producing negative « and m has been
discussed by Smith et al. (Smith et al., 2002, 2003, 2004a,b,c; Kolinko and Smith, 2003; Pendry
et al., 2003). However, until recently, the NIMs produced have been experimental samples, suitable
only for proof-of-concept demonstrations.
Based on the calculation of the effective EM properties of a medium containing period-
ically distributed very thin conducting wires and electric resonators, the authors at UCSD have
introduced into structural composites electromagnetic enhancements in the form of tunable index
of refraction, radio frequency (RF) absorption, and when desired, a negative index of refraction
(Starr et al. 2004). Such properties are the result of embedding periodic metal scattering elements
into the material to create an effective medium response over desired RF frequency ranges. We have
identified two wire architectures, namely thin straight wire arrays and coiled wire arrays, that are
suitable for direct integration into fiber-reinforced composites (Nemat-Nasser et al., 2002). These
arrays act as inductive media with a plasma-like response to control the electric permittivity. As a
result, the dielectric constant may be tuned to negative or positive values. Such a medium may be
used as a window to filter electromagnetic radiation. When the dielectric constant is negative, the
material does not transmit incident radiation. As the dielectric constant approaches to and exceeds
the turn-on frequency , the incident EM radiation is transmitted through the composite. Further-
more, over a desired frequency range, the dielectric constant may be tuned to match that of the
surrounding environment. For instance, the dielectric constant may be tuned to match that of air,
with a dielectric constant of unity, such that incident radiation does not experience a difference
when encountering the composite.

12.2.1.1 Thin-Wire Plasmonic Composites

The ionosphere is a dilute plasma. Many artificial dielectrics are plasma analogs. In 1996, Pendry et al.
suggested an artificial plasmon medium composed of a periodic arrangement of very thin conducting
wires, predicting a plasma frequency in the microwave regime, below the diffraction limit. Recently,
other researchers have presented examples of artificial plasmon media at microwave frequencies
(Smith et al., 1999). The dielectric constant k of a dilute neutral plasma is given by

2

f p
f

k ¼ 1

(12 : 1)

where f p is the plasma frequency and f is the electromagnetic excitation frequency. This parameter
must be evaluated empirically for any configuration, but analytical and numerical results can