AAWSAP DIRD, Metamaterials for Aerospace Applications, April 2010

Text

This text came with the file. It may have mistakes. When the text and the image disagree, the image is right.

UNCLASSIFIED/:£FQB OFEICI\k 1!18!! eNtf
sections, such as shown in Figures 15 and 16, will be primary candidates for
implementing optical and microwave non-reciprocity. Such metamaterials would be
comprised of a unit cell containing a non-radiative element (that is, a two-strip
capacitor-loaded antenna supporting a "dark" magnetic mode) and a single "bright"
dipole antenna. Such a system exhibits EIT when the frequencies of the "dark" and
"bright" resonances coincide. EIT results in energy compression and enhanced
nonlinearity. The source of the nonlinearity could be, for example, a variable
capacitance diode (varactor) used as a capacitive load of the double-strip antenna. The
second (linear) mode could have an orthogonal polarization, and the coupling between
the two could be accomplished via spatially-periodic displacement of the single-strip
and double-strip antennas with respect to each other.
Tunable Switchable Metamaterials
Electromagnetic properties of most metamaterials are "hard-wired", meaning they are
determined during fabrication. That can be a serious impediment to using them in the
context of space exploration. Being able to change optical/electromagnetic properties of
a metamaterials-based device without having to re-manufacture it would be highly
desirable. Therefore, this survey is concluded by describing some of the recent progress
in making reconfigurable/switchable metamaterials. This is a new exciting area of
metamaterials research that is worth watching for applications. One of the first
electrically-controllable THz metamaterials has been reported in Reference 36, where
resonant properties of the electric split ring were controlled by applying reverse bias
between metal and highly-doped in GaAs layer. The schematic of the experiment is
shown in Figure 25. Without reverse bias there is no resonant response of the split ring
to incident THz pulse because highly-conductive electrons of the n-GaAs layer are
shorting the gap of the resonant split ring as schematically indicated in Figure 26(b).
With the applied reverse bias, electron density is depleted inside the gap. The resulting
transmission spectrum shows spectral dips which were converted into the effective
dielectric permittivity &eff (w) that exhibited resonant peaks. The strongest of the peaks
corresponded to the Inductance-Capacitance (LC) resonance of the split ring. One
possible application of such electrically tunable metamaterial suggested in Reference 36
was a modulator. The authors claim that the performance of their device as a THz
modulator already exceeds current state-of-the-art electrical THz modulators (based on
semiconductor structures) by one order of magnitude on resonance. Moreover, their
device operates at room temperature. Needless to say, this metamaterial-based
modulator can be improved. For example, configurations exploiting EIT could result in
stronger modulation strength.
Another interesting possibility for tuning microwave metamaterials has been suggested
in Reference 37. Ferroelectrics (such as BST) can be tuned by applying DC voltage
which changes their dielectric permittivity. This property of BST was utilized to develop
frequency tunable magnetic metamaterials using metallic split rings loaded with
barium-strontium titanate thin film capacitors. The resonant frequency of this medium
is voltage tunable across a 140 MHz band centered at 1. 75 GHz. The effective relative
permeability of the slab was shown to have Lorentzian shape that reaches minimum
values between -2 and -3 for biases from Oto 5 V. Therefore, permeability of the slab
can tune between positive and negative values, making it useful in applications
requiring a state switchable magnetic permeability.
UNCLASSIFIED/ {EAR OEEICl.t.ls: 1!181!! eNtf
30