AAWSAP DIRD, Metamaterials for Aerospace Applications, April 2010

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One possible design for the microwave frequency band is shown in Figure 17. High
absorption is accomplished by reducing reflections to zero. This is accomplished by
choosing metamaterials parameters such that &eri@,.) =µ.1i@,. ) at the resonant
frequency co,. . Note that both the real and imaginary parts of the permittivity and
permeability must be equal to each other, and that the imaginary parts don't
necessarily need to be small for absorber applications. In fact, it is desirable that they
are not too small, thereby enabling 100 percent absorption within a single layer of
metamaterial. The above design can be scaled down to the THz range, as was later
demonstrated in Reference 34. It is difficult to find strongly absorbing materials at THz
frequencies that are compatible with standard photolithography. Thus, a potential
application of these metamaterial structures is as absorbing elements in thermal
detectors. A strong absorption coefficient is also necessary to have a small thermal
mass. This is important for optimizing the temporal response of thermal detectors. The
metamaterial presented here has a 6 micron thick film (that is, )\/50 thickness for THz
radiation) and 70 percent absorptivity, which yields an absorption coefficient of 2000
cm- 1 .
One drawback of the original design was the narrow angular range of the absorber. The
absorption dropped dramatically when the incidence angle was as small as 20 degrees.
The reason for that is a relatively large unit cell of the metamaterial. In fact, when the
unit cell size is larger or comparable to )IJ2n, where n is the refractive index of the
substrate, it is inappropriate to call such structure a "metamaterial". A true impedance­
matched metamaterial would, in fact, always have a very broad angular response.
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Figure 19. Wide-Angle Plasmonic Absorber Based on Negative Index Metamaterial. Right panel:
schematic of an absorption-measuring experiment. A generic metamaterial with 8 YY = f.l zz = - 1+i, f.lx., = l is
assumed. Left panel : angular dependence of the absorption for a generic and specific (shown in the inset)
metamaterial. Both exhibit wide-angle absorptivity. (Reference 20)
This fact can be expressed by a simple formula for the absorption coefficient
A (Reference 20):
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