AAWSAP DIRD, Metamaterials for Aerospace Applications, April 2010

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UNCLASSIFIED/;'fOll Offl@lsltt HS! OHLY
room temperature radiates 460 W. Earth-based powerful mid-infrared sources can be
used for nighttime powering of airborne platforms.
All these applications would benefit from the following components: (a) perfect
absorbers of infrared radiation that can be installed on the receiving platforms, (b)
efficient sources of thermal mid-infrared radiation. These two components are related
to each other. Therefore, developing an ultra-thin perfect absorber of infrared radiation
would be highly desirable. The ultra-thin aspect is important because it enhances the
radiation-to-electricity conversion efficiency. For example, it is well known that carrier
separation-collection efficiency in a solar cell improves as the cell gets thinner. The
challenge is to combine this carrier separation-collection efficiency with sufficient
absorption. Unfortunately, the absorption length of many semiconductors in the
infrared is fairly small. Recent experiments provide the new metamaterials-based
concept for increasing the absorptivity of otherwise semi-transparent materials. Below
some of the (still unpublished) experiments and theoretical developments that might
result in new metamaterials-based perfect absorbers are described.
Figure 20 (left) indicates a very instructive experimental result: reflectivity R from an
ultra-thin (500 nm) SiC film backed up by a metal mirror. At w = 762 cm- 1 (or 'A = 13.1
µm) reflectivity from the structure is less than 3 percent. That implies 97 percent
absorption in a film of thickness d = 'A/25 . This remarkable absorptivity can be
explained using the well-known microwave concept: the Salisbury screen. It turns out
that at 'A = 13.1 µm film thickness is d = 'A/4n, where n = n,.e + in;111 is the complex
refractive index of SiC. A simple formula for the reflectivity from a metal-backed thin
film can be derived:
2
r, _ e2;0
0
R ­_1.....::....--2;,5-1
(5)
1- r0e
where r0 = n - l is the reflection coefficient from the air/SiC interface and 5 =nf<od is the
n+l
complex phase shift across the film which includes losses. Fixing the laser frequency
w =cf<o and the sample's thickness d, we can plot the reflectivity as a function of re(n)
and im(n). As can be clearly seen from Figure 20 (right), there is a "sweet spot"
corresponding to specific values of re(n) and im(n) that results in vanishing reflectivity
(or perfect absorption). R vanishes when the quarter-wavelength condition is
approximately satisfied:
re(n)k0d =(2m+l)/2
(6)
where mis an integer. Finite reflection and imperfect absorption result from lower (or
higher) values of im(n), Note that, coincidentally, for the case of heavily doped SiC
predicted reflection is only 3 percent. However, for most materials (that is, Si for visible
light) absorption is too low for perfect absorption. Therefore, a question is posed: Can
one modify the structure of the metal screen to enhance absorption? It is CM Ms that
enable such functionality?
UNCLASSIFIED//FOR QFFI€ililik HS! OHL I
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