Defense Intelligence Reference Document Metamaterials For Aerospace Applications

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described in Figure 9. A sub-wavelength object (for example, two slits) is located at the
bottom of a multi-layer super-lens. Another sub-wavelength grating is cJeposited on top
of the super-lens. Because the super-lens (and for that matter, any indefinite
permittivity material) is capable of propagating sub-diffraction waves, the
electromagnetic perturbations created by the object are propagated through the super-
lens upwards, until they encounter the sub-wavelength grating. At that point these sub-
wavelength perturbations arediffracted on the image-releasing grating and converted
into the far-field electromagnetic' waves. Those far-field waves are collected by the
objective of a microscope and observed through the eyepiece. The schematic is shown
in Figure 9(a). Note that, again, there is no need for NSOM. The actual implementation
of the FSL used the following object: a nanowire pair with 50 nm wide slit and 70 nm
gap inscribed by focused ion beam on a 40 nm thick Cr film on the quartz substrate.
Diffraction-limited image from a conventional optical microscope cannot resolve the two
nanowires (NA= 1.4, Ao= 377 nm) as can be seen in Figure 9(c), but the FSL-equipped
microscope can as shown in Figure 9(d).
Despite the success of this demonstration, there are serious issues involved in imaging
sub-wavelength objects. Specifically it is pointed out in Reference 27 that multiple
diffractive orders can become entangled, (that is, launched in the same direction into
the far field). Disentangling these diffraction orders is very important. The payoff would
be imaging of fully 2-D (flat) objects with a resolution smaller than the period of the
image-releasing grating. More precisely, this ambiguity is illustrated by the right panel
in Figure 10. If the sub-wavelength object is represented by the continuous spectrum
(blue line), then the spectrum can be sampled within the discrete set of "zones" which
are defined by the diffractive orders of the image-releasing grating. The width of each
zone is 2(i)/<:, and they are labeled as 151: order, 2nd order, and so forth. Wave numbers
belonging to the different zones can be diffracted onto the same far-field detector as
explained in Figure 10. In order to disentangle the 1st and the 2nd zones, a single
detector cannot provide sufficient information. It turns out that using two detectors (A
and B) and two laser beams (Beam A and Beam B} provides additional information that
is sufficient to disentangle the two zones. This additional information is obtained by
comparing the Intensity on the two detectors A and B. Another advantage of this •
imaging technique is that it is interferometric in nature. Therefore, even if the
contribution of some of these spectral zones' orders is very weak, it can still be
detected because of the high sensitivity of the interferometric techniques. What makes
this interference special is that it involves sub-diffractive waves propagating through
the indefinite permittivity meta material. Below some of the experiments conducted in
the laboratory that dem,onstrate such interference are discussed.
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