3. Applications to Antennas, Scattering, Imaging and Cloaking Devices
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admin   |SAdministrator |2008-06-12 11:38:15
Prof. Raj Mittra:

A. Achieving superlensing by using DNG media that magnify evanescent waves.

Perhaps no topic has drawn more attention of metamaterial researchers as that of fabricating a superlens by using a DNG slab. The figure that has appeared more than any other in the metamaterial literature is the picture of refracting rays that originate from a point source, undergoing negative refraction through the DNG slab, and then focusing on the other side to form the image in a manner predicted by Vesalago, back in 1968.
The important question we raise is: Does the effective medium approach to characterizing a DNG-type slab, which is physically realized by employing periodic inclusions such as split rings and dipoles in a background medium, accurately describe the refraction of wave in the medium. Or is the real-life behavior of the propagating fields substantially different from that predicted by using the eps_eff and mu_eff parameters to describe the DNG medium?
We have carried out extensive numerical simulation of this type of DNG lens configuration and have found that the behavior of the fields inside the slab (or even outside in the image region) is very different from the refraction of rays predicted by the effective medium description of the DNG slab, which is supposed to exhibit negative refraction.
Has anyone else studied the field propagation inside a DNG slab, comprising of SRRs and dipoles and, if so, would care to share their findings with us? In particular, have they found any evidence of magnification of evanescent waves in actual physical (not effective) medium?
mittra  - b. Negative Refraction in a DNG slab made of SRRs   |Author |2008-06-16 10:14:55
Let us assume that we have constructed a DNG slab by using SRRs and dipoles and have evaluated its effective medium properties from its reflection and transmission characteristics at normal incidence. Next let us follow the usual practice of replacing the slab by its effective ε and
μ and let us consider a half-space of this negative index medium with free-space above it. If we now come in from the top with a ray (or a Gaussian beam) from the left of the normal to the interface, we would expect the ray to bend toward the left of the normal in order to satisfy Snell’s law at the interface and to match the projections of the k-vectors of the two media along the interface. We have carried out a rigorous numerical simulation of the physical structure of this geometry using a Gaussian beam, and have found that the field behavior inside the slab is very different from that predicted by its effective medium model.
We point out an interesting fact which may be helpful for explaining the discrepancy alluded to above. When we come in with a plane wave at normal incidence, its H-field is normal to the planes of the SRRs and, hence, the wave interactions with the medium to yield negative mu and epsilon. However, we note that the phase matching condition is imposed on the tangential components of the k-vectors, and that the propagation characteristic of a wave traveling along the interface is very different for this medium than it is along the normal direction, since the SRRs do not react with the H-fields of the wave in the former case. But once we replace the physical structure with its effective medium characteristics, as is the common practice, we implicitly assume that the medium is homogeneous and the propagation characteristics of waves traveling through the homogenized medium are independent of the direction of propagation. This is obviously not true for the SRR+dipole structure we are considering here.
mittra  - c. Performance enhancement of microstrip patch ant   |Author |2008-06-16 10:23:14
A large number of recent publications have proposed performance enhancement of conformal antennas, e.g., microstrip patch antennas (MPAs), by covering them with a DNG superstrate (planar), whose function is to act as a Veselago-type lens, and focus the energy emanating from the MPA at infinity to realize increased directivity. It has been convincingly demonstrated that directivity enhancement can indeed be achieved by using DNG superstrates, comprising of SRRs+dipoles, as well as other DNG materials that are periodic structures. If we think only in terms of their effective medium descriptions, we would restrict ourselves solely to DNG media and rule out the use of DPS or ENG type of superstrates. This is because planar DPS slabs wont exhibit lensing properties, and ENG slabs wont let the radiation from the MPA through (because its refractive index would be pure imaginary). However, we have found that both DPS and ENG superstrates can not only achieve substantially higher directivities than realized via the use of DNG slabs, but have much lower losses as well. We would like to hear from others who have similar experiences. Comments, questions and explanations, all are welcome.111
mittra  - d. Cloaking   |Author |2008-06-16 10:23:48
Another topic which is currently drawing considerable attention of the metamaterial community is cloaking. The objective is to coat a target with metamaterials such that it becomes invisible to the interrogating wave. The issue here is the realizability of the desired profile for the effective and that would provide the designed shielding characteristics. This is especially true when we wish the cloak to be effective for arbitrary polarizations and angle of incidence, and to be effective over a wide bandwidth as well. We have seen pictures of missiles and aircrafts in several presentations with the authors claiming that they would make such structures invisible by using cloaking.
Let us first recognize up front that the radar world wants a coating which would work over a wide band, preferably 2-18 GHz, or at least a substantial fraction thereof. In addition, they would like to have the shield working for all incidence angles and both polarizations. Despite all the buzz in the literature and media announcements that we have seen about cloaking, no one but no-one comes close even to realizing these goals. Not only are the cloaks designed to-date very narrowband, their RCS may even be higher than the case without the cloak, when the frequency, the incident angle, the polarization, or combinations thereof are changed.
sergei  - Cloaking - reply to R. Mittra   |Publisher |2008-07-29 05:23:37
Yes, the military would like to hide targets at all frequencies, all incident angles, and all polarizations. However, the conventional stealth techniques reduce only RCS but not the TOTAL scattering cross section. Shaping of an object only redistributes scattered power over angles. Even if we cover a target with an IDEAL absorber (zero reflection at all incident angles and polarizations), the total cross section is reduced only by 50 percent (the shadow is not affected).

Cloaking is a very different technique that reduces the TOTAL scattering cross section, and this can have many different (from stealth) applications. Yes, the first realization based on resonant particles is very narrow band. But there are alternative approaches which show wideband effects. Transmission-line cloaks are very broadband. A known example shows at least 75 percent reduction of the total scattering cross section from very low frequencies up to 2 GHz (although this is still 2D and single-polarization device). Once again, shaping and absorbers cannot reach so high reduction levels due to fundamental restrictions!
rmarques  - Cloaking - reply to Sergei   |Author |2008-08-14 09:25:09
Dear Sergei

There is a very fundamental argument against wideband cloaking of electrically big objects (like aircrafts): Since the electromagnetic signal must go around the object, the phase velocity near the object surface must be higher than the velocity of light in free space c. However, group velocity can not be higher than c. This means that the medium around the object must be highly dispersive. And this necessarily implies narrow band.
sergei  - Cloaking of electrically large objects   |Publisher |2008-09-11 07:54:56
Dear Ricardo,

Yes, I fully agree with your comment. To overcome this fundamental difference we suggested to let the signal go THROUGH the object. Of course, this implies making holes in it, so this method will apparently be useless for hiding aircrafts. But one can make invisible, for instance, a huge metal mast (if the mast is made not of a solid piece of metal, but of inter-connected metal rails).
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