Taking the Wraps Off Cloaking

This is an excellent review article by John Pendry on left-handed material and its application to "cloaking". Pendry, as everyone should already know, revived this field of study with his work on metamaterial. So this article compliments quite well with the earlier article in Physics Today.

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Reverse Cherenkov Radiation Detected In Metamaterial

A group has claimed the detection of the reversed Cherenkov radiation in left-handed metamaterial. However, they didn't pass any charge particle through the metamaterial structure.

The second innovation is to emulate a charged-particle beam by means of a waveguide with a periodic array of slots, instead of using real charged particles [Fig. 1, right]. By doing so, Xi et al. solved the problem of extremely weak Čerenkov radiation in the microwave frequencies associated with moving charged particles. As an electromagnetic wave travels inside the waveguide, it emerges at each slot with a fixed phase retardation relative to the neighboring slots. This leaking radiation from the waveguide is equivalent to the radiation from a phased antenna array. The Fourier transform of the electric current carried by a moving charged particle results in a broad spectrum in the frequency domain. But as long as a single frequency is concerned, the current of the charged particle is equivalent to that of a phased dipole array, as mathematically proved by the authors. In this analog, the phase velocity of the electromagnetic wave propagating inside the waveguide corresponds to the moving speed of the charged particles in a regular Čerenkov radiation configuration.

In the experiment, the authors designed a waveguide with comparatively low refractive index of n ~0.5, which is equivalent to a charged particle moving with a speed twice as great as that of light in a vacuum. With this new experimental configuration, the radiation signal can be many orders of magnitude stronger than the traditional Čerenkov radiation induced by a fast charged-particle beam, and thus the Čerenkov radiation was directly observed along the backward direction within the left-handed frequency range of the metamaterials. It is worth noting that Grbic and Eleftheriades carried out an experimental attempt earlier at the University of Toronto, in which they observed the backward radiation from a low-index left-handed microwave transmission line into free space [8]. However, considering the fact that it is the phase velocity rather than group velocity of the electromagnetic wave propagating in the transmission line that corresponds to the speed of an equivalent moving charge, the Čerenkov radiation observed by Grbic et al. was indeed in the forward direction with respect to the direction of the equivalent moving charge.

I guess this is fine, but I'd say that one still needs to show this with actual particle beams. If this can't be done, then the claim that such phenomenon can be used as beam diagnostics doesn't quite hold.

Ironically, in the same issue of PRL, another paper gave a theoretical analysis of the detection of this reverse Cherenkov radiation of an electron beam passing into a left-handed metamaterial[1]. So it would be nice if one can actually detect this direction from electron beams, rather than simulated ones.

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[1] S.N. Galyamin et al. Phys. Rev. Lett. v.103, p.194802 (2009).

Invisibility Umbrella Would Let Future Harry Potters See the Light

This is a report on a very clever scheme where an object is placed next to a left-handed material, resulting in both having a "cloaking" property.

The trick, they report in a paper to be published in Physical Review Letters, is to embed a matching "antiobject"--the metamaterial equivalent of a voodoo doll--in the outer layer of the post. The scattering from the embedded antiobject exactly cancels the scattering from the object, Chan says, "so it looks as if there is nothing there." Because the hidden object remains outside the post or umbrella, it can detect light from its surroundings.

The scheme does have limitations, Pendry notes. The umbrella works for only a single frequency, has to be specifically tailored to the object to be hidden, and won't completely hide something that absorbs light. Still, Pendry says, "that's carping on my part--it's really a neat idea."

The application for left-handed materials is very vast. I just wish that we don't attach it to such popularization and suffer from the same misunderstanding as the "teleportation" phenomenon.

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Semiconductor Bends Light The "Wrong" Way

This is rather interesting. This is a report on a new material having a negative index of refraction by using layers of semiconductors. Before this, these so-called left-handed material can only be found in metamaterials, which consist of conducting rods and split-ring resonators.

The metamaterial is made by depositing alternating layers of two semiconductors -- indium gallium arsenide and aluminium indium arsenide -- onto a substrate using molecular beam epitaxy. Each layer of the metamaterial is about 80 nm thick, which is much smaller than the wavelength of the infrared light.

This might provide a better, more controlled way of fabricating such material. Before this, it has been a tedious process to assemble one of these metamaterial, especially if one wants it to work in the viable region of microwave or shorter.

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Left-Handed Natural Material

Left-handed material are those that have a negative index of refraction. It means that both the permittivity and permeability of the material are both negative.

Till now, such material have been man-made, and they are called meta-material. This is because the material consists of structures made that is considerably smaller than the wavelength of interest. In this way, the EM wave does not see the material as consisting of discrete structures.

However, there is now a new report that showed for the first time that a natural material can have a negative refraction. Physicists in Germany has shown that metallic ferromagnet can exhibit such property up to the GHz range. This astounding result was published in PRL[1]. A summary of this discovery can be found at PhysicsWeb (requires free registration)

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[1] A. Pimenov et al. Phys. Rev. Lett. 98, 197401 (2007).

Hopes Dim for Perfect Lens

This could be a major setback for those working in trying to perfect "lenses" using the negative refraction phenomenon. A theorist has published a paper (link is open to all only for a limited time) arguing that there is an inherent problem with such a device - substantial absorption that cannot be overcome with current metamaterial structure.

Mark Stockman, a theoretical physicist at Georgia State University, argues that...

The electrons in the materials slosh in response to the electromagnetic fields in the light, and causality says that the precise arrangement and motion of the electrons can depend on light that has already passed through them--not on the light that has yet to arrive. Starting from that point, Stockman has shown mathematically that negative refraction and absorption are intertwined so that you cannot have one without the other, as he reports in a paper to be published in Physical Review Letters. "There is no way to decrease the losses," Stockman says. "The [negative refraction] effect will disappear." In particular, simple amplification won't do the trick, he says.

Of course, this will not stop others working in this field to either prove him wrong, or find a way around this.

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Negative Index of Refraction

A discussion elsewhere made me realize that, even though this area is taking off like mad and it has been known for quite a while, a number of people are still unfamiliar with it, especially the concept. So I thought I'd give what I think is the best "tutorial" on this area. It is a Physics Today article co-written by John Pendry, so I believe is one of the discoverer/inventor of the metamaterial that exhibit such property.

I'm a bit familiar with this area because one of our graduate student is working in it. He's trying to build a set of these metamaterial into a waveguide structure. We're hoping to have electron beam bunches going through these material and maybe generate a reverse Cerenkov radiation. If we can do that, it will be a major accomplishment, not just in terms of the physics, but also for electron beam diagnostic. This is because the Cerenkov radiation will not be in the forward direction as in ordinary medium, along with other forms of radiation that can add to the noise of the detected signal, but rather in the backward direction, which is less noisy.

However, the task in doing this is formidable. The structures on printed circuit boards, and have to be alligned very carefully. We also don't know how well it will do when it encounters our electron beam. Previous results have also shown that the beam will excite several different modes that might drown out what we want.

Oh well. We shall see....

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More "Cloaking" Device

This is one of those things that get a lot of milage in the media. A "cloaking" device in the microwave region has been "successfully" demonstrated using a metamaterial consisting of split ring resonators.

http://physicsweb.org/articles/news/10/10/12/1

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