Event Cloak

I just bumped into this funny stuff [http://physicsworld.com/cws/article/indepth/46376]:

But imagine if we could make a cloak that operates not only in space but in time as well. To understand how such a "space–time" cloak might work, consider a bank housing a money-filled safe. Initially, all incoming light continuously scatters off the safe and its surroundings, revealing the rather dull scene of an undisturbed safe visible to surveillance cameras. But imagine, near some specified time, splitting all the light approaching the safe into two parts: "before" and "after", with the "before" part sped up, and the "after" part slowed down. This would create a brief period of darkness in the stream of illuminating photons. If the photons were a stream of cars on a motorway, it is as if the leading cars were to speed up and those trailing behind were to decelerate, creating a gap in the traffic edged by bunches of cars (a dark period with bright edges – see t₃ in figure 1).

Now imagine that during the moment of darkness, a safe-cracker enters the scene and steals the money, being careful to close the safe door before he leaves. With the safe-cracker gone, the process of speeding up and slowing down the light is reversed, leading to an apparently untouched, uniform illumination being reconstituted. As far as the light reaching the surveillance cameras is concerned, everything looks the same as it did beforehand, with the safe door firmly shut. The dark interval when the safe was cracked has literally been edited out of visible history.

To complete our motorway analogy, it is as if the cars have acted to first open up and then close a gap in traffic, leaving no disturbance in the flow of vehicles. There is now no evidence of that temporary car-free interlude, during which the proverbial chicken may even have crossed the road without getting squashed. So by manipulating how light travels in time around a region of space, we can, at least in principle, make a space–time cloak that can conceal events – an "event cloak", if you will.

Cloaks work with a mirror

Cloaks are such inspiring objects that intrigue both physical and atheistic innovative can only be quested by physicists to the end. Now a work from Singapore reported a cloak working with a mirrow: the thing of a 2mm to be hided is placed on a mirror and covered by a cloak made of calcite. One only perceives the mirror but not thing under carpet [PRL 106, 033901 (2011)].

Invisibility cloaks, a subject that usually occurs in science fiction and myths, have attracted wide interest recently because of their possible realization. The biggest challenge to true invisibility is known to be the cloaking of a macroscopic object in the broad range of wavelengths visible to the human eye. Here we experimentally solve this problem by incorporating the principle of transformation optics into a
conventional optical lens fabrication with low-cost materials and simple manufacturing techniques. A transparent cloak made of two pieces of calcite is created. This cloak is able to conceal a macroscopic object with a maximum height of 2 mm, larger than 3500 free-space-wavelength, inside a transparent liquid environment. Its working bandwidth encompassing red, green, and blue light is also demonstrated.

Cloak for sounds

The idea of cloaks that make objects disappear is really capturing. It has been realized at least in the lab by a theory called "transformation optics", which relates to the transformation properties of Maxwell's equations. However, this idea not only blossoms in optics but also in acoustics, where similar equations exist in 2D. A new paper in PRL designs a cloak for ultrasonic waves[Phys. Rev. Lett. 106, 024301 (2011) – Published January 10, 2011]. The difficulty lies in varying the density of materials in a desired way[Physics 4, 2 (2011)].

Invisibility devices based on coordinate transformation have opened up a new field of considerable interest. We present here the first practical realization of a low-loss and broadband acoustic cloak for underwater ultrasound. This metamaterial cloak is constructed with a network of acoustic circuit elements, namely, serial inductors and shunt capacitors. Our experiment clearly shows that the acoustic cloak can effectively bend the ultrasound waves around the hidden object, with reduced scattering and
shadow. Because of the nonresonant nature of the building elements, this low-loss ( 6 dB=m) cylindrical cloak exhibits invisibility over a broad frequency range from 52 to 64 kHz. Furthermore, our experimental study indicates that this design approach should be scalable to different acoustic frequencies and offers the possibility for a variety of devices based on coordinate transformation.

Cloak enough to cover visible objects

A team in UK recently fabricated a cloak that works under green light to make optical delusions [http://arxiv.org/ftp/arxiv/papers/1012/1012.2783.pdf]. This cloak is large and able to cover an object as large as 10mm^3. The physics is governed by Maxwell's equations. The substance is calcite, which has anisotropic optical properties.

Insights of the decade from Science

Now we are coming to the end of not only this year but also the first decade of this century. Science has its list of the insights of this decade in science. In materials physics, the meta-material and the related conformal optics which underlies the operation of these materials are enlisted. The ground breaking papers are as follows: [http://www.sciencemag.org/site/special/insights2010/]

• U. Leonhardt, "Optical Conformal Mapping," Science 312, 1777 (2006).
• J. Pendry et al., "Controlling Electromagnetic Fields," Science 312, 1780 (2006).
• D. Schurig et al., "Metamaterial Electromagnetic Cloak at Microwave Frequencies," Science 314, 977 (2006).
• T. Ergin et al., "Three-Dimensional Invisibility Cloak at Optical Wavelengths," Science 328, 337 (2010).

Invisible gateway

Manipulating light with meta-materials is under intensive study recently. In a work published in PRL, the researchers realized an invisible gateway, which is an open channel but appears not there for certain frequency light. The produces optical illusions. [Phys. Rev. Lett. 105, 233906 (Published December 2, 2010)]

In 2009, a team of researchers led by Che-Ting Chan at the Hong Kong University of Science and Technology theorized on using transformation optics and complementary media to produce optical illusion devices that change the optical response of an object into that of another object. Illusion optics, the science of making an object appear as something else, or reappear elsewhere in space, or even disappear altogether (cloaking) is full of exciting possibilities, pending experimental realization.

In a paper in Physical Review Letters, Chao Li and co-workers at the Chinese Academy of Sciences, Beijing, and colleagues at Soochow University, China, and Hong Kong University of Science and Technology, experimentally demonstrate the first illusion-optics device. They trick light to miss an open channel across a slab at a frequency range of interest, rendering the channel into an electromagnetically invisible gateway. Li et al.’s design involves carving out an open channel across a metamaterial slab that behaves as a perfect electric conductor, then replacing a trapezoidal region of the slab adjacent to the channel with another metamaterial having the exact opposite dielectric properties. This “double-negative” region complements the dielectric space inside the channel into an optically equivalent region that behaves as a perfect electric conductor, thereby giving the appearance of a blocked gateway to light that attempts to go through.

Li et al. use a transmission-line approach that allows them to design metamaterials with the desired optical properties and with minimal losses. Their illusion-optics prototype works at around ~50 MHz and has a ~15 MHz bandwidth. [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.105.233906]

Invisibility in visible light has to remain tiny?

Manipulation of light rays has entered a new era since the first proposal based on matamaterials. Such materials feature negative refraction index. The reason is because they have both negative dielectric constant and magnetic permittivity. This double-fold negativeness makes an unusual left-handed system of electric field, magnetic field and the energy current vector (which defines the light propagation direction), thus, when considering the continuity of these fields along the tangent at the interface of two meeting media, the incident light shall be deflected in a counter-intuitive manner. This behavior has been utilized to make cloaks that enables the undetection of things beneath it. Up to now, all designs apply to only longwavelength detecting rays. Is it plausible to make a cloak that works under visible light, and how ?

A recent work published in PRL seems detering the interest. It claims that, cloaks for visible light, if feasible, shall be very tiny and can not cover large objects. Nonetheless, there are oppositions to this claim, arguing that, it is rational only for resonant-type devices. So, how will this dubious topic advance further ? Let's see.

Science 25 June 2010:
Vol. 328. no. 5986, p. 1621
DOI: 10.1126/science.328.5986.1621-a

Even so, a broadband cloak cannot be much bigger than the wavelengths at which it works, Johnson and colleagues argue. In a paper in press at Physical Review Letters, they consider a simple scenario in which a pulse of light with a range of wavelengths descends on a flat object covered by a cloaking layer. If the object were not there, the light pulse would take more time to reach the surface and bounce back. So to hide the object, the cloak must delay the light pulse. And for the cloak to do that correctly over the entire wavelength range, its thickness must increase in proportion to the height of the hidden object, Johnson argues.

The thicker the cloaking layer, however, the longer the light pulse will remain in the material and the more light the cloak will absorb or scatter. If the cloak is too thick, that light loss becomes noticeable. Johnson and colleagues estimate that researchers might someday beat down the losses enough to cloak a meter-sized object at microwave wavelengths. At optical wavelengths, the losses are orders of magnitude too high to conceal such a large object, they say. A cloak for infrared or visible light cannot be more than a few micrometers across, they conclude.

Not everyone is convinced. Johnson's argument applies only to resonant systems, Pendry contends; it does not prove you cannot make a large nonresonant cloak. "It's not Moses descending from the mountain and saying you can't do it," Pendry says. "It's a rider saying that there may be some complications." Johnson says the result is general.

Cloaking is only one application for the concept of "transformation optics" that Pendry has pioneered, and others could prove more important. Still, it would be disappointing if all you could hide in your personal invisibility cloak were an eyelash.

Chiral Swiss rolls show a negative refractive index

The following article also comes from JMCP 2009 highlights and is also interesting:

M C K Wiltshire, J B Pendry and J V Hajnal, J. Phys.: Condens. Matter 21 (2009) 292201 (5pp) doi:10.1088;

Chiral Swiss rolls, consisting of a metal/dielectric laminate tape helically wound on an insulating mandrel, have been developed to form the basis of a highly chiral metamaterial. We have fabricated these elements using a custom-built machine, and have characterized them. We find that the permeability, permittivity, and chirality are all resonant in the region of 80 MHz. The chirality is so strong that it can be directly measured by observing the magnetic response to an applied electric field, and is larger than either the permeability or the permittivity. We have estimated the refractive indices from these data, and find both strong circular.

A review on cloaking theory

Scientists and novelists have been intrigued for centuries by the possibility of hiding an object so completely that neither trace of the object nor of its cloak is to be found. Recent theoretical developments show that cloaking is, in principle, possible for electromagnetic waves and to a limited extent for other types of wave, such as acoustic waves. An energetic program of experimental research has shown some of the schemes to be realizable in practice.

We have a touching faith in the ability of our eyes to tell the truth. No other sense has such confidence invested in it, so when our eyes deceive us the result is bewilderment, giving rise to appeals to magic or even the supernatural. This explains the enormous interest aroused by recent work on invisibility and the cloaking of objects from electromagnetic radiation. In this article we review the theories and experiments behind the hype and suggest what devices might realistically be expected in the near future and what is likely to prove impossible.

Hard wired into our brains is the expectation that light travels in straight lines. Mostly this is true, but there are well-known exceptions, such as mirages, which occur when a hot surface heats the air above, reducing its density and hence creating a refractive index gradient immediately above the surface (Fig. 1, top). Such a gradient bends the trajectories of light rays so that an observer misinterprets where the light is coming from. Typically, light from the sky is refracted by the gradient, giving the appearance of water shimmering in the distance—hence a cruel illusion seen by a thirsty traveler in the desert or, more prosaically, the appearance of a wet road on a hot day.

It is the ability of refractive index gradients to bend light that the invisibility engineer exploits. Light is steered around the hidden object by a cloaking device, and then returned to the same straight line trajectory, rather as a skier would make a chicane around a tree (Fig. 1, bottom). The observer’s brain is unaware of the possibility of chicanes and sees only that which is behind the cloak and nothing of the cloak itself or of its contents. The real challenge of cloaking lies in deriving a theoretical prescription for the optical properties of the cloak and, even more challenging, realizing these properties in a material. Transformation optics provides the theoretical background and metamaterials provide the means of achieving the prescribed parameters.

shielding earthquakes

Earthquakes tend to cause disasters to humans. It is desirable to screen them. Seismic waves are generally composed of two components, the transverse one (i.e., the S waves, which represents the up-down vibrations of crust) and the longitudinal one (P waves, left-right vibrations). The latter travels faster and can reach more distant places, while the former is more fierce. Here is a piece of work coming up with a design, which assumes a concentric structure, to shield P waves. Their numeric simulations show that, this design is efficient with a broad frequency band.
(1)Ultrabroadband Elastic Cloaking in Thin Plates
(2)brief introduction to seismic waves