Generalized Snell's law

http://www.sciencemag.org/content/334/6054/333.full

Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface, which are in excellent agreement with generalized laws derived from Fermat’s principle. Phase discontinuities provide great flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer metallic interfaces.

Subwavelength focus of sound

In focusing waves, one is often faced with the so-called diffraction limit as a result of the wave nature, which limits the resolution when seeing objects using waves. Now there came an interesting study beating this limit by focusing sound into a 1/25th wave length spot. Remarkably, this is attained with Coke cans !

Sound, like light, can be tricky to manipulate on small scales. Try to focus it to a point much smaller than one wavelength and the waves bend uncontrollably — a phenomenon known as the diffraction limit. But now, a group of physicists in France has shown how to beat the acoustic diffraction limit — and all it needs is a bunch of soft-drink cans.

Scientists have attempted to overcome the acoustic diffraction limit before, but not using such everyday apparatus. The key to controlling and focusing sound is to look beyond normal waves to 'evanescent' waves, which exist very close to an object's surface. Evanescent waves can reveal details smaller than a wavelength, but they are hard to capture because they peter out so quickly. To amplify them so that they become detectable, scientists have resorted to using advanced man-made 'metamaterials' that bend sound and light in exotic ways.

Some acoustic metamaterials have been shown to guide and focus sounds waves to points that are much smaller than a wavelength in size. However, according to Geoffroy Lerosey, a physicist at the Langevin Institute of Waves and Images at the Graduate School of Industrial Physics and Chemistry in Paris (ESPCI ParisTech), no one has yet been able to focus sound beyond the diffraction limit away from a surface, in the 'far field'. "Without being too enthusiastic, I can say [our work] is the first experimental demonstration of far-field focusing of sound that beats the diffraction limit," Lerosey says.

Lerosey and his colleagues took a similar approach to an experiment they performed in 2007 and later described theoretically for electromagnetic waves^1,2. The group generated audible sound from a ring of computer speakers surrounding the acoustic 'lens': a seven-by-seven array of empty soft-drink cans. Because air is free to move inside and around the cans, they oscillate together like joined-up organ pipes, generating a cacophony of resonance patterns. Crucially, many of the resonances emanate from the can openings, which are much smaller than the wavelength of the sound wave, and so have a similar nature to evanescent waves.

To focus the sound, the trick is to capture these waves at any point on the array. For this, Lerosey and his team used a method known as time reversal: they recorded the sound above any one can in the resonating array, and then played the recording backwards through the speakers. Thanks to a quirk of wave physics, the resultant waveform cancels out the resonance patterns everywhere — except above the chosen can.

After the playback, the can continues to resonate by itself, scattering out the sound energy left inside. Normal waves scatter efficiently, so they disappear quickly. However, the evanescent-like waves are less efficient at scattering, and take roughly a second to make it out of the can — a prolonged emission that allows the build up of a narrow, focused spot. In fact, Lerosey's group found that the focused spot could be as small as just 1/25th of one wavelength, way beyond the diffraction limit. The results are due to be published in Physical Review Letters³.

There is some debate among acoustic scientists as to whether this is the first time anyone has truly beaten the acoustic diffraction limit. Mechanical engineer Nicholas Fang at the Massachusetts Institute of Technology in Cambridge thinks that the results are a first because the focal point is away from the lens, in the far field. But John Page, a physicist at the University of Manitoba in Winnipeg, Canada, who has published evidence for sub-wavelength focusing in the near field⁴, disagrees. "Super-resolution is super-resolution, no matter in what regime it is obtained," he says.

Still, Page calls the Lerosey group's work "a very important accomplishment" and believes it could find many applications, such as feeding energy to tiny electromechanical devices so they can operate.

Lerosey himself thinks that the simplicity of the apparatus is what bodes so well for applications. "To me, this experiment says, 'we can do it easily, even with Coke cans,' and it opens a door."

[http://www.nature.com/news/2011/110708/full/news.2011.406.html]

A One-Way Wall of Silence

In a previous entry I highlighted this work. Here is another citation of it [http://www.sciencemag.org/content/331/6022/twil.full?sa_campaign=Email/toc/3316022twil]:

The recent development of metamaterials and photonic crystals has provided a route to control the propagation of electromagnetic waves through the engineered structure of a material. Combined with transformation optics, such control is rewriting the expected rules of behavior governing the propagation of electromagnetic waves, and offers myriad possibilities ranging from imaging to communications and stealth applications. Sound is also a wave, and so the manipulation of acoustic waves may be expected to carry over by analogy to their electromagnetic counterparts. Li et al. present a sonic crystal composed of a periodic array of steel rods, the geometry of which was selected to give rise to a band gap, whereby the transmission of sound waves in a specific frequency range is prohibited in one direction but allowed in the opposite direction. The authors also show that by mechanically changing the spacing of the array (by rotating the square steel rods), the diode-like behavior can be switched on and off. A range of applications might be expected to follow, from acoustic isolation and filtering to ultrasound imaging.

Phys. Rev. Lett. 106, 84301 (2011).

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]

Toward engineering the color of metals by carving rings on the surface

The electrons in the metal are nimble and mobile and able to conduct electricity. The behaviors are controlled by two things: the band structure and the coulomb interactions. The elementary excitations of this sea of electrons may not be simply fermionic quasi-particles that partially resemble the original electrons. They can also be bosonic, for example, plasma. Such plasmons are hardly excitable by low energy probes, such as visible light. But they can indeed be created by X-ray. What controls the colors are basically visible light. To understand the colors of a particular metal, one needs know how the visible light interacts with which kind of elementary excitations of the similar energy scales. To describe this interaction, one may assume quantum mechanics, but the usual Maxwell equations will suffice, because the visible light has a wave length between 400nm to 760nm, which are indeed very long in comparison with the metallic band gaps of the order of nm (and hence, only the single partially filled band needs be considered). Basically, one has to treat an entangled system of light and electrons, the exact solution of which is a considerable problem. Usually, one treats the metal as a medium that is characterized by a complex dielectric function of frequency. This function determines which photon will be absorbed and which can be transmitted and which will be reflected. The reflected light decides the color. Most naturally occurring metals bear silver color. This is because, the spectrum encoded in the imaginary part of the dielectric function is a continuum in the visible light energy window, rather than a discrete set of resonances. Is it possible to tune the color of a metal without affecting its conductivity? The answer is yes. Due to the complex part of the dielectric function, visible light can hardly enter the bulk metal and can penetrate only a very thin layer near the surface, an effect called "skinning effect". Thus, the colors are actually controlled by the skin. By manipulating the surface electron spectrum, one should be able to tune the color. This has been achieved in a latest work by Jianfa Zhang at the University of Southampton and a few pals [arXiv:1011.1977v1 ]. See a review from Arxiv Blog [http://www.technologyreview.com/blog/arxiv/]:

Their idea is to carve a different type of repeating pattern on to the surface of a metal.

These patterns are smaller than the wavelength of visible light. Instead of causing the light to interfere, they work by changing the properties of the sea of electrons in the metal--in particular its resonant frequency. This alters the frequency of light it absorbs and reflects.

This is the same technique that researchers have been using for some time to build invisibility cloaks . The idea is that by carefully building repeating patterns of subwavelength structures, researchers can tailor the way a "metamaterial" can steer light.

But instead of creating 3D structures that steer light as it passes through the material, Zhang and co carve the relevant structures onto the surface to control the way light is absorbed and reflected.

The structures that do the trick are tiny rings carved into the surface. The team calculate that they can make gold or aluminium appear almost any colour simply by varying the size and depth of these rings. They've even demonstrated the technique on a thin layer of gold.

Simulating metric signature effects with metamaterials

A funny work here.

We demonstrate that the extraordinary waves in indefinite metamaterials experience an effective metric signature. During a metric signature change transition in such a metamaterial, a Minkowski space-time is created together with a large number of particles populating the space-time. Such metamaterial models provide a tabletop realization of metric signature change events suggested to occur in Bose-Einstein condensates and quantum gravity theories.

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.

Cherenkov radiation (CR) in negative refractive materials

Nature provides infinite number of substances with amazing properties. That is why scientists go back from time to time to learn from nature. The design of solar cells may benefit a great deal from inspecting how a plant makes food via photo-induced chemical reactions. What underlies the colorful butterfly wings is at all a marvelous texture, which is now known as photonic crystals. Nevertheless, sometimes the nature seems wanting in its diversity. An example is the so-called negative refraction materials (NRM), which are up to now not available naturally. All known NRM are made artificially, going under the name 'composite left-handed meta-materials'.

NRM is special in that, it has a negative refractive index, which means the refracted ray will lie on the same side as the incident ray, relative to the normal of the interface. This property stimulates novel ways of light manipulation. Many application are bound to take place. Years ago[1], it was utilized to make perfect lens, which has a remarkable resolution that is smaller than light wavelength, an impossible thing with conventional positive refraction materials. More innovations will surely come out soon.

Again due to this negative refraction, Cherenkov radiation will also be quite different. Actually, it is the reversed CR that happens. Namely, as a charged particle passes through an NRM dielectric at a speed greater than the light speed in this dielectric, backward radiation will be experienced. A direct experimental observation of this is much rare. In Ref.[2], a vivid simulation, however, was conducted. Despite this, it is still highly desirable to observe what will happen if a real beam is passed through the NRM.

A question: can we someday find a naturally available NRM ? This would be very interesting and exciting ... ...

[1]PRL, 85:3966(2000)
[2]PRL, 103:194801(2009)