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]

Giant Electroresistance

There are many devices that are of fundamental interest. The most latest and famous may be the one invented by Fert et al., which displays giant magnetoresistance. In the past few years, there have appeared a flurry of work exploiting ferroelectrics as the barrier layer. I have mentioned quite a number of them in this blog. Here comes a new one. It is made up by sandwiching a BTO layer with two LSMO at different dopings. All these efforts are clearly directed to manipulate the interweaving properties of spin, charge and orbital degrees.

A giant tunneling electroresistance effect may be achieved in a ferroelectric tunnel junction by exploiting the magnetoelectric effect at the interface between the ferroelectric barrier and a magnetic La1 xSrxMnO3 electrode. Using first-principles density-functional theory we demonstrate that a few magnetic monolayers of La1 xSrxMnO3 near the interface act, in response to ferroelectric polarization
reversal, as an atomic-scale spin valve by filtering spin-dependent current. This produces more than an order of magnitude change in conductance, and thus constitutes a giant resistive switching effect. [PRL 106, 157203 (2011)]

Accoustic diode

This is a device that allows one-way propagation of sound. Obviously, such device must break time reversal symmetry. [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.084301]

The device consists of a two-dimensional sonic crystal arranged in a mesh of square steel rods. By rotating the steel rods, Li et al. are able to manipulate the unit cell of the sonic crystal element to turn the diode on (sound waves only propagate one way) and off (sound waves can move back and forth). Furthermore, Li et al. make their device entirely from linear acoustic materials, which allows them to control sound propagation with a simpler and more efficient process, over a broader bandwidth, and with lower power consumption, compared to existing nonlinear sonic-crystal-based devices.

Inventing new physical systems

Particle physicists can hardly invent new physical systems but only to discover the already existing matter and energy world. Condensed matter physicists tend to create their own systems to meet fundamental intelligent challenges and practical ends. The as-created new physical systems are surely immense and the underlying physics are also diverse, but the mathematical models seem not so diverse: it is frequently the case that the model constructed for one system may be transplanted to describe another system, with proper interpretation of the symbols. In other words, there exists some kind of universality. This scenario offers opportunities for both theorists and experimentalists: (1) the theorists can make predictions about system A via the knowledge of system B if A and B are found sharing the same mathematical structure; while (2) the experimentalists can simulate system A by measuring system B. Such possibility drives the emergence of a host of artificial systems. The following is a list:
(1) p-n junction and transistors;
(2) 2DEG;
(3) Optical lattice and Ultra-cold atoms;
(4) Photonic crystals;
(5) Metamaterials;
(6) Circuit QED;
(7) Cavity QED;
(8) Trapped ions;
(9) Graphene, and CNTs;
(10) Topological insulators;
(11) ...

What may come out of a two-species-fermionic systems ?

The following is from a recent paper: Physics 3, 58 (2010) DOI: 10.1103/Physics.3.58, which is a comment on the work [Phys. Rev. A 82, 011605 (2010) ].

Caption of the above figure:

Parallel layers of fermionic atoms offer rich new physics. One species of fermionic atoms (A) is constrained to move only on two thin layers separated by distance d. Another species (B) is free to move in three-dimensional space. (a) At large interlayer separation, the A atoms only interact within a layer as they are dominated by p-wave interaction, forming a BCS-type pairing as shown by the rotating atoms. (b) If the interlayer spacing is small, the A atoms will pair up across layers. (c) If the interaction between A and B species is strong, they will form “molecules” or (d) three-body Efimov states involving A atoms in each layer and a B atom.