June 18, 2009

Sound Technology Roundup: Sonar Cloaking, Sound laser, Black hole for Sound

1. A new invisibility cloak for sound could help doctors find tiny tumors or hide submarines from enemy sonar.

"Our focus is not about dampening noise, but to guide sound waves around structures," said Nicholas Fang, a professor a the University of Illinois at Urbana Champaign and coauthor, along with Shu Zhang and Leilei Yin, on a paper that appears in the journal Physical Review Letters.

Sound waves are larger than electromagnetic/optical waves. To manipulate either wave requires structures many times smaller than the size of the wave. Because the properties of the material are determined by their physical structure and not their chemical make-up, as they traditionally are, they are called metamaterials.

"If you need to build an ultrasonic metamaterial, the dimension of the physical structure is tens or hundreds of microns," said Fang. "Compare that with optical metamaterials, and you are talking about hundreds of nanometers. That makes it a lot more amenable for research."

The sonic metamaterial uses cubes and octagons to create holes that can then bend the wave around the structure. The most obvious application would be as a coating for submarines that want to avoid detection from enemy sonar.

Acoustic shield

2. UK and Ukrainian physicists have built the first "saser", or sound laser, able to generate terahertz-frequency sounds.

The saser produces a highly focused beam that is similar to the way a laser pointer produces a red spot when it hits a wall (Image: Tony Kent and American Physics Society)

The new saser is a semiconductor stack, made from thin, alternating layers of gallium arsenide and aluminium arsenide.

To fire the device, the upper part of the sandwich is exposed to an intense light beam. That excites electrons in the material, which then release sound waves, or phonons.

Those reach the lower part of the sandwich where they bounce off the interfaces between different layers. The spacing of the layers has been carefully chosen so that the weak echoes combine into stronger sounds in which all of the phonons are synchronised.

Those strong phonons reflect back into the upper sandwich where they interact with the light-excited electrons – causing them to release further phonons and amplify the signal.

The result is the formation of an intense series of synchronised phonons inside the stack, which leaves the device as a narrow saser beam of high-frequency ultrasound.

Although light is currently used to "pump" the saser, it should be possible to achieve the same effect electrically too, says Kent.

Saser beams that operate at much lower frequencies, in which the phonons oscillate a billion times per second (gigahertz) rather than a trillion times per second (terahertz), have been made before.

However, they have had little impact because there are other methods of generating sound at such frequencies, says Kent. "The saser could have a much bigger impact at terahertz frequencies, where other methods of generating coherent sound waves are not as well developed."

3. A sonic black hole in a density-inverted Bose-Einstein condensate.

We have created the analogue of a black hole in a Bose-Einstein condensate. In this sonic black hole, sound waves, rather than light waves, cannot escape the event horizon. The black hole is realized via a counterintuitive density inversion, in which an attractive potential repels the atoms. This allows for measured flow speeds which cross and exceed the speed of sound by an order of magnitude. The Landau critical velocity is therefore surpassed. The point where the flow speed equals the speed of sound is the event horizon. The effective gravity is determined from the profiles of the velocity and speed of sound

17 page pdf paper on the sonic black hole.

In conclusion, a sonic black hole/white hole pair has been realized in a Bose-Einstein condensate. The supersonic flow field results from the repulsion of the condensate by an attractive potential. The Landau critical velocity is exceeded by an order of magnitude. The effective surface gravity is extracted from the in-situ velocity and sound speed profiles. The flow field contains regions of negative and positive phonon energies. This property, combined with increased velocity gradients, could be used in a study of Hawking radiation.

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