By Daniel Cressey

Theoretical suggestions for creating a one-way mirror for sound have finally been turned into reality by a device that blocks acoustic waves moving in a certain direction.

In the same way that electronic diodes permit current to move in only one direction, the team's "acoustic rectifier" converts a sound input to a new frequency and allows this frequency to move through the device, while preventing the original frequency from moving in the opposite direction. In certain fields, it could become as useful as electronic diodes have proved in computing.

Jian-chun Cheng and his colleagues at Nanjing University proposed a theoretical version of an "acoustic diode" last year. Now, in a paper published online on October 24 in Nature Materials, they show that theory can be turned into practice.

"With the first experimental realization of an acoustic rectifier, acoustic waves should no longer be considered to always travel easily in both directions in a given path, as perceived traditionally," they write.

One-way traffic

The device consists of two parts: a layer of ultrasound gel and a lattice comprising alternating layers of water and glass.

For sound to pass through the diode, sound waves must hit the gel layer first. The gel is a "non-linear medium" that converts some of the energy of the acoustic wave into a wave of double the frequency. The lattice then screens out all frequencies except this second, higher-frequency harmonic of the original wave, allowing some of the sound through--albeit at a higher pitch.

However, if the original acoustic wave attempts to pass the other way back through the device, it will first encounter the lattice. Because this wave has not passed through the gel layer--and thus has only the original frequency--it will be totally screened out by the lattice.

Such a device could be useful for medical ultrasound applications, for example, which can require extremely focused sound waves of specific frequencies, the authors write.

Sound idea

Chiara Daraio, an assistant professor of aeronautics and applied physics at the California Institute of Technology in Pasadena, describes the Nanjing team's realization of theoretical proposals as "a clever and original experimental setup."

However, she points out that the rectifier works only at predefined frequency ranges--a potential limitation of the device. In addition, it does not transmit the original frequency but increases it: "So it is like a one-way mirror that changes the color of the transmitted light," she explains.

Daraio's group are working on a diode that uses different types of acoustic crystal that would be more tuneable, she adds.

Nicholas Fang, who works on nanophotonics at the University of Illinois at Urbana-Champaign, says the invention is as important to the field as the invention of the electronic diode was. "It's definitely a great inspiration for people like me working in this field," he says.

Fang says the next major step will be to integrate devices and to go "from the diode to an integrated circuit." Whether this will be possible with the type of diode designed by Cheng and his team remains to be seen.