Diodes, in the form of thermionic valves and semiconductor devices have helped revolutionize electronics. Their defining property is that they only allow current to flow through them in one direction. Over the last 10 years several studies have been published on producing rectifying diodes for other propagating phenomena, including sound. Such a system was experimentally demonstrated late last year by a group of Chinese researchers [Liang et al., Nature (2010) 9, 989]. Now, just a few months later a separate Chinese team has succeeded in producing an alternative acoustic diode system, which can also be switched on and off [Li et al., Phys Rev Lett (2011) 106, 084301].
Both systems rely on acoustic metamaterials; so called sonic crystals that only allow certain frequencies of sound to propagate. In the original acoustic diode, one side of the sonic crystal was combined with a highly non-linear (NL) material, capable of changing the frequency of incident sound waves. As the sonic crystal only lets certain frequencies propagate through it, only sound waves converted by the NL materials could pass through.
The new system uses a slightly different process. One side of the sonic crystal is given a different periodicity, producing a corrugated diffraction structure. This results in a change of the spatial frequency. Lu and Chen told Materials Today that “Compared to the previous acoustic diode, our device leads to a totally linear unidirectional effect with higher efficiency, broader bandwidth, and much less power consumption.”
The diode consist of square steel rods in air, each having a width of 4 mm and a spacing of 3 mm. Toward one side of the sonic crystal, rods were removed to create a stepped diffraction structure, such that the rods were eventually separated by 38 mm. As the square rods broke the rotational symmetry of the unit cell, it was possible to tune the acoustic scattering by rotating the orientation of the blocks. A rotation of 45o allowed the diode to be switched off, such that sound was able to propagate in both directions.
The researchers believe that a similar methodology could be used to construct an on-chip isolator for acoustic waves, but also “extended to hypersonic frequencies whereby the device could be applied to manipulate the heat flow”. The researchers are also considering the possibility of producing “an acoustic transistor to manipulate, amplify and switch the incident waves by [using] acoustic or light waves”.


Stewart Bland