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An introduction to phononic crystals (4)

Any application for phononic crystals?

The phononic crystal concept is much too recent to have already impacted any object of everyday life. The following applications are up to now purely speculative. The first application suggested by Kushwaha and colleagues is acoustic insulation. Indeed, for all frequencies contained in a complete band gap, a phononic crystal behaves like an acoustic shield, sending back all incident waves. It is then perfectly possible to conceive soundproof fences for motorways in accordance with this principle. For such an application, the involved periodicities are of the order of the meter, and the design game amounts to shrinking this dimension while making sure that the band gap frequency range remains sufficiently low, the human hear being sensitive to frequencies between 20 and 20000 Hertz, typically.

A second potential application of phononic crystals, maybe less intuitive, is the realization of breakwaters. Indeed, the waves over seas and oceans can be understood as waves propagating over the surface of water. When waves strike an obstacle they are partially scattered in all directions. The interferences resulting from multiple scattering on a periodic lattice of obstacles could thus lead to an important reduction of the amplitude of waves reaching a shore. It would be needed to periodically anchor poles to the sea bottom, which would create a haven of peace behind a barrier astonishingly efficient though not filled. Hu and Chan from the Technology University of Hong-Kong have proposed in 2005 to use such structures to focus waves towards a plant in order to convert mechanical energy to electrical energy. For this last point obviously, everything still needs to be demonstrated!

Phononic crystal for surface acoustic waves

Phononic crystal for surface acoustic waves

This phononic crystal made of air holes in a silicon wafer was obtained with a scanning electron microscope. The holes have a diameter of 6 microns and are 100 microns deep. Such a phononic crystal presents a complete band gap around 500 MHz.

On our side, we believe that the most of the potential of phononic crystals will be revealed at the micro or nanoscopic scale. For periodicities of a few microns or less, band gaps appear at frequencies in the few hundred of MHz up to a few GHz range, exactly inside the field of wireless communications. New devices mixing microelectronics and acoustic waves could be designed and extend the capabilities of mobile phones and wireless networks, for instance. Microelectronics has begun approximately fifty years ago; photonics was born about twenty years ago and is becoming mature these days. Time will tell is phononics will follow these glorious examples.