Some sound barriers, like those along motorways in built up areas, have dimension in the metre scale and yet the operating principle is the same as the one of stopping sound in the micro- and nanometre range. The impact in smaller scales is relevant for dissipation mechanisms taking place in laboratory- scale devices used in, e.g., quantum technologies, highly sensitive position sensors as well as photo- acoustic devices used in medical imaging, all of which are usually affected by the thermal energy background arising from regularly vibrating atomic chains, called phonons.
The idea is simple: design a structure with the right distribution of mass density in a judiciously designed pattern so that destructive interference of sound or mechanical waves cancels the unwanted frequencies. However, the physical realisation and study of such a structure is nothing but simple, requiring state-of-the-art nanofabrication and imaginative experimental methodology to extract the information needed to fully characterise the structures supporting a mechanical frequency gap.
This is precisely what we have achieved: a design based on the geometry of etched clover leaves realised in a free-standing silicon membrane, measured by Brillouin light scattering following a novel methodology which yields unambiguously the spectrum of mechanical frequencies in the GHz range.
The core concept is phononic crystals, a structure with a periodic distribution of masses and well- defined symmetry. By means of sound waves interference ranges of frequencies are selected forming a band structure which shows frequency gaps – the mechanical band gaps. No sound wave can travel in the material at those frequencies. In fact, our structure was a phononic waveguide supporting two modes in the gap, akin to optical modes travelling in an optical fibre, only that here the mechanical modes are in the hypersonic range. We are now turning to modes with minimum or no losses, in the jargon, topological protected modes.
The results from the collaboration between the ICN2 and the Danish Technical University appeared in Nature Nanotechnology accompanied by a News and Views article highlighting the work.