Researchers from the University of Jyväskylä in Finland have demonstrated theoretically that acoustic waves can tunnel completely across a vacuum gap between two piezoelectric crystals. This phenomenon, known as acoustic phonon tunneling, was analytically proven and verified through simulations.
Acoustic waves are vibrations that propagate through a solid material medium as phonons, the quantum mechanical description of vibrations. However, it was previously thought impossible for acoustic waves to transmit energy across a vacuum gap, since no solid medium exists to carry the vibrations.
In piezoelectric materials like crystals, mechanical strains are coupled to electric fields. The researchers showed that when an acoustic wave hits the surface of a piezoelectric crystal, it generates an evanescent electric field that decays into the vacuum gap. If another piezoelectric crystal is brought within a wavelength’s distance, this electric field can couple to it and transmit the acoustic wave energy across the gap.
Through theoretical derivations, the researchers identified the precise condition to achieve complete tunneling, where 100% of the incoming acoustic wave’s power is transmitted to the second crystal. This relies on exciting a resonant coupled leaky surface wave at the interfaces. Simulations verified this effect for acoustic waves propagating in arbitrary directions in ZnO crystals, a common piezoelectric material.
The results prove that efficient acoustic phonon tunneling is possible between piezoelectric crystals separated by a vacuum gap up to hundreds of nanometers wide. This demonstration of strong coupling of mechanical and electrical effects could impact nanoscale heat transfer, optomechanics, and other areas involving manipulation of phonons. If experimentally realized, the complete tunneling effect could enable new applications like precise nanoscale gap distance control. Overall, this research significantly advances the fundamentals of acoustic wave physics across vacuum gaps.