Acoustic waves can tunnel over a vacuum gap beyond the charge-charge interaction distance because the mechanical displacements in piezoelectric materials transmit along macroscopic electric fields. Complete acoustic wave tunneling hasn’t been rigorously shown, though, and the conditions needed for achieving complete tunneling haven’t been determined.
In a new study, physicists demonstrate analytically that sound can be transmitted strongly across a vacuum region! Physicists have shown that if the materials are piezoelectric, a sound wave can jump or “tunnel” fully across a vacuum gap between two solids.
Since an electric field can exist in a vacuum, vibrations (sound waves) in such materials also result in an electrical response, which allows the sound waves to be transmitted. It is necessary for the gap to be smaller than the sound wave’s wavelength. As long as the vacuum gap is made smaller as the frequencies grow, this effect operates not only in the audio range of frequencies (Hz-kHz) but also in ultrasound (MHz) and hypersound (GHz).
Professor Ilari Maasilta from the Nanoscience Center at the University of Jyväskylä said, “In most cases, the effect is small, but we also found situations where the full energy of the wave jumps across the vacuum with 100 % efficiency, without any reflections. As such, the phenomenon could find applications in microelectromechanical components (MEMS, smartphone technology) and heat control.”
Additionally, scientists demonstrate that to achieve complete tunneling experimentally, the piezoelectric material’s surface electric impedance and effective surface permittivity can be connected to the complete transmission condition.
Scientists noted, “We support our findings with numerical results for the maximum power transmittance of a slow transverse wave tunneling between identical ZnO crystals. The results show that complete tunneling can be achieved for many orientations.”
Journal Reference:
- Geng, Z., Maasilta, I.J. Complete tunneling of acoustic waves between piezoelectric crystals. Commun Phys 6, 178 (2023). DOI: 10.1038/s42005-023-01293-y