The effect of a magnetic field on electrons has a broad impact. It could lead to many unique phenomena in materials. For many applications, it would be helpful if the same could be achieved for vibrations and sound waves. For a very long, scientists have wanted to know whether an effect similar to a magnetic field on electrons could be achieved on sound, which has no charge.
In a new study, scientists from AMOLF used a network of vibrating nano-strings controlled with light to control the sound flow. They were able to make sound waves move in a specific irreversible direction and attenuate or amplify the waves in a controlled manner for the first time. This gives rise to a lasing effect for sound.
Surprisingly, scientists came up with new mechanisms called ‘geometric phases.’ This mechanism allowed them to control and transmit sound in systems that were thought to be impossible.
Group leader Ewold Verhagen said, “This opens the way to new types of (meta)materials with properties that we do not yet know from existing materials.”
The magnetic field for sound
As mechanical vibrations have no charge, they do not respond to magnetic fields. But, they are sensitive to the radiation pressure of light.
Scientists hence used laser light to influence mechanical nano-resonators.
Verhagen said, “We have now shown that if we make a network of multiple vibrating nano-strings, we can realize a range of unconventional vibrational patterns by illuminating the strings with laser light. For example, we managed to get sound particles (phonons) to move in a single direction in the same way as electrons in the quantum Hall effect.”
Scientists also realized that the radiation pressure could also be used to control the amplification and attenuation of the sound.
Verhagen said, “Such amplification or attenuation is impossible for electrons in a magnetic field.”
The scientists are the first to carry out experiments where the driving light amplifies sound waves while also assuring that they are subjected to a magnetic field-like effect.
Verhagen said, “We discovered that amplification and breaking the time-reversal symmetry leads to a range of new and unexpected physical effects. First of all, laser light determines the direction in which the sound is amplified. In the other direction, the sound is blocked. This is caused by a geometric phase: a quantity that indicates the extent to which the sound wave is shifted as it passes through the network of nano-strings, which in this case is caused by the radiation pressure.”
“Our experiment allowed us to control and alter that geometric phase fully. In addition, we used the radiation pressure of the laser light to amplify the sound. That sound can even spontaneously oscillate, like a light in a laser. We discovered that the geometric phase we apply determines whether that happens or not, and with what strength of amplification.”
“We discovered that new geometric phases could be realized in systems where that was not considered possible. In all of these, the phases influence the sound waves’ amplification, direction, and pitch.”
“Geometric phases are important in many branches of physics, describing the behavior of different systems and materials. When combined with magnetic fields, they can lead to a topological insulator for electrons, but which properties a ‘sound’ variant based on the discovered principles could have is something we still need to learn. However, we do know that this will not be similar to anything we know.”
“We could further investigate the effects by linking more nano-strings in acoustic ‘metamaterials’ that we control with light. But the effects that we have observed should apply to a range of waves without charge, including light, microwaves, cold atoms, et cetera. We expect that with the new mechanisms we have discovered, it will be possible to produce new (meta)materials with properties that we do not yet know from existing materials.”
“Such materials and systems have unusual properties that might have useful applications. It is still too early to provide a complete overview of the possibilities. However, we can already recognize some potential directions. For example, a unidirectional amplifier for waves could have useful applications in quantum communication. We could also make sensors far more sensitive by breaking the time-reversal symmetry.”
Journal Reference:
- Javier del Pino, Jesse J. Slim, Ewold Verhagen, Non-Hermitian chiral phononics through optomechanically-induced squeezing, Nature, 2 June (2022). DOI: 10.1038/s41586-022-04609-0