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New topological materials dramatically amplify sound waves

New topological materials dramatically amplify sound waves

In an international collaboration, scientists at the Dutch research institute AMOLF have developed a new metamaterial through which sound waves can propagate in a completely new way. Metamaterial amplifies mechanical vibrations in a previously unknown form, which has the potential to improve sensing and information processing technologies. This metamaterial is the first representative of the so-called Kitaev Bossonian series, whose special properties arise from its nature as a topological material.

Artist's impression of Kitaev's bosonic string: Several mechanical string resonators are connected in series using light. Mechanical vibrations (sound waves) are transmitted along the string and amplified.

To achieve this, the research team made nanomechanical resonators interact with laser light using radiative pressure forces. The discovery, published in the journal Nature, is the result of an international collaboration between AMOLF, the Max Planck Institute for the Physics of Light in Erlangen, the University of Basel, the ETH Zurich, and the University of Vienna.

The “Kitaev series” is a theoretical model that describes the physics of electrons in superconducting materials, especially nanowires. This model is particularly important because it predicts special excited states at the ends of such a nanowire: Majorana zero modes. They are of great interest because of their potential use in quantum computers. AMOLF group leader Ewald Verhagen: “We were interested in a model with an identical mathematical structure, but which describes waves such as light or sound and not electrons. Because these waves are made of bosons (photons or phonons) and not fermions (electrons), it was assumed that they would behave differently Exactly. As early as 2018, it was thought that the “Bossian Kitaev string” would exhibit remarkable behavior that was not yet known from any natural matter or metamaterial. Many researchers were very interested in showing this, but experimental implementation seemed elusive.

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Visual feathers

The “Kitaev bosonic series” is basically a series of coupled resonators. It is a metamaterial, that is, a synthetic material with artificial properties. The resonators represent the “atoms” of matter, and the way they are coupled together controls the collective behavior of the metamatter – in this case, the propagation of sound waves along the string. “Couplings – Kitaev’s bosonic chain links – must meet special requirements and cannot be produced using conventional springs,” says first author Jesse Slim. “We realized that we could create the necessary connections between nanomechanical resonators – tiny vibrations” of silicon strings on a chip – experimentally: we connect them using forces exerted by light and thus create “optical” springs. “By carefully varying the laser intensity, we were able to connect five resonators together and create a ‘Kitaev Bosson string.’”

Huge gains

The result amazed scientists. “Optical coupling is mathematically similar to superconducting bonds in the Kitaev fermionic series,” says Verhagen. “However, uncharged bosons are not superconducting; instead, optical coupling amplifies nanoscale mechanical vibrations. As a result, sound waves, or mechanical vibrations, propagating through the arrangement are dramatically amplified from one end to the other. Interestingly, the transmission of oscillations in the opposite direction is prohibited. And what is even more remarkable: if the wave is delayed a little – by a quarter of the period of oscillation – the behavior is completely reversed: the signal is amplified backwards and inhibited forwards. The “Kitaev Bossonian series” thus acts as an exceptional directional amplifier It could enable promising applications for signal processing, especially in quantum technology.

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Topological metamaterials

The special properties of Majorana zero modes in the Kitaev electronic series are related to the fact that the material is topological. In topological materials, some phenomena are inevitably related to the general mathematical description of the material. These phenomena are topologically protected, meaning they are guaranteed to exist even if the material has imperfections and defects. The 2016 Nobel Prize in Physics was awarded for understanding topological materials. However, these were materials that had no reinforcement or damping. The description of the topological stages involved in amplification remains the subject of intense research and discussion.

In collaboration with theorist Clara Wangora (Max Planck Institute for the Physics of Light), theorists Matteo Brunelli (University of Basel), Javier Del Pino (ETH Zurich), and Andreas Nonenkamp (University of Vienna), the AMOLF researchers showed that the “Bosonic Kitaev series” actually embodies a A new topology of matter. The observed directed amplification is a topological phenomenon related to this phase of matter, as researchers had already predicted in 2020. “For us, the experiments we report here are a confirmation of the considerations of the last few years,” says Andreas Nonenkamp of the University of Vienna. : “We have been able to discover a unique signature of this phase: when the chain is ‘closed,’ similar to a closed pendant, the amplified sound waves continue to circulate in the ring of resonators and reach a very high intensity, similar to the generation of powerful light beams in lasers.”