Karlsruhe Institute of Technology: New materials: When sound waves run backwards

Acoustic waves in gases, liquids and solids usually have an almost constant speed of sound. So-called rotons are an exception: the speed of sound changes significantly with the wavelength, and waves moving backwards are also possible. Researchers at the Karlsruhe Institute of Technology (KIT) are working on using rotons in artificial materials. These metamaterials, designed on the computer and produced with ultra-precise 3D laser printing, could in the future manipulate or direct sound in unprecedented ways. The scientists are currently reporting on their work in Nature Communications. 

Rotons are quasiparticles, which means that they behave in a similar way to free particles. In contrast to normal acoustic waves in gases, liquids and solids, the speed of sound changes significantly with the wavelength. In addition, three different partial waves occur for certain frequencies. “The slowest of them is a backward wave: the energy flow and the wave fronts run in exactly opposite directions,” explains Professor Martin Wegener from the Institute for Applied Physics (APH) and the Institute for Nanotechnology (INT) at KIT. Understanding and using quasiparticles like rotons is one of the great challenges of quantum physics. The physicist Lew Landau, who received a Nobel Prize in 1962 for his pioneering work, she predicted in connection with superfluidity, a state in which a liquid loses its internal friction and acquires an almost ideal thermal conductivity. Up to now, rotons could only be observed under special quantum physical conditions at very low temperatures – and therefore eluded technical use.

Rotons without any quantum effects

That could change in the future: In the Cluster of Excellence 3D Matter Made to Order at KIT and Heidelberg University, a group of researchers is working on metamaterials that, as it were, breed rotons. Metamaterials have optical, acoustic, electrical or magnetic properties that do not occur in nature. The scientists propose such an artificial material that shows rotons without any quantum effects under normal ambient conditions and at almost freely selectable frequencies or wavelengths. This could make it possible in the future to better manipulate sound waves in air or in materials, for example throwing them back, redirecting them or generating echoes. These materials have not yet been demonstrated experimentally; However, it should be possible to produce them with the help of ultra-precise 3D laser printing, for example. “In the meantime we have even made some of these metamaterials,” reports Professor Martin Wegener. “We are currently working intensively on the direct experimental detection of rotons.”

3D printing as the gateway from the digital to the physical world

The researchers came to the computer-aided virtual design of materials with such novel properties through a mixture of reflection, many discussions, and numerical simulations and optimizations, as Dr. Yi Chen reports, the first author of the publication, whose work as a postdoctoral fellow at KIT is financed by the Alexander von Humboldt Foundation and is also embedded in the Helmholtz “Material Systems Engineering” program, which was launched in 2021. “In general, we have the dream of designing materials on the computer and then translating them directly into reality – without years of trial and error. 3D printing is then, as it were, just an automated converter from the digital to the physical world, ”explains Professor Martin Wegener.

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