Scientists make breakthrough in preserving the integrity of sound waves

In a breakthrough for physics and engineering, researchers from the Photonics Initiative at The Advanced Science Research Center at The Graduate Center, CUNY (CUNY ASRC), and Georgia Tech presented the first topological order demonstration based on time modulations . This advance allows researchers to propagate sound waves along the boundaries of topological metamaterials without the risk of the waves traveling backwards or being frustrated by material defects.

The new findings, which appear in the journal. Scientific advancesIt will pave the way for cheaper and lighter devices that use less battery power and can operate in harsh or dangerous environments. Andrea Alù, founding director of the CUNY ASRC Photonics Initiative and professor of physics at The Graduate Center, CUNY, and postdoctoral research associate Xiang Ni authored the article, along with Amir Ardabi and Michael Leamy of Georgia Tech.

The topology field examines the properties of an object that are not affected by continuous deformations. In a topological insulator, electrical currents can flow along the boundaries of the object, and this flow is resistant to being interrupted by imperfections in the object. Recent progress in the field of metamaterials has expanded these characteristics to control the propagation of sound and light following similar principles.

In particular, previous work by Alù Labs and City College of New York professor of physics Alexander Khanikaev used geometric asymmetries to create a topological order in 3-D printed acoustic metamaterials. In these objects, sound waves were shown to be limited to traveling along the object’s edges and around sharp corners, but with one significant drawback: these waves were not completely restricted: they could travel forward or backward with the same properties. This effect inherently limits the overall soundness of this approach to the topological order of sound. Certain types of clutter or blemishes would actually mirror back the sound that propagates along the boundaries of the object.

This latest experiment overcomes this challenge, showing that breaking of time inversion symmetry, rather than geometric asymmetries, can also be used to induce topological order. Using this method, sound propagation becomes truly one-way, and strongly robust to clutter and blemishes.

“The result is a breakthrough for topological physics, as we have been able to show the emerging topological order of temporal variations, which is different and more advantageous than the large body of work in topological acoustics based on geometric asymmetries,” said Alù. “Previous approaches inherently required the presence of a backward channel through which sound could be reflected, which inherently limited its topological protection. With time modulations we can suppress backward propagation and provide strong topological protection.”

The researchers designed a device made of a series of circular piezoelectric resonators arranged in repetitive hexagons, like a honeycomb network, and attached to a thin disk of polylactic acid. They then connected this to external circuits, which provide a time modulated signal that breaks the time inversion symmetry.

As an added benefit, its design allows for programmability. This means they can guide the waves along a variety of different reconfigurable paths, with minimal loss. Alù said that ultrasound imaging, sonar and electronic systems using surface acoustic wave technology could benefit from this advance.