Jan 2 2019
Scientists at the City College of New York (CCNY) and at the Advanced Science Research Center (ASRC) at The Graduate Center of The City University of New York have created a novel metamaterial that is capable of transporting sound in extraordinarily robust ways along its edges and localizing it at its corners.
A recent paper published in Nature Materials revealed that the newly developed metamaterial forms a strong acoustic structure that can regulate the propagation and localization of sound in unusual ways, and even when fabrication imperfections are present. This special property can possibly enhance technologies using sound waves, like ultrasound devices and sonars, and thus make them more impervious to defects.
The study is a joint effort between the lab of Alexander Khanikaev, a professor at CCNY’s electrical engineering and physics departments and also affiliated with the ASRC, and the lab of Andrea Alù, director of the ASRC’s Photonics Initiative. The duo’s development is based on studies that ushered in a field of mathematics known as topology into the materials science realm. The term topology refers to the study of an object’s properties that are unaffected by constant deformations. For example, a donut can be said to be topologically equivalent to a plastic straw, because both of them have a single hole. It is possible to mold one into the other by either adding new holes or deforming and stretching the object, and without tearing it.
Topological insulators—unique materials that conduct electric currents merely on their edges and not in the bulk—were first predicted and subsequently discovered by researchers with the help of topological principles. Their extraordinary conduction properties arise from the topology of their electronic band gap, and hence they are remarkably impervious to constant changes, like imperfections, noise, or disorder.
There has been a lot of interest in trying to extend these ideas from electric currents to other types of signal transport, in particular to the fields of topological photonics and topological acoustics. What we are doing is building special acoustic materials that can guide and localize sound in very unusual ways.
Andrea Alù, Director, Photonics Initiative, ASRC, The Graduate Center, The City University of New York.
In order to develop their new acoustic metamaterial, the researchers 3D-printed a series of minute trimers, organized and joined in a triangular lattice. Three acoustic resonators were included in each trimer unit. The trimers’ rotational symmetry as well as the lattice’s generalized chiral symmetry imparted special acoustic properties to the structure; these properties originate from the topology of their acoustic bandgap.
The resonators’ acoustic modes hybridized, resulting in an acoustic band structure for the entire object. Consequently, when sound is played at frequencies beyond the band gap, it can propagate via the material’s bulk. However, when sound is played at frequencies within the band gap, it can simply travel along the edges of the triangle or be localized at its corners. According to Alù, this property is unaffected by disorder or fabrication errors.
“You could completely remove a corner, and whatever is left will form the lattice’s new corner, and it will still work in a similar way, because of the robustness of these properties,” stated Alù.
If scientists want to break these properties, they would need to reduce the material’s symmetry by, for instance, altering the coupling between resonator units, which, in turn, alters the band structure’s topology and thereby alters the properties of the material.
We have been the first to build a topological metamaterial for sound supporting different forms of topological localization, along its edges and at its corners. We also demonstrated that advanced fabrication techniques based on 3D printed acoustic elements can realize geometries of arbitrary complexity in a simple and flexible platform, opening disruptive opportunities in the field of acoustic materials. We have been recently working on even more complex 3D metamaterial designs based on these techniques, which will further expand the properties of acoustic materials and expand capabilities of acoustic devices.
Alexander Khanikaev, Professor, Departments of Electrical Engineering and Physics, The City College of New York.
“We’re showing, fundamentally, that it is possible to enable new forms of sound transport that are much more robust than what we are used to. These findings may find applications in ultrasound imaging, underwater acoustics and sonar technology,” said Alù.
The study is the result of a joint effort funded by the Defense Advanced Science Research Projects Agency (DARPA) Nascent program and the National Science Foundation (NSF) EFRI program.