Square-shaped structures can topologically guide water waves

Topological insulators have protective properties that allow for the efficient guided transmission of energy in both quantum and classical wave systems and are robust against disorder. While canonical graphene-like structures are the most common topological insulator candidates, Makwana et al. demonstrated the use of square lattices to obtain strong topological confinement in a highly dispersive water wave system.

In the past, applications of topological insulators have been limited to nondispersive waves, such as electromagnetic or acoustic waves. In order to be implemented in a highly dispersive system, active components that require complex experimental setups should be avoided in order to preserve time-reversal symmetry. Using a passive design consisting of a grid of square-shaped aluminum pillars, the researchers overcame this limitation and induced quasi-topological modes in water by avoiding the need to feed any energy into the system.

“We still obtain many desirable properties of topological insulators while using a straightforward and simple design that leverages other abstract areas of mathematics,” said author Mehul Makwana.

To test their setup, the group placed the array in a water tank and used a paddle to generate waves. In the recorded images, they observed topological states originating at the interface of the array. Based on these results, they were able to extend the design to numerically create a three-way topological splitter – which cannot be done in the canonical hexagonal system – to confine and direct wave energy.

The authors note these results can be used to develop efficient energy harvesters in water-wave systems. By changing the size or spacing of the pillars, different components of the water channel can be selected, allowing for localized energy extraction.

Source: “Experimental observations of topologically guided water waves within non-hexagonal structures,” by Mehul Makwana, Nicolas Laforge, Richard Craster, Guillaume Dupont, Sébastien Guenneau, Vincent Laude, and Muamer Kadic, Applied Physics Letters (2020). The article can be accessed at https://doi.org/10.1063/1.5141850 .