Focusing of flexural waves achieved by pillared metasurface

Metasurfaces are an artificial structural material that can be employed to precisely manipulate mechanical waves through an array of unit cells with a nonlinear phase response. The overall thickness of such materials is usually less than one wavelength of the incident wave, so they provide a broad prospect in the miniaturization or high integration of functional devices.

For industrial applications, a metasurface should ideally be compact, lightweight, and simple in structure with robust mechanical properties. Wang et al. designed and fabricated a pillared metasurface that fulfills these practical requirements while maintaining its wave functionalities. It consists of a transverse line of graded height resonant pillars with identical subwavelength diameters arranged on a flat plate.

“At the micron scale, this working mechanism of pillared metasurfaces can be used to design on-chip devices, microelectromechanical systems, wearable devices, and opto-mechanical coupling devices,” said author Yabin Jin. “At the millimeter scale, this work has a great significance for energy harvesting, nondestructive testing, and so on.”

The researchers employed the finite element method for numerical simulation and 3D printing to fabricate the sample. Experiments involved the excitation of flexural waves with piezoelectric ceramic transducers arranged on the upper surface of the plate. Their out-of-plane displacement was measured with a laser Doppler vibrometer.

The pillared metasurface was able to successfully focus an incident plane flexural wave into a spot. The focusing effect was robust to both disordered perturbation of geometric parameters and frequency fluctuations. The phase response of the pillared units spans a 2pi shift range, which is the basis for an arbitrary manipulation of the flexural wavefronts in the transmitted region. In addition, the resulting transmitted wave maintained a high amplitude.

This paper is part of the Phononic Crystals at Various Frequencies collection; learn more here .

Source: “Experimental realization of a pillared metasurface for flexural wave focusing,” by Wan Wang, Julio Iglesias, Yabin Jin, Bahram Djafari-Rouhani, and Abdelkrim Khelif, APL Materials (2021). The article can be accessed at http://doi.org/10.1063/5.0052278