Extraordinary ultrasonic wave manipulation with reflective elastic metasurface

Abstract

This thesis aims to investigate and propose new ideas to achieve novel wavefront manipulation techniques with reflective elastic metasurface for various ultrasonic applications, which have been impossible previously. Elastic metasurfaces, artificially and engineered subwavelength surfaces whose properties cannot be discovered in nature, have attracted great attention for their special performances to open new breaks in engineering. However, the studies on elastic metasurface, which has to consider various wave modes, have been relatively less than those of acoustic or electromagnetic metasurfaces. Accordingly, the elastic metasurfaces are still at toddler level from the point of view of real-life applications. Although a large variety of innovative wave physics have been realized by using the concept of metasurfaces, in the field of elastic waves, there has been an ambiguous gap between elastic metasurfaces and their utilizing for real-life applications, which may severely hinder their further improvement.

In this thesis, by utilizing the concept of metasurfaces, new ideas that can revolutionize the key technologies required in a huge variety of ultrasonic applications were proposed, which have been impossible or difficult previously. Priority, we newly developed and revisited the background physics of metasurface, such as the generalized Snell’s law and time delay of higher-order diffraction, in elasticity, allowing it possible to utilize the concept of metasurfaces in the field of elastic waves. Based on these background physics, we carried out three individual studies using elastic metasurfaces that can be actively applied to key technologies in various ultrasonic applications, such as mode conversion, wave absorption and scattering.

First, we were able to achieve the total mode conversion over a wide incident range using a metasurface in which a sufficiently high phase gradient was introduced. This metasurface has special point that it forbids the formation of reflected longitudinal wave and redirects shear wave only in highly redirectional range of longitudinal wave incidence over the critical angle. The total mode conversion with this simple condition in the proposed metasurface was fully demonstrated and verified by numerical simulations and ultrasonic experiments. Second, an elastic metasurface for effective wave absorption, namely dissipation-amplifying metasurface, was achieved. By introducing two different physics, local resonance and conversion of surface waves, the energy of elastic waves can be significantly concentrated in metasurface region, and the reflected longitudinal and shear waves can be remarkably reduced for longitudinal wave incidence by the proposed metasurface with a small loss. It was also supported by numerical simulations and experimental validations. Finally, an elastic metasurface that can achieve the effective loss of elastic wave energy, which we call elastic meta-scatterer, was realized and numerically validated by exhibiting compact and high quality scattering.

Since the various extraordinary elastic wave manipulations are possible as desired with thin artificial layer and simple structure, we believe that proposed ideas of reflective elastic metasurfaces can not only be effectively applied in numerous ultrasonic applications, but can also fill the ambiguous gap between elastic metasurfaces and their utilizing for real-life applications.