Phase-guided approach for elastic metasurface through achromatic principle

Abstract

The purpose of this thesis is to focus flexural waves for multi-frequencies and multi-incident angles through achromatic metasurface. With the recent emergence of wave control theory through phase gradients on surfaces, there has been active research on wave control technologies using metasurfaces, which are thin films composed of artificial structures much smaller than the wavelength. By employing artificially engineered structures to implement the desired phase gradients, metasurfaces can create boundary conditions that are not typically observed, enabling various wave control phenomena. Examples include controlling transmittance through impedance matching, adjusting specific refraction and reflection angles, and even achieving complete transmission and refraction between different media. Particularly, wave focusing technology, which converges the propagation direction of circular waves to a specific direction, is gaining attention for its potential applications in fields such as ultrasound and speaker sound field control. However, wave focusing technology using metasurfaces has a significant drawback, which is that its performance changes drastically with varying frequencies. Consequently, wave focusing can only be implemented within a very narrow frequency range, making practical applications extremely difficult. To overcome this limitation, recent proposals of achromatic metasurface technology have provided a breakthrough, allowing consistent wave-focusing performance across a range of frequencies. Nonetheless, previous theoretical approaches have a limitation where they can only be designed within a very limited space due to geometric substitutions. Therefore, we propose a phase-based design approach from the principle of achromaticity to overcome these limitations. Additionally, to verify the validity of the theory, numerical analysis will be conducted for both normal and oblique incidence across various frequencies. It is expected that the newly proposed achromaticity-based design approach will be used more efficiently in various fields, such as ultrasound sensing and energy harvesting.