Manipulating reflected acoustic wave via Helmholtz resonators with varying-length extended necks

Abstract

In this work, we develop a deep subwavelength metasurface which is capable of reflected wave manipulation arbitrarily. Each unit cell of the metasurface is constructed of a Helmholtz resonator with an extended neck. The possibility of creating a phase shift offered by different unit cells is analytically explored based on characteristic mode analysis and demonstrated by the finite element method. It is found that the phase shift of the reflected wave ranging from 0 to 2π in a supercell (consists of eight inhomogeneous unit cells) can be engineered by tuning the length of the extended neck. A periodical array of the supercell is used to construct the designed metasurface. The reflection performance of the proposed metasurface is investigated both numerically and experimentally, and good agreement is achieved. Anomalous phenomena such as converting an incident wave to a surface wave and negative reflection are demonstrated using the designed metasurface. The key features of the proposed metasurface are the thin thickness ≈λ/30 (λ is the operation wavelength), simple configuration, and easy fabrication, making it possess a promising potential in miniaturization and integration in acoustic devices.