Flexible broadband acoustic metamaterials

Abstract

Metamaterials are artificial structures designed to provide exotic properties and unconventional functionalities for wave control. In the past two decades, the concept of acoustic metasurfaces has been developed as an evolution of acoustic metamaterials, allowing realization of wavefront modulation or absorption with meta-lenses in a planar profile. This thesis focus on how to broaden the bandwidth of acoustic metasurfaces and match their impedance to that of the surrounding materials. It may as well provide strategy that enhance the flexibility of metasurfaces. The work aims to optimize the properties of metasurfaces for practical application scenarios. After a review of metamaterials and metasurfaces, the research work in this thesis starts with a thorough investigation of gradient helicoid metamaterials both with theoretical exploration and experimental demonstration. By using relatively larger pitch at both end of the helicoid unit-cell, the interface between metamaterial and the background media gets better impedance matching over a large frequency range and weakens the Fabry-Perot resonance. The phase shift over spectra is also in a gentle slope. The "V"-shaped pitch distribution of helicoid metamaterial brings broadband focal lens, which has been verified in experiment. The monotone distribution of pitch offers an ideal braodband sound amplifier. The following chapter focuses on the modular design of metasurface for anomalous refraction to provide adjustable refractive angle. This work opens a new freedom of metasurface design based on the rearrangement of unit-cells. A modular-designed metasurface shows three different working modes when reordering their unit-cells. The phenomena have been experimentally demonstrated. To show the practicality of anomalous refraction, a braodband Bessel beam launcher is realized. Final part is theoretical generalization of helicoid metamaterial. The formation of helicoid metamaterial has been demonstrated both for their shape and acoustic characteristics. A spiralling metamaterial is proposed as a general model. This model gives an analytical description to the effective impedance and refractive index. Furthermore, this new developed general model can perfectly match the impedance to the background media and inherits all the advantages of helicoid metamaterials. A three-dimensional wavefront converter has been designed and experimentally measured to show its functionality and broadband performance.