Advanced Wave Control Techniques with Metasurfaces for Low-Frequency Elastic/Acoustic Wavefront Shaping

Abstract

Metasurfaces are wavefront shaping devices that offer compact and versatile wave control capabilities by modulating the phase and amplitude of impinging waves within subwavelength-scaled patterns. Initially developed for electromagnetic waves, the metasurface concept has been extended to acoustic and elastic regimes. Unlike traditional elastic wave control approaches based on phononic crystals that result in larger structures for low frequencies (longer wavelength) due to its unit cell dependence on the wavelength, or locally resonant metamaterials that involve heavy mass attachments to operate at low ambient vibration frequencies, elastic metasurfaces offer an elegant, compact and lightweight solution for controlling wave propagation, making them especially beneficial for low-frequency applications. While metasurfaces excel in controlling optical and acoustic waves, mapping their success to the elastic domain is still challenging due to differences in their intrinsic nature. Elastic waves possess a variety of polarizations, complex interactions with boundaries, and typically feature longer wavelengths and lower frequencies compared to optical and acoustic waves, making it challenging to efficiently couple them to thin metasurfaces. Therefore, there are research gaps that impede the integration of metasurfaces into elastic wave control. For instance, few studies have explored metasurfaces for shaping low-frequency elastic wavefronts. Additionally, most existing designs have fixed configurations and rely on linear structural properties to modulate the wavefront, restricting their effectiveness to narrow bandwidths, specific dimensions, and limited wavefront control capabilities. Addressing these challenges prompts key research questions: (i) how to efficiently modulate elastic waves within thin metasurfaces; (ii) how to design reconfigurable metasurfaces for tunable wavefront control; (iii) how to unlock new functionality. To address these inquiries, this thesis explores new metasurface designs for achieving broadband, reconfigurable, versatile, and unconventional wavefront shaping. The research vision is realized by synthesizing wavefront control techniques, local resonance, mode conversion phenomenon, origami art, electromechanical coupling, circuit shunt techniques, and structural nonlinearity. This dissertation presents five novel metasurfaces. First, an elastic metasurface composed of slender beams is proposed to control Lamb wave propagation. Unlike previous studies relying on numerical approaches for mechanism exploration, this study establishes an analytical model to guide the elastic metasurface design and facilitates the understanding of wave-material interactions inside the metasurface. Next, this research endows metasurfaces with tunable capabilities by exploiting different mechanisms. One approach involves rod-nut resonators, enabling unconventional wavefront control tailored for various polarizations with and without mode conversion. Another approach draws inspiration from traditional origami art, utilizing an array of zigzag-base folded sheets to achieve different performances based on folding angles. Aside from mechanically reconfigurable elements, an electromechanical metasurface is synthesized that leverages single- and multi-resonant piezoelectric shunts for tunable wavefront tailoring without structural modification, enabling multiband control. Finally, this research harnesses the nonlinearity in the metasurface design. Given the complexity of elastic waves, this dissertation launches the study in the acoustic regime, proposing a nonlinear acoustic metasurface concept composed of the locally resonant unit cells formed by curved beams. By exploiting the nonlinearity and the nature of mode shapes, this nonlinear metasurface can effectively generate a second-harmonic wave in the transmitted region and simultaneously modulate its wavefront for various functions. Overall, this research explores unconventional approaches to shape low-frequency elastic/acoustic wavefronts, uncovering fundamental insights into the intricate interplay between metasurface and the underlying wave physics. The findings hold promise for diverse engineering applications, e.g., lensing, energy harvesting, imaging, signal processing, and wave-based computing.