Research And Applications On The Control Of Acoustic Waves By Transmissive Metasurfaces

As two-dimensional(2D)acoustic metamaterials,acoustic metasurfaces have gained more and more attention in recent years.acoustic metasurface is generally a planar metamaterial system with subwavelength thickness formed by arranging a variety of microstructured elements in a special sequence,which can be used to flexibly and effectively control the acoustic waves at various positions of the interface to achieve a variety of novel phenomena and functions.Acoustic metasurface has become a challenging and important cutting-edge research topic because of its great application prospects in many acoustic fields.In the first chapter,we reviews the basic theory and research background of metasurfaces,introduces the research content,progress and status quo of metasurface to control acoustic waves,and summarizes the main contents of this research work.In the last Chapter,we make a simple summary and outlook.In the other parts of the thesis,we study the control and application of transmissive metasurfaces to acoustic waves.The main topics cover four sections,including phase control of ultra-high transmission,pure amplitude control,decoupling control of phase and amplitude,high-efficiency broadband control of acoustic waves.For the phase control,in the second chapter of this thesis,the introduction of the coated labyrinthine structure(CLS)units greatly improve the transmission efficiency of the phase-controlled metasurfaces.CLS units provide a phase-controlled range of 0?2? while still maintain the ultrahigh transmission of nearly 100%.Based on the generalized refraction law,the high-efficiency transmissive metasurfaces constructed by the 2D CLS units realize the application of phase control such as acoustic anomalous refraction,Bessel beam generation,acoustic focusing and acoustic beam bending along any convex trajectory.Based on the iterative angular spectrum algorithm,the highly efficient transmissive metasurfaces constructed by the three-dimensional(3D)CLS units realize the acoustic imaging by controlling the phase.For the amplitude control,in the third chapter of this thesis,the amplitudes of the acoustic waves are controlled by the transmissive metasurfaces through the introduction of apertured structure(AS)units.The AS unit can provide a change in transmittance in the range of 0-100%to modulate the transmitted acoustic amplitude while it does not change the transmission phase.The simple structures of the AS units make them possible to neglect their own viscous losses during application,which is conducive to miniaturization of the device design.Based on genetic algorithm,the metasurface constructed by 2D AS units achieves multi-focal focusing by pure amplitude modulation.Based on the improved genetic algorithm,the metasurface constructed by 3D AS units achieves 3D multi-focus acoustic focusing and acoustic imaging by pure amplitude modulation.For the holographic control of acoustic field,in the fourth chapter,we theoretically discuss the general design of structure unit that can decoupling control for phase and amplitude.Through the combination of the CLS and the AS units in series,we obtain a series of CLS&AS units that can control the phase and amplitude of acoustic waves separately.Based on the 2D CLS&AS units,the holographic applications such as multi-directional transmission of acoustic waves and multi-focus focusing are realized and verified by simulations and experiments.Based on the 3D CLS&AS units,the acoustic holographic imaging is realized.Compared with the conventional acoustic imaging using complex algorithms,the acoustic holographic imaging realized by the CLS&AS units not only significantly improves the imaging performance,but also results in that the acoustic energy and phase distributions are consistent with the target image.In the fifth chapter,the generalized Snell's law is extended to broadband,and an impedance-matching high-efficiency broadband metasurface model is proposed.The phase gradient in the generalized Snell's law becomes a refractive index gradient in this model,which no longer contains frequency term and thus has a broadband characteristic.Under normal circumstances,pentamode materials(PM)have fluid-like properties and their acoustic parameters such as effective density and acoustic velocity are non-dispersive over a relatively wide band of frequencies.Therefore,a series of PM units with different refractive indices but matching the background medium are designed to construct the broadband acoustic metasurfaces.Based on the PM metasurfaces,broadband and high-efficiency acoustic applications,such as anomalous refraction,Bessel beam generation and acoustic focusing,are respectively realized.Previous studies on transmissive metasurfaces mainly focused on phase control,while few studies on other control modes were reported,and this work fills the gap in this aspect.We also discuss in the aspects of improving the efficiency,exploring the further miniaturization,and expanding the working frequency of the acoustic metasurface.In this thesis,a variety of theories and methods are used to design the acoustic metasurfaces,which enriches the theoretical framework and expands the application range of the acoustic metasurfaces.