Acoustic Information Processing And Applications Based On Acoustic Metasurfaces

Acoustic metasurfaces with sub-wavelength thickness are two-dimensional artificial materials.By arranging the basic units,the acoustic metasurfaces can flexibly control the acoustic fields,and can realize many novel phenomena,such as anomalous reflection/refraction,absorbing,holography,ultra-sparse reflection,and cloaking.Compared with the cubic acoustic metamaterials,acoustic metasurfaces feature smaller sizes and lower loss,and have shown great application prospects in the field of designing acoustic functional devices.In this dissertation,the research content involves the realization of acoustic computing system and ordinary differential equation solving system,tunable coding metasurface and its effect of perfect negative reflection,the asymmetric coding metasurface and the realization of controllable projection of acoustic images.In the first chapter,we mainly introduce the research content,progress and current situation of metasurfaces,and summarize the main research contents of this paper.In the second chapter,based on the totally transmitting focusing metasurface(TFM)and reflective computing metasurfaces(RCM),we propose the acoustic analog computing system.By designing the acoustic coating unit cells(CUCs),we construct the TFM.The CUCs can provide the full phase control and almost I00%transmittance.By combining the CUC and perforated panel(CUC&PP),we construct the RCM,whose reflection amplitudes and phases can independently vary.When an acoustic signal is incident,the proposed analog computing system can perfo4rm mathematical operations such as spatial differentiation,integration,and convolution.In the third chapter,we extend the application range of the acoustic computing metasurface to solve the nth-order ordinary differential equations(ODEs).Based on the layered labyrinthine units,we can realize the focusing metasurface and computing metasurface(CM),and realize the ODE solving system.The layered labyrinthine units can provide broad modulation ranges on the phase and amplitude of the input acoustic signals,which can construct the CM with complex transmission coefficients,and solving the arbitrary order ODEs.The proposed ODE solver with fixed geometry can be designed for arbitrary nth-order ODE in principle.Besides,the constructed ODE solving system can perform spatial differential,integration and convolution operations on the incident acoustic signal.In the fourth chapter,based on the acoustic labyrinth structure,an acoustic coding metasurface(ACM)with ultra-thin thickness(0.11λ0)is designed,where λ0 is the wavelength in background medium.By designing the coding sequence,the coding metasurface can control the beam of transmission waves.Besides,we design an ACM consisting of a series of rectangular strips.Each rectangular strip has either;"0" or "1"response when it is pushed up or pressed down.By artificially pushing and pressing,the rectangular strips can be converted between the two responses,thus we can artificially adjust the coding sequence of the ACM.We demonstrate both theoretically and experimentally,by adjusting the coding sequence of the ACM,we can realize the negative reflection with tunable directions,and the reflection efficiency is approaching 100%.Since the rectangular strip has simple structure and is not resonant the ACM has a broadband performance,and can spread an incident white noise into different directions.In the fifth chapter,based on the PP&CUCs,we design the asymmetric coding metasurfaces for controllable projection of acoustic images.The PP&CUC has tuable transmission coefficient reflection coefficient and reflection phase asymmetry.When the carefully designed PP&CUC is in the regular and inverse forms,it can function as the coding elements "0" and "1",respectively.Two coding states can be converted by flipping the PP&CUC,therefore,an arbitrary coding sequence can be realized based on the coding metasurface consisting of the fixed PP&CUCs.By introducing two coherent acoustic waves,the coding metasurface can realize the controllable projection.The simulation and experiment results demonstrate that the coding metasurfaces will not project the acoustic images during the OFF state,will project the desired acoustic images during the ON state.Besides,based on the acoustic coding metasurface,we design the acoustic metasurfaces,and realize the acoustic images with encryption and decryption.The simulation and experiment results demonstrate that the acoustic metasurfaces can project the encrypted images during the OFF state,but can project the original images during the ON state.The experimental results are consistent with the simulation results.In the sixth chapter,we present a brief summary of this dissertation and some prospects of the future work.