The Study And Application Of Acoustic Orbital Angular Momentum Based On Acoustic Metamaterials

In recent years,acoustic metamaterials has aroused great attention of researchers at home and abroad.Acoustic metamaterials with the unit element of the sub-wavelength scale,has shown some special properties not realizable in traditional nature materials,such as the anisotropic mass density,negative mass density,negative modulus,etc.With these unique properties,acoustic metamaterials has prominent ability in the manipulation of acoustic wave to realize the arbitrary control of the incident wave front for obtaining the targeted pressure field distribution,which enriches the free manipulation of sound.Based on that,more and more functional devices have been designed,and supposed to have broad and attractive application prospects,such as acoustic negative refraction,acoustic unidirectional transmission,acoustic cloaking,etc.On the other hand,the investigation of acoustic angular momentum has become a hot research topic recently.The wave front of the acoustic vortex beam bearing orbital angular moment is helically twisted,which is determined by the topological charge(or called topological order).In this situation,the dimensions of frequency,amplitude and phase are not enough for describing the wave feature completely,and a independent new degree of freedom,orbital angular momentum,should be introduced into the representation.Since the acoustic orbital angular momentum is able to work as an additional and independent dimension,the study of orbital angular momentum would provide new possibility in the free manipulation of acoustic wave.In this sense,it strongly proves the fundamental theoretical value of the study of acoustic orbital angular momentum.On the other hand,in the practical applications,acoustic vortex beam carrying the orbital angular momentum is able to act as the acoustic tweezer,similar to the optical tweezer,to realize the precise manipulation of the tiny particle in the local position.Meanwhile,we can use the radiation force induced by the acoustic vortex beams to realize the non-touch push,pull or drag of particles.Moreover,we can use the acoustic torque acquired in the field to transfer the orbital angular momentum from the wave to the matter,and achieve the accurate rotation of the object.These important and interesting practical implementations show that the study of acoustic orbital angular momentum would also offer profound potential in the real applications.Most of the recent realizations of sound wave controlling based on acoustic metamaterials,including the design of numerous special acoustic lens,acoustic cloaking and acoustic illusion,are still limited in the manipulation of linear momentum.This dissertation combine the unique acoustic properties not available in natural materials and sub-wavelength geometric advantage of the acoustic metamaterials,to realize the arbitrary and free control of both the linear and orbital angular momentum of acoustic wave.Furthermore,we introduce the acoustic orbital angular momentum as a new degree of freedom into the acoustic multiplexing communication.This thesis systematically study the combination of acoustic metamaterials and acoustic orbital angular momentum.The dissertation is divided as the following sections:In Chapter Ⅰ,theoretical and experimental works on the electromagnetic and acoustic metamaterials are reviewed,which serve for the background of the research,and the progress of the investigations on these topics is introduced.Also,for a better understanding of this dissertation,the related theories and calculation methods of acoustic metamaterials and acoustic vortex beams have been briefly described.In Chapter Ⅱ,we first introduce the physical concept of the acoustic orbital angular momentum and acoustic vortex beams,and the significant value of acoustic orbital angular momentum in various research fields.The traditional active methods for generating acoustic vortex beams is briefly discussed.For overcoming the limitation of the conventional method,we propose a sub-wavelength metasurfaces with the multiple spiral-armed slit to generate the stable acoustic vortex beams in a broad bandwidth.The incident plane wave with a uniform wave front can be effectively converted to the vortex beams bearing acoustic orbital angular,which has a helical wave front.Through changing the number of the spiral-armed slits,acoustic vortex beams of arbitrary topological order can be efficiently produced both in theoretical calculation and experimental measurement,showing the flexibility of this method.In addition,we present the performance of the spiral artificial structure in different operating frequency,showing the broadband functionality of the scheme which is not available in traditional method.Furthermore,the vortex beams produces with this spiral-armed metasurface can maintain the stability of the topological charge over a long distance in the propagation direction,which offers great potential in the practical applications.In Chapter Ⅲ,we first briefly introduce the design of the transmission acoustic metasurface based on the hybrid resonance comprising of the Helmholtz resonance and the Fabry-Perot resonance,and the realization of generating the non-paraxial self-accelerating beam along arbitrary trajectory in experiment.Emphatically,we first theoretically proposed a new physical mechanism for introducing the acoustic orbital angular momentum and demonstrate an implementation of the mechanism through be hybrid resonance acoustic metasurface.Through producing the effective wave vector in the azimuthal direction by the planar resonant acoustic metasurfaces with the dimensions much smaller than wavelength,we propose to convert the acoustic resonance into acoustic orbital angular momentum.Remarkably,we successfully generating the first order acoustic Bessel vortex beam both through numerical simulations and experimental measurements,providing the theoretical innovation and the technical breakthrough in the generation of acoustic orbital angular momentum.Meanwhile,the unique advantages of high efficiency,compactness,easy fabrication,planar shape and non-helical geometry are presented,which makes up for the deficiency of the previous methods and opens a new route in the study of the relevant fields.In Chapter Ⅳ,the application background of the information transfer based on acoustic wave and the research progress of acoustic multiplexing communication are briefly introduces,together with some representative works.The development of the existing acoustic multiplexing technologies,including the innovations in the dimensions of frequency,amplitude and phase,could not satisfy the highly increased demand in the high-capacity information transfer with acoustic wave.We propose a new orbital-angular-momentum-based multiplexing to tremendously boost the capacity of acoustic multiplexing,by introducing acoustic orbital angular momentum which is a new and independent degree of freedom comparing with the previous dimensions.Through encoding the information into the acoustic vortex beams of various topological and transfer the multiplexing signal simultaneously,and passively decoupling the overlaid modes and efficiently decoding the information with the acoustic metamaterials in the receiving end,we can dramatically increase the capacity of the acoustic communication link in a low cost manner,without posting extra burden into the pre-existing communication system.For the first time,we experimentally demonstrate the real-time signal transmission based on the acoustic orbital angular momentum in a dynamic manner by reconstructing several images in the receiving end.The introduction of acoustic orbital angular momentum into the acoustic communication technology could not only expand the application field of acoustic orbital angular,but also provide a new avenue in the development of acoustic communication.Finally,the main conclusions of the present study and the prospect for the future work are drawn in Chapter Ⅴ.The innovation points of the thesis can be concluded in the following three parts:(1)Extends the control ability of acoustic metamaterials into the acoustic orbital angular momentum.Most of the existing acoustic metamaterials can only realize the manipulation of the sound with the linear momentum.This thesis proposes a ultra-thin metasurfaces with multiply spiral arms to generate the acoustic vortex beams in a broad band and stable manner.Through tuning the parameters of the structure,the topological charge of the vortex beams can be freely controlled.The broadband functionality and stability of the topological charge are demonstrated in experiments,which are not available in traditional methods.(2)Proposes a new physical mechanism:convert acoustic resonance to orbital angular momentum.Based on acoustic resonance,we effectively transform the acoustic plane wave with the uniform phase distribution into the vortex beams carrying the orbital angular,.Strictly deduce the generation of acoustic vortex by introducing the effective wave vector within a sub-wavelength non-helical and planar acoustic resonator.In experiments,successfully synthesis the first order acoustic Bessel beam.The unique properties of the designed metamaterial:high transformation efficiency,sub-wavelength geometric size,simple fabrication,planar shape and non-helical geometry,would make up for the deficiency of traditional methods and provide significant importance to relevant fields.(3)Introduces the acoustic orbital angular as a new degree of freedom into the acoustic communication system,and proposes the acoustic multiplexing technology based on orbital angular momentum.Using the acoustic metamaterial to realize the passive vortex modes decoupling and the efficient readout of the data.In experiments,realizes the real-time dynamic information transmission with the OAM-based acoustic multiplexing,and successfully demonstrates the transmission of images with this method.It would helps to significantly improve the transmission rate at a low cost,open a new avenue for development of the high speed and large capacity acoustic communication and more potential to the relevant fields.