Ultrathin Acoustic Parity-Time Symmetric Metasurface Cloak

For hundreds of years,invisibility has always been the aspiration of human beings,which may only be found in science fiction films or novels.The stealth technology,which is now widely used in the military,can only reduce the probability of detection by eliminating the scattering echo through absorption or leading it to other directions.With the emergence of metamaterials,invisibility cloak has become one of the hottest issues in scientific research in recent years.The physical mechanism of invisibility cloak,which can be explained simply,is to eliminate the sound scattering effectively so that the object can not be detected by electromagnetic waves or acoustic waves.Based on the theory of coordinate transformation,various kinds of optical and acoustic invisibility cloaks have been constructed with metamaterials.However,the design theory requires that the equivalent parameters of the cloak strictly meet the complex spatial distribution,which inevitably leads to the large volume of these stealth devices and is more vulnerable to the error and loss of the materials,making it difficult to realize these thin-layer cloaks which can hide large objects in free space.In this paper,based on Parity-time symmetry,we put forward the design concept that the invisibility cloak is constructed with acoustic metamaterials,which solves the problem of traditional stealth devices which usually have complex fabrications and bulky volumes.By setting the loss and gain media which satisfy the condition of PT-symmetry,an ultrathin invisibility cloak that possesses some important characteristics such as unidirectional diffraction has been successfully realized in the experiment for the first time.The dissertation is divided as the following sections:In Chapter One,the preface briefly introduces the research background and application value of stealth technology,as well as analyzes the advantages/disadvantages of typical stealth researches.Chapter Two introduces the theoretical background in two parts.One part briefly introduces the development of Parity-time symmetry,and the other part shows the early work of acoustic stealth by PT-symmetry and discusses its limitations.Chapter Three proposes a new solution based on the limitations of past stealth work.By combining PT-symmetry and acoustic metamaterials,a two-dimensional ultrathin acoustic invisibility cloak is actually constructed.This chapter mainly introduces the theoretical work of this paper.In analytical results,the distribution characteristics of the surface acoustic impedance of the model are derived with cylindrical coordinates,and the characteristics of the distribution of sound intensity under PT-symmetry are verified.Chapter Four shows the results of the simulation and experiment of our work.In the simulation,Helmholtz resonators and loudspeakers are selected as the Loss and Gain of the invisibility cloak.By adjusting the geometric parameters of the artificial metamaterials and designing the corresponding control circuits,requirements of the surface impedance distribution under PT-symmetry are met.The design scheme is then optimized by numerical simulations.On the basis of the theoretical and simulation work,the metasurface cloak is built in a 2D waveguide and the unidirectional diffraction effect is verified.Chapter Five gives the main conclusions of this study and the outlook for the future.