Research On Generating Method Of Broadband Collimated Beam Based On Artificial Gradient Material

During the past decade,investigations on the acoustic wave manipulation via metamaterials has been such a hotspot that has attracted large amount of researchers’extensive attention.Based on the analogy of the acoustic wave equation and Maxwell’s equations,it is always possible to find feasible acoustic counterparts to the optical novel phenomena,e.g.negative refraction,one-way transmission,invisibility cloaking,and so on.Correspondingly,the acoustic gradient-index(GRIN)media is of extensive functionality like its optical counterparts.A common and effective way of implementing the GRIN media is to utilize phononic crystals.This implementation is inherently nonresonant,which leads to easy feasibility of the broad working bandwidth.By tuning the competitive parameters of the phononic crystals,the required effective refractive index can be achieved.This provides possibilities to manipulate acoustic wave trajectories and wave fronts,which give rise to remarkable applications in medical ultrasonics and vibration and noise control.In this thesis,utilizing the effective medium theory,we study how to achieve acoustic collimation based on the GRIN material.The devised configuration is omnidirectional and broadband for the incident waves.The thesis is divided into the following sections:Section I is the introduction.The background and current research status of the metamaterials,phononic crystals and acoustic collimation are briefly discussed.Section II is the theory and methods.The effective medium theory,the geometric acoustics approximation and the GRIN phononic crystals are introduced,which are to be exploited in the following sections.Section III proposes a design of the acoustic lens which generate acoustic collimating beams inspired by the non-scattering imaging of the Maxwell fish-eye.The lens has a complex distribution of the refractive index,consisting of a constant refractive index and the Maxwell-fish-eye-like distribution.The constant part contributes to the narrowness of the beam while the Maxwell-fish-eye-like distribution accounts for the focusing point shifting towards the vicinity of the lens.In this section,it is demonstrated that the transmitted acoustic beam can render the two characteristics simultaneously via the superposition of the two parts of the refractive index.By further adjusting the refractive index properly,the acoustic collimating beam is obtained.Finally,the full-wave simulations demonstrated the broad working bandwidth of the acoustic GRIN system.Section IV propose an implementation method of the system by exploiting the GRIN phononic crystals.The specific geometric symmetry leads to the tiny discrepancy of the focusing property when the incident waves impinges towards different positions on the boundary of the system.Section V gives the summary of the thesis and proposes the outlook of the current works.