Design Of Functional Lenses Based On Acoustic Metamaterials

Acoustic metamaterials are an artificial metamaterial that have been proposed in recent years and studied extensively and deeply.These metamaterials open a door for improvement or new applications because they have characteristics that are difficult or impossible to achieve in natural materials.For example,acoustic metamaterial lenses can exceed the diffraction limit and realize sub-wavelength imaging results.The principle is to use the negative refractive index displayed by the unit structure in resonance to make the decay wave participate in the imaging,thus improving the resolution ability.However,the bandwidth at resonance is extremely narrow and the loss is large,so the metamaterial devices designed by this way will inevitably exhibit the characteristics of narrow frequency band and large loss.The purpose of this paper is to design some non-resonant broadband acoustic lens,which makes artificial acoustic metamaterials play a greater role in the design of acoustic devices and provides new ideas for future applications.In this paper,we study a design method of acoustic metamaterial lens based on wavefront transform.Three broadband acoustic lenses with different functions are designed and verified by numerical simulation.The main work includes the following parts:(1)Based on the method of general lens wavefront transformation,an acoustic two-dimensional broadband low-loss planar lens is designed,which can be realized by a sub-wavelength artificial structure with a gradient refractive index and a hard boundary.In this paper,the wavefront transformation idea is applied to the design of other functional lenses.By minimizing the anisotropy factor,the synthetic material is approximately isotropic,and a unit structure that meets the needs is designed.The results show that the functional lens have good wavefront adjustment performance in a certain frequency range.(2)We design two gradient index lenses,which respectively realize the deflection and divergence of plane waves.Through equivalent medium simulation and numerical simulation of the cell structure filling into the lens,the designed deflection lens and divergence lens can realize the functions of plane wave deflection and divergence adjustment in the wideband frequency range.This technology can be extended to different acoustic devices and has a wide range of applications.(3)A gradient index lens is designed to realize Bessel acoustic beam,which can reshape the wave from the point source into Bessel acoustic beam with energy concentrated near the axis and almost no divergence.Through the analysis of impedance matching theory,the two-dimensional distribution of gradient refractive index is obtained.Due to the impedance matching with air,the interface reflection is significantly reduced.The simulation by finite element method proves that the Bessel acoustic lens can work in the wide band.This method is expected to provide new ideas for medical ultrasound imaging and signal detection.