| Because of the sub-wavelength dimensions and the powerful manipulation capabilities for sound wave front,acoustic metasurfacs(AMs)have received more attention in in the past decade.By constructing different AM structural units,the propagation characteristics of sound waves can be effectively modulated.Thereby,various novel and peculiar physical effects and applications are realizied,such as sound absorption and noise reduction,acoustic imaging,particle manipulation,acoustic focusing,acoustic stealth,and signal processing,and so on.Acoustic focusing effect can concentrate the sound wave energy in a local space,and a high-energy density sound intensity area is formed.As an important science and technology,it has been widely used in detection and detection,ultrasonic medical treatment,non-destructive testing and other fields.Compared with the traditional acoustic focusing lens,the AM lens has the advantages of small size,simple structure,flexible adjustment and so on.This paper focuses on the high-efficiency focusing effect based on AMs,including high-efficiency acoustic focusing based on Airy beams,acoustic bottle focusing and acoustic needle focusing based on self-accelerating beams,and focused vortex ultrasound in water.In addition,we propose an acoustic coding metasurface(ACM)lens and an asymmetric transmission system(ATS)with two flat AMs to achieve adjustable focusing and bidirectional asymmetric acoustic focusing(BAAF),respectively.The main content and innovations are as follows:(1)Two acoustic metasurfaces are proposed to achieve acoustic Airy beam(AAB)focusing.One is composed of space-coiling subunits,and the other is composed of two coding units.The self-accelerating,self-healing,and non-diffracting features of AABs are demonstrated using finite element simulations.It is found that this highly efficient acoustic focusing can circumvent obstacles in the propagating path.By changing the working frequency of the incident sound wave,the focal length of acoustic focusing can be modulated.(2)Acoustic bottle beam focusing and acoustic needle focusing are realized based on the AMs.Two intersecting self-accelerating beams are generated on a preset trajectory by using AM,thereby forming an acoustic bottle beam focusing.The shape and area of the acoustic bottle beam can be modulated by changing the phase distribution of the metasurface or the angle of the incident sound wave.Two back-symmetrical parabolic self-accelerating beams are generated by using AM,thereby forming an acoustic needle focusing.The focal intensity,depth and position of needle focusing can be modulated by changing the incident direction or tailoring the trajectory of two accelerating beams.(3)A reconfigurable ACM lens is designed.The ACM lens is composed of two coding units,which can achieve broadband acoustic focusing in the air.By adjusting the coding sequence of the ACM,the focus location of the ACM lens can be modified efficiently.Numerical simulation and experimental verification are used to verify the focusing effect of the ACM lens in the frequency range of 3.1 ? 4.6 k Hz.(4)An ATS with two flat AMs is designed to yield BAAF.The acoustic waves can be focused on both sides of the ATS with different focal lengths and intensities.It is found that by adjusting the distance between two flat AMs,the focal length and intensity of the bidirectional focusing could easily be modulated.When the distance between two flat AMs is large enough,the BAAF can be converted into a unidirectional acoustic focusing.(5)The focused vortex ultrasound is realized in the water by using the acoustic artificial structure plate.The generated focused vortex ultrasounds are verified by theoretical analysis,numerical simulation and experimental verification.The focal length and depth of the focused vortex ultrasound can be modulated by simply changing the initial radius of the Archimedean spiral.In addition,by changing the number of Archimedes spiral slits,focused vortex ultrasound with different topological charges can also be realized. |
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