Research On Orbital Angular Momentum Based On Acoustic Artificial Structures And Its Applications

With the unique helical phase distribution and acoustic field characteristics,acoustic vortex beams carrying orbital angular momentum have wide application prospects in fields such as particle control,acoustic torque,acoustic communication,and information processing.In recent years,the rapid development of acoustic artificial structures has provided a powerful support platform for the control of acoustic vortex beams.Compared with traditional metasurface,metagratings based on acoustic artificial structures not only have the characteristics of a flat structure and subwavelength size,but also effectively overcome the critical mode limitations of phase gradients metasurface and the propagation of acoustic vortex beams,providing new method for the modulation and control of acoustic vortex beams and the design and application of multiple functional acoustic devices based on orbital angular momentum.This research designs and implements multiple new types of acoustic vortex beam-related devices based on the theory of metagratings and the extended of generalized Snell’s law,realizing the effects and regulation of Bessel beams,acoustic vortex beams,and Bessel vortex beams.The main research content is divided into the following four parts:（1）the basic theoretical methods of acoustic vortex beams;（2）the design and regulation of Bessel beam lenses based on metagratings;（3）the design and experimental verification of multifunctional acoustic vortex beam lenses based on metagratings;and（4）the design and regulation of Bessel vortex beam lenses based on metagratings.In the second chapter,the basic theoretical methods of the acoustic vortex beam and its control are introduced,including the basic equation of the acoustic vortex beam,the propagation mode of the acoustic vortex beam,the generalized Snell’s law and its extension,which provides a theoretical basis for the design of multi-functional acoustic devices based on the acoustic vortex beam.In the third chapter,a Bessel focusing lens based on a two-dimensional metagrating structure is studied.By changing the tube width and opening width of the phase control unit,phase delay covering the entire 2πrange can be achieved,and two-dimensional metagratings with phase gradients of 3.42k₀ and 5.13k₀ can be designed to achieve transmitted and reflected Bessel sound beams with a convergence angle of 15°,respectively.The results show that the relative operating bandwidth of the transmitted Bessel beam lens can reach 23.3%,and considering air loss,it still maintains good Bessel beam performance.Finally,a double-layer metagrating structure was designed,which achieved the conversion of transmitted and reflected Bessel beams,as well as the conversion of convergence angles of 15°to 20°,by adjusting the spacing between the metagratings.In the fourth chapter,we study the acoustic vortex beam lens based on a two-dimensional phase gradient metagrating structure and its performance regulation.It is found that,based on the extended generalized Snell’s law,different phase gradient metagratings can be designed to generate the same order of acoustic vortex beam excited by the same incident sound wave,and the experiment verifies that the relative working bandwidth of the designed metagrating is 8.4%.On this basis,an acoustic vortex beam converter based on phase gradient metagrating structure was designed,which can convert the incident acoustic vortex beams with orders 1 and-1 in the same direction into output acoustic vortex beams with orders-2 and 2,respectively.Finally,a multifunctional acoustic vortex beam lens was designed and prepared.By separating and merging two metagrating structures with the same phase gradient,the regulation of reflected and transmitted acoustic vortex beams can be achieved.In the fifth chapter,a Bessel vortex beam lens using a two-dimensional metagrating structure and its performance regulation are presented.Design two-dimensional metagratings with phase gradients of 7 and-11,respectively,to excite and generate transmission Bessel vortex beams with order 1 and convergence angle of 15°,and reflection Bessel vortex beams with order 2 and convergence angle of 20°.Research has shown that in the range of propagation distance of 4λ,the acoustic energy of the Bessel vortex beam is significantly higher than that of the acoustic vortex beam without introducing the Bessel beam effect.In addition,by separating and merging two metagratings with the same phase gradient,a transmission and reflection Bessel vortex beam conversion with order 1 and convergence angle of 15°is achieved.