Localization of off-axis multiple acoustic vortices formed by two coaxial vortex beams

Acoustic Vortex (AV) is a special acoustic field with a helical wave front and a pressure-zero phase singularity at the center.The Orbit Angular Momentum (OAM) carried by an AV beam can be transferred to objects,exhibiting good application potential in particle manipulation.Whereas,only an AV beam with controllable topological charges can be formed along the center transmission axis,which limits the flexibility in practical applications.Based on the circular array of point sources and the phase-coded approach,two coaxial AV beams with different topological charges are generated by the even and odd numbered sources at a same frequency,and several off-axis multiple AVs are formed on a circumference.Based on the radial distributions of pressure and phase,the number and the topological charge of the generated off-axis multiple AVs are analyzed with an accurate radius between the vortex center and the center axis,and the accurate azimuthal angles of AVs are derived based on the spatial angle difference of the sources.The generation of off-axis AVs overcomes the limit of only one center vortex trap along the center axis of the traditional AV technology.The favorable results provide the basis for off-axis multi-point trapping and open a new area of effective precise particle manipulation in biomedical engineering.