Focused acoustic vortex by an artificial structure with two sets of discrete Archimedean spiral slits

Focused acoustic vortex (FAV) beams can steadily trap particles in three-dimensions. Previous FAV emitters are mainly based on the active device arrays. Here, we design a passive artificial structure engraved with two sets of discrete Archimedean spiral slits to generate the FAV in water. The acoustic intensity and phase distributions of FAVs are investigated theoretically and demonstrated using the finite element method. The constructive interference between two transmitted acoustic vortices through the inner and outer spiral parts achieves the FAV. It is found that the focal length and depth of the FAV can be modulated by simply changing the initial radius of the Archimedean spiral. Furthermore, we implement the Schlieren imaging experiment to verify the generation of the FAV by the artificial structure. Our design may offer potential applications in particle trapping, biomedical therapy, and medical imaging.