Quasiperfect fractional ultrasonic vortices by monolayer transmission metasurfaces

A monolayer transmission metasurface (MTMS) combining a radial phase shift plate (RPSP) and a spiral phase plate is developed to realize the quasiperfect fractional ultrasonic vortex (FUV). The theoretical prediction for the quasiperfect FUV generation by the MTMS has been demonstrated by numerical simulations. The MTMSs produce transmitted quasiperfect FUVs with different topological charges, which display annular distributions with a constant radius and a low-intensity gap. Additionally, it is found that the radial phase shift factor of the RPSP can effectively modify and expand the intensity ring radius of the obtained quasiperfect FUV. Finally, we fabricate the discretized MTMS, and experimentally verify that the suggested MTMS can generate the quasiperfect FUV. The quasiperfect FUV generated by the simple MTMS includes the advantages of both the fractional vortex and the perfect vortex, which may have potential applications in underwater communication, imaging, and particle manipulation.