Stability of Fractional Vortex Ultrasound via a Piezoelectric transducer

The ultrasound vortex beam showed the potential for applications including biomedical imaging, therapeutic ultrasound, acoustic trapping, and manipulations, etc. However, most existing vortex ultrasound generation are based on theory with integer topological numbers and highly relied on transducer arrays, which have limited control over orbital angular momentum. Meanwhile, the fractional vortex beam has a gapped donut distribution of acoustic pressure and is composed of complex phase chain, which provides detailed modulation of orbital angular momentum. Thus, in this work, we demonstrated the design and fabrication of the transducer integrated with a spiral phase plate to generate vortex waves with fractional topological charge (FTC) and validated the stability of fractional vortex ultrasound within 10 mm, which may serve as a new platform for diagnostic and therapeutic ultrasound applications. Diffraction, media nonlocality, and Gouy phase will be analyzed in this work during propagation. The transducer is expected to have an overall dimension of 2.0 × 2.0 × 5.4 mm³ and 0.05 MPa acoustic pressure under 13 Vpp voltage. The generated fractional vortex maintains its phase pattern of FTC and gapped toroidal amplitude distribution.