# Multi-channel ultrasonic Bessel vortex beams by spatial multiplexing metalens

## TL;DR

A metalens design generates multiple ultrasonic vortex beams with precise control, enabling advanced applications in wave-based technologies.

## Contribution

A spatial multiplexing metalens enables multi-channel ultrasonic Bessel vortex beams with controllable topological charge and orientation.

## Key findings

- A four-channel metalens was experimentally designed with 0.2 mm pixel accuracy.
- Generated vortices showed directional errors less than 1° and matched simulation results.
- Vortex intensity can be tuned by combining multiple channels.

## Abstract

The generation of acoustic vortexes sparks intense research interest since they have applications in modern wave-based technologies, such as underwater communication and particle manipulation. However, the existing schemes mainly rely on a phase mask to excite a single vortex beam, thereby lacking the functionality and adaptability for practical scenarios. In this article, we propose a feasible methodology to realize multi-channel ultrasonic Bessel vortex beams at megahertz. By leveraging the concept of spatial multiplexing, the adjacent pixel of the metalens can be assigned to independently generate non-diffraction ultrasonic vortices with different topological charge and spatial orientation, without losing the characteristics of the helicoidal wavefront. We experimentally designed a four-channel metalens with a high fabrication accuracy of 0.2 mm pixel size and measured the far-field ultrasound distribution in the water. Both topological charge and radiation direction of the generated vortices can be precisely controlled as predicted, showcasing great agreement with simulation results with a directional error of less than 1°. Moreover, the intensity of the vortex can be tuned by gradually combining multiple channels into one. The proposed scheme enhances the flexibility of manipulating ultrasonic vortex and offers more possibilities in designing multi-functional ultrasound devices.

Yinjie Su and colleagues present a spatial multiplexing metalens that generates four independent acoustic vortex beams. This efficient scheme provides a compact platform for multi-vortex generation, enhancing integration and application capabilities.