Ultrasonic radial Airy array beams generated by phase modulation metasurfaces

Airy beams, known for their non-diffraction, self-acceleration, and self-healing properties, provide great opportunities for ultrasound-beam manipulation. However, the absence of simple and practical methods to achieve these beams in water hinders their widespread application. This work presents the generation of ultrasonic radial Airy array beams (URAABs) by phase modulation metasurfaces (PMMs) theoretically, numerically, and experimentally. The autofocusing and self-healing properties of URAABs during underwater propagation are also demonstrated. The designed PMM, composed of arranged photosensitive resin (PR) elements, implements a phase superposition approach to generate URAABs by combining multiple centrosymmetric two-dimensional (2D) ultrasonic Airy beams. Systematic case studies of radial four-beamlet and six-beamlet Airy array configurations reveal that the inherent non-diffraction and self-accelerating characteristics of Airy beams enable progressive beamlet coalescence, resulting in autofocusing formation during propagation. Furthermore, the PMM is fabricated using three-dimensional (3D) printing technology and experimentally verified to achieve the radial four-beamlet Airy array beam underwater. Experimental results agree with the theoretical and numerical calculations. The demonstrated URAABs exhibit potential for diverse applications including particle manipulation in acoustic tweezers, modulation in acoustic communications, and targeted energy delivery in ultrasound therapy.