Acoustic needle focusing induced by metasurface-generated accelerating beams

¹ Jiangsu Key Laboratory of Opto- Electronic Technology, School of Physics and Technology, Nanjing Normal University - Nanjing 210023, China
² Key Laboratory of Modern Acoustics of MOE, Department of Physics and Collaborative Innovation Center of Advanced Microstructures, Nanjing University - Nanjing 210093, China
³ School of Design and Environment, National University of Singapore - Singapore 117566, Singapore

^(a) weiqi@njnu.edu.cn
^(b) wudajian@njnu.edu.cn

Received: 14 November 2019
Accepted: 18 February 2020

Abstract

Acoustic needle focusing, which features elongated depth of focusing and narrow lateral size, is of great importance in applications requiring acoustic convergence in a large region, such as acoustic fast imaging. Here, a method is proposed to design an acoustic metasurface lens (AML) composed of space-coiling subunits for shaping a needle focusing. By manipulating the transmitted phase delay, the designed AML can generate two symmetrical parabolic accelerating beams along the designed convex trajectories, finally forming the acoustic needle focusing. Numerical simulations demonstrate that the focal intensity can reach roughly 17 times that of the incident wave while keeping the depth of focusing 18.2 times the wavelength. The working bandwidth of the highly efficient needle focusing is from 3.042 to 3.842 kHz. In addition, the focal intensity, depth and position of needle focusing can be freely modulated by simply changing the incident direction or tailoring the trajectory of two accelerating beams. The proposed acoustic needle focusing may have potential applications in acoustic imaging, acoustic energy delivering and particles trapping and manipulation.