High-Frequency Self-Focusing Ultrasonic Transducer With Piezoelectric Metamaterial

Focused ultrasonic waves are widely used in medical imaging, fibroids ablation and acoustic tweezers due to its high sensitivity and large energy at the focal spot. Although, acoustic metamaterials have emerged for acoustic focusing, they have limited to low frequency, narrowband and less acoustic transmission. Herein, an active gradient metamaterial (AGM) based on composite structure on traditional piezoelectric materials is proposed to achieve the acoustic impedance matching and underwater acoustic focusing. Based on the principle that the acoustic wave emitted from the piezoelectric plate reaches the focal spot at the same time, an aperiodic axisymmetric composite with gradient depth is established. A $200 ~\mu \text{m}$ kerf is selected, and the depth ranges from $44 ~\mu \text{m}$ to $146 ~\mu \text{m}$ . The acoustic field pressure intensity is simulated with finite element software, and measured by a needle hydrophone with fabricated transducer. Based on the good agreement between the simulation and experimental results with the excitation frequency range of 4 MHz-20 MHz, the active self-focusing and broadband focusing behaviors of the AGM are demonstrated. It is expected to apply the proposed acoustic metamaterial in biomedical and industrial applications.