Ultrasonic transducers with broad bandwidth are considered to have high axial resolution and good ultrasound scanning flexibility for the clinical applications. The limitations of spatial resolution due to bandwidth are of great concern in ultrasound medical imaging. The method of acoustic impedance matching between the piezoelectric element and medium is commonly used to obtain broad bandwidth and high resolution. In this study, an optimized backing layer design was proposed to broaden the bandwidth by adding a tunable acoustic impedance matching layer of backing (AIMLB) between the backing layer and the piezoelectric ceramic element. The Mason equivalent circuit method was used to analyze the effect of the backing material composition and its structure on the bandwidth of the transducer. The optimized transducer was simulated using the finite-element method with the PZFlex software. Based on the PZFlex simulations, a 20-MHz ultrasonic transducer using the AIMLB with a bandwidth of approximately 92.29% was fabricated. The experimental results were in good agreement with the simulations. The ultrasonic imaging indicated that the designed ultrasonic transducer with an additional AIMLB had high performance with good imaging capability.

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.

Acoustic micro-beams produced by highly focused ultrasound transducer have been investigated for micro-particle and cell manipulation. Here we report the selective trapping of microspheres via the acoustic force using the single acoustical beam. The forbidden band theory of acoustic radiation force trapping is proposed, which indicates that the trapping of particles via the acoustic beam is directly related to the particle diameter-to-beam wavelength ratio as well as excitation frequency of the ultrasonic acoustic tweezers. Three tightly focused LiNbO3 transducers with different center frequencies were fabricated for use as selective single beam acoustic tweezers (SBATs). These SBATs were capable of selectively manipulating microspheres of sizes 5–45 μm by adjusting the wavelength of acoustic beam. Our observations could introduce new avenues for research in biology and biophysics by promoting the development of a tool for selectively manipulating microspheres or cells of certain selected sizes, by carefully setting the acoustic beam shape and wavelength.