Needle-like focusing and longitudinal depth imaging based on high-throughput acoustic metamaterials.

Acoustic focusing and imaging have emerged as a significant and highly discussed topic in scientific research. However, the axial localized point-like focal spots formed in previous studies cannot provide clear imaging of curved objects with finite spatial thickness. Here, we propose an acoustic needle metalens (ANM) composed of high-throughput spiral units with a specific phase distribution. According to the caustic theory, the designed ANM can generate two parabolic accelerating beams along the pre-designed path, ultimately forming a non-localized needle-like focal spot. We numerically simulate and experimentally verify the focusing property of the ANM at 8.5 kHz. The results indicate that the focal depth of the ANM is 50.63 times the wavelength while maintaining a transmission coefficient above 0.9 for each spiral unit constituting the ANM. Moreover, non-axial acoustic needle focusing has been achieved by altering the trajectory equations of the two parabolic accelerating beams. We further experimentally examine the excellent imaging capability of the designed ANM by conducting acoustic imaging on curved objects with a thickness of three wavelengths. The proposed ANM will have potential applications in microfluidic manipulation and medical ultrasound imaging.