3D Acoustic Imaging Hitting the Diffraction Limit via Fully Parameter-Optimized Meta-Lens and Frequency-Domain...

Super-resolution imaging is the long-sought goal pursued in acoustics. In the past decade, many solutions have been proposed by utilizing various metamaterial devices. For example, in acoustics, the holographic meta-lens is employed to improve the lateral resolutions of ultrasonic imaging. However, the filtering effect of meta-lens broadens the acoustic wave pulses, which severely degrades longitudinal resolutions, making the high-resolution 3D imaging hitherto quite challenging. Here, an innovative solution is proposed by designing and implementing a fully parameter-optimized meta-lens. At the fundamental frequency, this meta-lens can achieve a lateral resolution of 80 µm (≈0.37λ₀), successfully breaking the diffraction limit. Through the frequency-domain reconstruction, the longitudinal resolution is increased to 296 µm (≈1.36λ₀). Utilizing the nonlinear effect of ultrasound in water, the acoustic meta-lens can furtherly optimize its imaging quality at the harmonic frequency, where the lateral resolution is increased to 44 µm (≈0.2λ₀) with the longitudinal resolution to 148 µm (≈0.68λ₀). The work breaks the bias on meta-lenses with deteriorated longitudinal resolution for broad applications in functional ultrasonic imaging.