Deep-subwavelength ultrasonic imaging by MHz column-structured metalens: First evidence of quantitative visualization of subsurface defects.

This article focuses on the design, fabrication, and experimental validation of a silicon-based periodic column-structured metalens operating at 0.5 MHz for deep-subwavelength ultrasonic imaging in nondestructive evaluation (NDE) of solid structures. Utilizing Fabry–Pérot resonance theory, the metalens enables transmission of evanescent waves and achieves super-resolution imaging beyond the diffraction limit, demonstrated by resolving two subsurface defects spaced at λ/30 in a stainless-steel specimen. The work addresses challenges of contact ultrasonic detection in solids, differing from prior metalenses designed for fluids, and shows potential for enhanced defect detection in NDE and biomedical diagnosis without immersion in liquid media. Experimental results with continuous and pulsed ultrasonic waves confirm the metalens’s capability to improve imaging resolution and contrast, with prospects for further enhancement by reducing the lattice constant and applying post-processing techniques.