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

High-resolution ultrasonic imaging, which is highly demanded in nondestructive evaluation, is inherently limited by the detection wavelength. Acoustic metamaterial is an emerging technique to achieve subwavelength-resolution ultrasonic imaging beyond the diffraction limit due to its unprecedented acoustic properties. However, existing reports focus on metalenses for manipulating acoustic waves propagating in fluids like air and water, typically at a low-frequency range below 10 kHz. In this paper, a 0.5 MHz periodic column-structured metalens is designed and fabricated to realize deep-subwavelength ultrasonic imaging for quantitive visualization of subsurface defects in solid structures. The silicon-based metalens is designed based on Fabry–Pérot resonance theory. It consists of silicon columns arranged periodically with a lattice constant of 0.2 mm. The Fabry–Pérot resonance frequency is analyzed theoretically and the wave fields of the metalens at resonance mode are verified numerically. The subwavelength ultrasonic imaging performance of the proposed metalens is numerically proved and experimentally demonstrated. As a result, super-resolution ultrasonic imaging (λ/30, with λ being the wavelength) with a high resolving contrast is realized to identify two separated subsurface defects in a stainless-steel structure experimentally with the designed column-structured metalens. This work demonstrates a valuable deep-subwavelength imaging method that beyond traditional diffraction limits and paves the way for enhanced applications in nondestructive evaluation and biomedical diagnosis.