Deep-subwavelength-optimized holey-structured metamaterial lens for nonlinear air-coupled ultrasonic imaging

Marco Boccaccio, Pasquale Rachiglia, Gian Piero Malfense Fierro, Giovanni Pio Pucillo, Michele Meo

Research output: Contribution to journalArticlepeer-review

9 Citations (SciVal)


Ultrasound non-destructive testing (NDT) is a common technique used for defect detection in different materials, from aluminium to carbon-fiber-reinforced polymers (CFRPs). In most cases, a liquid coupling medium/immersion of the inspected component is required to maximize impedance matching, limiting the size of the structure and materials. Air-coupled inspection methods have recently been developed for noncontact inspections to reduce contact issues in standard ultrasonic inspections. However, transmission of ultrasound in air is very inefficient because of the enormous impedance mismatch between solids and air, thus requiring a signal amplification system of high-sensitivity transducers. Hence, the captured signal amplitude may not be high enough to reveal any wave distortion due to defects or damage. This work presents a design of a holey-structured metamaterial lens with a feature size of λ/14 aiming at improvement of acousto-ultrasonic imaging using air-coupled transducers. The required effect is obtained by matching geometrical parameters of the proposed holey-structured metamaterials and the Fabry–Perot resonance modes of the structure. Transmission tests have been conducted on different fabricated metamaterial-based structures, to assess the frequency component filtering of the proposed method in both acoustic (f = 5 kHz, 20 kHz) and ultrasonic range (f = 30 kHz, 40 kHz). Results showed an improved sensitivity of damage imaging, with an increase in amplitude of the design frequencies of the lens by 11 dB. Air-coupled inspections were conducted on a stress-corrosion cracked aluminum plate and impacted CFRP plate using the holey-structured lens. Results showed an improvement in the damage-imaging resolution due to a wave-amplitude increase across the defective features, thus demonstrating its potential as an efficient and sensitive inspection tool for damage-detection improvement in geometrically complex components of different materials.

Original languageEnglish
Article number1170
JournalSensors (Switzerland)
Issue number4
Publication statusPublished - 7 Feb 2021

Bibliographical note

Funding Information:
Funding: This research has been funded by National Nuclear Laboratory (NNL), UK, grant agreement no. NNL/UA/029.

Publisher Copyright:
© 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Copyright 2021 Elsevier B.V., All rights reserved.


  • Acoustics
  • Air-coupled
  • Deep-subwavelength
  • Holey lens
  • Metamaterial
  • Non-destructive testing
  • Nonlinear ultrasonics

ASJC Scopus subject areas

  • Analytical Chemistry
  • Biochemistry
  • Atomic and Molecular Physics, and Optics
  • Instrumentation
  • Electrical and Electronic Engineering


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