Abstract
Metamaterials exhibiting Fabry-Pérot resonances are shown to achieve ultrasonic imaging of a sub-wavelength aperture in water immersion across a broad bandwidth. Holey-structured metamaterials of different thickness were additively manufactured using a tungsten substrate and selective laser melting, tungsten being chosen so as to create a significant acoustic impedance mismatch with water. Both broadband metamaterial behavior and sub-wavelength imaging in water are demonstrated experimentally and validated with finite element simulations over the 200-300 kHz range.
Highlights
Holey-structured metamaterials of different thickness were additively manufactured using a tungsten substrate and selective laser melting, tungsten being chosen so as to create a significant acoustic impedance mismatch with water
Sub-wavelength acoustic imaging is possible within the near field of an object, provided the evanescent component of the scattered field, carrying details with high spatial frequency, can be recovered via Fabry–Perot (FP) resonances within a metamaterial structure.[1]
Enhancement of the evanescent wave magnitude has been reported as a consequence of highly anisotropic equi-frequency contours.[4]
Summary
Sub-wavelength acoustic imaging is possible within the near field of an object, provided the evanescent component of the scattered field, carrying details with high spatial frequency, can be recovered via Fabry–Perot (FP) resonances within a metamaterial structure.[1]. This limits the transmission of acoustic signals into the solid structure, potentially avoiding complicated acoustic interactions In air, this condition can be satisfied by many polymers and metals, since their impedance differs by several orders of magnitude from that of air.[1,2,3] Fewer experiments have been performed in water, mostly because of the lower impedance mismatch between water and commonly used materials, an example of characterization of metallic components in water can be found in Ref. 12. Heff for an open-ended pipe in water with h 1⁄4 6 mm and a 1⁄4 0.68 mm is heff 1⁄4 6.45 mm, expected to resonate at a frequency of 230 kHz. When k is much larger than both a and K, the presence of resonant standing waves leads to a prediction of a transmission coefficient that is unity for both propagating and evanescent waves in the perfect case of plane waves at normal incidence. The aim is to demonstrate that individual well-designed metallic metamaterials can be used to record images with sub-wavelength features across a range of frequencies
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