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. 2017 Feb 17;7:42863. doi: 10.1038/srep42863

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Copyright © 2017, The Author(s)

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Under the optical microscope, the bottom side of the matching layer (a) is hexagonal close-packed silica columns. This side of the matching layer has the maximum acoustic impedance, which is approximately equal to that of silica. (b) On the top side of the matching layer, barely visible tiny dots, which represent the tips of the silica fibres, were regularly spread in the epoxy resin matrix. This side has the lowest acoustic impedance, which is equal to that of epoxy resin. (c) The cross-sectional image of the metamaterial matching layer was acquired via an industrial CT test. The pyramidal cone array has a cycle length of 122 μm, and the tip length of the pyramidal cone is 610 μm. (d) The 3-D topography of the pyramidal cone arrays embedded in the matching layer.