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\bibitem{Zhu_etal2011} Zhu, J., Christensen, J., Jung, J., Martin-Moreno, L., Yin, X., Fok, L., et al. (2011). A holey-structured metamaterial for acoustic deep-subwavelength imaging. \textit{Nat. Phys.}, \textit{7}(1), 52--55.

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