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Acoustic metamaterials

The concept of metamaterials can be extended to acoustic waves in fluid and in solid. Radically different designs of structural unit cells, supprisingly rich physics and applications arise.

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Willis metamaterial on a structured beam
Willis coupling is the analogy of electromagnetic bianisotropy in elastic waves. Here, we provide a realization on flexural waves.
https://arxiv.org/abs/1807.02285

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Source illusion for flexural waves 
Metasurface based on space-coiling unit cells for elastic waves. Source illusion with target phase profile = original excitation + phase discontinuity by metasurface
Phys. Rew. Lett. 119, 034301 (2017)

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Negative refractive index
Coiling up space for airborne acoustic waves.
Negative index, cone-like dispersion and large index without local resonances,  
Phys. Rev. Lett. 108, 114301 (2012)
Sci. Rep. 3, 1614 (2013) (arxiv)
Highlight: APS
Exper. from Cummer: Phys. Rev. Lett. 110, 175501 (2013)

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Coherent perfect absorption
Perfect absorption with coherent input waves when we have either resonating dynamic density or modulus.
Appl. Phys. Lett. 104, 121902 (2014)
Expriment from Yang: Sci. Rep. 7 43574 (2017)

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Subwavelength imaging to the far-field
Capturing subwavelength imaging resolution and project to far-field using highly anisotropic dispersion surface.  
Nature Materials 8, 931 (2009) 
Highlight: PopSci, nature.com
Elastic wave expr. from Kim: Appl. Phys. Lett. 98, 241912 (2011)

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Meaning of an acoustic negative index
Resonating breathing mode to negative modulus and resonating oscillating mode to negative density.
This double negativity gives a negative index
Phys. Rev. E 70, 055602(R) (2004)

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