Acoustic metamaterials offer great flexibility for manipulating sound waves and promise unprecedented functionality, ranging from transformation acoustics, super-resolution imaging to acoustic cloaking. However, the design of acoustic metamaterials with exciting functionality remains challenging with traditional approaches using classic acoustic elements such as Helmholtz resonators and membranes. Here we demonstrate an ultraslow-fluid-like particle with intense artificial Mie resonances for low-frequency airborne sound. Eigenstate analysis and effective parameter retrieval show two individual negative bands in the single-size unit cell, one of which exhibits a negative bulk modulus supported by the monopolar Mie resonance, whereas the other exhibits a negative mass density induced by the dipolar Mie resonance. The unique single-negative nature is used to develop an ultra-sparse subwavelength metasurface with high reflectance for low-frequency sound. We demonstrate a 0.15λ-thick, 15%-filling ratio metasurface with an insertion loss over 93.4%. The designed Mie resonators provide diverse routes to construct novel acoustic devices with versatile applications.

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The plasmon coupling in a three-layered Au/SiO2/Ag nanoshell has been investigated by means of Mie scattering theory. The dipole–dipole and dipole–quadrupole Fano resonances have been observed in the extinction spectra of Au/SiO2/Ag nanoshells. With increase of the thickness of the middle layer, the dipole–dipole Fano resonance shows a blue shift and the magnitude of this Fano profile enhances while the dipole–quadrupole Fano resonance shows a red shift and reduces. With increase of the thickness of the outer shell, the dipole–dipole Fano minimum shows a blue shift and its magnitude decreases while both the energy and magnitude of dipole–quadrupole Fano resonance increase. All of the behaviors have been discussed with the plasmon hybridization model. In addition, the Au/SiO2/Ag nanoshell is found to show strong near-field enhancements in several regions, especially in the infrared region, which may provide effective applications in surface enhanced spectroscopy.